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Lateral load paths in historic truss bridges

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Title:
Lateral load paths in historic truss bridges
Creator:
Rutz, Frederick R
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English
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xxviii, 337 leaves : ; 28 cm

Subjects

Subjects / Keywords:
Lateral loads ( lcsh )
Truss bridges -- Design and construction -- History -- United States ( lcsh )
Lateral loads ( fast )
Truss bridges -- Design and construction ( fast )
United States ( fast )
Genre:
History. ( fast )
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )
History ( fast )

Notes

Bibliography:
Includes bibliographical references (leaves 319-337).
General Note:
Department of Civil Engineering
Statement of Responsibility:
by Frederick R. Rutz.

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Source Institution:
|University of Colorado Denver
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|Auraria Library
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
61503239 ( OCLC )
ocm61503239
Classification:
LD1190.E53 2004d R89 ( lcc )

Full Text
LATERAL LOAD PATHS
IN
HISTORIC TRUSS BRIDGES
by
Frederick R. Rutz
BSCE, Tufts University, 1970
MSCE, University of Washington, 1971
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
Civil Engineering
2004


This thesis for the Doctor of Philosophy
degree by
Frederick Robert Rutz
fyS 2 l
Date
Yunping Xi


Rutz, Frederick R. (Ph.D., Civil Engineering)
Lateral Load Paths in Historic Truss Bridges
Thesis directed by Associate Professor Kevin L. Rens
ABSTRACT
This thesis examines lateral load paths in historic truss bridges. The
objective of the thesis is to ascertain if the deck of a representative historic truss
bridge could be rationally accounted for in structural analysis under wind load.
The historical background of truss bridge development was reviewed, as was
the historical development of design wind loads for bridges. Preservation issues
were reviewed and examples of adaptive reuse of historic truss bridges were
studied. Analytical studies were undertaken to investigate the influence of a deck
under wind loading, and field tests were completed to verify the analyses.
It was found that prevailing codes mandate higher wind pressures that those
used in the original design of historic truss bridges, which hampers efforts at historic
truss bridge rehabilitation. The traditional basis for lateral structural analysis was
found to be based on the presumption that the bridge structure acts as a skeleton
framework. This thesis explores the stiffening effect of the deck system, which has
the effect of reducing forces in the bottom chords when compared to analyses based
iii


on a skeleton. An analytical model was developed that demonstrated the stiffening
effect of the deck. The analytical model was verified by two field tests, one on a
truss bridge with timber stringers and a timber deck, and the other on a truss bridge
that had no stringers or deck, making it a true skeleton structure.
This abstract accurately represents the content of the candidates thesis. I
recommend its publication.
Signed
SEvin L. Rens
IV


DEDICATION
I dedicate this thesis to my wife, Denise Rutz, and to my parents, Richard and Mary
Rutz.


ACKNOWLEDGEMENT
A great many people assisted with this effort. I wish to acknowledge the
efforts of my advisor, Kevin Rens, who has supported this endeavor over the past
several years. Next, my thanks go to my graduate committee: George Hearn,
Michael Holleran, Tom Noel, Judy Stalnaker, and Yunping Xi. The instrumentation
used for the field testing was partially funded by a State Historical Fund grant award
from the Colorado Historical Society. Finally, and at the risk of omitting a worthy
contributor, I wish to acknowledge the help and support in one way or another of
Mohammad Abu Hassan, Dawn Arge, Ben Allen, Will Babbington, Steve Banks,
Dan Bechtold, Annette Beck, Bogus Bienkiewicz, Ben Blackard, Don Brown, Doug
Brown, Sam Brown, Don Carroll, Jack Cermak, Fred Chambers, N.Y. Chang,
Finley Charney, Ken Cobb, Saeedeh Chavoos, Joe Cullen, Eric DeLony, Nolan
Doeskin, William Edmands, Clayton Fraser, Doug Fredericks, Pete Gaby, Dario
Gasparini, Geraldine Gold, Mark Hamouz, Jim Harris, Frank Hatfield, Dale
Heckendorn, Teby Herrero, Tom Huston, Jim Ingls, Taewan Kim, Stan Kobiashi,
Diana Litvak, Teymor Manzouri, Mary McCahon, Paul Miller, Val Moser, Ed
Moss, Brent Norris, Sharon Norris, Lisa Schoch, Jon Peterka, Dean Peterson, Henry
Petroski, Yvonne Piquette, Jim Pringle, Judy Prosser-Armstrong, Joe Pularo, Bill
Rossman, Bill Rutz, Brett Rutz, Carl Rutz, Denise Rutz, Jeff Rutz, Jody Rutz, Mary


Rutz, Richard Rutz, Tracy Rutz, Ray Selbe, John Sinnreich, Jim Van Liere, and
Arn Womble.


CONTENTS
Figures................................................................... xvii
Tables.................................................................... xxvi
Chapter
1. Overview................................................................ 1
1.1 Introduction............................................................ 1
1.2 Application to Historic Truss Bridge Preservation....................... 1
1.3 Goal.................................................................... 2
1.4 Organization............................................................ 2
2. Historical Background................................................... 7
2.1 Introduction............................................................ 7
2.2 Evaluation.............................................................. 8
2.3 Historical Background.................................................. 11
2.3.1 Trajans Bridge........................................................ 11
2.3.2 Roof Trusses........................................................... 11
2.3.3 Palladio............................................................... 12
2.3.4 Early Timber Trusses................................................... 13
2.3.5 Engineering Education.................................................. 16
2.3.6 Academic Progress, 1800- 1865 ......................................... 16
viii


2.3.7 Railroads......................................................... 18
2.3.8 Ithiel Town........................................................ 18
2.3.9 Stephen Long....................................................... 20
2.3.10 William Howe....................................................... 24
2.3.11 Rider and Moulton.................................................. 26
2.3.12 Thomas and Caleb Pratt ............................................ 26
2.3.13 Squire Whipple..................................................... 27
2.3.14 Herman Haupt....................................................... 33
2.3.15 Early Texts........................................................ 34
2.3.16 The Coming Era of Engineered Bridges............................... 38
2.3.17 Wrought Iron....................................................... 39
2.3.18 Professional Organizations......................................... 41
2.3.19 Latrobe, Bollman, and Fink......................................... 41
2.3.20 John Murphy........................................................ 44
2.3.21 S.S. Post.......................................................... 45
2.3.22 New Challenges..................................................... 46
2.3.23 Steel.............................................................. 48
2.3.24 Dominance of the Pratt Truss....................................... 50
2.3.25 Growth and Stabilization of the Bridge Industry.................... 53
2.3.26 J.A.L. Waddell..................................................... 55
2.3.27 Pratt Truss Variants............................................... 56
IX


2.3.28 End of an Era...................................................... 57
3. Preservation Issues................................................. 60
3.1 Introduction........................................................ 60
3.2 Why Preservation? .................................................. 60
3.3 Legal Framework..................................................... 61
3.4 National Register of Historic Places ............................... 67
3.5 The Vanishing Truss Bridge.......................................... 69
3.6 The 5th Street Bridge Story......................................... 70
3.7 Conclusion.......................................................... 72
4. Adaptive Reuse...................................................... 75
4.1 Introduction........................................................ 75
4.2 Demolition.......................................................... 75
4.3 Abandonment......................................................... 77
4.4 Remain in Use ...................................................... 77
4.5 Relocation.......................................................... 80
4.6 Preserve in Park.................................................... 82
4.7 Convert to Pedestrian Use........................................... 85
4.8 Issues with Conversion to Pedestrian Use............................ 87
4.8.1 Damage and Deterioration............................................ 88
4.8.2 AASHTO Requirements................................................. 91
4.8.3 Secretary of the Interiors Standards............................... 93
x


5. Design Wind Load: 19th Century and Today............................ 95
5.1 Introduction ....................................................... 95
5.2 Wind Pressure: Unknown.............................................. 95
5.3 Whipple on Lateral Loads ........................................... 96
5.4 Wind and Bridge Disasters .......................................... 99
5.5 19th Century American and British Concepts of Wind Load............ 104
5.6 Summary of Wind Pressure Recommendations........................... 108
5.7 A Century of Wind Research..........................................Ill
5.8 Wind Load and Model Codes.......................................... 112
5.9 AASHTO Guidelines for Design of Pedestrian Bridges..................112
5.9.1 Todays Requirements............................................... 113
5.9.2 AASHTO Load Combinations........................................... 115
5.10 Literature Review of AASHO and AASHTO Specifications................116
5.10.1 1924 .............................................................. 117
5.10.2 1928 ............................................................. 118
5.10.3 1931 ............................................................. 119
5.10.4 1935 ............................................................. 120
5.10.5 1941 ............................................................. 121
5.10.6 1953 ............................................................. 121
5.10.7 1961 ............................................................. 122
5.10.8 1965, 1969, and 1973...............................................123
xi


5.10.9 1977 123
5.10.10 1992 124
5.10.11 1997 124
5.10.12 1998 125
5.10.13 Summary.........................................................126
5.10.14 Conclusion...................................................... 127
5.11 Comparison of 19th Century Design Load Criteria
to AASHTO Guide Specifications......................................127
5.12 Wind and Rehabilitation Project Examples............................128
5.13 Conclusion..........................................................130
6. Load Tests and Observations ........................................131
6.1 Introduction........................................................131
6.2 Gravity Loads: Load Tests vs. Analyses..............................132
6.2.1 Hubby Bridge........................................................132
6.2.2 Chestnut Ford Bridge................................................133
6.2.3 Walnut Street Bridge............................................... 134
6.2.4 Calhoun Street Bridge.............................................. 135
6.2.5 Six Truss Bridges in Maryland...................................... 136
6.2.6 Rock Creek Bridge.................................................. 136
6.2.7 Neligh Bridge ..................................................... 137
6.2.8 Valley Bridge ......................................................138
xii


6.2.9 Little Nemaha Bridge .................................................139
6.2.10 Perley Bridge .......................................................139
6.2.11 Pratt Truss in Ohio ..................................................140
6.2.12 Bridge Testing in Ontario ...........................................142
6.2.13 Bridge Rating Through Load Testing ..................................144
6.3 Lateral Analyses of Historic Truss Bridges ...........................145
6.3.1 Carrolls Investigations ............................................ 145
6.3.2 Seyednaghavis Investigations ........................................150
6.4 Fruita Bridge ....................................................... 153
6.5 Additional Reports ...................................................159
6.6 Skeletal Load Path .................................................. 159
6.7 Tension and/or Compression Effects from Gravity Load
vs. Wind Load ....................................................... 161
6.8 Current Practices ................................................... 161
6.9 Possible Alternative Load Paths ..................................... 163
6.10 Hypothesis .......................................................... 163
7. 3D: Models: Skeleton vs. Deck ....................................... 165
7.1 Introduction..........................................................165
7.2 Example.............................................................. 166
7.3 Traditional vs. Modern Analysis.......................................166
7.4 Loads................................................................ 167
xiii


7.5 Materials......................................................... 168
7.6 Modeling Considerations............................................168
7.7 Comparison.........................................................169
7.8 Conclusions........................................................180
7.9 Verification Testing...............................................180
8. Field Experimentation..............................................181
8.1 Introduction.......................................................181
8.2 Instrumentation System............................................ 182
8.3 Instrumentation Components.........................................182
8.3.1 Strain Transducers................................................ 183
8.3.2 Wheatstone Bridge..................................................184
8.3.3 Anemometers........................................................185
8.3.4 Wind Direction Sensor..............................................185
8.3.5 Interval Timer.................................................... 186
8.3.6 Data Logger....................................................... 187
8.3.7 Laptop Computer....................................................189
8.3.8 Software...........................................................189
8.3.9 Cables............................................................ 189
8.3.10 Thermometer....................................................... 190
8.4 Test Set-Up at Fruita Bridge...................................... 191
8.5 Data from Fruita Bridge Test.......................................195
xiv


8.5.1 Wind Speed Data................................................... 195
8.5.2 Strain Data....................................................... 198
8.5.3 Strain Computation.................................................205
8.5.4 Rolling Averages...................................................206
8.5.5 Zeroing the Strain Data............................................209
8.6 Test Set-Up at Four Mile Bridge....................................212
8.7 Data from Four Mile Tests..........................................221
8.7.1 Wind Speed Data....................................................221
8.7.2 Temperature Variation............................................. 223
8.7.3 Strain Computation................................................ 225
9. Analysis and Discussion............................................228
9.1 Introduction.......................................................228
9.2 Fruita Bridge: Test Data Reduction................................ 229
9.2.1 Forces in Bottom Chord Eyebars.................................... 229
9.2.2 Moments in Portal Frames...........................................231
9.2.3 Wind Pressure General Considerations.............................235
9.3 Fruita Bridge Analysis............................................ 236
9.3.1 Wind Load by Quadrants.............................................237
9.3.2 Truss Boundary Conditions......................................... 240
9.3.3 Member Releases................................................... 242
9.4 Variations in Pressure vs. Strain................................. 244
xv


9.5 Fruita Bridge Parametric Study.................................. 247
9.5.1 Boundary Conditions............................................. 247
9.5.2 Fruita Bridge Comparisons....................................... 247
9.5.3 Comments on Figures 9.12 9.21 ................................ 257
9.5.4 Discussion...................................................... 259
9.6 Four Mile Bridge: Test Data Reduction .......................... 262
9.6.1 Forces in Bottom Chord Eyebars.................................. 262
9.6.2 Moments in Portal Frames.........................................262
9.7 Four Mile Bridge: Analysis.......................................263
9.7.1 Wind by Quandrants.............................................. 264
9.7.2 Truss Boundary Conditions........................................268
9.7.3 Member Releases..................................................272
9.8 Four Mile Bridge: Parametric Study.............................. 273
9.8.1 Four Mile Bridge Comparisons.................................... 274
9.8.2 Comments on Figures 9.30 9.39 ................................ 282
9.8.3 Four Mile Bridge Discussion..................................... 284
10. Summary, Conclusions, and Recommendations for Future Research.. 286
10.1 Introduction ................................................... 286
10.2 Summary of Findings..............................................286
10.3 Conclusions......................................................288
10.4 Related Findings ............................................... 289
xvi


10.5 'Recommendations for Future Research
290
Appendix
A. Lost Truss Bridges of Colorado..................................293
B. Program for Datalogger..........................................314
References.............................................................319
xvii


FIGURES
Figure
1.1 Flow chart summarizing major events................................... 6
2.1 A busy 19th century bridge fabricating shop ......................... 10
2.2 Trajans bridge over the Danube ..................................... 12
2.3 Example of a truss bridge by Palladio ............................... 13
2.4 Burrs 1804 Hudson River Bridge at Waterford ........................ 15
2.5 Town lattice truss .................................................. 19
2.6 A wedge in a Long truss ............................................. 22
2.7 Excerpt from Stephen Longs 1830 patent drawing ..................... 23
2.8 Howe truss .......................................................... 24
2.9 Excerpt from the 1844 patent drawings for a Pratt truss ............. 27
2.10 Whipple bowstring truss, excerpted from the 1840 patent drawings ... 30
2.11 Whipple bowstring truss, from his 1847 book, A Work on Bridge
Building ............................................................ 30
2.12 Whipple trapezoidal truss, from an 1880 advertisement
in Engineering News ................................................. 31
2.13 Bollman truss ....................................................... 42
2.14 Fink truss .......................................................... 44
xviii


2.15 Post truss ........................................................... 45
2.16 A utilitatian, economic Pratt truss .................................. 52
2.17 Thirty bridge truss types ............................................ 59
3.1 Fifth Street Bridge over Grand River, Grand Junction, Colorado........ 71
3.2 A photograph showing both the original 1886 Fifth Street Bridge
and its 1933 replacement............................................ 72
3.3 San Miguel Bridge..................................................... 73
4.1 Eagle River Bridge ................................................... 76
4.2 Eagle River Bridge ................................................... 78
4.3 Dotsero Bridge in 1983 in a state of partial demolition .............. 78
4.4 Howe truss at State Bridge, Colorado ................................. 79
4.5 Collapsed truss at State Bridge, Colorado ............................ 79
4.6 Costilla Crossing Bridge over Rio Grande River ................. 80
4.7 Del Lado Rio Bridge over the Piedra River ............................ 81
4.8 An 1890s Pennsylvania truss bridge at Klamath, California ........... 83
4.9 Klamath Bridge relocated to Folsom, California ................. 83
4.10 Ft. Laramie Bridge ................................................... 84
4.11 Charlotte Highway Bridge ............................................. 84
4.12 The Keystone Bridge at Park County Historical Park
in Bailey, Colorado ................................................ 85
4.13 19th Street Bridge over South Platte River, Denver, Colorado ......... 86
xix


4.14 Walnut Street Bridge, Chattanooga, Tennessee ............................ 87
5.1 Collapse of the Tacoma Narrows Bridge................................... 101
5.2 Collapse of the Tay Bridge.............................................. 102
5.3 Forth Bridge over the Firth of Forth in Scotland.........................108
6.1 Influence line from load test vs. analysis example for the Hubby Bridge.. 133
6.2 Rock Creek Bridge after installation in Structures Laboratory
at University of Nebraska Lincoln.................................... 137
6.3 Eight-panel Pratt through truss tested by Aktan et. al.................. 141
6.4 View of buckled bottom chord eyebars at Blue River Bridge,
Silverthorne, Colorado................................................. 150
6.5 Fruita Bridge, Fruita, Colorado..........................................154
6.6 View of underside of Fruita Bridge, looking from south to north..........158
7.1 3D model of Fruita Bridge............................................... 167
7.2 Representation of superimposed gravity loads (D+L).......................170
7.3 Graphical representation of relative axial forces in the bottom chord
eyebars due to gravity loads for the skeleton structure................170
7.4 Wind pressure on the bridge............................................. 171
7.5 Graphical representation of axial forces in the bottom chord eyebars
due to wind for the skeleton structure..................................172
7.6 Shear and moment diagrams for the propped cantilever condition........ 173
7.7 Skeleton model with stringers added..................................... 174
xx


7.8 Skeleton model with stringers and deck............................... 175
7.9 Rendering of stringers on a steel floor beam..........................176
7.10 Offset members and release location.................................. 176
7.11 Rendering of the timber deck planks on timber stringers
on steel floor beam...................................................177
7.12 The deck treated as a single diaphragm, shown here as part of each bay.. 178
8.1 Schematic diagram of instrumentation system.......................... 182
8.2 Strain transducer.................................................... 183
8.3 Terminal Input Module (TIM) and full bridge wiring diagram........... 184
8.4 Anemometer and wind direction sensor................................. 186
8.5 Interval Timer....................................................... 187
8.6 Campbell Scientific CR5000 Data Logger............................... 188
8.7 Fruita Bridge over the Colorado River................................ 191
8.8 Location of Fruita Bridge............................................ 192
8.9 Diagram of Fruita Bridge, illustrating the locations of
anemometers (WS1 WS5) and wind direction sensor (WD)...............192
8.10 Diagram of Fruita Bridge, illustrating the locations of the
strain transducers....................................................193
8.11 Anemometer (WS1) and Wind Direction sensor (WD) installed
at Fruita Bridge .................................................... 193
8.12 Strain transducers installed on bottom-chord eyebars at Fruita Bridge ... 194
xxi


8.13 Strain transducer installed on portal at Fruita Bridge................ 194
8.14 Wind speed from two of the five anemometers over a
10-minute time period at Fruita Bridge............................... 196
8.15 Fruita Bridge: Wind direction over the same 10-minute period
as shown in Figure 8.14.............................................. 197
8.16 Detailed wind speed from the five anemometers......................... 199
8.17 Air temperature during Fruita Bridge test..............................200
8.18 Temperature effect on strain transducer................................202
8.19 Raw data from G5, unfiltered...........................................207
8.20 Same data from G5 filtered by use of a 0.5-second rolling average..... 208
8.21 The same G5 data, filtered by use of a rolling average over 3 seconds ... 208
8.22 The same G5 data, filtered by use of a 10-second rolling average.......209
8.23 Fruita Bridge: Differential strain data at leeward eyebars
over a 40-second period .............................................210
8.24 Fruita Bridge: Differential strain data at windward eyebars........... 211
8.25 Fruita Bridge: Differential strain data at south portal............... 211
8.26 Fruita Bridge: Differential strain data at north portal............... 212
8.27 Location of Four Mile Bridge ......................................... 213
8.28 Diagram of Four Mile Bridge, illustrating the location
of anemometers (WS1 WS5) and wind direction sensor (WD)............214
xxii


8.29 Diagram of Four Mile Bridge, illustrating the locations
of the strain transducers............................................ 214
8.30 Photograph of Four Mile Bridge........................................215
8.31 Strain transducers G5 G8 installed on the windward eyebars at
Four Mile Bridge .....................................................215
8.32 Strain transducer G15 and anemometer WS5 .............................216
8.33 One of two low anemometers......................................... 216
8.34 Wide flange beam at base of south portal............................. 217
8.35 Lifting beam at the north portal of Four Mile Bridge..................217
8.36 Overview of sail in place at Four Mile Bridge........................ 218
8.37 Sail subject to wind pressure from the west...........................219
8.38 View behind the sail..................................................219
8.39 Cable X-bracing in the plane of the bottom chords.................... 220
8.40 Four Mile Bridge: Wind Speed from two of the five anemometers
over a period of seven minutes...................................... 222
8.41 Four Mile Bridge: Wind direction over the same seven-minute
period as figure 8.40................................................ 222
8.42 Four Mile Bridge: Wind speed from five anemometers at the spike....224
8.43 Four Mile Bridge: Air temperature during the test period.............224
8.44 Four Mile Bridge: Differential strain data at leeward eyebars........226
8.45 Four Mile Bridge: Differential strain data at windward eyebars..... 226
xxiii


8.46 Four Mile Bridge: Differential strain data at south portal............227
8.47 Four Mile Bridge: Differential strain data at north portal............227
9.1 Strain transducer arrangement on bottom-chord eyebars................ 230
9.2 Strain transducer locations on the north portal...................... 232
9.3 Section of end diagonal with strain transducer....................... 232
9.4 Idealized portal frame................................................233
9.5 Quadrants subjected to different uniformly distributed
wind pressures.......................................................239
9.6 Wind pressure applied to the four quadrants for analysis..............240
9.7 Illustration of boundary conditions.................................. 241
9.8 End conditions of lateral X-bracing members.......................... 243
9.9 Nine spikes selected from the average wind velocity trace............ 245
9.10 Change in force in the leeward bottom chord eyebars vs. change
in average pressure due to wind......................................246
9.11 Member numbering at Fruita Bridge.....................................247
9.12 Fruita Bridge comparison for member M13.............................. 252
9.13 Fruita Bridge comparison for member M5................................252
9.14 Fruita Bridge comparison for moment at the top of member M23 .........253
9.15 Fruita Bridge comparison for moment at the bottom of member M23 .... 253
9.16 Fruita Bridge comparison for moment at the top of member M17..........254
9.17 Fruita Bridge comparison for moment at the bottom of member M17 .... 254
xxiv


9.18 Fruita Bridge comparison for moment at the top of member M26............255
9.19 Fruita Bridge comparison for moment at the bottom of member M26 .... 255
9.20 Fruita Bridge comparison for moment at the top of member M20............256
9.21 Fruita Bridge comparison for moment at the bottom of member M20 .... 256
9.22 Four Mile Bridge: North portal model................................... 264
9.23 Quadrants subjected to different uniformly distributed
wind pressures..........................................................267
9.24 Wind pressure applied to the four quadrants plus the sail for analysis .... 268
9.25 Four Mile Bridge: South portal model....................................269
9.26 Southwest bearing...................................................... 271
9.27 Southeast bearing...................................................... 271
9.28 Effect of rotational stiffness at south supports........................272
9.29 Four Mile Bridge member numbering...................................... 274
9.30 Four Mile Bridge comparison for member M16..............................277
9.31 Four Mile Bridge comparison for member M7...............................277
9.32 Four Mile Bridge comparison for moment at top of member M18............... 278
9.33 Four Mile Bridge comparison for moment at bottom of member M18 ... 278
9.34 Four Mile Bridge comparison for moment at top of member M9.................279
9.35 Four Mile Bridge comparison for moment at bottom of member M9... 279
9.36 Four Mile Bridge comparison for moment at top of member M17............... 280
9.37 Four Mile Bridge comparison for moment at bottom of member M17 ... 280
xxv


9.38 Four Mile Bridge comparison for moment at top of member M8.......281
9.39 Four Mile Bridge comparison for moment at bottom of member M8 281
xxvi


TABLES
Table
2.1 Growth of Bridge Companies in the United States ................... 8
2.2 Notable Timber Trusses of the Late 1700s and Early 1800s ....... 15
2.3 Dates when Civil Engineering Courses were Introduced at Various
American Colleges ................................................ 17
2.4 Partial List of Truss Bridge Patents.............................. 53
3.1 Breakdown of Properties Listed on the National Register
of Historic Places................................................ 68
5.1 Implicit Wind Pressure Based on Whipples Recommendations
for Sway Rods and Braces....................................... 98
5.2 Some Truss Bridge Failures Attributed to Wind.....................103
5.3 Design Wind Pressure Examples in 1881 ........................... 106
5.4 Summary of Wind Pressure Recommendations from the
Late 19th Century and Early 20th Century .........................109
5.5 Summary of Basic Wind Pressure Requirements for Truss Bridges
at Normal Levels of Stress....................................... 126
7.1 Summary of Maximum Axial Compressive Forces in Bottom Chord
Eyebars.......................................................... 179
xxvii


8.1 Resistance of Lead Wires............................................... 204
9.1 Fruita Bridge WindVelocities, Quadrant Average Velocities,
and Quadrant Pressures.................................................239
9.2 Boundary Conditions.....................................................242
9.3 Releases at the Ends of Internal Members............................... 243
9.4 Change in Wind Pressure vs. Change in Force for Various Spikes..... 246
9.5 Fruita Bridge: Summary of Member Forces.............................248
9.6 Fruita Bridge: Summary of Member Forces.............................249
9.7 Fruita Bridge: Summary of Member Forces.............................250
9.8 Legend for Graphs Shown in Figures 9.12 - 9.21 ...................... 251
9.9 Four Mile Bridge: Wind Velocities, Quadrant Average Velocities,
And Quadrant Pressures.................................................267
9.10 Four Mile Bridge: Summary of Member Forces........................ 275
9.11 Four Mile Bridge: Summary of Member Forces........................ 275
9.12 Four Mile Bridge: Summary of Member Forces........................ 276
9.13 Four Mile Bridge: Summary of Member Forces........................ 276
9.14 Legend for Graphs Shown in Figures 9.30 - 9.39 ...................... 276
xxviii


1.
Overview
1.1 Introduction
The basis for structural analysis of truss bridges has remained fundamentally
the same for over a century. A skeleton frame is analyzed, without taking credit for
non-structural items such as the stringers and the deck. While the techniques -
manual calculation vs. computer analysis have changed, the basis a skeleton
structure has remained essentially the same. Yet alternative load paths present
themselves: The deck will tend to stiffen the bridge in both the vertical and the
lateral directions. The stringers will tend to supplement the bottom chords in
carrying both gravity loads and lateral loads.
1.2 Application to Historic Truss Bridge Preserv ation
Engineers attempting to preserve historic highway truss bridges by
conversion to pedestrian use often discover that the old structures appear to have
insufficient lateral strength to resist the wind load requirement imposed by the
current code, the AASHTO Guide Specifications for the Design of Pedestrian
Bridges (AASHTO 1997). However, increased lateral strength can be ascertained
if non-traditional load paths are considered in the analysis. Research at the
University of Colorado at Denver has been initiated to examine these possibilities
1


(Rutz and Rens 2004; Carroll 2003; Herrero 2003). This thesis addresses the issue
by examining non-traditional lateral load paths, which could be used
advantageously in historic truss bridge rehabilitation.
1.3 Goal
The overall goal of this study is to examine the effect of a deck on the
response to wind load for representative historic truss bridges, both analytically and
by tests on actual structures. In order to accomplish this goal, a number of steps,
summarized in the flow chart of Figure 1.1, were undertaken.
1.4 Organization
This thesis contains ten chapters. This first chapter introduces the goal and
scope of the project. The second through sixth chapters comprise the background
portion of this work. The seventh through tenth chapters describe the research and
findings in detail.
Chapter 2 reviews historical background for metal truss bridges. It contains
a narrative of events significant to the development of the historic truss bridge types
that survive today.
?


Chapter 3 deals with preservation issues. It contains a summary history of
the legal framework significant to historic preservation in general, and to bridges
specifically. An example of the ongoing cycle of construction, service, removal and
replacement is presented.
Chapter 4 discusses options for adaptive reuse of truss bridges. Some of the
challenges to preservation of the truss bridges are presented.
Chapter 5 presents a history of wind loads used in bridge design from the
19th century through today. 19th century concepts for design wind pressure are
discussed because these are the bases for which the bridges were originally
designed. A historical review of AASHTO and predecessor wind load requirements
is presented. Some examples of the significance of wind load and its effect on
preservation of truss bridges is presented.
Chapter 6 focuses on load tests that have been conducted on truss bridges.
While these tests have investigated gravity loads, inferences for lateral responses
can be drawn. Observations of the Fruita Bridge, near Fruita, Colorado, are
presented. The concept of the skeletal load path vs. actual load path for lateral loads
is presented.
3


An example of the problem of an existing historic truss bridge that cannot
satisfy modern wind load requirements is presented in Chapter 7. The traditional
skeleton analytical model for lateral loads is compared to a non-traditional
analytical model that includes the stringers and deck.
Reports of two field experiments, conducted to verify the analytical
methodology of Chapter 7, are made in Chapter 8. One test was at the Fruita
Bridge, a truss bridge with timber stringers and a timber deck. The other was at the
Four Mile Bridge, an abandoned truss bridge without any stringers or deck;
physically it is a true skeleton.
In Chapter 9 the field data from both experiments is reduced and compared
to analyses that were based on the method of Chapter 7, but adjusted to account for
specific test conditions.
Chapter 10 provides summaries of the findings and makes recommendations
for future research.
Supplementary information is included in appendices. A photographic
record of Colorado truss bridges that have been removed during the past twenty
4


years is included in Appendix A. Appendix B contains the software program that
was used with the data logger in the field experiments.
5


NCPTT Grant ;
Awarded (2004) 1
Continuing
Research
Figure 1.1. Flow Chart Summarizing Major Events
6


2.
Historical Background
2.1 Introduction
1876 was a busy year for the Wrought Iron Bridge Company of Canton,
Ohio (Wrought Iron Bridge Co. 1876). Their plant was running at full capacity
with 160 men at work 11 hours a day with up to 100 additional employees busy
erecting bridges at various places around North America. Among the bridges then
underway were: a 440 foot-long truss bridge at Ottawa, Canada; a 500 foot-long
truss bridge at Columbus, Ohio; a 240 foot-long truss bridge at Newcastle,
Pennsylvania; a 700 foot-long truss bridge at Junction, Ohio; a 680 foot-long truss
bridge at Utica. Illinois; a 300 foot-long double-arch bridge at Iowa City, Iowa; a
200 foot-long swing bridge at Houston, Texas; and a 500 foot-long truss bridge at
Salamanca. New York. The future looked bright: during the month of June
alone, they received 80 contracts for new bridges. A typical bridge fabrication
shop is shown in Figure 2.1.
The Wrought Iron Bridge Company, swamped with new orders, was only
one of about one hundred bridge companies in 1876. The bridge business was
7


booming in late 19th century America (DeLony, 1994a). Table 2.1 summarizes the
growth.
Table 2.1 Growth of Bridge Companies in United States (Fraser,
1986a)._______________________________________________
Year Number of Bridge Companies
1860 5 companies
1870 75 companies
1890 137 companies
1900 190+ companies
2.2 Evaluation
How had the bridge industry come to be so busy? The answer can be found
in three powerful forces that came together in 19th century United States, resulting
in unprecedented advances in a new technology truss bridges. These forces are
summarized below:
Materials Initially, timber bridges were suitable for wagon traffic
and livestock loadings; timber was abundant in the forested eastern
United States, and timber trusses offered an economic solution.
Later, as timber supplies were reduced and as loadings increased,
the new materials of cast iron, then wrought iron, and finally steel
became available. They were rapidly utilized as they satisfied the
demand for ever-stronger bridges of ever-increasing length.
8


Theoretical Advances in mathematics of the 18th century
continued and were applied to engineering structures in the 19th
century. Analytical techniques for rational analysis of trusses were
developed. These methods became available to students through
technical colleges and to practicing engineers through the
distribution of books and periodicals.
Economic A huge demand for bridges came from westward
expansion in the 19th century United States. This led to the need for
many roads with crossings for thousands of rivers and waterways
over a vast area. Truss bridges fulfilled the need for economical
crossings. The first American railroad, the Baltimore & Ohio, was
introduced in 1829 at Baltimore (Jacobs 1989). As railroad engines
became larger and heavier, this was coupled with the need for
bridges of ever-increasing strength1. Further, the expanding
railroads provided a means of transport for heavy iron and steel
fabrications.
1 Railroad loads increased from 22,000 pounds for early locomotive engines on the
Baltimore & Ohio Railroad of the 1830's to 840,000 pounds for a Mallet locomotive plus
tender by 1915.
9


The convergence of economical materials, structural theory, and large
demand led to rapid advances in the development of truss bridge technology, which
exploded in the United States during the second half of the 19th century.
The Pratt truss and its several derivatives would evolve into the most
common of all the truss types used during this period. The steel Pratt truss remains
the most common type to have survived to the present day (Comp and Jackson
1977).
Figure 2.1. A busy 19lh century bridge fabricating shop in East Berlin, Conneticutt.
Bridge members can be seen in various stages of fabrication. Numerous shop-
installed rivets were used to connect the pieces of built-up members. Many
overhead cranes and hoists facilitate the work. Narrow gage railway tracks serve
the interior. Large windows along the walls provide lighting. (Plant of Berlin Iron
Bridge Co., 1891).
10


2.3
Historical Background
Events significant to the development of the truss bridge are discussed
below.
2.3.1 Trajans Bridge
Construction technology in the West may be summarized as stone masonry
arches, stone masonry posts and lintels, and timber construction. There is one
example of timber trussing in a bridge from early in the 2nd Century. Apollodoros
of Damascus, engineer for the Roman Emperor Trajan (Kirby et. al. 1990a), used
timber cross bracing (trussing) in his bridge over the Danube, shown in the
background of Figure 2.2.
2.3.2 Roof Trusses
The stone masonry bridge tradition was kept alive in Europe by the Fratres
Pontifices (Brothers of the Bridge) Order of Benedictine monks (Hayden 1976a).
Trusses constructed of timber were used to support roofs. Notable among them
were roofs of cathedrals. However, because no one knew how to analyze a truss,
they were often cluttered with inefficient extra members that made no contribution
to strength (de Camp, 1963a). Instead, truss designs came from successful
experience, not mathematical analysis.


Figure 2.2. Trajans bridge over the Danube. The illustration is from a relief in
Trajans Column in Rome (Schreiber 1895). The bridge appears to be solidly built,
with five timber arches springing from the tops of stone piers. The arches appear
to contain truss elements. The approach on the left is a viaduct, of which two
arches can be seen. The right approach is the embankment of the river. In the
foreground, the Emperor offers a sacrifice. The bridge was built to support Trajan's
conquest of Dacia (present day Romania).
2.3.3 Palladio
In the 1570s the Renaissance architect Andrea Palladio designed and built
several truss bridges in Italy. He described his work in II Quatro Libri
deUArchitettura (Four Books on Architecture), written in 1581 (Palladio 1965).
One of Palladios bridge trusses is shown in Figure 2.3.
12


1)1
Pallidios truss designs became more widely known when his II Quatro
Libri dellArchitettura was, after nearly 200 years, translated into English in 1740.
2.3.4 Early Timber Trusses
Timber truss bridges began to appear in the 1700s. The famous
Grubenmann brothers of Switzerland built a timber truss bridge in 1758 which,
combined with an arch, spanned 61 meters (200 feet)2. The Grubenmanns longest
span was 119 meters (390 feet). It was clearly not analyzed, because it consisted of
a ponderous combination of long and short braces extending from many points to
the abutments. Their work has been described as a maze of timbers, scarfed,
bolted, strapped, and clamped together to form nondescript trusses (Fletcher and
2 It was burned by the French during the Napoleonic War in 1799.
13


Snow 1932a). Nevertheless, the brothers Grubenmann were pioneer builders in the
use of timber trusses in bridges.
The first significant timber truss bridge built in America was the
Geometry-work Bridge, built across Shetucket River near Norwich, Connecticut
by John Bliss in the 1760s.
Three notable American bridge builders built timber trusses in combination
with timber arch bridges. They were Timothy Palmer, Louis Wernwag, and
Theodore Burr. All three built magnificent bridge structures, all used trusses in
combination with arches, and all based their designs on successful experience,
rather than mathematical analysis. Palmers bridges were really shallow arches
with trusses to stiffen the arch. Burrs trusses were self-supporting; for longer
spans he added a solid arch in combination with the truss, because the truss sagged
too much without an arch. Wernwag used both methods. Wernwag. who had an
eye for permanence, adopted the practice of using only well-seasoned heartwood
and spaced the members of built-up sections so air could circulate for drying.
Table 2.2 lists some of the notable bridges by these early builders. A combination
arch truss is shown in Figure 2.3.
14


Table 2.2. Notable Timber Trusses of the Late 1700s and Early 1800's. This is a
partial list.
Builder Date Bridge Max. Span
Palmer 1792 Essex-Merrimac, at Newburyport, MA 49 meters (160 feet)
1794 Piscataqua Bridge (The Great Arch) at Portsmith, NH 55 meters (180 feet)
1804-06 Schuylkill River (Permanent Bridge) at Philadelphia, PA 59 meters (195feet)
Burr 1804 Hudson River at Waterford, NY 55 meters (180feet)
1804-06 Delaware River at Trenton, NJ 62 meters (203 feet)
1812-16 Susquehanna River at Harrisburg, PA 64 meters (210 feet)
Wernwag 1812 Schuylkill River (Colossus Bridge) at Philadelphia. PA 104 meters (340 feet)
1814 Delaware River at New Hope, PA 53 meters (175 feet)

Figure 2.4. Burrs 1804 1 ludson River Bridge at Waterford, New York.
It is a timber combination arch-truss bridge (Cooper, 1889a).
15


2.3.5 Engineering Education
Schools devoted to training engineers began to be established in the 18th
century. The French embraced formal education in contrast to the British
emphasis on apprenticeship and experience so it is logical that the first technical
schools were founded in France. The year 1747 brought the Ecole des Ponts et
Chaussees, the oldest academic institution in the world for engineering education.
It sprang from the far-sighted French government policy of 1716, which
established the Corps des Ingenieurs des Ponts et Chaussees, the first national
department of transportation (DeLony 1992a). In 1794 Napoleon established
LFxole Polytechnique, the French national technical university. The French
example was copied throughout the world, and became the model for engineering
programs in the United States (Turner 1996).
2.3.6 Academic Progress, 1800 1865
In 1802 the U.S. Military Academy was founded at the former
Revolutionary War fort at West Point, New York. By 1823 the Academy offered
civil engineering topics such as the construction of bridges, roads, and canals.
Graduates of West Point did much of the engineering work in the United States
during the first half of thel9th century (Turner 1996). The first non-military
school to offer a significant engineering program was the Rensselaer School, now
known as Rensselaer Polytechnic Institute, or RPI. founded in 1824 at Troy New
16


York. The demand for civil engineers was such that more schools began to follow
this lead and civil engineering programs began to appear at other institutions.
Examples are summarized in Table 2.3.
Table 2.3 Dates when Civil Engineering Courses were
Introduced at Various American Colleges (Watkins 1891a;
Turner, 1996)_________________________________________
School Year
West Point 1823
Rensselaer School 1824
University of the City of New York 1834
University of Alabama 1837
Virginia Military Institute 1839
Union College 1845
Harvard College 1846
Yale College 1846
Dartmouth College 1851
Lehigh University 1864
Columbia College 1865
Massachusetts Institute of Technology 1865
Mathematics continued to advance. 1826 brought Claude Naviers theory
of beam deflection and his double integration method (Timoshenko 1953a).
Experimental work brought progress too. Eaton Hodgkinsons 1840
experiments on cast and wrought iron columns led to constants for the Euler
equation, used to determine the buckling load for columns. Eewis Gordon
developed simplified equations for columns, which were widely adopted because
17


of their ease of use, even if they were not theoretically accurate (Merrill 1870a).
In the 1850's the firm of Plymton & Murphy made a model truss with spring
balances so loads could be measured. Barre de Saint Venant, a student of Navier,
developed an accurate analysis of elastic beams at yield and ultimate strength in
1857 (Timoshenko 1953b).
2.3.7 Railroads
The first railroad- using horse-drawn cars was built in 1805 in England
(Watkins 1891b). Steam locomotives, notably George Stephensons Rocket, were
first developed in England. Steam locomotives were introduced into the United
States in 1829 at Baltimore with the Baltimore and Ohio Railroad, widely known
as the B&O Railroad (Jacobs 1989). The first railroad bridge for the B&O
Railroad was for an engine that weighed 22,000 pounds. The advent of railroads
accelerated the westward expansion of the United States, creating demand for more
bridges of increasing strength. For the remainder of the century, new
developments in bridges came in large measure from the demands of railroads.
2.3.8 IthielTown
Ithiel Town, a New Haven, Connecticut inventor, developed a timber lattice
truss, and was granted a patent for it in 1820. This is significant as the first all
truss bridge, that is, a truss bridge that stood without a supplementary arch. It
18


was comprised of numerous sawn boards oriented diagonally to each other. Figure
2.5 shows a Town lattice truss. Even if anyone knew how to perform a structural
analysis, there was no practical way to do so because it was highly indeterminate.
While the Town lattice truss had a tendency to become unstable owing to its
relatively thin cross section, it became popular because of its simplicity. It could
be constructed from readily available materials by unskilled labor using ordinary
tools. This was an asset in the rapidly growing country, where bridges were
needed far from centers of skilled mechanics and fabrication facilities. Town was
more of a promoter and salesman than a builder, and made his living selling rights
to build his design. The Town lattice truss proved economical for many years, and
was still being built in the 1890s (Fletcher and Snow 1932b). This is often
considered the first all American truss bridge type.
19


CtDAR SHtN-'AE ROC* boards
root ^hc^**ng boards
LUMPtR C0U* ne (1YPJCAU
cjubcr raptr
i.UUBE TOP OORO (TtPCAl)
M£Ul ROC- T0P i.*T£R*L
. ypt>f BOTTOM CHORD
-LOWfR BOTTOM i>0*0
- BtO *Ma'.S (''C*l :
flfAR-Nt, TIMBER OOo&iC lATfR MOOT
P-wANK OtC*
RQURE 1
FLOOR SYSTEM
supeemxmjfc fkamnc
TOWN LATTICE TWJ88
Figure 2.5. Town lattice truss. Towns truss was patented 1820. (Rogers and
Canham 2001a).
2.3.9 Stephen Long
Stephen Harriman Long brought his Dartmouth college education to the
Armys Corps of Topographical Engineers for nearly 50 years. Me taught
mathematics at West Point in 1814-15. The Army assigned him to lead
explorations of the west, which he did for seven years (1817- 1824)3. Long
worked on roads, canals, river navigation, railroads, bridges, buildings,
locomotives, and steamships. When appointed to the board of engineers of the
3 One of these expeditions brought him to northern Colorado. Longs Peak, the highest
mountain in Rocky Mountain National Park, bears his name, as does the City of Longmont,
Colorado.
20


B&O Railroad in 1827, he began to direct his attention to truss bridges. (Gasparini
and Simmons 1997a).
For the construction of truss bridges, timber was still the practical material
of choice in the United States. It is presumed that Long, the mathematician,
conducted some early analyses of trusses. The X-bracing of trusses of the
Palmer/Burr/Wernwag type presented a shortcoming: the members in tension
tended to be ineffective because they pulled away from their connections. Among
Longs contributions was precompression of the tension members. (This is an
early from of prestressing, although Long did not call it that.) Precompression was
achieved by driving wooden wedges into the connections at either end of the
tension members. Long noted that the wedges were essential to the trussing and
stiffening of the bridge, and that the builder should drive them as hard as they
may be driven, with an ax or sledge weighing 4 or 5 pounds (Gasparini and
Simmons 1997a). Once a member was compressed to a level beyond that of the
tension developed under load, the bridge could rely on it as an effective load-
carrying component, allowing the truss to function as a cohesive unit. This
strengthened the timber truss. Long eliminated the arch, which had formerly been
combined with the trusses of his predecessors. He obtained patents for truss
bridges in 1830, 1836, and 1839. Figure 2.6 shows a portion of Long's patent
21


drawings from 1830. One of his wedges is shown in Figure 2.6. A long truss is
shown in Figure 2.7.
Figure 2.6. A wedge in a Long truss. It was used to precompress a
member which would be otherwise in tension. (Gasparini and Simmons
1997a).
22


Figure 2.7. Exerpt from Stephen Longs 1830 patent drawing. (Gasparini and
Simmons 1997a).
Although Longs trusses were statically indeterminate, he proportioned the
chords using Naviers analogy of a parallel-chord truss as a beam. That he was
familiar with Navier's work suggests Long was probably the first to base actual
truss designs on analytical methods. (Gasparini and Simmons 1997a).
23


2.3.10 William Howe
The Long truss had drawbacks. The precompressing depended on the
wedges, and the wedges could work loose. Consequently, frequent redriving of the
wedges was necessary, which made a Long truss high-maintenance.
William Howe, in 1840, patented his improvement over Longs design.
Instead of wedges, Howe prestressed the diagonal members by means of wrought
iron rods. Tensioning of the rods resulted in precompressing the tension diagonals.
This precompressed the compression diagonals too, but because wood was
cheap, that didnt matter larger compression members could be provided. A
Howe truss is shown in Figure 2.8.
Figure 2.8. Howe truss. (Gasparini and Simmons 1997a)
24


Like Longs truss, the Howe truss was statically indeterminate. The
structural behavior of the two trusses is similar the tensioning by rods served
exactly the same purpose as Longs wedges. However, prestressing by tightening
nuts was easier and more permanent than driving wedges. Howes design made the
Long truss obsolete (Gasparini and Simmons 1997a). Further, components of the
new truss timbers and rods could be standardized and even prefabricated.
Another advantage was that damaged members could be replaced with comparative
ease. The Howe truss became adopted as the dominant form of its time. Howe
trusses continued to be built into the 20th century. They were used where
construction simplicity and speed of erection time were paramount criteria, such as
by the U.S. Army in France during World War I (Jonah 1921).
The Howe truss also had the advantage that it could be adapted into all-iron
bridges. Railroads began experimenting with all-iron versions. For these, the
compression members were fabricated from cheaper cast iron, which was suitable
for compression forces only and the rods from the more expensive wrought iron,
which could resist tensile forces. The tension diagonals were precompressed by
the rods, so all the diagonals could be made from cast iron. Thus, an economical
all-iron truss came about.
25


Howe sold the patent rights to his brother-in-law, Amasa Stone, who
formed a new bridge company around the new design.
2.3.11 Rider and Moulton
Several variations appeared in short order. Nathaniel Rider, in 1847,
developed an all-iron single-intersection version whereby the verticals were
prestressed by wedges. Because of the similarity to the Long Truss, a patent
infringement squabble ensued. The trusses became known as Long-Rider Trusses
after Riders death in 1848 (Gasparini and Simmons 1997a).
Stephen Moulton, also in 1847, introduced an all-iron double-intersection
version, also with the verticals prestressed by wedges. It is not clear if he avoided
patent infringement because his truss was double-intersection (Longs was not) or
because he obtained his patent in England. However, neither of these trusses
achieved the wide popularity of the Howe Truss. (Gasparini and Simmons 1997a).
2.3.12 Thomas and Caleb Pratt
In 1844, Thomas and Caleb Pratt created a Howe variation whereby the
vertical members became compression elements, prestressed by diagonal rods.
This is a Howe truss in reverse. An early Pratt truss is shown in Figure 2.9.
Wood was initially used for the vertical compression members. But there were
26


twice as many nuts to tighten as on the Howe Truss, and the nuts were inclined,
which resulted in a troublesome detail. Consequently, the Pratt development was
initially less attractive than the simpler Howe, at least for wood/iron combinations
Although the Pratt truss was off to a slow start, it would ultimately surpass all of
the others.
Figure 2.9. Excerpt from the 1844 patent drawings for a Pratt truss. (Brock 1882).
2.3.13 Squire Whipple
Squire Whipple has been called the father of iron bridges (Griggs 2002a).
This sentiment is warranted for the inventor of the bowstring truss and the
trapezoidal truss. But his introduction, for the first time, of a practical method for
structural analysis of trusses was an even more lasting contribution. Whipple has
been described as the retiring and modest instrument maker, who, without
27


precedent or example, evolved the scientific basis of bridge building in America
(Schneider 1905).
Whipple, whose father ran a cotton spinning mill, graduated from Union
College, Schenectady, New York, in 1829 after taking the one-year scientific
curriculum, lie worked variously for railroads, on the enlargement of the Erie
Canal, and in building instruments such as theodolites and drafting equipment.
Thus he had both a scientific education and practical experience in engineering and
construction when he entered the bridge building business (Griggs 2002a).
His 1841 patent includes three truss bridges, one of which is his famous all-
iron bowstring truss. It is not clear if Whipple was aware of Palladios arch
designs, but his three patented bridges are all geometrically similar to timber
designs by Palladio (Griggs 2002b). Is this the result of independent thinking by
two men of genius? Or had Whipple been exposed to, and influenced by, the work
of Palladio? Either way, Whipple revolutionized the field of bridge building.
The new bridge utilized cast iron for the compression-carrying top chord
and verticals, and wrought iron for the tension diagonals and lower chords.
Whipples cast and wrought iron bowstring truss was intended for crossings of the
newly enlarged Erie Canal, and a number of them were built for that purpose.
28


Whipples 1841 patent drawing of his bowstring truss, shown in Figure
2.10, differs from the drawing shown in Figure 2.11, taken from his 1847 book, A
Work on Bridge Building (Whipple 1847a). The truss from the patent drawing,
with double verticals, is statically indeterminate. It also shows a foundation thrust
block as if the abutment were subject to horizontal thrust (it is not). The drawing
from his book shows single verticals and no thrust block. Could it be that Whipple
had not developed his method of joints in 1841 and that his bridge patent design
followed previous experience but by 1847 he had analyzed the bridge using his
method of joints, and convinced himself that the thrust block was unnecessary?
Both drawings have rod X-bracing or panel ties. Today we would call them
tension only members; they do not introduce significant indetermancy. Whipple
notes an understanding of this fact in his discussion of the analysis of a canceled
(X-braced) truss: the diagonals ... act by thrust ... in this condition of the load,
while the rest are relaxed and useless (Whipple 1847b).
29


Figure 2.10 Whipple bowstring truss, excerpted from the 1841 patent drawings
(Griggs, 2002c). The pairs of vertical members make the structure statically
indeterminate. The drawing shows a seemingly unnecessary foundation thrust
block. A timber variation is included in his Fig. 9, Side View of a Wooden
Truss. Note the similarity of that design to Palladios timber truss of 1570, shown
in Figure 2.3.
Figure 2.11. Whipple bowstring truss, from his 1847 book, A Work on Bridge
Building (Whipple 1847c). By 1847 the double verticals from the 1841 patent
drawing were gone, as was any suggestion of a foundation thrust block.
30


In 1846 Whipple introduced a trapezoidal truss, which had a double
intersection system of diagonals and inclined end posts. Again, it was a first for its
type, and his design served as a starting point for the use of this general type of
truss (Edwards 1933). Similar to his bowstring truss, it featured cast iron for the
compression components and wrought iron for those in tension. It was intended for
heavier use on railroad bridges. The first such bridge was built in 1853 near Troy,
New York.
KING IRON BRIDGE & MANUFACTURING CO,
CLEVELAND, OHIO.
ZEN AS KING. President. PAID-IN CAPITAL i HARLEY Ii. GIIJBS. Secretary.
JAMES A. KJN'o. Vice-Pres. T $225,000. < ALBERT II. PORTER. Engineer.
Figure 2.12 Whipple trapezoidal truss, from an 1880 advertisement in
Engineering News. By then the truss was known as the Whipple-Murphy truss.
Murphy had added pinned connections to Whipples trapezoidal truss. (King Iron
Bridge 1880).
Squire Whipple did more than build bridges. In 1847 he wrote the first
published book on structural analysis of trusses, A Work on Bridge Building. His
method was both elegant and practical, and its influence was widespread. He


treated all members of a truss as if connected by hinges, so each member was
subject to axial forces alone, without flexure. His method permitted rational
analysis of what had formerly been empirical knowledge or estimates. Today, we
call Whipples technique the method of joints. It is still used. It is considered to
have ushered in the era of scientific bridge building (Plowden 2002a). Whipple
used his method in his own designs, but clearly recognized the broader significance
of his work (Whipple 1869):
The Original Publication was a pioneer effort.. .It is
believed that no previous attempt had been
successfully made...to determine, by exact calculation,
the forces acting upon the various parts of such
structures; & to deduce thence, the proper sizes and
proportions of
such parts, upon known and reliable principles.
The method of joints is not a general method for analysis. It was later
recognized that for certain configurations the analyst must cut a section to create a
free body diagram, this to supplement the method of joints for particular cases.
But it does constitute the necessary first step, and does work unassisted for many
truss configurations. In his 1847 book, Whipple applied his method to trusses of 2,
3, 5, and 7 panels. All of his analyses for these trusses were correct.
32


2.3.14 Herman Haupt
Like Whipple, engineer and bridge designer Herman Haupt was also
interested in finding a rational basis for bridge design. Upon entering the bridge
building field he apparently looked for a mathematical basis for design but found
none. In his 1841 pamphlet (which was published anonymously), he stated
(Plowdon 2002a):
To my great surprise I found no attempts were made to
make calculations and the strain sheets showing
documentation and magnitude of strains were entirely
unknown.
Haupt also wrote a book in 1847, but was unable to get it published until
1851. His General Theory of Bridge Construction, like Whipples book, presented
a rational method for analysis. It appears that neither Whipple nor Haupt knew of
the other's efforts. Haupt, in his preface wrote (Haupt 1851):
If ... any work exists, containing an exposition of a
theory sufficient to account generally for the various
phenomena observed ... the author has never heard of
it.
It must be concluded that each man developed the analytical methodology
independently. While Whipples treatment is the more clearly presented, Haupt
deserves great credit for his contribution.
33


During the Civil War (1861-1865), timber was used extensively for bridges
because of the need for fast construction. Haupt served the Union as chief of the
bureau in charge of military railroads. The rebuilding of destroyed bridges in
record time was one of his principal duties.4 (McPherson 1988).
In 1864, while the Civil War was still raging, Haupt wrote Military Bridges,
which had designs for trestle and truss bridges. Haupt included designs for Howe-
type bridges, a combination arch-truss bridge, and a curious combination half
parabolic arch-truss bridge (Haupt 1864). Clearly all of these bridges were
intended for quick assembly. Military Bridges, unlike his 1851 text, is easy to
read, clear, and practical.
2.3.15 Early Texts
Following the lead of Whipple and Haupt. others took up the method of
joints. Texts began to appear with the effect that the rational method of analysis
reached the classroom and the practicing engineer. The days of empirical design,
from its beginnings in ancient times, were coming to an end. The design of bridges
4 One of Haupts constructions was a trestle bridge 80 feet high and 400 feet long, built by
unskilled soldier labor using green logs and saplings in under two weeks. Abraham Lincoln
admired it. saying, That man, Haupt, has built a bridge... over which loaded trains are
running every hour, and upon my word, gentlemen, there is nothing in it but beanpoles and
cornstalks. (McPherson 1998).
34


would leave the realm of the craftsman. Engineering would rise to the status of a
profession.
Robert Henry Bow's work, A Treatise on Bracing with its Application to
Bridges and Other Structures of Wood or Iron, was published in 1851 in
Edinburgh. By bracing, he meant what we would call trussing. His
mathematics was simplistic and is hard to follow at times. His principal thesis
seemed to be to promote triangular geometries, having sides that are
unchangeable...is a completely braced form. He presented a large number of
figures illustrating many different braced geometries (i.e. trusses) for roofs and
bridges, all with triangular forms, as well as a collection of connection details for
iron construction. (Bow 1851). Presumably all of these details came from actual
practice in the British Isles.
In Skeleton Structures, published in 1867. Olaus Henrici endorsed what we
would call pin-connected trusses. In his plans the structures were composed of
elementary bars, which were connected at their extremities by round bolts. This
book, which appears to have been intended as a text, contains relatively elementary
explanations of the determination of axial forces in truss members. He expounded
on the advantages of the pin-connected system: Axial tension or compression
members were efficient, calculation of forces could be accomplished with utmost
35


exactness, large structures could be erected, and such structures were well suited
for iron and steel (presumably a reference to ease of manufacture of iron and steel
connections, compared to those of wood). To help support his arguments, he
discussed the material inefficiencies of flexural members (Henrici 1867). The
book was technically simple, and seems to have been directed toward the student.
James Clerk Maxwell of Scotland also found a method to determine the
forces in each member of a truss. It would appear that he had heard of neither
Whipple nor Haupt. In 1864 he took this further; he developed a method for
determining deflections of trusses and, using it, analyzed statically indeterminate
trusses. Maxwell presented this work in an abstract form, without figures or
illustrations. Engineers apparently ignored it. The method was independently
developed by Otto Mohr of Germany ten years later, whereupon it began to find
practical application. It has become known as the Maxwell-Mohr method.
(Timoshenko 1953c).
William Merrill in his 1870 book, Iron Trusses for Bridges and Railroads,
compared the major truss varieties of his day. He applied the same live load to
trusses of the same span to find the type with the least weight. His conclusion, at
least for his hypothetical bridge parameters, was that the Post truss had the least
36


weight and thus, by inference, was the most economical (Merrill 1870b).
Merrill's conclusions were later refuted by J.A.L Waddell (Waddell 1915).
John A. Roebling, famous for the Brooklyn Bridge among others, wrote
Long and Short Span Railway Bridges in 1869, the year of his death. He advocated
the parabolic (also known as lenticular) truss for a hypothetical 500-foot span.
Arches were added for stiffness and cable stays for strength. (Roebling 1869).
Samuel Shreves A Treatise on the Strength of Bridges and Roofs was
published in 1873. He included a treatment on analysis based on Whipples
method of joints. It was intended to be a text and reference for students and
engineers. His methods are simple and straightforward; he uses simple
mathematics in his analysis to determine forces in the various bridge and roof types
(Shreve 1873). Like his contemporaries, his emphasis was on determining
strength.
In 1872, twenty-six years after his pioneering A Work on Bridge Building,
Squire Whipple wrote An Elementary and Practical Treatise on Bridge Building,
an Enlarged and Improved Edition of the Author's Original Work. He promoted
both the Whipple arch-truss bridge (also known as the Whipple bowstring truss)
and the parallel-chord Whipple trapezoidal bridge (known by then as the Murphy-
37


Whipple truss). The method of joints was used for analysis, and in addition, he
offered much practical advice on design and construction details. (Whipple 1872).
2.3.16 The Coming Era of Engineered Bridges
By mid-century, the Howe Truss was still dominant, and a summary of its
salient features is:
Wood was the principal material.
The wood diagonals were set on castings
It was prestressed by iron rods, which held the assembly together.
It required maintenance to keep the prestressing rods tight.
It was statically indeterminate.
Analysis, if any was performed, was limited to approximations based on
Naviers analogy.
The designers were craftsmen.
All this was to change. Within a twenty-year period the Whipple-Murphy
Truss, itself soon to be replaced by a new version of the Pratt Truss, exhibited the
new features. The changes can be summarized as:
Iron became the principal material. Cast iron was used for compression
members and wrought iron for the tension members.
Pins were used for connections.
38


Tension members, now made of iron and pin-connected, no longer
necessitated precompression. Prestressing was abandoned.
The structure was statically determinate^. Accurate analysis was
possible. Navier's analogy was no longer needed.
The method of joints had been published and was being promoted in
new texts.
Engineering was rising to the status of a profession.
2.3.17 Wrought Iron
While cast iron could be manufactured by pouring molten iron into molds,
wrought iron added the expensive step of forging the metal. This reduced its
carbon content and introduced ductility. Ductile wrought iron had about twice the
strength as its brittle parent, cast iron. But wrought iron was more expensive, so its
initial use was limited to members subject to tension. Those components to remain
in compression were manufactured from the less expensive cast iron. This practice
would change. 5
5 Strictly speaking, they were statically indeterminate if the effect of the rod panel ties is
included in the analysis. As a practical matter, that effect was not included (Haupt 1851;
Bow 1851; Whipple 1847a; Whipple 1873a; Whipple 1873b; Henerici 1867; Shreve 1873).
39


Compression elements necessitated larger cross sections than tension
members. While iron could be cast to manufacture members of large cross-section,
the early rolling mills could produce wrought iron shapes of only relatively small
cross-section. It was in 1864 that David Reeves of the Phoenix Bridge Company
introduced the Phoenix column of wrought iron. This was an ingenious invention
by which multiple small rolled sections were fastened together to form a single
member of sufficient size to serve as a compression member in large trusses. He
retained cast iron bearing blocks. The Phoenix column permitted longer columns,
thus deeper trusses, thus longer spans, all for the same working stress.
Use of wrought iron contributed to longer spans. By 1865, Jacob Linville
had built the first bridge across that formidable barrier, the Ohio River, at
Steubenville, Ohio (Plowden 2002b). It was a double intersection Whipple-
Murphy type, with a main span of 320 feet (Cooper 1889b). It retained cast iron
for the top chords and compression verticals. Albert Fink designed a Whipple-Pratt
hybrid bridge to cross the Ohio River at Louisville, KY in 1870 (DeLony 1994a).
Finks longest span was 390 feet. By 1876, a Linville-designed 519-foot span
wrought iron truss was built over Ohio River at Cincinnati in 1876. (Cooper
1889b).
40


2.3.18 Professional Organizations
It was in the 19th century that engineers began to consider themselves
professionals, and began to demand professional respect from others. The
(English) Institution of Civil Engineers (ICE) was organized in 1818 and received a
Royal Charter in 1828. In the United States, civil engineers were associated with
the Franklin Institute from 1824. A number of railroad engineers organized the
National Society of Civil Engineers, but the attempt was short-lived. The Boston
Society of Civil Engineers was founded in 1846. The American Society of Civil
Engineers (ASCE) organized itself in 1852 in New York City. Prominent among
its early membership were several notable bridge engineers including Stephen
Eong and John A. Roebling. The Transactions of the ASCE became a repository
for reports on developments in the field. It remains so to this day.
2.3.19 Latrobe, BoIIman and Fink
Benjamin Latrobe, son of U.S. Capitol architect Benjamin Henry LaTrobe
and chief engineer of the progressive Baltimore & Ohio Railroad, decided to use
iron in truss bridges on an extension of the B&O line to Wheeling, Virginia. On
Latrobes staff were two engineers of vastly different backgrounds, who would
write the next chapter in the story of truss development.
41


Wendell Bollman, a former pharmacist, rose through the ranks of railroad
work to a position under Latrobe. Parallel top and bottom chords, vertical
compression members, and a series of diagonals characterize Bollmans 1852 truss.
The diagonals all originate at the top of the end post, and run to the base of each
and every vertical. There are also X-bracing rods, also known as panel ties, in
each panel to keep the top chord in line. A Bollman Truss is shown in Figure 2.13.
While the Bollman Truss is statically indeterminate, it was designed as if
statically determinate by omitting the panel ties from the analysis (Whipple 1847a;
Whipple 1873a). The load at each panel point was transmitted to the end post and
thus to the abutment independently of all the other forces. The bottom chord did
not enter into the calculation, and so it was furnished with light rods or members
with sliding connections. The drawback of this system was that the horizontal
component of force from each diagonal is applied at the end of the top chord.
Thus, the full length of the top chord must carry the sum total of all the horizontal
components of forces from the long diagonal rods (Merrill, 1870c). This led to
42


strong, large, and expensive top chord members. Despite this inefficiency,
Bollman's truss was a true invention, representing new thinking and a new
approach to the truss problem. Bollman founded his own bridge company after the
Civil War(DeLony 1994b).
Albert Fink, another of Latrobe's engineers, came from Germany in 1848
with a formal technical education. After Bollman turned him down for a drafting
position, Latrobe hired him and he became Latrobes assistant (DeLony, 1994b).
Finks truss bridge, patented in 1854 and shown in figure 2.14, was similar to
Bollmans. Again it was statically indeterminate and again, it was treated as if
statically determinate. Both utilized cast iron for compression elements and
wrought iron for tension elements. Both used tension rods to direct the load from
the panel points to the abutments. Both used diagonals to support the verticals
directly to the tops of the end posts. But here the similarity ended.
43


Fink then supported the verticals at quarter points by diagonals to the center
post and the end posts. Diagonals that ran to the adjacent quarter points supported
the verticals at eighth points. Likewise diagonals that ran to the adjacent eighth
points supported the verticals at sixteenth points. For cases where the deck was
above the truss, the Fink truss needed no bottom chord, and the seemingly complex
arrangement of diagonals gave it the awkward, ungainly appearance that can be
seen in Figure 2.14. Still, the Fink Truss was more efficient than the Bollman, and
represented a technical advancement.
2.3.20 John Murphy
It was in 1858 that John W. Murphy introduced the idea of using eyebars
connected by physical pins throughout the structure. He designed an all-wrought
iron version on the Whipple trapezoidal truss geometry, connected by pins (cast
44


iron being used only for joint blocks). The pin had two advantages: First, it
provided a fast and economical connection. Second, it made the real joint the
theoretical equivalent of that used in computations so that Whipples method of
joints could be readily used for analysis. This was important to designers. The
resulting design has been called the Whipple-Murphy truss, much to the chagrin of
Squire Whipple, but rightly gives credit to the contributions of both men.
2.3.21 S.S. Post
The Post truss was patented and first used on the Erie Railroad in 1865. It
is included here as one example of the great variety of truss configurations that
were developed during this period. Named not for any bridge component but for
its inventor, S.S. Post, it featured non-vertical compression members, but was
touted as a very economical assembly. See Figure 2.15 for a Post Truss. Merrill,
in his 1870 study of truss types, found the Post truss to be the lightest of all eight
types that he examined (Merrill 1870b).
Figure 2.15. Post truss. (Cooper, 1889e)
45


This determination undoubtedly was parlayed into a marketing advantage, at least
for a while. (Merrills study was later discredited by Waddell). While the Post
truss is an offshoot from the mainstream of evolutionary developments that led
to the Pratt Truss, it is presented here as one example of the great variety in truss
developments.
2.3.22 New Challenges
Technological and theoretical advances were not to cease. The railroads,
with ever heavier engines, reached first the Ohio River, and soon afterward, the
Mississippi and Missouri Rivers. According to Eric DeLony, the railroads, by the
end of the Civil War, had become the most powerful economic and political force
in the United States (DeLony 1994c). Their demand for longer and stronger
bridges continued unabated.
There were setbacks too. A cast and wrought iron truss of five spans over
the Rock River at Dixon, Illinois collapsed in the spring of 1873. 150 people had
gathered at one rail to watch a baptism in the river below. Suddenly the loaded
span snapped, and span after span dislodged from their piers, all sagging downward
until they literally folded up. Forty five people had been confirmed dead when
Scientific American reported the accident in its May 24, 1873 issue, and 25 more
were still missing, presumed to be entangled in the debris. The bridge, erected by
46


L.E. Truesdell & Co., was not the first of its type to fail. At least two others had
collapsed within the prior six years, although neither with so great a loss of life
(Dixon Bridge 1873).
There were 20 inches of snow on the ground and winds gusting up to 45
mph on a December night in 1876, when a cast and wrought iron bridge collapsed
into the Ashtabula River in Ohio, taking with it a passenger train (Simmons 1985).
92 people were killed. The bridge at Ashtabula was a Howe truss that had been
built eleven years earlier. Three years later the crossing at the Firth of Tay in
Scotland the Tay Bridge collapsed in a windstorm. A train plunged into the
water below. None of its 100 passengers survived.
The Ashtabula disaster shook the American engineering community. A
public outcry launched investigations. The construction weekly Engineering News
was filled with articles describing the collapse, reports on the investigations,
editorials, and expressions of opinion contained in letters to the editor. Review of
the periodicals issues reveals a huge interest in the collapse throughout much of
the subsequent year. A similar response to the Tay collapse is documented in its
issues seemingly throughout the year 1880.
47


Investigations of these disasters brought lessons to light: The use of cast
iron was abandoned altogether. The railroads realized that reliance on the bridge
companies alone was insufficient; specialists in bridge construction were needed.
Procedures for material testing and quality control were introduced. Studies of
wind bracing were made. (Watson 1975).
2.3.23 Steel
Steel is not a new material, having been manufactured in small quantities
for swords and tools since ancient times. Steel was used in eye-bar chains for a
334-foot span suspension bridge at Vienna, Austria in 1828. However,
manufacture on a scale necessary for structural members had to wait until the mid-
19th Century.
Cast iron, wrought iron, and steel are all solid solutions of iron and carbon
(i.e. iron alloyed principally with carbon). They are differentiated by the amount of
carbon: cast iron has 2.5% to 4.0% carbon, steel generally has up to 1.7% carbon,
and wrought iron has on the order of 0.1% carbon, but also 1% to 2% slag (Kirby,
et. al. 1990b).
As late as 1850 liquid pig iron (i.e. cast iron) could be produced in a blast
furnace, but the processes necessary to refine it into steel permitted batches of only
48


a few hundred pounds each. Structural members on a large scale could not be
produced. The growing need for large tonnages of both wrought iron and steel
brought the inventive mind of man to work. It was in 1856 that Henry Bessemer,
an Englishman, developed a process for steel manufacture. Also in 1856, the
German-born brothers William and Friedrich Siemens, living in England, invented
a method for preheating the air that was introduced into a furnace. By 1864, Pierre
and Emile Martin of France improved the Siemens furnace. This became known as
the Siemens-Martin, or open-hearth, furnace.
The new processes made commercial production of steel possible. Their
use spread rapidly, and Bessemer steel was in production in the United States by
1864 (at Wyandotte, Michigan). In 1868 steel was being manufactured in Trenton,
New Jersey by the open-hearth method. Soon, steel batches, called heats, were
being produced in quantities of up to 15 tons or more. The ever-expanding
railroads in the United States demanded steel for rails in large quantities. Steel for
structural shapes, known as structural steel, could now be produced competitively
in relatively large amounts. (Elliott 1975).
Structural steel was new. Despite its advantages of strength, ductility, and
economy, many bridge designers were reluctant to abandon the more familiar
wrought iron for an untried material. Visionary engineering pioneers realized its
49


benefits and utilized steel in their projects. One was James Buchanan Eads, who
used steel in the ribs of his mighty arches of the first Mississippi River bridge at St.
Louis, completed in 1874. Another major advance for steel was its use in the five-
span truss bridge across the Mississippi River at Glasgow, Missouri in 1878,
designed by William Sooy Smith. This was the first all steel truss bridge in the
United States. Washington Roebling specified steel for the cables of the East River
Bridge, now known as the Brooklyn Bridge, which opened in 1883. In the late
1880s, steel was used on a huge scale in the bridge over the Firth of Forth, known
as the Forth Bridge, in Scotland.
The decade of the 1880's is considered a transition period in truss bridges,
as the change from wrought iron to steel was phased in. By 1890, all the structural
shapes produced in wrought iron could be produced in steel. Virtually all truss
bridges manufactured in the United States were now fabricated of steel.
2.3.24 Dominance of the Pratt Truss
By 1880, the double intersection Whipple-Murphy had evolved into a
similar single intersection truss, also of trapezoidal shape the Pratt truss. Like its
1844 namesake, the new Pratt truss utilized parallel top and bottom chords, had
vertical members carrying compression and diagonal members carrying tension.
Unlike the original, which used the tension diagonals to prestress the timber
50


verticals, this new improved Pratt had pin connections and needed no prestressing,
so only one diagonal per panel the one which carried tension was needed. In
the earlier period of prestressed wood (recall that the original Pratt truss offered a
prestressed alternate to the Howe) the Pratt did not compete well against the Howe
truss. It was with the abandonment of prestressing schemes for pin-connected
members that permitted engineers to analyze the structures using rational analyses.
With these developments, the Pratt truss proved very economical, and it began to
find favor in the 1870s. By the 1880s it had become the dominant form. It is
telling that the replacement bridge at Ashtabula was a Pratt (Simmons 1985).
Of all the varieties preceding it, the Pratt truss had evolved into the optimal
form. See Figure 2.16. It could be characterized as follows:
Construction was entirely of steel.
Built-up members were shop-riveted.
Major truss connections were pins6.
It was statically determinate7. The use of pins made analysis using
the now established method of joints a standard practice.
Erection time in the field was kept to a minimum.
6 Bolts and field rivets were also used for field connections.
7 Excepting the effect of the panel ties, by this time called counterbracing.
51


In 1889, Theodore Cooper estimated that the combined length of all the
railroad bridges in the Unites States totaled slightly over 3,000 miles. He estimated
there were over 39,000 spans greater than 20 feet, and over 700,000 spans of less
than 20 feet. (Cooper 1889f).
Figure 2.16. A utilitarian, economical Pratt truss. This is the Four-Mile Bridge
over the Elk River near Steamboat Springs, Colorado, built in 1900. It was
fabricated by the Wrought Iron Bridge Company of Canton, Ohio (Fraser 1986b).
Called the Baalhorn Bridge locally, after an adjacent family farm, it has since
been removed. (Photo courtesy of Clayton Fraser)
As the Pratt truss was becoming the dominant form, textbook authors began
to emphasize the Pratt. A particularly notable text from 1884 was J.A.L. Waddell's
The Designing of Iron Highway Bridges. His book treats only the Pratt and
Whipple Trapezoidal (a.k.a. the Murphy-Whipple, which is similar to the Pratt
except for double intersection panel geometry), because ... it must be remembered
52


that at least ninety percent of all American iron highway bridges are built on these
systems. (Waddell 1884a).
2.3.25 Growth and Stabilization of the Bridge Industry
As shown in Table 2.1 at the beginning of this chapter, the bridge business
was booming in the late 19th century. The early and middle parts of the century
were exemplified by a period of experimentation. Idea followed upon idea, and
patent upon patent. By the century's end, more than 800 patents had been granted
(DeLony 2002). Table 2.3 lists some of the many hundreds of bridge patents from
this period, focusing on truss types.
A review of Table 2.3 reveals that a great number of patents were issued
from about 1830 through about 1870. It is reasonable to take the large number of
patents as indicative of the large numbers of innovations that were then being tried.
This may be considered the period of experimentation. Inventors were
experimenting with materials, analytical methodologies, connection details,
tooling, manufacturing processes, and construction procedures.
Table 2.4. Partial List of Truss Bridge Patents
Year of Patent Patent Holder
1796 Palmer
1817 Burr
1820 Town
53


Table 2.4. (Cont.)
Year of Patent Patent Holder
1829 Wernwag
1830,36,69 Long
1833 Canfield
1839 Haupt
1840 Price and Phillips
1840 Howe
1841 Cottrell
1841 Whipple (Bowstring)
1844 Pratt
1846 Childs
1846 1larbach
1846 Gay
1847 Rider
1847 Moulton *
1847 Whipple (Trapezoidal)
1847 Warren-Monzani
1851 Pennington
1852 Bollman
1852 Gridley
1853 Fink
1854 Thayer
1854 Baldwin
1854 Champion
1855 Osborn
1856 Guion
1856 I lervey
1856 Huggins
1856 Rogers
1857 Brown
1857 McCallum
1857 Mosley
1864 King
1866 Hammond
1867, 69 Smith
* The Moulton Truss patent was granted in England
54


After the Ashtabula disaster of 1876, followed by the Tay Bridge collapse
of 1879, the reduction of new patents was dramatic. David Simmons, in a 1985
study of the bridge industry in Ohio, notes that in the 16-year period ending with
Ashtabula, 69 bridge patents were issued to Ohioans. This is an average of four per
year. For the 23-year period following Ashtabula to the end of the century, the
number of patents dropped to only 24, for an average of only one per year. Further,
the post-Ashtabula patents issued to Ohioans (and by inference to the rest of
Americans) were no longer for bold and sweeping changes, but rather small
improvements for small, short span, predominately timber bridges. All but one of
the post-1876 patents were granted to individuals with no association to the large
bridge companies. These patentees were from small rural villages and were
apparently people who were seeking solutions to old problems associated with
small timber spans. On the other hand, the major bridge companies responded to
the disasters by limiting their liability; they tended to standardize the designs with
which they had had the greatest success. The great era of experimentation and
innovation was coming to an end. (Simmons 1985).
2.3.26 J.A.L.Waddell
John Alexander Low Waddell (1854 1938), an 1875 graduate of
Rensselaer Polytechnic Institute, was born in Ontario, Canada. He became one of
the preeminent bridge engineers of the late 19th century and early 20th century in
55


the United States. I le was an early practitioner of that new breed of engineer the
independent consultant. Waddell was a theoretician, an educator, a practical
designer, a great contributor to the vertical lift bridge, but is best remembered today
as the author of many papers and texts on bridge design. His three books are
noteworthy, and provide us with an insight into the practices of his day:
The Designing of Ordinary Iron Highway Bridges, 1884
De Pontibns, 1898
Bridge Engineering, 1915
These three texts bound a 31 -year period, which saw the end of timber and
cast iron truss members, the advent of wrought iron, and its replacement by steel.
Connection details evolved from iron pins to steel rivets. His books are practical,
complete, and are sprinkled with his opinions, making for informative and
enjoyable reading. From these writings, insight into the views of the independent
consultant, not the patent-holder, can be gained.
2.3.27 Pratt Truss Variants
Several geometric variants of the Pratt were soon developed. The Parker
truss is a Pratt truss with a polygonal top chord. Because the geometry of the top
chord approximates that of a moment diagram for the entire span, an economy in
cross-section is achieved. A variant of the Parker is the Camelback, which is a
56


Parker with a top chord of exactly five slopes. As spans became longer, the panel
dimensions began to exceed the economical lengths of stringers. As a remedy, the
Baltimore truss (or Baltimore-Petit) included a sub-strut at the panel mid-dimension,
from which floor beams are connected. It was developed by the Baltimore and Ohio
Railroad, hence its name. The Pennsylvania Railroad developed a similar variant of
the Parker, which has become known as the Pennsylvania truss (or Pennsylvania-
Petit). These along with many other types are found on Figure 2.17, which shows
thirty truss types used for bridges.
2.3.28 End of an Era
In 1900 J.P. Morgan formed the American Bridge Company out of a merger
of 24 bridge firms. American Bridge built thousands of bridges across the country.
The remaining, smaller, bridge companies found it increasingly difficult to
compete with this gargantuan enterprise.
Following passage of the Federal Road Act in 1916, bridge design began to
become the responsibility of state highway departments. By the 1920s, state
highway departments were organized and many of these developed their own pre-
engineered bridges for set spans.
57


These developments made it even more difficult for small bridge firms to
compete. Faced with competition from the huge American Bridge Company and
from state highway departments, the smaller bridge firms began to disappear from
the scene.
The individual standards of numerous small bridge companies were
replaced by Coopers General Specifications for Steel Highway Bridges and
Viaducts (Cooper 1896), which was revised several times from the 1890s through
the early 1900s, and set standards for the industry. 1910 1920 is considered a
transitional period from pin-connected trusses to riveted connections. By 1920, the
all-riveted bridge was the norm; the era of pin-connected truss bridges was past.
58


L/l
Figure 2.17. Thirty bridge truss types. Taken from a poster prepared by the Historic American Engineering
Record. (Fraser 1986c)


3.
Preservation Issues
3.1 Introduction
This chapter discusses preservation-related issues including altruistic
arguments for bridge preservation; the legal framework for preservation; the
National Register of Historic Places, and its effect on bridge preservation; and a
case study of an historic truss bridge as an example of the cycle of bridge
construction, demolition, and replacement.
3.2 Why Preservation?
Truss bridges are disappearing at a rapid rate. Once numbering in the tens
of thousands, now only a few hundred remain nationwide. Why preserve historic
truss bridges?
Truss bridges are engineered structures. American innovators, utilizing
advances in materials and mathematical analyses, developed the truss bridge as a
new technology, a technology developed to respond the tremendous pressure of
westward expansion in the 19th century. A record of this technological achievement
is found embodied in metal truss bridges. Material evidence is present in physical
60


structures, often in a form unavailable in documentation. Truss bridges represent
uniquely American developments, not found to such extents in other countries.
They are not small artifacts, concentrated in museums, but large structures dispersed
throughout the country, presenting these innovations in a forum readily accessible
for public viewing. This legacy is significant to American history, to the history of
science and technology, and to engineering history and heritage.
3.3 Legal Framework
Preservation is also a Congressional mandate. The awareness of a rich
historic heritage has grown over the decades. Likewise, public pressure to protect it
under the law has gained momentum. Preservation in general, and that of bridges in
particular, has been the subject of an intermittent but ultimately continuing body of
law. It is not surprising that as the resource dwindles and awareness of the
significance of the remnant is raised, the laws have become ever more exacting.
Several of the more fundamental events that have shaped the preservation
landscape are briefly discussed below (Eilers and Vedder, 1996):
1899: The Rivers and Harbors Act contains two provisions that were to
become significant for bridge preservation in later years. Section 10 of
the Act requires the U.S. Army Corps of Engineers (COE) to regulate
61


activities that take place in navigable waters. Section 404 of the Act
requires COE to issue a permit for any discharge or fill material into any
United States waters. This goes far beyond the navigable waters of
Section 10, to include virtually any body of water at all. Thus nearly all
bridge projects built over water (as opposed to highway overpasses, etc.)
need what has come to be called a 404 permit. An unintended
consequence of this law would help propel bridge preservation efforts in
1966.
1906: The Antiquities Act granted the President the power to designate
as national monuments areas of great historical significance. Examples
are Ellis Island National Monument and Edison Laboratory National
Monument.
1916: The National Park Service was established. A part of the
Department of the Interior, the functions of the National Park Service
include management of the national parks, national monuments, national
landmarks, and, since 1966. administration of the National Register of
Historic Places.
1929: In this year total freight tonnage by motor truck exceeded that of
the railroads. The railroads period of significance was coming to an
end; that of the highway was gaining momentum.
62


1930s: The Historic Architectural Buildings Survey (HABS) was
established and partly to document historic properties that were then
recognized as threatened before they disappeared and partly to create
work for unemployed architects.
1935: The Historic Sites Act declared it a national policy to preserve
national historic sites, and authorized the Secretary of the Interior to
acquire and manage such sites.
1949: The National Trust for Historic Preservation was chartered by
Congress to administer donated sites, buildings, and objects that are
significant to United States history' and culture. It has since become a
private philanthropic organization.
1966: The National Historic Preservation Act was the first federal
program to explicitly require special effort to protect historic properties.
Further, Congress broadened the protection from national sites to include
state, local and private properties. The National Register of Historic
Places was established. Section 106 of the Act extended protection to
properties affected by federal projects of any sort. This has come to be
particularly relevant for bridges because projects affecting existing
bridges removal, replacement, or widening for example nearly
always involve some form of federal participation. That participation
may take the form of federal monies or permits including 404 permits
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issued by the COE as discussed above. Should the proposed project
adversely affect'5 a historic property such as a bridge, then the impact
on the historic property is to be avoided or mitigated. While Section 106
does not forbid destruction of historic places; it does impose a
comprehensive review before destructive action is taken.
1966: The Department of Transportation Act enacted stricter
stipulations on federal transportation undertakings. Bridges fall within
its jurisdiction. Section 4(f) requires highway agencies to avoid harm to
historic properties unless there is no prudent and feasible alternative.55
1967: The Point Pleasant Bridge, (the Silver Bridge55) collapsed into
the Ohio River, killing 46 people. The collapse was attributed to crack
propagation as a result of the joint action of stress corrosion and
corrosion fatigue55 (Highway Accident Report, 1970) in an eyebar link of
a suspension chain. A public outcry for bridge safety improvements
ensued.
1968: The Federal-Aid Highway Act established the National Bridge
Inspection Program. This act came about in large measure as a response
to the Silver Bridge collapse of the previous year. It was essentially a
federal mandate for bridge safety.
1969: The Historic American Engineering Record (HAER) was
established. HAER, similar to its architectural counterpart I IABS,
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emphasizes documentation of engineering and industrial sites before they
are demolished or decay into ruin. This is referred to as preservation by
documentation.'
1970: The National Environmental Policy Act regulates against
environmental damage. The Act requires that Environmental Impact
Statements be prepared to describe the proposed action and potential
adverse effects. It includes, along with its emphasis on environmental
goals, a mandate to preserve important historic ...aspects of our
national heritage.
1970: Another Federal-Aid Highway Act established the Bridge
Replacement Program. This program assists states in the replacement of
unsafe bridges.
1978: The Surface Transportation Assistance Act established the
Highway Bridge Replacement and Rehabilitation Program. Preservation
seemed to gain added status when rehabilitation became a stated goal.
Funds were made available for rehabilitation and replacement of bridges
with low sufficiency ratings. But the word rehabilitate has been
subject to multiple interpretations. Preservationists consider it to mean
the act or process of returning a property to a state of utility through
repair or alteration. Highway planners and engineers have taken it to
mean bringing about compliance with the AASHTO Standard
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Specifications for Highway Bridges (AASHTO, 1992). Thus if an older
bridge did not conform to AASHTO, it should be removed and replaced
with a new, conforming bridge (Often the conformance dealt with issues
such as width, and geometry of approaches and roadway alignment.)
Consequently, almost all projects funded by the Bridge Program have
replaced old bridges with new ones (Snyder, 1994a).
1986: The Federal Water Pollution Act has become known as the Clean
Water Act. or CWA. When an application for a permit is considered by
the COE, a public interest review is conducted. Among the many
factors to be considered are the effects on historic properties.
1987: The Surface Transportation Uniform Relocation Assistance Act
included the Historic Bridge Program. That program singles out historic
bridges for special treatment under the Bridge Replacement and
Rehabilitation Program in a manner that encourages the inventory,
retention, adaptive reuse, and future study of historic bridges. Each
state was directed to conduct an inventory of historic bridges. Efforts to
preserve the historic integrity of these bridges became eligible for federal
funds. Bridges that become slated for demolition are to be made
available for donation if a recipient can be found.
1991: The Intermodal Surface Transportation Act (also known as
ISTEA) made funds available for transportation enhancement projects.
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10% of the funds were to be applied specifically to projects such as bike
trails and bridge rehabilitation. Further, the Act permitted greater
flexibility in design standards. The Secretary of Transportation may
approve historic preservation bridge projects that do not meet the
AASHTO Standard Specifications for Highway Bridges (AASHTO
1992), provided safety is ensured (Snyder, 1994b).
It can be seen that the federal mandate for historic preservation has
sporadically, but with ever increasing forcefulness, become a reality in the United
States. While all of these laws affect historic bridges to one degree or another, the
most important of them with regard to historic bridges are the 1966 National
Historic Preservation Act. Section 106; the Department of Transportation Act of
1966, Section 4(f); and the 1987 Historic Bridge Program.
3.4 National Register of Historic Places
Since its inception under the 1966 National Historic Preservation Act, over
75,000 properties have been registered (National Register of Historic Places, 2003).
It has become the definition of historic for regulatory purposes. It applies to
districts, sites, buildings, structures, and objects that are usually over 50 years of
age and meet specific guidelines of criteria, historic context, and historic integrity.
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The information in Table 3.1 is from the National Register of Historic Places, or
NRHP (Byrne, 2003).
Table 3.1. Breakdown of Properties Listed on the
National Register of Historic Places. The quantities
are as of 2003.
Property Type Number of Listings
Buildings 54,219
Districts 11,870
Sites 4,174
Structures 5,240
Objects 336
Within the Structures of Engineering or Industrial Significance category,
2,346 bridges are found. The NRHP maintains the following sub-groups within the
Structures category, (U.S. Dept, of the Interior, 2003):
Railroads, subways, and related resources
Roads, highways, and parkways
Canals and waterways
Airports
Bridges
Trestles and viaducts
Tunnels
Lighthouses
Water supply and control systems
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Power dams
Power generating plants
Electrical systems
Heavy power machinery'
Sanitary systems
Mines and other extraction facilities
Mills, factories, and other processing facilities
3.5 The Vanishing Truss Bridge
Despite the numerous laws cited above and National Register status, historic
truss bridges continue to be destroyed. None of the laws require preservation; they
require effort at preservation. At this point, early in the first decade of the 21st
century, it can be observed that, except for covered bridges which have a popular
following, virtually all wooden truss bridges are gone. Iron and steel truss bridges,
the next most vulnerable, continue to be lost.
For example, consider through truss bridges in Colorado. Twenty-eight that
were listed on the National Register of Historic Places can be counted from the first
historic bridge survey conducted in the mid-1980s. Today, slightly less than
twenty years later, only eleven of these remain. These bridges have suffered a 60%
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attrition rate over the past 20 years. While one could count lost bridges in other
ways, the point is that by any measure, truss bridges are vanishing at an rapid rate.
3.6 The 5th Street Bridge Story
The common cycle of bridge construction and replacement is illustrated by
the following example. Three successive bridges have followed an earlier ferry
crossing over the Colorado River in Grand Junction, Colorado. This story serves as
an ordinary example with an uncommon twist: part of the first bridge has been
preserved.
Grand Junction. Colorado had only been settled five years when the need for
a crossing over the Grand River (todays Colorado River) was brought to fruition
with a new wrought iron truss bridge. This bridge consisted of five spans, all with a
timber deck on timber stringers. The Fifth Street Bridge, shown in Figure 3.1, was
the first major bridge project undertaken by the Colorado State Engineers Office. It
was designed by State Engineer E.S. Nettleton and fabricated by the Phoenix Bridge
Co., Phoenixville, Pennsylvania, using wrought iron members manufactured by its
parent company, the Phoenix Iron Co. (Winpenny, 1996). It first served as a wagon
bridge, and then as a vehicular bridge through the late nineteenth and early twentieth
centuries. The Fifth Street Bridge was dismantled upon completion of a two-lane
replacement steel through-truss bridge in 1933, shown in Figure 3.2. This through-
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truss served the community until the early 1990s when it was demolished and
replaced by the present four-lane, prestressed concrete girder bridge.
Figure 3.1. Fifth Street bridge over Grand River, Grand Junction, CO. Built in
1886, this Pratt truss was of wrought iron construction. (Photo courtesy of Loyd
Files Research Library, Museum of Western Colorado).
One of the spans from the 1886 bridge was dismantled and moved about 100
miles to the south, where it was reassembled over the San Miguel River near Uravan
in western Montrose County. There it served until the early 1990s, when a
replacement span was constructed a short distance upstream. The San Miguel
Bridge, a rare example of a wrought iron truss is shown in Figure 3.3, now awaits its
fate as an abandoned bridge.
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Figure 3.2. A photograph showing both the original 1886 Fifth Street Bridge and
its 1933 replacement. The 1886 bridge is a pin-connected wrought iron truss. The
1933 bridge is a riveted steel truss, typical of state bridges of its day, so called
because the design was one of several standardized designs prepared by the state
highway department. See also Figure 4.8. (Photo courtesy of Loyd Files
Research Library, Museum of Western Colorado).
3.7 Conclusion
The story of the various Fifth Street bridges serves as an example of
decisions made by thousands of communities, affecting hundreds of thousands of
bridges all across the nation. New bridges, functional and satisfactory, become old
bridges. The old bridges are no longer satisfactory. They may have become
functionally obsolete because of insufficient width, height, or of proper roadway
alignment. They may have become structurally deficient, no longer of sufficient
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