Citation
Wind analysis of the historic San Miguel Bridge

Material Information

Title:
Wind analysis of the historic San Miguel Bridge
Creator:
Elias, Kazwan M
Place of Publication:
Denver, Colo.
Publisher:
University of Colorado Denver
Publication Date:
Language:
English
Physical Description:
xvii, 137 leaves : illustrations ; 28 cm

Thesis/Dissertation Information

Degree:
Master's ( Master of Science)
Degree Grantor:
University of Colorado Denver
Degree Divisions:
Department of Civil Engineering, CU Denver
Degree Disciplines:
Civil Engineering
Committee Chair:
Rens, Kevin L.
Committee Members:
Durham, Stephan
Janson, Bruce

Subjects

Subjects / Keywords:
San Miguel Bridge (Colo.) ( lcsh )
Wind-pressure ( lcsh )
Bridges -- Aerodynamics ( lcsh )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaves 134-137).
General Note:
Department of Civil Engineering
Statement of Responsibility:
by Kazwan M. Elias.

Record Information

Source Institution:
|University of Colorado Denver
Holding Location:
Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
166391432 ( OCLC )
ocn166391432
Classification:
LD1193.E53 2007m E54 ( lcc )

Full Text
;
WIND ANALYSIS OF THE HISTORIC SAN MIGUEL BRIDGE
by
BSCE, Metropolitan State College, 1993
Thesis submitted to the
University of Colorado at Denver and Health Science Center
in partial fulfillment
of the requirements for the degree of
Kazwan M. Elias
Master of Science
Civil Engineering
2007


This thesis for the Master of Science
degree by
Kazwan M. Elias
has been approved
by
Bruce Tanson
2
Date


Elias, Kazwan (M.S., Civil Engineering)
Wind Analysis of the Historic San Miguel Bridge Thesis directed by Associate
Professor Kevin L. Rens
ABSTRACT
This thesis examines non-traditional load paths as an alternative to the
conventional skeleton approach analysis. It is concluded that alternate load
paths do exist and could be utilized by designers using commonly available
computer software. San Miguel Historical Bridge, located in Montrose County
was researched for conversion from a vehicular bridge to pedestrian usage. The
study finds todays design vertical live loads to be on the same order as those
used by the designer from a century ago. Material allowable stresses are also
similar (or better). But lateral design wind load is significantly higher. The
study included monitoring the existing bridge in the field under real wind
conditions and comparing its behavior to a 3D analytical model program.
Equipment utilized in the laboratory and field studies included Strain
Transducers, Anemometers, and Data acquisition computers. The overall bridge
behavior due to wind loads of 3.0 to 4.6 psf (0.144 to 0.221 kpa) were measured
in the field and show that the deck provides lateral stiffness and help resists
wind.


This abstract accurately represents the content of the candidates thesis. I
recommend its publication.


DEDICATION
I dedicate this thesis to the following people:
My brother Aban M. Elias and his family.
My parents Thamira M. Alsaloum and Abdul M. Elias.
My wife Athraa A. Mahmood.
My daughter Noor K. Elias.


ACKNOWLEDGEMENT
I would like to acknowledge my professor, Kevin L. Rens, who directed
the investigation of San Miguel Bridge and made it a reality. His direction and
support during my master course study at the University of Colorado at Denver
were tremendous. In addition, I would like to show my appreciation to
Frederick R. Rutz, the principal investigator of the University of Colorado at
Denver historic bridge research, who was the first to study and investigate the
historic bridges and presented results in his doctoral dissertation in 2004. I
would like to extend my thanks to my graduate committee; Stephan Durham and
Bruce Janson. Additional thanks goes to the funding agencies including Grant #
MT-2210-04-NC-12 from the National Center for Preservation Technology and
Training and Grant # 2004-M1-019 from the State Historical Fund of the
Colorado Historical Society. Furthermore, acknowledgement is given to
Montrose County, CO for allowing access to the bridge.
The assistance of the following University of Colorado at Denver
students is equally acknowledged: Shohreh Hamedian, Veronica Jacobson,
William Swigert, Sam Brown and Chris Kline. These students assisted in
laboratory and field investigations throughout the project.


TABLE OF CONTENTS
Figures.............................................................xii
Tables............................................................xviii
CHAPTER
1. Overview........................................................1
1.1 Introduction.................................................. 1
1.2 Goal............................................................2
1.3 Former to Modem Procedures......................................9
1.4 Preservation...................................................10
1.5 Loads..........................................................10
2. San Miguel Bridge..............................................12
2.1 The History Behind the Bridge..................................12
2.2 San Miguel Bridge Location.....................................14
2.3 San Miguel Bridge Description..................................17
2.4 Bridge Members.................................................20
2.4.1 Top Chords and Diagonal Portals................................20
2.4.2 Cross Bracing..................................................22
2.4.3 Bottom Chords and Diagonal Bracings............................24
vii


2.4.4 Vertical Columns................................................26
2.4.5 Abutments.......................................................28
2.4.6 Deck............................................................29
2.4.7 Floor Beams and Stringers.......................................31
2.4.8 Railing.........................................................33
3. Field Testing...................................................35
3.1 Introduction....................................................35
3.2 System Arrangement..............................................36
3.3 Strain Transducers..............................................37
3.4 Wheatstone Bridge...............................................40
3.5 Wind Direction Sensor...........................................41
3.6 Anemometers.....................................................43
3.7 Interval Timer..................................................44
3.8 Data Logger.....................................................45
3.9 Software........................................................47
3.10 Laptop Computer.................................................49
3.11 Cables..........................................................49
4. San Miguel Modeling and Analysis................................51
4.1 Introduction....................................................51
4.2 Modeling........................................................52
4.3 Skeleton Model..................................................53
viii


4.3.1 Skeleton Model Due to Gravity Loads............................53
4.3.2 Skeleton Model Subjected to Lateral Loads......................55
4.4 Deck Model.....................................................57
4.5 Comparison.....................................................61
4.6 Conclusions....................................................63
5. San Miguel Bridge Verification Method..........................64
5.1 Introduction...................................................64
5.2 Data Reduction.................................................64
5.2.1 Bottom Chord Eyebars...........................................65
5.2.2 Moment in Portal Frames........................................65
5.2.3 Wind Pressure Theory...........................................66
5.2.4 Wind Load by Quadrants.........................................67
5.3 Boundary Conditions............................................71
5.4 Member Releases................................................72
5.5 Verification Analysis..........................................72
5.6 Conclusions....................................................78
5.6.1 Correlation....................................................78
5.6.2 Drift..........................................................79
6. Summary, Conclusions, and Recommendations for Further
Study..........................................................81
6.1 Introduction...................................................81
IX


6.2 Summary of Findings
81
6.3 Conclusions..................................................84
6.4 Recommendations for Future Research..........................85
APPENDIX
A. San Miguel Photographs.......................................87
A. l Introduction.................................................87
B. Data Logger Program..........................................96
B. l Program Input................................................96
C. Supporting Material.........................................106
C. 1 T emperature................................................106
C.2 Strain Data.................................................106
C.2.1 Cable Resistance............................................106
C.2.2 Strain Computation..........................................108
C.2.3 Rolling Average.............................................108
C.2.4 Zeroing the Strain Data.....................................110
C.3 Wind Pressure...............................................Ill
C.4 Forces in Bottom Chord Eyebars..............................112
C. 5 Moments in Portal Frames....................................113
D. Spreadsheet Instructions....................................117
D.l Introduction................................................117
D.2 Screen Captures.............................................118
x


E. Disregarded Wind Study...................................127
E.l Introduction.............................................127
E.2 Spreadsheet Screen Captures..............................131
REFERENCES.....................................................134
xi


FIGURES
FIGURE
1.1 Thirty Bridge Truss Types.........................................3
1.2 San Miguel Bridge Over San Miguel River Near Uravan, Colorado.....5
2.1 Fifth Street Bridge Over Grand River.............................13
2.2 A Photograph Showing the Original 1886 Fifth Street Bridge and its
1933 Replacement.................................................13
2.3 The 1990 Prestressed Concrete Bridge as it Appears Today.........14
2.4 San Miguel Bridge................................................15
2.5 Colorado Map.....................................................16
2.6 Location Map.....................................................17
2.7 Fishtail Girder Supporting Steel Stringers and Deck..............18
2.8 San Miguel Bridge, Site Plan.....................................19
2.9 San Miguel Bridge, Front Elevation...............................20
2.10 San Miguel Bridge, Portals and Top Chord Detail..................21
2.11 Diagonal Portal at the Frame End of San Miguel Bridge............22
2.12 Lattice Bars at End Portal Elevation.............................23
2.13 Broken Lattice Bars..............................................24
2.14 Bottom Chord Eyebars at San Miguel Bridge........................25
xii


2.15 Bracings at Different Locations on the San Miguel Bridge............26
2.16 Vertical Column Cross Sections at San Miguel Bridge.................27
2.17 Vertical Post at San Miguel Bridge..................................28
2.18 San Miguel Bridge, Detailed Elevation and Section...................29
2.19 San Miguel Bridge Deck Corrosion....................................30
2.20 Top of San Miguel Bridge Deck.......................................30
2.21 Underside of San Miguel Bridge Deck.................................31
2.22 San Miguel Bridge, Detailed Elevation and Section...................32
2.23 Floor Beams and Stringers at San Miguel Bridge......................33
2.24 Current Railing at San Miguel Bridge................................34
3.1 Field Experimentation of San Miguel Bridge..........................36
3.2 System Arrangement Diagram..........................................37
3.3 Three Dimensional Drawing of a Strain Transducer Device.............38
3.4 Strain Transducer Device Prior to Installation......................39
3.5 Diagram Illustrating the Locations of the Strain Transducers........40
3.6 Bridge Wiring Schematic and Terminal Input Module (TIM).............41
3.7 Wind Direction Sensor Installed on San Miguel Bridge................42
3.8 One of the Anemometers is Located Directly Below the San Miguel
Bridge Deck........................................................43
3.9 Diagram Illustrating the Locations of Anemometers (WS1 WS5) and
Wind Direction Sensor (WD).........................................44
xiii


3.10 Interval Timer......................................................45
3.11 Campbell Scientific CR5000 Data Logger..............................46
3.12 Campbell Scientific CR5000 Data Logger and Time Interval at San
Miguel Bridge Being Connected by K. Elias..........................47
3.13 PC 9000 Figure of Software..........................................48
3.14 Cable Management at San Miguel Bridge...............................50
4.1 Illustration of the Traditional Skeleton Structure..................53
4.2 Gravity Loads Applied to Skeleton Structure.........................54
4.3 Diagram of Axial Force in Bottom Chord Eyebars Due to Gravity
Loads for the Skeleton Structure...................................54
4.4 Wind Load Applied to Skeleton Structure.............................55
4.5 Diagram of Axial Force in Bottom Chords Due to Wind Load............56
4.6 Shear and Moment Diagrams for a Cantilever Beam Condition...........56
4.7 San Miguel Bridge Deck..............................................57
4.8 Skeleton Model with Stringers and Deck..............................58
4.9 RISA Rendering of Floor Beams and Stringers.........................59
4.10 Offset Members and Release Locations................................60
4.11 Rendering of Floor Beams, Stringers, and Deck.......................61
5.1 Strain Transducer Arrangement on Bottom-Chord Eyebar................65
5.2 Diagram Illustrating the Locations of the Strain Transducers........66
xiv


5.3 Diagram Illustrating the Locations of Anemometers (WS1 WS5) and
Wind Direction Sensor (WD)......................................68
5.4 Quadrants Subjected to Different Uniformly Distributed Wind Pressures
.......................................................................69
5.5 Wind Pressure Applied to the Four Quadrants for Analysis.........71
5.6 Wind Speed as Measured by the Five Anemometers...................73
5.7 Wind Direction as Measured During the Test.......................73
5.8 Strain Measurements for the Windward and Leeward Eyebar..........74
5.9 Enlargement of the Trace for Windward and Leeward Bottom Chord
Eyebar Measured Strains.........................................75
5.10 Measured Strains at the South Portal.............................76
5.11 Measured Strains at the North Portal.............................76
A. 1 Location of San Miguel Bridge....................................87
A.2 The Site is Over San Miguel River................................88
A.3 San Miguel Bridge as it Appears Today............................88
A.4 Generator Used for Electrical Supply.............................89
A.5 San Miguel Bridge Side view......................................89
A.6 San Miguel Bridge Fishtail Girder Style Floor Beam and Stringers.90
A.7 The Deck Made of Road Base Gravel on an Unusual System of
Galvanized Pipe Culvert Halves..................................90
A.8 San Miguel Bridge Abutment.......................................91
xv


A.9 San Miguel Bridge Steel Welding................................91
A. 10 All the Vertical Posts at San Miguel Bridge are Free of
Bending.......................................................92
A. 11 Bridge Railing is a W Type Guard Rail........................92
A. 12 Connections between Bottom Chord Eyebars, Floor Beams, Bracing and
Vertical Posts................................................93
A. 13 The Pin Connection Between the Floor Beam and Vertical Posts...93
A. 14 San Miguel Bridge Test Set-up During the Experiment............94
A. 15 Strain Transducer Installed on Bottom Chord Eyebars with Thermal
Reflector Insulation Rapid Around it..........................94
A. 16 Strain Transducer Arrangement on Bottom Chord Eyebar...........95
A. 17 Anemometer Mounting............................................95
C.l Strain Measurements for the Windward and Leeward Bottom Chord
Eyebars......................................................110
C.2 Strain Transducer Locations on the South Portal...............114
C.3 Idealized Portal Frame........................................115
E. 1 Wind Pressure Applied to the Four Quadrants for Analysis......128
E.2 Wind Speed as Measured by the Five Anemometers................128
E.3 Wind Direction as Measured During the Test....................129
E.4 Strain Measurements for the Windward and Leeward Bottom Chord
Eyebars......................................................129
xvi


E.5
Measured Strains at the South End Post
130
xvii


TABLES
TABLE
4.1 Summary of Maximum Axial Forces in Bottom Chord Eyebars........62
5.1 Wind Velocities, Quadrant Average Velocities and Quadrant
Pressures.......................................................70
5.2 San Miguel Bridge Verification Summary..........................77
E. 1 Verification Summary...........................................131
xviii


1.
Overview
1.1 Introduction
Wrought Iron bridges were being erected at different locations around
the United States in the mid 1800s to the early 1900s. As many as 190 bridge
companies were working rapidly to fulfill the need of the truss bridges in North
America (Rutz, 2004). The vast growth of bridge companies was due to the
economic demands of railway tracks and river and waterway crossings. Cast
and wrought iron bridge trusses started replacing timber bridge trusses for their
proven strength and increased spans.
The Pratt truss and its several derivatives would evolve into the most
common of all 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). Several geometric variants of the Pratt were soon developed. One of
the well known Pratt trusses was the Parker truss with a polygonal top chord.
Because the geometry of the top chord approximates that of a moment diagram
for the entire span, an economical cross-section is achieved. A variant of the
Parker is the Camelback, which is a Parker with a top chord of exactly five
slopes. As spans became longer, the panel dimensions of the Parker began to
exceed the economical lengths of stringers. 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 bridge
types are found in Figure 1.1, which shows thirty truss types used for bridges
(Rutz, 2004).
Many of the iron bridges were destroyed leaving approximately half of
the historic bridges which have been relocated and serve as pedestrian bridges.
In some cases, they have been abandoned (Rutz, 2004).
1.2 Goal
The goal of the research was to study the historic bridge and its behavior
to wind. The main concentration of the work at the University of Colorado at
Denver has been focusing on the lateral wind loads. The results provide insight
into actual performance of pin-connected truss bridges under lateral wind loads.
Both the stiffening effect of different deck types and the actual flexural response
of portal frames were examined. The actual responses were compared to results
from analyses. These findings were offered as a first step toward experimental
investigations of lateral load response on ordinary truss bridges, the type of
bridges now of interest for historic preservation (Rutz, 2006).
Since 2001, a total of six historic truss bridges were studied and
analyzed. Five of these bridge studies were sponsored by the National Center
2



LoncoM nuuaouci double ntosctoi mwen i
v rt(> w'< c§* *+
aw mi &*(***'
t>M '<*1 I **t MM
v^ttes. /wwv
T"

^ zM!B&
m# jj
M **!* *#' '* **
MVA ,1 #1 #Mr*#i
PENNSYLVMM KT1T)
(Mr tomtim****
I I MM(( (* M !'#*
I 4 Mi/< WWWEN
' KMTC4I
ft# MM MMr
GflQNER
# MrMMMTW
PCQftAM
Mk (Mlf ##* CMWWM < ', ot>- -M *,
k( MICA MS'*** JMMMT 4C/. TT* *#/
,| i
-L=
M>*f Mwil* m MWlMM
Ml# I ^M MM ('

EMEEti
axai HTTWECTot autt
POST
m *#rr maiw
#'M
A-.
* .* IM/#

9CMWQUP
.< £#**
OOUiWN
W ./l k* M 4 m7 MHM iM
.-r (M#'
V'VMWI
/HUTTV
*'<£*1* *Ci .
M ' r ( #(r
MTU
II SttU.
HIS
KELLOGG
K-T1U5S
I'
FIW
raw*)
i
TOWN LATTICE
*10 ,< *<*#
#M0
i '/ J' MM(< M0M.J ft
c *ur*AJ * S*i w/ #v*
r nmi**
/TTTTTT\ MAM
papkep n

PAPKE*
> .* -rrt /#/ rt*n**
WIOCPT
M MX (
tSLmfm**j£m* '
MW M# '/ <
Mum
AAAAA1 /fVfZN
wmn
>1<
rt-****** M#
STEAM
BALTMOPE (PETIT) STW
Mh /Mir mm c#rw> L MTII ( MMT MM MM I mm *<*fm t* * ^Tlr ?7 I (M (' *rrMW M. rJ
< ( M/rr Vi tWem
tf i WW ( *># r#j
iM't (M ## ill'
i# ie r
(Ml (MW'
Figure 1.1. (Thirty Bridge Truss Types). Taken from a poster prepared by the Historic
American Engineering Record. (Fraser 1986c)
£
5
5
era
Z
o
ns
H
H
fO
c
N
&
a
D
y>
m
S*
acobson, Hamedian, Swigert, 2005).


The six historic truss bridges studied are located in Colorado as listed below:
1. San Miguel Bridge, built in 1886, located in Montrose County (Elias, et. al.,
2006).
2. Fruita Bridge, built in 1907, located in Mesa County (Rutz, 2004).
3. Blue River Bridge, built in 1895, located in Summit County (Hamedian, et.
al., 2006).
4. Prowers Bridge, built in 1909, located in Bent County (Jacobson, et. al.,
2006).
5. Rifle Bridge, built in 1909, located in Garfield County (Swigert, et. al.,
2006).
6. Four Mile Bridge, built in 1900, located in Steamboat Springs (Rutz, 2004).
This research will be focusing on San Miguel Bridge. As shown in
Figure 1.2.
Several other papers, reports and publications regarding the historic
bridge research were published and some are in press. These publications are
listed below:
Jacobson, V.R., Rutz, F.R., and Rens, K.L (2006). Prowers bridge
study: experimental and analytical techniques of wind loading analysis
to an historic truss bridge, Proceedings of the 4th ASCE Forensics
Congress, ed. by P.A. Bosela and N.J. Delatte, Technical Council on
4


Forensic Engineering of the American Society of Civil Engineers, Oct. 6-9,
Cleveland, OH, ASCE, Reston, VA.
Figure 1.2. (San Miguel Bridge Over San Miguel River Near Uravan,
Colorado). This wrought iron Pratt truss was built in 1886 as part of a five-span
Fifth Street Bridge over the Grand River (Colorado River today) at Grand
Junction, Colorado. It has a roadway of gravel on semi-circular lengths of
corrugated metal pipe set between steel stringers. It has wrought iron eyebar
bottom chords and diagonals and wrought iron rod counterbracing. A steel
vehicular rail has replaced the original railing. It served the mining industry in
western Colorado until the 1980s. Abandoned since 1990, it remains the oldest
bridge originally built in Colorado.
Hamedian, S., Rutz, F.R., and Rens, K.L (2006). Analysis and testing
of the historic blue river bridge subjected to wind, Proceedings of the
5


4th ASCE Forensics Congress, ed. by P.A. Bosela and N.J. Delatte,
Technical Council on Forensic Engineering of the American Society of
Civil Engineers, Oct. 6-9, Cleveland, OH, ASCE, Reston, VA.
Swigert, W.B., Rutz, F.R., and Rens, K.L (2006). Wind load analysis
of a truss bridge at Rifle, Colorado, Proceedings of the 4th ASCE
Forensics Congress, ed. by P.A. Bosela and N.J. Delatte, Technical
Council on Forensic Engineering of the American Society of Civil
Engineers, Oct. 6-9, Cleveland, OH, ASCE, Reston, VA.
Elias, K. M., Rutz, F.R., and Rens, K.L (2006). Analysis and
verification testing of San Miguel historical bridge, Proceedings of the
4th ASCE Forensics Congress, ed. by P.A. Bosela and N.J. Delatte,
Technical Council on Forensic Engineering of the American Society of
Civil Engineers, Oct. 6-9, Cleveland, OH, ASCE, Reston, VA.
Jacobson, V.R. (2006). Analytical techniques and field verification
method for wind loading analysis of the historic prowers bridge, MS
Thesis, Civil Engineering, University of Colorado-Denver, Denver, CO.
Hamedian, S, (2006). Analysis and testing of the historic blue river
bridge subjected to wind, MS Thesis, Civil Engineering, University of
Colorado-Denver, Denver, CO.
6


Rutz, F.R., Rens, K.L., Jacobson, V., Hamedian, S., Elias, K., and
Swigert, W.B. (2006). Response of pin-connected truss bridges to
wind, Proceedings of the 2006 Structures Congress, ed. by B. Cross
and J. Finke, Structural Engineering Institute of the American Society of
Civil Engineers, May 18-21, St. Louis, MO, ASCE, Reston, VA.
Herrero, T., Rutz, F.R., and Rens, K.L. (2006). Field testing of historic
truss bridges using modular strain transducers, Proceedings of the 2006
Structures Congress, ed. by B. Cross and J. Finke, Structural
Engineering Institute of the American Society of Civil Engineers, May
18-21, St. Louis, MO, ASCE, Reston, VA.
Rutz, F.R., Rens, K.L., Jacobson, V., Hamedian, S., Elias, K., and
Swigert, W.B. (2005). Load paths in historic truss bridges, 2004-25,
prepared by Dept, of Civil Engineering, University of Colorado at
Denver for National Center for Preservation Technology Transfer,
Natchitoches, LA.
Rutz, F.R. (2004). Lateral load paths in historic truss bridges, PhD
Thesis, Civil Engineering, University of Colorado-Denver, Denver, CO.
Rutz, F.R. and Rens, K.L. (2004). Alternative load paths in historic
truss bridges: new approaches for preservation, Proceedings of the
2004 Structures Congress, May 22-26, Nashville, TN, ASCE, Reston,
VA.
7


Work accepted for publication (in press):
Herrero, T.V., Rutz, F.R. and Rens, K.L., Wind pressure and strain
measurements on bridges part II: strain transducer development,
Journal of Performance of Constructed Facilities, American Society of
Civil Engineers, Reston VA. (in press).
Rutz, F.R. and Rens, K.L., Wind pressure and strain measurements on
bridges part I: instrumentation/data collection system, Journal of
Performance of Constructed Facilities, American Society of Civil
Engineers, Reston VA. (in press).
Rutz, F.R., Rens, K.L., Jacobson, V.R., Hamedian, S., Elias, K.M., and
Swigert, W.B. (2006). Structural modeling for improved lateral
stiffness in historic truss bridges, APT Bulletin The Journal of
Preservation Technology, The Association for Preservation Technology,
Mt. Ida Press, Albany, NY (in press).
Rutz, F.R., Rens, K.L. (2006). Wind loads for 19th century bridges:
design evolution, historic failures, and modem preservation, Journal of
Performance of Constructed Facilities, ASCE, Reston, VA (in press).
University of Colorado Denver, Civil Engineering Department Reports:
Carroll, D. (2003). Analysis of historic pin-connected through truss
bridges for conversion into pedestrian use, Dept, of Civil Engineering,
Univ. of Colorado at Denver, Denver, CO.
8


Herrero, T. (2003). Development of strain transducer prototype for use
in field determination of bridge truss member forces, Dept, of Civil
Engineering, Univ. of Colorado at Denver, Denver, CO.
1.3 Former to Modern Procedures
Computer analyses versus manual calculations have changed. The basis
for these calculations has stayed the same. In addition, engineers today
distribute the loads using the skeleton model and do not utilize alternative
load paths. The deck and stringers represents an alternative load path that is in
most cases ignored. Adding load paths such as the deck and stringer system will
add strength and stiffness to any bridge. Engineers in the past and even today
overlook the deck and stringer calculations due to the limited time and budget
situations. Neglecting these essential load paths can lead to the incorrect
conclusion that wind load results in structure overstress.
1.4 Preservation
After studying many cases, observations had proven no physical
evidence of that wind damage to any of the exposed bridges, even after a
century of exposure (Rutz, 2006). Since these bridges are on the order of 100
years old, they have undoubtedly been subjected to many severe windstorms.
Historic bridges are standing icons of the American engineering development.
9


If these bridges can be converted to pedestrian use, the public will have access
to view the great American engineering innovations and admire the progress that
was accomplished in the late 19th and early 20th centuries.
1.5 Loads
Superimposed dead and superimposed live loads are still computed
manually, the same way as the 19th century design. Self-weight can computed
manually, or may be determined by software. The AASHTO Guide
Specifications for the Design of Pedestrian Bridges (AASHTO, 1997) prescribes
the live load value. It varies between 3.11 kPa to 4.207 kPa (65 psf to 85 psf),
depending on the area of the walkway. The late 19th century and early 20th
century designer selected design wind loads on a case-by-case basis, which
varied typically from 1.44 kPa to 2.39 kPa (30 psf to 50 psf) applied to the
projected area of the components (Smith, 1881; Waddell, 1898; and Cooper,
1905). However, todays AASHTO Guide Specifications for the Design of
Pedestrian Bridges mandates 3.59 kPa (75 psf) applied to the same area (Rutz,
Rens, Elias, Jacobson, Hamedian and Swigert, 2005).
This research will build on the original skeleton analysis of the San
Miguel Bridge which was completed earlier by Carrol (2003), Carrol used RISA
3D software to analyze the bridge truss members under AASHTO wind loads.
Carrol was specifically looking for applied wind forces and their stress effect on
10


truss bridges. The authors observation of overstressed portals and eyebars at
San Miguel Bridge will be discussed. The skeleton load paths versus alternative
load paths will also be discussed. In conclusion, the authors assumed load
paths for skeleton structures versus actual load paths for an existing truss bridge
will be presented. Refer to Chapter 4 and 5 for the field measurements and
RISA 3D analysis.
11


2.
San Miguel Bridge
2.1 The History Behind the Bridge
The Fifth Street Bridge in Grand Junction, Colorado was the first major
bridge project to be designed and managed by the Colorado State Engineers
Office. It was designed by State Engineer E.S. Nettleton and manufactured by
the Phoenix Bridge Co., Phoenixville, Pennsylvania. The Fifth Street Bridge
was one of three bridges to be constructed to serve as a cross way over the
Grand Mesa River in Grand Junction, Colorado, which is known as Colorado
River today (Rutz, 2004). The bridge consisted of new wrought iron truss with
five approximately equal timber deck and stringer spans. The Fifth Street
Bridge was built in 1886 and is shown in Figure 2.1. The bridge served as a
cross bridge for wagons and then vehicles through the late nineteenth and early
twentieth centuries. In 1933 a new steel truss bridge was constructed adjacent to
the existing structure over the Colorado River, shown in Figure 2.2. The steel
bridge served in that location for approximately sixty years. In the early 1990s
a four-lane, prestressed concrete girder bridge was built. Figure 2.3 shows the
prestressed concrete bridge along with the 1933 bridge as it appears today in
Grand Junction.
12


Figure 2.1. (Fifth Street Bridge Over Grand River), Grand Junction, Colorado.
Built in 1886, this Pratt truss was of wrought iron construction. (Photo courtesy
of Loyd Files Research Library, Museum of Western Colorado).
Figure 2.2. (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. (Photo courtesy of Loyd Files
Research Library, Museum of Western Colorado).
13


Figure 2.3. (The 1990 Prestressed Concrete Bridge as it Appears Today). The
bridge crosses over Colorado River close to the Botanical Garden near
downtown Grand Junction adjacent to the 1933 steel bridge. (Photo by K.
Delaney, 2002).
2.2 San Miguel Bridge Location
One of the five spans from the original 1886 Colorado River Bridge was
moved 87 miles to the south. It was reassembled over the San Miguel River,
approximately 2.0 miles northwest of Uravan in Montrose County, Colorado, at
an elevation of approximately 4,900 feet. The bridge as it appears today shown
in Figure 2.4, was abandoned in early 1990s, when a replacement span was
constructed a short distance upstream.
14


Figure 2.4. (San Miguel Bridge). When the Fifth Street Bridge was replaced in
1933, one of its spans was dismantled and reconstructed over the San Miguel
River in western Montrose County. It serviced ore hauling trucks for Vanadium
and Uranium mining operations until 1984, when the last mine was shut down.
It was abandoned in the early 1990s when a replacement bridge was
constructed on a new roadway alignment. Here it awaits its fate as an
abandoned bridge. (Photo by F.R. Rutz, 2002).
The site is over the San Miguel River, approximately 2.5 miles upstream
(east) of its junction with the Dolores River. The bridge, which is now closed is
on an abandoned Colorado County Road approximately 600 feet long, which
formerly connected State Highway 141 with County Road 110. Figures 2.5 and
2.6 show the exact road map location of the San Miguel Bridge.
15


Riv&Mt
kimtwH tWw
Baggy
PwdW.* ^ K^r*t*j <^Fe
O,^ 0Cra£
1 Sunbam
t,
Steam M
Q
OtkCfe*
S ~ 0
^oR*s't,@ fetter HotS1*ftj^prin9o
Rnj Blanco
e Beque
Mrd j5*^" -^Central Cit/
MU ^ ^
ElJebd
ort C^is
Greeley Bucknghaa
Chappel "^Ogf
^JU stxaj___
** ir
CteMIe
Erwters
'HovtWggm
0
Yuma
W iy
fleton
* o^*" ct
lef Tiai
si Fn turn
O
Aspen 3^?riH' :, Lsrtapw|cse Rock
i Park
1 Junction Vii
cP,0n J, ^CatoitoSpfings^^^^ f)L O3*1*'
w cw ------- \ MCanon 0 5 ;(.
?iJ> OWc WWfwe o i. V7 Caflon Qlty Hwwel 0Ea* £ S
u.^1 -li^on .Colons 5apirw)0 (Pueklo _ Brmdon /T^:
^ ^ syt; ^
a Ple^tView
0,.
Overton greeds ^
V o
jfmf* **Â¥>on
Rocky Ford * L*nW
1 o UJuftla
0 Carton q^j g
3yta<
. Rockwood
b
- . - of"05*
Durango PagosaSj^gs IMnci

o o
1 tti w*^.
-7rr7) Urafr
o La V*
LSan Luis
Priteftett
in. X;_______ *____JL.
Jfew Mexico
Figure 2.5. (Colorado Map). The red star shows the current location of San
Miguel Bridge.
16


Figure 2.6. (Location Map). One of the five original spans was relocated from
Grand Junction, CO to a now abandoned County Road northwest of Uravan in
Montrose County, CO spanning the San Miguel River (DeLorme 1997). The
bridge still resides in this location, but was closed to vehicular traffic in 1988
(Fraser, 2000).
2.3 San Miguel Bridge Description
The original bridge in Grand Junction, Colorado, consisted of a wrought
iron truss with approximately five equal spans. The total length of each span is
142-feet (43-meters) resulting in a total bridge span of 740-feet (226-meters),
and a nominal clear road width of 14-feet (4-meters). The superstructure is a
classic pin-connected Pratt through-truss.
17


The bridge consists of top chords and end diagonals of back-to-back
channels with a cover plate on the top and battens on the bottom. The bottom
chords consist of rectangular eyebars; the vertical members of back-to-back
channels with lacing on each side; the diagonal bracings consist of rectangular
eyebars, square eyebars, and round bars. All of the built-up members are riveted
and major connections are pins. The floor beams are hung from the vertical
members shaped as a fishtail girder style and support the steel stringers. The
stringers are steel wide flange members with a corrugated half-culvert deck and
gravel overlay as shown in Figure 2.7.
18


There is round rod X-bracing in each bay in the plane of the top chords
and also in the plane of the bottom chords under the deck. The upper portion of
each portal is filled with diagonal lattice. There is a W type guardrail attached
to the trusses and also to the steel posts midway between the truss verticals.
All member sizes and dimensions for this structure were primarily
obtained from field measurements made available by Carroll (2003), which was
originally collected from the Colorado Department of Transportation Bridge
Inspection Report (Range, 1991). Some damaged lattice bars of the cross
bracing on one end of the portals were found and noted. The site plan and front
elevation schematics of the structure are shown in Figure 2.8 and 2.9.
\
kcA-O. HUT 14-1
\
S^rrKO A£H KAIL. 6
PLAKl
Figure 2.8. (San Miguel Bridge, Site Plan). (Range, 1991)
19


SYMMETRICAL
Ul U2 Ui U4 U5 U6 U7 U8
/ \ \ * 1
\ Y Y : i / -1
-*----------
Ld Li L2 Li L4 L? L6 L7 Ls
142'-2"
Lo
NOTE: PIN TRUSS W/ 2 0 PINS AT PANEL POINTS
Figure 2.9. San Miguel Bridge, Front Elevation.
2.4 Bridge Members
2.4.1 Top Chords and Diagonal Portals
The top chords Ui Ug and the diagonal portals L0Ui and LgUg of the San
Miguel Bridge consist of 2 C9 x 2 !4'x 5/16 inches (22.86 x 5.72 x 0.79
centimeters) steel channels. The channels are spaced 11 % inches (29.85
centimeters) apart and connected with a 5/16 x 12 inch (0.79 x 30.48 centimeter)
continuous steel plate on the top and a 14 x 4 x 12 inch (0.64 x 10.16 x 30.48
20


centimeter) batten steel plate at 3-2 (96.52 centimeters) on center at the
bottom. Figure 2.10 and Figure 2.11 shows the diagonal portal detail schematic
and the diagonal portal at the frame end of San Miguel Bridge respectively.

x 12"
(0.79x30.48 CM.)
RIVETS 4" C).C\
(10.16 CM. O.C.) TYF.
C 9" x 2 y*" x vi"
(C 22.86 x 5.72 x 0.79 CM.)
1/4" x 4" x 12" (it 5'-l" O.C.
(0.64 x 10.16x30.48 CM.
a 96.52 CM. O.C.) BATTEN
STEEL PLATE
NORTH & SOlTTH PORTAL
(LoUi LoUs) & TOP CHORD
(ITi THRlT Ifr) DETAIL
Figure 2.10. San Miguel Bridge, Portals and Top Chord Detail.
21


Figure 2.11. Diagonal Portal at the Frame End of San Miguel Bridge.
2.4.2 Cross Bracing
In Figure 2.12 and Figure 2.13 the end portal elevation schematic is
shown along with the damaged 1 14 x 14 x 12 inch pitch (3.18 x 0.64 x 30.48
centimeter pitch) lattice bars on one side of the span. The lattice bars are located
diagonally on both ends at the top of the bridge frame. The cross bracing is
located perpendicular to the main Pratt frame across the top of the bridge and
consists of L2 x 2 x 14 inch (L5.08 x 5.08 x 0.64 centimeter) with 1 14 x % x 13
inch (3.18 x 0.64 x 33.02 centimeter) pitch lattice bars held together with L2 x 2
x 14 inch (L5.08 x 5.08 x 0.64 centimeter) brace kickers on both sides of the
22


cross bracing. The cross bracing is attached with 7/8 inch diameter X-bracing
bars at top cross bracing.
BROKEN LATICE
Figure 2.12. Lattice Bars at End Portal Elevation.
23
16'-8" (5.08 M)


2.4.3 Bottom Chords and Diagonal Bracings
The San Miguel Bridge consists of four different double steel plate
bottom chord eyebars. The four different members are listed below
individually:
L0L1, L1L2- (2) PL 7/8 x 2 Vi inch (2.22 x 6.35 centimeter) Eyebars.
L2L3- (2) PL 1 14 x 3 inch (3.18 x 7.62 centimeter) Eyebars.
L3L4- (2) PL 1 14 x 4 inch (2.22 x 10.16 centimeter) Eyebars.
L4L5- (2) PL 1 3/8 x 4 inch (3.49 x 10.16 centimeter) Eyebars.
Figure 2.14 shows the bottom chords of the structure.
24


The six different diagonal bracings at San Miguel Bridge are also listed below:
U1L2- (2) PL 15/16x3 inch (2.38 x 7.62 centimeter) Eyebars.
U2L3- (2) PL 3/4 x 2 1/2 inch (1.91 x 6.35 centimeter) Eyebars.
U3L4- (2) PL 3/4 x 2 inch (1.91 x 5.08 centimeter) Eyebars.
L2U3- (1) 3/4 inch. (1.91 centimeter) diameter bars.
L3U4 (1) 1 inch (2.54 centimeter) diameter bars.
U4L5, L4U5 (1) 1 1/4 inch (3.49 x 10.16 centimeter) square Eyebars.
The various types of bracing at different locations are shown in Figure 2.15.
Figure 2.14. Bottom Chord Eyebars at San Miguel Bridge.
25


Figure 2.15. Bracings at Different Locations on the San Miguel Bridge.
2.4.4 Vertical Columns
Posts (vertical columns) are located approximately 15 feet-10 inches (4.6
meters) on center and attached to the floor beams on either side of the deck.
Two out of three vertical columns consist of double lattice channels. The double
channels are 8 inches (20.32 centimeters) apart and connected together by lacing
bars. The third post type consists of two continuous L shape channels. Figure
2.16 shows the cross-sections and sizes of each column type. The double lattice
channels of the vertical columns on San Miguel Bridge are shown in Figure
2.17.
26


C 7"x2 "x%"
RIVETS @ 12" O.C.
(30.48 C'M. O.C.) TYP.
"x 12"PITCH
<04)4x381x3048 CM.
w TOP&BOT.LATICE
BARS STACKED
(2 LOCATIONS)
VERTICAL MEMBER U:L:
C 6"xl %"x34i"
RIVETS @ 12" O.C.
(30.48 CM. O.C.) TYP.
lA" x 114" x 12" PITCH
(0jf4 x 318 x 30.48 CM.
w TOP^BOT.LATICE
BARS STACKED
(2 LOCATIONS)
VERTICAL MEMBER UjL) ILL,
% O
(1P1 CM.)
HOLE
(2) L 3" x 2" x M"
((2>L 762 x5J08 x
0.64 CM.) TYPICAL
XTIRTICAL MEMBER UiLi
Figure 2.16. Vertical Column Cross Sections at San Miguel Bridge.
27


Figure 2.17. Vertical Posts at San Miguel Bridge.
2.4.5 Abutments
The north abutment is a reinforced concrete cap set on a short solid stone
wall, on top of solid native rock. The south abutment consists of steel piles,
which directly support the main bearings, along with a reinforced concrete
backwall. A full detail of the front elevation of the bridge along with the
abutment details are shown on Figure 2.18.
28


1991).
2.4.6 Deck
The bridge deck consists of road base gravel on an unusual system of
galvanized pipe culvert halves, semi-circular lengths of corrugated metal 3/4 x 2
% x 3/32 inches (1.91 x 6.99 x 0.24 centimeters), set between steel stringers.
Some of the corrugated metal culvert halves have been repaired using timber
planks. As shown in Figure 2.19, numerous holes and heavy rust at joints were
found mainly at the south and south west end of the bridge. Figure 2.20 shows
the top of the deck and Figure 2.21 shows the galvanized pipe culvert halves on
the bottom of the deck.
29
HU
O-L MU


Figure 2.20. (Top of San Miguel Bridge Deck), as it appears today near Uravan,
Colorado).
30


Figure 2.21. (Underside of San Miguel Bridge Deck), as it appears today near
Uravan, Colorado.
2.4.7 Floor Beams and Stringers
The two different types of stringers are shown in Figure 2.22. The first
type is an Exterior Stringer that consists of (2) C15 x 3 3/8 x 7/16 inches (38.10 x
8.57 x 0.44 centimeters) which are located on each side of the bay. Seven more
Interior Stringers consisting of 12 x 6 Vz inch (30.48 x 16.51 centimeter) I-beams
with a 3/8 inch (0.95 centimeter) thick flange are located between the galvanized
pipe culvert halves. The I-beams are supported by a 17-0 (5.18 meter) fishtail
31


girder style floor beam hung from the vertical members and consist of (4) L3 x 3 x
5/16 (L7.62 x 7.62 x 0.31 centimeters) with a !4 inch (0.64 centimeter) steel plate
as illustrated in Figure 2.22. A Photo of the floor system as it appears today as
shown in Figure 2.23.
spasms (F)
UZ 'Ux U'TH ,
LATTICE S>ARS W *
'/A' IS' PVT^H
* lA &kace.
Y-ZXKACIkI'S : Ve* f 3ARA
AT sfeA^HS
BR-'.PSE PEgg: SALV.
PlpE CULVERT HALVES
"VAi r. /S2
&ETUEEW STRIU^EPS
With Pirrr pill r TZT CT= EXTERIOR STR.
EXTEEkTK SrRljUSfR :
o)Ti&i5i=F7?r^'
LI/ ->/!<>> ti?E
I LITER ICR STRkJ^EK
rn lir peep- c^/z.
w/w PLAj^e
e '/it.' shape stcxJw
I,,
U*V
T^AMeVEKeE:
IV4.1 4> ears
AT FUa^r: seams
^ht. *1 ^
Figure 2.22. (San Miguel Bridge, Detailed Elevation and Section). (Range,
1991)
32


2.4.8 Railing
A steel vehicular type of rail has replaced the original railing. The
current San Miguel vehicular railing is a W type guard rail attached to the
trusses and the 6 inches x 4 inches (15.24 x 10.16 centimeters) I-shape steel
posts midway between truss verticals. Figure 2.24 shows the bridge railing that
exists today.
33


34


3.
Field Testing
3.1 Introduction
Various devices were utilized to complete the field work. The devices
were installed at different locations on the bridge to measure wind speed, wind
direction, and strains in the selected chord members.
To collect strain data under different wind speeds, sixteen transducers
were utilized. Two sets of four transducers were placed by clamping the
transducers adjacent to each other on the windward and leeward bottom chord
eyebars at mid-span where the axial wind force was the highest. Four more
transducers were installed on each end diagonal at the south and north portals
where the highest moment regions were predicted. Figure 3.1 shows the pre-test
set-up of San Miguel Bridge. Refer to Appendix A for additional test set-up
photographs. Five anemometers were also used to obtain wind speed during
data collection. In addition, a wind sensor instrument to measure wind direction
was installed directly upwind approximately at the center of San Miguel Bridge.
35


Figure 3.1. Field Experimentation of San Miguel Bridge.
3.2 System Arrangement
The system as shown in Figure 3.2 consists of sixteen strain transducers,
a wind direction detector and five anemometers. All of these instruments are
connected to the time interval and data logger which is powered by an
uninterruptible supply and a deep cell battery or a generator. The application
software (PC9000) was programmed into the laptop computer to decode the
field data. Each of the instruments in the schematic diagram is described below.
36


STRAIN TRANSDUCERS
WIND DIRECTION
ANEMOMETERS
LEEWARD WINDWARD SOUTH NORTH
EYEBAR EYEBAR PORTAL PORTAL WD WS1 WS2 WS3 WS4 WS5
SCHEMATIC DIAGRAM
TIM = TERMINAL INPUT MODULE (WHEATSTONE BRIDGE)
UPS = UNINTERUPTABLE POWER SUPPLY
Figure 3.2. System Arrangement Diagram.
3.3 Strain Transducers
The strain transducers were initially developed by Herrero (2003). Most
of the transducers used in the field work were already functional. Nevertheless,
the field team had to adopt Herrero producers to assemble extra needed
transducers to conduct the research testing. The transducer is a device consists
of a 3 inches (7.62 cm) steel ring attached to two steel angles which is then
clamped to a bridge element to study its deformation. Figure 3.3 shows a three
dimensional finite element analysis rendering transducer.
37


Figure 3.3. Three Dimensional Drawing of a Strain Transducer Device.
(Herrero, 2003).
The strain gage is adhered to the inside surface of the strain transducer
ring as shown in Figure 3.4. The strain gage in the transducer is considered a
quarter bridge strain gage because it is one of four resistors in the Wheatstone
bridge circuit, which is explained in greater detail in the following sections.
Model CEA-06-250VW-12c strain gages, manufactured by Vishay Micro
Measurements Group, were used.
38


The gage factor for the strain gages was either 2.065 or 2.095. When the
bridge member is under axial strain the transducer ring will deform, the true
strain is obtained by multiplying the transducer strain by a factor determined
theoretically and confirmed experimentally by Herrero (2003). Figure 3.5
shows the approximate locations of the sixteen transducers that were installed on
San Miguel Bridge.
39


*015
*013 *016
*G14
Figure 3.5. (Diagram Illustrating the Locations of the Strain Transducers).
North is to the left. The wind direction was from the west, orthogonal to the
bridge. Strain transducer numbers Gl, G2, G3 & G5 were clamped to the
leeward bottom chord eyebars. G4, G6, G7 & G8 were clamped to the
windward bottom chord eyebars. G9 G12 were clamped to the end diagonals
at the north end post. G13 G16 were clamped to the end diagonals at the south
end post.
011
G12
*08
*010
t
=t
-----*GT/32,03&05
I.G6.G7&G8
3.4 Wheatstone Bridge
Wheatstone bridge module is connected directly to each of the sixteen
strain channels. The model is a 4-wire full bridge device called 4WFB120
Terminal Input Modules (TIMs) that was provided by Campbell Scientific to
run the bridge circuits. The bridge wiring schematic and Terminal Input Module
are shown in Figure 3.6. The resistor in the bridge circuit had a resistance of
120 ohms plus or minus a tolerance of 0.01%, which matches the nominal
resistance of the quarter bridge strain gage (Campbell Scientific, 1996b).
Because the actual change in resistance of the strain gage is small, a full bridge
configuration was used to give the maximum resolution. A quarter bridge
40


strain gage is named because the strain gage is considered one of four resistors
that make up a full bridge. The TIM module provides the other three resistors.
Figure 3.6. (Bridge Wiring Schematic and Terminal Input Module (TIM)). The
voltage is supplied from the data logger through the terminal Vx- The TIM
connects to the data logger via three pins (labeled H, L, and AG in the
schematic) (Campbell Scientific, 1996b).
3.5 Wind Direction Sensor
The wind direction sensor is of corrosion resistant construction, with
stainless steel shafts, precision instrument ball bearings, and is lubricated with
wide temperature range high quality instrument oil. (Campbell Scientific
1996a). The R.M. Young Model 03301-5 Wind Sentry Vane rotates its pointer
position in the direction of the wind. Figure 3.7 shows the wind direction sensor
as mounted on the San Miguel Bridge.
41


As the vane shaft turns, the potentiometer changes electrical resistance
which creates an excitation voltage that determines the wind direction. The
wind direction sensor was aligned with the longitudinal direction of the bridge.
The north end of the bridge is considered to be zero degrees and the south end
180 degrees. Therefore, the wind from the west direction would be at 270
degrees which is transverse to the bridge. The local directions used herein for
San Miguel Bridge are different from the true directions. The bridge is aligned
east-west.
42


3.6 Anemometers
The anemometer is similar to the wind direction sensor as it is of
corrosion resistant construction, stainless steel shafts, precision instrument ball
bearings, and well lubricated. The anemometer rotates along its shaft to
measure wind speed. Five R.M. Young model 0310-5 anemometers were used
to obtain wind data in this experiment. As shown in Figure 3.8, they have three
cups connected to a wheel on a vertical shaft. The shaft drives an AC generator,
which produces a sine wave output voltage signal directly proportional to the
wind speed.
Figure 3.8. One of the Anemometers is Located Directly Below the San Miguel
Bridge Deck.
43


One complete sine wave cycle is produced for each cup wheel revolution
(Campbell Scientific 1996a). Figure 3.9 shows the approximate wind direction
sensor and anemometer locations on San Miguel Bridge.
WS2 *WS5
Figure 3.9. (Diagram Illustrating the Locations of Anemometers (WS1 WS5)
and wind direction sensor (WD)). North is to the left. WS1 was positioned
directly upwind of the approximate center of the wind intercept area. WS2 and
WS5 were located 1.5 meters (approximately 5 feet) above the top of the end
diagonal members in the end post. WS3 and WS4 were positioned 2 meters
(approximately 7 feet) below the bridge deck, at an elevation mid-height
between the bridge deck and the water surface below.
3.7 Interval Timer
The interval timer receives the sine wave voltage signals that were
generated by the anemometers, and to download processed data to the data
logger for logging. The data logger processes this information into wind
velocity in units of either meters per second or miles per hour. A Campbell
Scientific model SDM-INT8 interval timer, shown in Figure 3.10 was used
to download the processed data to the data logger.
44


3.8 Data Logger
The Campbell Scientific model CR5000 data logger was utilized for the
field work. The data logger sample intervals from different channels up to 5000
Hz. sampling rate. All outputs, logged at a sampling rate of 0.1 seconds (10
45


Hz). The experiment used 17 channels, one channel for each of the 16
transducers and one for the wind direction measurements.
The data logger provides excitation voltage for these devices, and measures the
voltage drop. The five anemometers were connected via the interval timer to
one of two pulse counters on the data logger. Figure 3.11 shows the Campbell
Scientific data logger and Figure 3.12 shows the data logger and the time
interval during the experiment at San Miguel Bridge.
Figure 3.11. Campbell Scientific CR5000 Data Logger (Rutz, 2004).
46


Figure 3.12. Campbell Scientific CR5000 Data Logger and Time Interval at San
Miguel Bridge Being Connected by K. Elias.
3.9 Software
To have manageable data tables, the PC 9000 software was utilized
along with the data logger. Each of the collected data consisted of wind speeds,
wind direction, strain transducer outputs and date and time of logging. This
software was accessed from the VALNEW5 program, which was developed
with the aid of Campbell Scientific to assist in the experiment. All of the data
were logged at a sampling rate of 0.1 seconds (10 Hz). Figure 3.13 shows a
47


screen capture of the PC 9000 software. A program listing for VALNEW5 is
included in Appendix B.
CPC9000 APPLICATION SOFTWARE CR5000 Station: 1696 [FIELD MONITOR 1]
p File Edit Realtime Analysis Tools Collect Options Windows Help
Flag 1 r
Flag 2 re
Flag 3
Flag 4 PS
Flag 5 re
Flag 6 m
Flag Z re
Flag 8 re
CR5000
DEMO
F? Flag
Verify
Help
Print
Quit
Units Table Time

FIELD MONITOR 1
STOP
BSME;
Fid NAME VAL
BBWindlnterval(7: O'
HB WindDir 0
|Q Strain 1 90
IE! Strain2 -479.
E3 Strain3 -82.
El Strain4 -23
El Strains -582.
El Strains -100.
El Stram7 -185.
El Strains 23.
El Strain9 -52
HStrainlO Range?
^Strain 11 1.
El Strain 12 -139.
E3 Strain 13 -452.
ElStrain14 321
E3Strain15 28.
ElStrain16 -202
fi
I
Record Number
1 Table Size: 1
£[Disk File
Reset Table
ile j
STOP
L
[
fid | NAME jVAL

Record Number
2g&Table Size:
W:}-
Reset Table
r
3
r
Figure 3.13. PC 9000 Figure of Software.
48


3.10 Laptop Computer
A Dell laptop computer was utilized along with the data logger for the
field work. The laptop was used for uploading the program to the data logger
and for downloading the data tables from the data logger. Windows 98 software
was the operating system utilized at the time of testing.
3.11 Cables
Each strain gage had copper lead wires of 26 American Wire Gauge
(AWG) soldered to its terminals. The strain wire length ranged approximately
between 2 ft (0.61m) to 5 ft (1.52 m). However, the strain gages had the same
wire resistance along the entire length. Refer to C.2.1 in Appendix C for more
of cable resistance details.
The lead wires were connected to the data logger by three 18 AWG
copper wires with PVC insulation, all three wires were wrapped inside a foil and
a PVC sheathing cover. The lengths of these wires varied between 78 ft (23.77
m) and 250 ft (76.2 m). All wires were labeled on each end to ensure the correct
installation of the transducers to the data logger channels. The foil shield was
grounded to the data logger to provide shielding from spurious electrical signals.
Figure 3.14 shows some of the cables that were used in San Miguel Bridge
49


50


4.
San Miguel Bridge Modeling and Analysis
4.1 Introduction
The traditional skeleton method analysis and calculations for gravity
loads on bridges has not changed; however, the code wind requirements have
changed. The AASHTO Guide Specifications for the Design of Pedestrian
Bridges (AASHTO 1997) require higher design wind load than was required in
the past. Adding load paths such as the deck and stringer system adds strength
and stiffness that traditionally is not included in any skeleton analysis. The
bridge was modeled and analyzed using a structural finite element software
program RISA-3D software (RISA 2002). RISA-3D is a multi purpose finite
element program software program that is capable of analyzing both frame
elements and plate/shell elements.
The superimposed dead loads were determined by the software. In
addition, the bridge was analyzed under wind plus dead loads. The AASHTO
Guide Specifications for the Design of Pedestrian Bridges (AASHTO, 1997)
prescribes the live load value. It varied between 3.11 kPa and 4.07 kPa (65 psf
to 85 psf), depending on the area of the walkway. The late 19th century and
early 20th century designer selected design wind loads on a case-by-case
51


basis, which varied typically from 1.44 kPa to 2.39 kPa (30 psf to 50 psf)
applied to the projected area of the components (Smith, 1881, Waddell, 1898,
Cooper, 1905). Todays AASHTO Guide Specifications for the Design of
Pedestrian Bridges mandates 3.59 kPa (75 psf) applied to the same area
(NCPTT, 2006).
4.2 Modeling
The two major models analyzed in this section were as follows:
Skeleton model.
Deck model.
The bridge boundary conditions for both models were chosen to be
pinned joints at one end and roller joints at the other end. These boundary
conditions were utilized for modeling purposes; the verification model in
Chapter 5 will have the actual field boundary conditions. The boundary
conditions on this model indicate that the roller joints are restrained from lateral
translation, but are free to translate in the longitudinal direction of the bridge.
All internal member connections were assumed to be pinned and released to
rotate which is common engineering practice.
The traditional structural pin connected skeleton frame shown in Figure
4.1 was analyzed under AASHTO loads for pedestrian bridges. This analysis
was compared to the deck model subject to the same loads. The skeletal model
52


is compared to the deck model in order to gather concrete data to verify that the
axial forces in the bottom chord eyebars compared to those calculated using a
traditional skeleton model are reduced. This will determine that the
combination of the skeleton model plus stringers plus deck stiffens the bridge in
the lateral direction.
Figure 4.1. (Illustration of the Traditional Skeleton Structure), based on steel
members only. The boundary conditions of pinned at one end and rollers at the
other end are indicated. The rollers are restrained from translation in the lateral
direction.
4.3 Skelton Model
4.3.1 Skelton Model Due to Gravity Loads
Figure 4.2 illustrates the gravity loads, superimposed vertical dead and
live loads, applied to the skeleton structure. Figure 4.3 illustrates the relative
axial force in bottom chords eyebars due to gravity loads for the skeleton
53


structure using RISA-3D software. Note relative force is larger at midspan, as
expected.
Figure 4.2. Gravity Loads Applied to Skeleton Structure.
Figure 4.3. Diagram of Axial Force in Bottom Chord Eyebars Due to Gravity
Loads for the Skeleton Structure.
54


4.3.2 Skelton Model Subjected to Lateral Loads
The same skeleton model structure used in the gravity load analysis was
utilized to study the lateral wind effect. When applying the wind pressure based
on the AASHTO criteria of 3.59 kPa (75 psf) as shown in Figure 4.4, the bottom
chord eyebars were in compression on the windward side and in tension on the
leeward side as illustrated in Figure 4.5. Note that the last bay of the bridge at
the pinned ends changes the axial force signs for both of the leeward and the
windward sides. The reversal sign in member forces on both sides near the
pinned ends could be compared to a cantilever beam condition as illustrated in
Figure 4.6. Therefore, the lateral wind force compression in the bridge is greater
than the gravity load tension on the windward side.
Figure 4.4. (Wind Load Applied to Skeleton Structure). The wind load shown
is based on the AASFITO criteria of 3.59 kPa (75psf).
55


Compression on
Figure 4.5. (Diagram of Axial Force in Bottom Chords Due to Wind Load).
The reversal in sign occurs near the pinned end.
R, viV l ^ wl //// 1
TTTTTT
-xJ Ttt>^j
Shear li l 4
u,\_
Moment \|^ "jvax
Figure 4.6. (Shear and Moment Diagrams for a Cantilever Beam Condition),
from the AISC Manual of Steel Construction (AISC 2001).
56


4.4 Deck Model
The deck shown in Figure 4.7, believed to have been installed in 1964,
consists of semi-circular corrugated metal pipe segments which bear on the
bottom flanges of steel stringers. The metal pipe has been replaced with timber
blocks in a few places where the corrugated metal pipe has rusted away. The
semi-circular corrugated metal pipe segments were topped with gravel roadbase.
57


This deck is much heavier than its original timber deck. The gravel
roadbase produces a heavy dead load of approximately 3.54 kPa (74 psf), much
higher than the original timber deck, which was estimated at approximately 0.62
kPa (13 psf). The deck was modeled using interconnected plate elements to
represent the gravel roadbase. All the elements were added to the skeleton
model along with the stringers and deck to form the model shown in Figure 4.8.
Figure 4.8. Skeleton Model with Stringers and Deck.
The steel stringers bear on the floor beams as shown in Figure 4.9. The
interior stringers were 12 inch deep wide flange beams while the outer stringer
was a 15 inch deep channel.
58


Figure 4.9. RISA Rendering of Floor Beams and Stringers.
The frame elements were offset from each other as illustrated in Figure
4.10 to represent the stacking of actual stringers on the floor beams. The plate
elements representing the deck were offset in a similar manner.
59


ROAD BASE
A..F'. -V\
FIXED END CONNECTION
RELEASED CONNECTION
FLOOR BEAM
FRAME ELEMENT
TYPICAL
FIXED END
CONNECTION
STEEL FLOOR
BEAM TYPICAL
SEMICIRCUUAR
v CORRUGATED METAL PIPE
VSTRINGER FRAME
ELEMENT TYPICAL
-STEEL
STRINGER
TYPICAL
Figure 4.10. Offset Members and Release Locations.
To analyze the deck in the offset elements, shown in Figure 4.11, were
modeled such that the deck elements were at the same elevation as the stringer
top flanges. This was done for modeling simplicity. Note that the deck is
represented as centered on the top flange of the stringers, intended to
approximate the actual roadbase material, the deck was modeled as an elastic
solid that enveloped the top flange of the stringers.
60


4.5 Comparison
San Miguel Bridge deck, with its large amount of gravel roadbase, has a
relatively heavy dead load. Dead load due to the gravel was approximately 75
psf, much higher than any of the other decks in the study. It is clearly much
heavier than the original (1886) timber deck.
When considering the deck as a diaphragm, predictably the axial forces
in the bottom chord eyebars reduced when compared to the values analyzed in
the skeleton model. This reduction in the axial forces was due to the added
lateral stiffness to the structure when adding the deck diaphragm. The results of
axial forces in the bottom chord eyebars are presented in Table 4.1.
61


Table 4.1. (Summary of Maximum Axial Compressive Forces in Bottom Chord
Eyebars). Forces are for windward side and are expressed in kN (kips),
followed by percent reduction (or increase in tension) compared to the
traditional skeleton value. (Positive = tension; negative = compression).
Model Axial force due to dead load only Axial compression due to wind load only Net axial force due to wind plus dead load*
Case 1:
Skeleton 399 -222 180
(89.7) (-49.9) (40.5)
Case 2:
Deck 317 -105 209
(71) (-23.7) (46.9)
21% 53% 16%
Case 3:
Diaphragm 316 -32.1 292
(71.1) (-7.2) (65.6)
21% 86% 62%
* Note values are not necessarily identical to (D+W) because tension-only
members may not be the same for the (D+W) case as for the individual D or W
cases.
Forces are for windward side bottom chord and are expressed in kN
(kips), followed by percent reduction (or increase in tension) compared to the
traditional skeleton value. (Positive = tension; negative = compression). The
percent change from the skeleton case was determined for the deck model from:
62


% change =100 x
Fskeleton Fdeck
Fskeleton
And for the diaphragm model from:
% change = 100 x
Fskeleton ~ Fdiaphragm
Fskeleton
(4.1)
(4.2)
Where:
F'skeleton = calculated force in windward bottom chord from the skeleton model.
Fdeck = calculated force in windward bottom chord from the deck model.
Fdiaphragm = calculated force in windward bottom chord from the diaphragm
model.
4.6 Conclusions
Studying the values in Table 4.1 indicate that the force in Case 3 for the
axial force due to dead load is decreased by 21 percent, while the axial
compression due to wind load is increased by 86 percent. It is concluded that
the combination of the skeleton model plus stringers plus deck stiffens the
bridge in the lateral direction, resulting in a significant reduction of axial forces
in the bottom chord eyebars compared to those calculated using a traditional
skeleton model.
63


5.
San Miguel Verification Method
5.1 Introduction
To verify the analysis, the forces gathered using the strain data from the
field testing of the bridge were compared to the RISA-3D model under actual
wind pressures. The strain data collected from field measurements were used to
verify if the implemented models were accurate.
To verify the modeling and analysis completed in Chapter 4, an
approximation of field conditions needed to be applied to a 3D analytical model.
The AASHTO Guide Specifications for the Design of Pedestrian Bridges 3.59
kPa (75 psf) which corresponds to approximately 194 km/hr (121 mph) were not
applied. Instead, the actual wind velocity was measured and actual pressures
were applied to the model.
5.2 Data Reduction
Implementing data reduction to the forces and moments was essential to
compare the results to the RISA-3D model. The low and high values measured
at 0.1 second intervals over 2-second wind event periods were averaged and this
64


pressure was determined for each quadrant and applied to the 3D modeling
program.
5.2.1 Bottom Chord Eyebars
Sixteen transducers and five anemometers were used to test the bridge.
Some of the transducers were positioned on the bottom-chord eyebars as shown
in Figure 5.1. The location of the gage within the transducer senses flexural and
axial strain. By using two transducers, one on each side, and averaging their
strain values, the flexural component of strain was compensated for, leaving
only axial strain. This is explained in greater clarification in Section C.4,
Appendix C.
Figure 5.1. Strain Transducer Arrangement on Bottom-Chord Eyebar.
5.2.2 Moments in Portal Frames
To compare the measured data to the 3D analyses, it is desired to know
the end moments in the members. Four transducers were located at each end
65


diagonal to measure moments in portal frames. The location of the transducers
were at the knee brace near the top of the end diagonals and above the pin
location at the bottom of the end diagonal as shown in Figure 5.2. The
measured data is a combination of axial plus flexural strain. Refer to Moments
in Portal Frames section in Appendix C for a step by step illustration on how to
isolate the flexural strain and determine the end moments.
015
G11
.------GT^2,<33S05
G12
09
rG6,G7&G8
G13 *016
014
*G10
Figure 5.2. (Diagram Illustrating the Locations of the Strain Transducers).
North is to the left. The wind direction was from the west, orthogonal to the
bridge. Strain transducer numbers Gl, G2, G3 & G5 were clamped to the
leeward bottom chord eyebars. G4, G6, G7 & G8 were clamped to the
windward bottom chord eyebars. G9 G12 were clamped to the end diagonals
at the north end post. G13 G16 were clamped to the end diagonals at the south
end post.
5.2.3 Wind Pressure Theory
The Bernoulli wind pressure equation on a structural member used in the
analysis may be expressed as:
66


for SI units
(5.1)
p = 0.6l2CdCaV2
p = 0.00256CdCaV2 (For US customary units)
Or in the form of:
Ap = 0.00256CdCa(Vhigh2 Vlm2) (5.2)
Where:
Ap = The difference between high and low wind pressures, Pa
(psf).
Q = Drag Coefficient.
Ca = Altitude Coefficient, from ASCE 7-02 (ASCE 7-02, 2002b).
V/ngh= 2-second average of high wind velocity, m/s (mph).
Vlow = 2second average of low wind velocity, m/s (mph).
Refer to Appendix C, Section C.3 for further discussion on wind
pressure theory.
5.2.4 Wind Load by Quadrants
The anemometers were utilized simultaneously to collect wind speed
data at different locations of the bridge as described in Chapter 3. The strain
transducers collected data in milli-volt per volts, and the anemometers collected
wind speeds in mph. The wind time for the events used were between
11:57:23.1 and 12:02:54.4 a total of 2400 wind events. The data logger
67


collected these events on 5/30/2005 at a wind direction approximately 270.
The wind events used were approximately orthogonal on the bridge, a non-
orthogonal wind events were also monitored. Figure 5.3 shows the approximate
location of the wind devices.
WS2 *WS5
Figure 5.3. (Diagram Illustrating the Locations of Anemometers (WS1 WS5)
and Wind Direction Sensor (WD)). North is to the left. WS1 was positioned
directly upwind of the approximate center of the wind intercept area. WS2 and
WS5 were located 1.5 meters (approximately 5 feet) above the top of the end
diagonal members in the end post. WS3 and WS4 were positioned 2 meters
(approximately 7 feet) below the bridge deck, at an elevation mid-height
between the bridge deck and the water surface below.
The bridge was modeled with pressure acting on four different
quadrants. The pressure in each quadrant was determined by using the wind
pressure equation discussed earlier. This was applied to data from all five
anemometers. As shown in Figure 5.4, wind velocities from WS2 through WS5
were averaged with that from WS1 at the center of the wind intercept area. This
procedure was completed to determine a pressure to be applied over each of four
quadrants in the 3D-RISA model.
68


Figure 5.4. (Quadrants Subjected to Different Uniformly Distributed Wind
Pressures). Wind pressure on Quadrant 1 was determined from a weighted
average from the velocities measured at WS2 and WS1. Wind pressure at the
other quadrants was similarly determined.
The average wind speed method Wind Load by Quadrant for high and
low velocities was utilized. Both the low and high wind velocities correspond to
the average of 62 consecutive data measurements over two second intervals.
Velocities were measured from time at 86.9 seconds to 88.9 seconds at the top
of the spike and from time at 103.1 seconds to 105.1 seconds at the bottom of
the spike. Each set of values were averaged to remove some of the very local
fluctuations. The square root of the difference of the velocity squares was taken.
As an example, consider the wind velocity at anemometer WS1:
Twenty low values were averaged, resulting in a wind velocity of 4.54
m/s (10.22 mph). Twenty high values were also averaged, resulting in a wind
velocity of 15.49 m/s (34.83 mph). The average velocity was then
69


^(l5.492 -4.542) = 16.14 m/s (36.29 mph). A summary of the wind velocities,
quadrant average velocities, and quadrant pressures are shown in Table 5.1.
Table 5.1. Wind Velocities, Quadrant Average Velocities, and Quadrant
Pressures.
Anemometer Location Velocity m/s (mph) Average velocity for quadrant m/s (mph) Average pressure for quadrant Pa (psf)
WS5 Central 16.1 (36.3)
WS1 South 14.8 15.5 179.1
upper (33.2) (24.3) (3.7)
WS2 South 15.7 15.9 220.7
lower (35.3) (24.5) (4.6)
WS3 North 9.1 12.6 143.6
lower (20.4) (20.0) (3.0)
WS4 North 18.1 11.2 194.9
upper (40.7) (21.0) (4.1)
The pressures applied to the bridge members on each quadrant in the
RISA-3D model need to be multiplied by the depth of each member. Figure 5.5
shows the actual wind loads applied to the bridge members on all quadrants.
70


Figure 5.5. (Wind Pressure Applied to the Four Quadrants for Analysis). North
is to the left.
5.3 Boundary Conditions
Most bridges are considered to be simply supported beams when
analyzed, with a pin support on one end and a roller on the other end. However,
the roller support is often restrained against lateral translation. The bearings are
usually rusted, and filled with dirt deposits and debris. This alters the boundary
conditions as they may not be truly fixed or truly pinned. This suggests a small
degree of partial fixity. As a result, some rotational restraint about the Z axes of
the portal frames has been included.
71


5.4 Member Releases
Common practice of truss analysis is based on the assumption that all
internal members are pinned at the joints. Due to the relatively heavy structure
and the low lateral wind pressure along with corrosion and dirt in the
connections, rotation may not exist.
5.5 Verification Analysis
The conditions discussed in the previous sections were implemented into
the RISA-3D model to develop an analytical structure that closely replicates the
San Miguel Bridge under natural wind pressure. The data gathered in the field
was downloaded and converted into a spreadsheet (Rutz, 2005) which is
illustrated in Appendix D. Figures 5.6 through 5.11 were captured from the
same spreadsheet that is included in Appendix D. The graphs are a direct result
of the measured data that was collected during the wind event. The figures
plotted represent the wind speed, wind direction, strains at the bottom chord
eyebars, and strains at the south and north portals.
72


WIND SPEED
San Miguel 5/30/2005
Figure 5.6. (Wind Speed as Measured by the Five Anemometers). The bold line
shows the average of all five anemometers.
WIND DIRECTION
San Miguel 5/30/2005
360
300
b 240
T3
C
180
120
240
Figure 5.7. Wind Direction as Measured During the Test.
73


BOTTOM CHORDS
San Miguel 5/30/2005
WS avg Leeward Strain Windward Strain Leeward filtered Windward filtered |
Figure 5.8. (Strain Measurements for the Windward and Leeward Bottom Chord
Eyebars). The bold trace at the top is the average wind speed, to an arbitrary
scale. Measured strain in the leeward eyebar is shown above measured strain in
the windward eyebar. Both the raw data and a filtered line that removes much
of the signal noise by taking a 2- second rolling average are shown.
74


40
BOTTOM CHORDS: Change in strain
San Miguel 5/30/2005
30
20
n
Si 10
0
-10
80
90 100
______________________________Time (seconds)___________________________________
WS avg Delta Leeward Strain filtered Delta Windward Strain- filtered
110
Figure 5.9. (Enlargement of the Trace for Windward and Leeward Bottom
Chord Eyebar Measured Strains). Both are baseline traces of the filter data.
Thus, they represent the change in measured strain starting from the same point
in time as the corresponding change in wind velocity. The wind velocity is
shown for reference at the top of the graph to an arbitrary scale.
75


SOUTH PORTAL
San Miguel 5/30/2005
60 --------------------------------------------------------
60 120 Time (seconds) 180 240
Average WS/2.5 G13 2-Sec Avg G13 G14 2-Sec Avg G14 G15 2-Sec Avg G15 G16 2-Sec Avg G16
Figure 5.10. Measured Strains at the South Portal.
NORTH PORTAL
San Miguel 5/30/2005
0 60 120 180 240
_________________________Time (seconds)________________________________
WSavg G9 G10 G11 G9 2-Sec Avg G10 2-Sec Avg G112-Sec Avg
Figure 5.11. (Measured Strains at the North Portal). Gage 12 was eliminated
from the graph due to the strange strain behavior, attributed to the gage being
mounted adjacent to a broken member on the north portal.
76


The bold trace at the top of Figure 5.11 is the average wind speed, to an
arbitrary scale. Measured strain for both leeward and windward portals is
shown. Both the raw data and a filtered line that removes much of the signal
noise are shown. (Note: The strange record to the strain value around the 60
second time is most likely due to some signal drift it does not appear to be due
to mechanically-induced strain because the wind pressure is diminishing in this
range).
The raw data and a filtered line are baseline traces of the filtered data.
Thus, they represent the change in measured strain starting from the same point
in time as the corresponding change in wind velocity. The verification study
data is summarized in Table 5.2.
Table 5.2. (San Miguel Bridge Verification Summary). Comparison of
calculated to measured forces expressed in kN (kips) and kN-m (foot-kips).
Member Calculated Force Measured Force Correlation: % difference
Windward bottom chord -5.25 kN (-1.18 kips) -6.89 kN (-1.55 kips) 31%
Leeward bottom chord 4.45 kN (1.00 kips) 6.27 kN (1.41 kips) 41%
North portal upper -4.57 kN-M (-3.37 kip-ft) Broken Member -0.18 kN (-0.13 kip-ft) Broken Member N/A
North portal lower -0.31 kN-M (-0.23 kip-ft) -0.14 kN-M (-0.10 kip-ft) 57%
South portal upper 2.25 kN-M (1.66 kip-ft) 0.79 kN-M (0.58 kip-ft) 65%
South portal lower -4.66 kN-M (-3.44 kip-ft) -0.58 kN-M (-0.43 kip-ft) 87%
77


5.6 Conclusions
The study was an approximate verification of the San Miguel Bridge.
The verification had to be analyzed to study the different load paths influencing
the bridge. The weak correlation in the south lower portal may be attributable to
a possible inconsistency in the field data. The next sections will discuss some of
the issues that may possibly alter the results.
5.6.1 Correlation
Good correlation except for the south lower portal of the data results is
revealed comparing the results of the calculated forces to the measured forces in
Table 5.2. Listed below are some issues that might be helpful in clarifying
results:
The existing boundary conditions at the bottom of the portals in the
bridge might have changed from its original conditions. Rust, dirt
deposits, debris, and weather impact the abutments. That transformation
altered the boundary conditions from being pinned to being fixed, vise
versa, or someplace in between. These distresses probably impacted the
measured strains. Although the bearings were assumed to be fixed in the
calculations, probably were not be truly fixed, and may have permitted
some small rotation giving it partial fixity.
78


Hidden elements like deck attachments and their connections, whether
fixed, pined, or hinged should be verified and utilized. These
attachments will impact the moment along the deck.
The amount of gravel to sand mixture and their assumed stiffness
modules values vary.
Non-orthogonal wind events impacting the bridge lead to poor
correlations at the end posts, which amounts to difficulty in modeling
the lateral component of wind pressure. The wind pressure was not
directly perpendicular to the bridge during the entire event (between
270 and 290), which reduces the measured force in the leeward bottom
chord.
5.6.2 Drift
The strange electrical noise records drift affecting the data as shown in
Figure 5.8, Figure 5.10 and Figure 5.11 for the strain measurements at the
bottom chords and north and south end portals can be explained by:
Slipping or movement of the C- clamps at the transducers following the
wind velocity spike.
79


The movement of the wires connected between the transducers and the
data logger under high wind pressure. The transducer strain result output
is in units of millivolts per volt of excitation. The wire resistance to the
high wind might have created some electrical noises that affected this
small voltage.
80


6.
Summary, Conclusions, and Recommendations for Further Study
6.1 Introduction
The main concentration of the study was focused on lateral loads. The
research objective is to preserve the San Miguel Bridge and rehabilitate it into
pedestrian use. The bridge history was reviewed along with the influence of the
deck under lateral wind pressure. The bridge strains and moments were
measured using transducer and anemometer devices. The deck was analyzed and
modeled using RISA-3D software. The actual responses were compared to
results from analyses. Summary of the steps followed in the procedures and their
conclusions are listed below.
6.2 Summary of Findings
The steel Pratt truss remains the most common type of historical bridge
to have survived to the present day.
After studying six different bridges, observations had proven no physical
evidence that wind had caused damage to any of the bridges, even after a
century of exposure.
81


The original bridge consisted of a wrought iron truss with five
approximately equal spans. The total length of each span was 142-feet
(43-meter) resulting in a total bridge span of 740-feet (226-meter), and a
nominal clear road width of 14-feet (4-meter).
To collect strain data under different wind speeds, sixteen transducers
were utilized. Five anemometers were used to obtain wind speed and
direction.
The deck is much heavier than its original timber deck. The gravel
roadbase produces a heavy dead load of approximately 3.54 kPa (74 psf),
which is much higher than the original timber deck estimated at
approximately 0.62 kPa (13 psf).
The deck was modeled using RISA 3D. Interconnected plate elements
were used to represent the gravel roadbase.
The traditional structural pin connected skeleton frame was analyzed
using the AASHTO Guide Specifications for the Design of Pedestrian
Bridges (AASHTO, 1997) live load value. Live load varies between
82


Full Text
Const BEXCIT1 = 1000 'Blockl excitation
mVolts 5000 to 1000 db 3-18 Const BSETL1 = 2000 'Blockl settling time
(usees) Const BINT1 = 250 'Blockl integration
time (usees) Const BMCJLT1 = 355 'Blockl default
multiplier Const BOSET1 = 0 'Blockl default offset
Public BBlkl(BREP1) ' Blockl dimensioned
source Units BBlkl = Volts ' Blockl default units
(Volts) Bridge Block 2 Const BRNG2 = 40 'Block2 measurement
range (mSecs) Const BREP2 = 16 'Block2 repetitions
Const BEXCIT2 = 2500 'Block2 excitation
mVolts Const BSETL2 = 400 'Block2 settling time
(usees) Const BINT2 = 500 ' Block2 integration
time (usees) Const BGF2 = 2.065 'Block2 gauge factor
Const BCODE2 = 1 'Block2 gauge code for
1/4 bridge strain Const BMULT2 = 1 'Block2 default
multiplier Const BOSET2 = 0 ' Block2 default offset
Public BBlk2(BREP2) 'Block2 dimensioned
source Public BBlk2mV(BREP2) Dim GBBlk2(BREP2) 'Block2 dimensioned
gauge factor Dim BBlk2ZeroMv(BREP2) 'Block2 zero mV
variable Dim BBlk2ZeroUs(BREP2) 'Block2 zero uStrain
variable Units BBlk2ZeroMv = mVperV 'Block2 default units
(mVperV) Units BBlk2ZeroUs = uStrain 'Block2 default units
(uStrain) Units BBlk2 = uStrain 'Block2 default units
(uStrain) Units BBlk2mV = mVperVolt WWWWWWWWW ALIASES & OTHER VARIABLES //////////////////
97