i\
SEISMIC DESIGN OF INTAKE TOWER
by
Mannie Simon
B.S. University of Colorado Denver, 2006
A thesis submitted to the
University of Colorado Denver
In partial fulfillment
of the requirements for the degree of
Master of Science
Civil Engineering
2010
This thesis for the Master of Science
degree by
Mannie Simon
has been approved
by
Li Cheng Yu
/
//
Date
Simon, Mannie (M.S., Civil Engineering)
Seismic Design of Intake Tower
Thesis directed by Professor NienYin Chang
ABSTRACT
The purpose of this thesis is to study the response of intake tower during earthquake
and the serviceability after the occurrence of design earthquake. Los Alamos and
Bloomfield Dams with one existing tower each are located in the state of New
Mexico. These two towers were selected in the study to assess their seismic response
and after shock serviceability. The tower structures: a lightly reinforced square
concrete at the Los Alamos Dam and a circular steel tower founded on reinforced
concrete block at the Bloomfield Dam were built in 1943 and 1956, respectively.
The seismic response analyses were performed and the design response spectrum for
an earthquake of a 5000year recurrence period obtained from the U.S. Geological
Survey (2002) was used in the analysis. This selected spectrum satisfies the current
seismic design criteria of the State of New Mexico. The criteria were adopted in the
performance of the linear response spectrum analysis. A twomode method and
commercial software, SAP2000, were used to obtain the design moment and shear
force. Based on the results obtained from these two analysis methods, the structural
strength and stability are evaluated to ensure the serviceability and safety of the dams.
The stability of towers against sliding, overturning, and foundation bearing failures
wasevaluated and the Bloomfield intake tower was found to be safe under seismic
shaking; whereas the Los Alamos intake tower failed to satisfy overturning stability.
Structural strength under the design earthquake was evaluated; the Bloomfield Dam
tower has the required strength for shear and flexure, and the Los Alamos Dam tower
satisfies the shear requirement, but the analysis result indicates a potential of brittle
failure. This is critically done following the comprehensiveresearch by following the
codes and standard of the United States Army Corps of Engineers (US ACE) and the
United Sates Bureau of Reclamation (USBR). It is, however, cautioned that the
structural material deterioration over the several decades since the dam construction
was not evaluated. This should constitute one aspect of future evaluation.
This abstract accurately represent the content of the candidates thesis. I recommend
its publications.
DEDICATION
Dedicated to my father Simon T. Azbite who has positively changed peoples life.
ACKNOWLEDGEMENTS
I am heartily thankful to my advisor NienYin Chang whose knowledgeable academic
excellence exceeds my words. His insights, advice, encouragements, and attention to
detail are part of my success. My supervisor, Mike Zusi is to whom my deepest
gratitude goes for his genuine and significant encouragement towards reaching my
academic destiny. My sincere thanks also goes to Ed. Toms, URS Corporation for his
generous and academic concern in providing me with Projects that rightfully fit to my
subject areas. Chengyu Li is my other resourceful person to whom I extend my deep
gratitude for his support on ACI and AISC Codes clarification. It is also an honor for
me to thank Larry Nuss for his corporation on providing me with Reclamation Design
Criteria used for tower design.
On a personal level, the completion of my Masters degree could have been a dream if
I had not had the continual and energetic support and encouragement of my wife
Emebet Wolde. The everlasting smile and love of my children, Bethanya and Caleb,
were also my means of complete strength. Finally, thank you my lively parents and
family members for your rooted concern and good wish for my wellbeing and
success.
TABLE OF CONTENTS
Figures........................................................................xi
Tables.......................................................................xiii
Notations.....................................................................xiv
Chapter
1. Introduction......................................................1
1.1 Problem Statement........................................................1
1.2 Research Approach and Tasks..............................................1
1.3 Engineering Significance.................................................2
2. Literature Review........................................................4
2.1 Overview.................................................................4
2.2 Evaluation of Existing Project...........................................6
2.3 Standard and Site Specific Seismic Studies...............................6
2.3.1 Standard Seismic Studies...............................................6
2.3.2 Site Specific Seismic Studies..........................................7
2.4 Choice of Design Earthquake..............................................7
2.4.1 Operational Basis Earthquake (OBE).....................................8
vii
2.4.2 Maximum Design Earthquake (MDE)........................................8
2.4.2.1 Critical Structures................................................8
2.4.2.2 NonCritical Structures............................................8
2.5 Structural Strength.....................................................9
2.6 Seismic Analysis Method ................................................9
2.7 Computer Analysis Model.................................................9
2.8 Analytical Procedure...................................................10
2.8.1 TwoMode Model.........................................................10
2.8.2 TwoMode Analysis Procedure............................................10
2.9 Stability Analysis.....................................................14
2.9.1 Sliding and Overturning Stability Criteria............................ 14
2.9.2 Flotation Stability Criteria...........................................17
3. Design Earthquakes.......................................................19
4. SAP2000 Structural Software............................................23
4.1 Defining...............................................................23
4.2 Material Properties....................................................24
4.3 Drawing................................................................27
4.4 Assigning..............................................................27
4.5 Analyzing..............................................................27
5. Bloomfield Dam...........................................................28
5.1 Overview...............................................................28
5.2 Description of the Intake Tower........................................30
vm
5.3 Earthquake History at Bloomfield Dam.................................33
5.4 Intake Tower Structural Strength and Stability Analysis .............34
5.4.1 Added Mass...........................................................33
5.4.2 Dynamic Analysis of Intake Tower ....................................34
5.4.3 Allowable Strength of Intake Tower...................................34
5.5 Summary and Conclusion..................................................34
5.5.1 Summary..............................................................34
5.5.2 Conclusion ..........................................................35
6. Los Alamos Dam..........................................................36
6.1 Overview.............................................................36
6.2 Intake Tower Description.............................................37
6.3 Earthquake History at Los Alamos Dam.................................39
6.4 Strength and Stability of Intake Tower Structure.....................41
6.4.1 TwoMode Model.......................................................52
6.4.1.1 Added Mass.........................................................52
6.4.1.2 Stiffness and Shape Functions......................................56
6.4.1.3 Natural Period, Frequency and Spectral Acceleration................57
6.4.1.4 Lateral Displacement, Shear Force and Moment...................37
6.4.2 Dynamic Analysis of Tower............................................59
6.4.3 Comparisons of Analysis Results .....................................64
6.4.4 Global Moment Capacity of Intake Tower Structure....................68
6.4.5 Stability of Intake Tower Structure..................................70
IX
6.4.6 Reinforcement Anchorage and Splice Failure......................69
6.5 Summary and Conclusion..........................................70
6.5.1 Summary.........................................................70
6.5.2 Conclusion .....................................................70
6.5.3 Further Study...................................................71
7. Recommendation for Future Studies..................................72
Appendix
A Bloomfield Dam
B. Los Alamos Dam
References..............................................................74
x
LIST OF FIGURES
Figure
1.1 Profile of Typical Outlet Works.....................................3
2.1 Intake Tower Design & Evaluation Chart..............................5
3.1 Seismic Zone Map for United States .................................3
3.2 Bloomfield Dam Earthquake Response Spectrum.........................5
3.3 Los Alamos Dam Earthquake Response Spectrum........................30
5.1 Bloomfield Dam Location............................................30
5.2 Bloomfield Dam Profile.............................................33
5.3 Earthquake Faults Locations around Bloomfield Dam..................37
5.4 Structure Idealization of Bloomfield Dam Intake Tower..............40
5.5 Bloomfield Intake TowerDeformed Shape Model 1 Period 0.1748
Second.............................................................30
5.6 Bloomfield Intake TowerDeformed Shape Model 2 Period 0.03699
Second.............................................................33
5.7 Bloomfield Intake TowerMoment Diagram from Dynamic Analysis
in kip ft Units....................................................37
xi
5.8 Bloomfield Intake TowerShear Diagram from Dynamic Analysis in
kip ft Units.....................................................40
6.1 Los Alamos Dam Location Map.......................................47
6.2 Earthquake Fault Locations around Los Alamos Dam..................50
6.3 Los Alamos Intake Tower Structural Idealization...................53
6.4 Shape Function....................................................55
6.5 Los Alamos Intake TowerDeformed Shape Model 1 Period 0.10359
Second...........................................................59
6.6 Los Alamos Intake TowerDeformed Shape Model 2 Period 0.0237
Second...........................................................60
6.7 Los Alamos Intake TowerMoment Diagram from Dynamic Analysis
in kip ft Units..................................................61
6.8 Los Alamos Intake TowerShear Diagram from Dynamic Analysis in
kip ft Units.....................................................62
6.9 Los Alamos Intake TowerDisplacement Values Comparison............64
6.10 Los Alamos Intake TowerShear Values Comparison...................65
6.11 Los Alamos Intake TowerMoment Values Comparison..................66
xii
LIS'! OF TABLES
Table
2.1 Intake Tower Loading condition classification..............................12
2.2 Sliding Stability Factor of Safety ........................................24
2.3 Limits of Resultant Location...............................................26
2.4 Flotation Factor of Safety.................................................27
3.1 Assumed Material Properties for Bloomfield Dam Intake Tower............12
3.2 Assumed Material Properties for Los Alamos Dam Intake Tower ...............24
5.1 Total Added Mass Location for Bloomfield Tower.............................26
6.1 Total Added Mass Location for Los Alamos Tower.............................27
6.2 Natural Period, Frequency and Spectral Acceleration........................24
6.3 Lateral Displacement, Shear Force and Moment...............................26
xm
NOTATIONS
acft acrefeet
AC I American Concrete Institute
ASTM American Standard Testing Method
cfs cubic feet per second
Corps United States Army Corps of Engineers
D Dead Load
dia. Diameter
DSHA Deterministic Seismic Hazard Analysis
E modulus of elasticity
El. elevation
fps feet per second
ft. feet or foot
ft2/day square per day
g gravity
Hi inside water depth
Ho outside water depth
hr hour
H:V horizontal versus vertical
xiv
Isase moment inertia of the base
ItoP moment inertia of the top
in. inch or inches
K stiffness
L overall height of tower
LL live load
m* with added mass
MDE Maximum Design Earthquake
min. minute
mph miles per hour
OBE Operating Basis Earthquake
OSE New Mexico Office of the State Engineer
OSE Rules OSE Rules and Regulations for Dam Design, Construction, and Dam
Safety
pcf pounds per cubic foot
PGA peak ground acceleration
PGD Peak ground displacement
psf pounds per square foot
PSHA Probabilistic Seismic Hazard Assessment
psi pounds per square inch
Reclamation United States Bureau of Reclamation
Rm Moment Reduction Factor
xv
SRSS Square Root of the Sum of Squares
sq.mi. square mile(s)
T period
TM technical manual
USACE United States Army Corps of Engineers
USAEC United States Atomic Energy Commission
uses Unified Soil Classification System
USGS United State Geologic Service
USWD United States War Department
yr year
XVI
1. Introduction
1.1 Problem Statement
Evaluating the performance of intake tower structures under earthquake shaking is
essential to ensure the dam safety as well as serviceability of the structure after the
occurrence of earthquakes. Most dams regulate the release of water through intake
towers built from either steel plate or concrete. The sudden ground motion caused by
earthquake moves the entire foundation of the dam. Taking proper precaution is very
crucial that catastrophic failure of the dam and sudden release of reservoir are
prevented to avoid a sever damage or loss of life and resources. Seismic studies done
by United States Corps of Engineer (USACE) on intake tower structures reveal that
the major causes of failure in the event of earthquake are due to lack of structural
strength and stability.
Two intake towers, i.e. Bloomfield and Los Alamos Dams, both located in the State
of New Mexico, were selected to study their dynamic responses to earthquake loads.
The intake towers were also evaluated for their structural strength and stability.
1.2 Research Approach and Tasks
This literature survey focused on the dynamic response of intake towers during
earthquake shaking. The specifications from the United States Corps of Engineer
1
(USACE) and the United States Bureau of Reclamations (USBR) form the basis for
the evaluation. The National Seismic Hazard Map from the United States Geological
Survey (USGS) was also used for developing the design response spectrum for an
earthquake of 5000year reoccurrence period.
Based on the available literature and drawings used for the construction of
Bloomfield and Los Alamos intake towers, the structural sizes, such as wall
thickness, structure height, material type and properties were used to develop
idealized analysis models of structures using SAP2000 structural software. This also
assisted in estimating the added mass for the dynamic response of structures. The
AISC (American Institute of Steel Construction) used for structural strength
evaluation of the Bloomfield Dam tower built of steel plate, whereas the ACI Code
(American Concrete Institute) was used to assess strength of the Los Alamos Dam
intake tower, a concrete structure.
1.3 Engineering Significance
Many of the existing dams and intake towers were built prior to the development of
advanced seismic design guidelines and analysis methods. Developing and utilizing
reliable and efficient techniques are, therefore, among the major steps required in the
evaluation of existing intake towers. Proper maintenance of dam appurtenant
structures is essential to the operation and safety of dams. Figure 1.1 shows the
typical location of an intake tower and profile through outlet works.
2
The survival and functionality of a dam after experiencing earthquake depends upon
the stability and strength of intake tower structure and outlet works. The structural
integrity and quality of the construction plus the materials used are the other
important factors for the maintenance of the dam safety when earthquake occurs.
Figure 1.1 Profile of Typical Outlet Works
3
2. Literature Review
2.1 Overview
The intake tower design recommendations and specifications from US Army Corps
and US Bureau of Reclamation were used extensively as guidelines for developing
seismic loads, structural strength and stability evaluation criteria for the two existing
intake towers at Bloomfield and Los Alamos Dams. (See Figure 2.1 for the evaluation
chart of the intake towers.) Besides, USGS maps pertaining to the seismic hazard
were used.
The major literature components are:
Evaluation of Existing Project
Standard Site Specific Studies
Choice of Earthquakes
Structural Strength
Analysis Methods
Computer Analysis Model
Analytical Procedure
Stability Analysis
4
Figure 2.1 Intake Tower Evaluation Chart
5
2.2 Evaluation of Existing Project
Evaluation of an existing project includes study of the areas such as conditions of the
structure foundation, construction materials used, structural design criteria and
regulations used and the records of structural performance [9]. It is also important to
check information resource that include acceleration contour maps, boring logs,
geological maps, standard response spectra and as built project records [10,20]. From
the existing information, the sufficiency of previous seismic evaluation was assessed.
2.3 Standard and Site Specific Seismic Studies
Seismotectonic, geologic, site, geotechnical and structural investigations are the main
features recommended for choosing the design ground motion for evaluation of the
structure response under an earthquake event at the particular site [10,14],
2.3.1 Standard Seismic Studies
Standard seismic studies use generic seismology, available site data and information
and simplified method of evaluation developed for similar projects or structures.
Ground motion obtained from published seismic zone maps was used in this study.
Final design or evaluation can be performed for seismic zones 1 or 2A using standard
seismic data [10].
6
2.3.2 SiteSpecific Studies
Site specific studies require detailed actual site exploration, which includes
geological, investigations of all faults that are active in the project area, seismological
and geotechnical parameters to evaluate the project hazard and the response of
foundation and structure for seismic loading [10]. Conducting such evaluation could
reduce the possible dynamic response of structure by considering the actual
geotechnical properties of that particular site foundation which influence the vibration
property and earthquake response of tower. Based on these actual site studies, the
final responses of foundation and structure are determined. This method is used for
final design for projects in the region of seismic zone 2A, 2B, 3 and 4 when the
structure design is governed by earthquake load case [10,12].
2.4 Choice of Design Earthquake
The choice of design earthquakes and the intake tower performance requirements
vary across the dam regulation adopted. The US ACE design earthquakes are chosen
based on the following two requirements [10,12,14],
Operational Basis Earthquake (OBE)
Maximum Design Earthquake (MDE).
7
2.4.1 Operational Basis Earthquake (OBE)
The OBE corresponds to a ground motion with a 50 percent probability of excedance
during the service life of 100year (a 144year return period) [17]. The structure is
expected to suffer a minor or no damage, while still operable without interruption.
2.4.1 Maximum Design Earthquake (MDE)
The MDE is the strongest of the ground motion chosen for the design and analysis of
a structure. During MDE, a structure is expected, while damaged, to survive the
earthquake without a disastrous failure caused by the failure of a reservoir that results
in severe damage or loss of life. The selection of MDE values further categorized in
to critical and noncritical structures [24].
2.4.1.1 Critical Structures
For a critical structure the MDE is equivalent to the Maximum Credible Earthquake
(MCE), the largest ground motion that the structure might experience based on
seismological and geological evidences [12,17].
2.4.1.2 NonCritical structures
For a noncritical structure, the MDE ground motion can be lower than the MCE.
This design method is the most economical based on a 1,000 year return period,
which is equivalent to a ground motion with 10 percent probability of excedance
during the service life of 100year [10].
8
2.5 Structural Strength
Structural strength of existing tower is evaluated according to the methods explained
in EM 111022104. Strength reduction factors of 0.9 for bending and 0.85 for shear
applied to the nominal strength. Load combination consists of dead load, live load
and earthquake load combined to obtain design load [2,3,17].
2.6 Seismic Analysis method
The three most common methods of analysis for earthquake forces includes seismic
coefficient, response spectrum and timehistory methods are the approaches
employed in analyzing the structural responses to earthquake motions [1,10,18].
These methods used idealized cross section and makes various assumptions
regarding the structures responses to the ground motion and its interaction with the
foundation and reservoir [1, 15].
The final design of any structure does not, however, require the seismic coefficient
method where the load case is controlled by an earthquake loading conditions. Time
history or response spectrum methods are used for final design in zone 3 and 4
seismic region [15].
2.7 Computer Analysis Model
The creation of a standardized model of the structure plays a significant role for the
evaluation purposes. The evaluation of a structure may extend from two dimensional
9
(2D) beam models for regular shape towers to complex three dimensional (3D) for
irregular shape tower [1].
2.8 Analytical Procedure
2.8.1 Two Mode Model
The twomode model is one of the analysis procedures for the earthquake resistant
and safety evaluations of intake tower structures. Computation can be performed
using spreadsheet. The maximum responses are obtained from the first two vibration
modes of tower computed directly from the earthquake design spectrum. The
structure is assumed to maintain rigid foundation to compute the first two vibration
periods and mode shapes [1]. Dynamic analysis for intake tower performed using
commercial software of SAP2000 conformed that the modal participation factors is
significantly higher for the first two modes which contributes the earthquake response
of tower [1,17].
2.8.1 TwoMode Model Analysis Procedure
The dynamic response of towerwater system is presented using the first two
vibration periods and mode shapes [1]. A twomode model simplified procedure can
represent the effect of added hydromantic mass associated with surrounding and
inside water interacting with the structure. The simplified procedure provides key
design values of deflection, shear and moment. The procedure is presented below as a
sequence of computational steps [1]:
10
1. Define the smooth sitespecific design spectrum for the tower. This may be an
elastic design spectrum or a reduced inelastic design spectrum to account for
the effect of ductility. The design ductility of towers generally should not
exceed two.
2. Compute the added hydro dynamics mass ma (z) associated with the
surrounding (outside) water.
3. Compute the added hydrodynamic mass ma(z) associated with the inside
water.
4. Define structural properties of the tower:
a) Virtual mass, ms (z) unit of height is given by the equation
ms (z) =ms (z) + ma (z) + m'a (z)
Where ms(z) is the mass of the tower by itself ma(z) is computed in
step 2, and in step 3.
b) Flexural stiffness, &sl(z), and sear stiffness C)'4 (z) per unit of
height.
c) Modal damping ratio f n.
d) Compute the periods Tn =2 /Â£> and mode shapes <Â£n (z) for the first
two modes. Where <*>n is nth mode vibration frequency of fixed 
base tower surrounding and inside water.
11
5. Compute the periods Tn =2U /<^n and mode shapes $n (z) for the first two
modes of Vibration (i.e. n = 1,2) with mass ms (z) replaced by the virtual
mass ms (z) the superscript r in con is included to be consistent with earlier
notation as corn includes the effect of water on the vibration frequencies, and
the notation n (z) is used to indicate that these mode shapes of tower with
mass ms (z).
6. Compute the vibration period T i and damping ratio
vibration mode of the tower including the hydrodynamic effects and the tower
foundation soil interaction effects. For this purpose, the period ratio T X[T%
7 Tf / f
and damping ratio s i are given by yy, and respectively, with % (z)
replaced by the virtual mass (z).
7. The vibration period T2 and damping ratio f 2 for the second vibration mode
are determined by standard procedures disregarding the effects of tower 
foundation soil interaction. Thus :
t 2 = n
Where T, was determined in step 5, and
<2 = t,2
Where the damping ratio Â£2 was estimated in, step 4 (C).
12
8. Compute the maximum response (shears and moments ) in individual modes
of vibration by repeating the following steps for the first two modes of
vibration (i.e. n = 1,2):
a) Corresponding to period T n and damping ratio
the pseudo acceleration from the design spectrum.
b) Compute equivalent lateral forces fn(z) associated with vibration of the
tower in its n th mode from:
Sa{Tn,^n)ms{z)^n{z)
in which the generalized mass and generalized excitation 4 terms,
including the added hydrodynamic mass, are :
Mn = jw, (*)[
"j
4 = J, (*)?(*) &
0
c) Compute the shear QnfcO and bending moment Â£2nCz)at any section by
static analysis of the tower subjected to equivalent lateral forces /(?);
qjz)= j/.tete
z
H,
m.(z)= jfeZ)/fete
z
13
9. Estimate the maximum shear Qnfc) and bending moment m(z) at any section
by combining the model maxima <2nCz) and <2n(~) in accordance with the
equation:
Q(z) = 4Qt{z) + Q22{z)
the square root of the sum of squares (SRSS) combination rule is
appropriate because the vibration periods Tt and of towers are well
separated. Essentially no improvement in accuracy will result by including
correlation of modal responses in equations.
2.9 Stability Analysis
The main requirements for stability of intake tower are sliding, location of resultant,
rotational stability and flotation according to US ACE.
2.9.1 Sliding and Overturning Stability Criteria
The minimum factors of safety satisfying sliding and overturning stability according
to EM 111022100 are presented in Table 22 and Table 23 [15].
14
Table 22
Sliding Stability Factors of Safety
Loading Minimum Allowable Sliding
Stability Factor of Safety
Usual Load Cases 2.0
Unusual Load Cases 1.5
Extreme Load Cases OBE 1.7
Extreme Load Cases MDE 1.3
Sliding stability factor of safety for the reinforced concrete structures will be
computed using the following Equation 61 [15]:
Equation 21
15
Where:
Q Sliding Factor of Safety
Tan(e) = Effective coefficient of friction
Fn Normal Force
U Uplift Force from Water
fd Driving Force
The computations for sliding stability neglected the effects of cohesion in the
foundation.
Overturning stability will be insured by the location of resultant force on the base of
the structure, as shown on Table 23 [15].
Table 23
Limits of Resultant Location
Loading Location of Resultant
Usual Load Cases 100% of Base in Compression
Unusual Load Cases 75% of the Base in Compression
Extreme Load Cases Resultant within the base
16
If the resultant location is outside of the base, the USACE EM 11102400 requires a
rotational stability analysis. Rocking occurs when the spectral acceleration of the first
model is greater than the product of gravitational acceleration multiplied by the ratio
of the onehalf of the base width to vertical distance from the base to the center of
gravity.
2.9.2 Flotation stability Criteria
The minimum required factor of safety for the flotation according to EM 111022100
is presented on Table 23 [15].
Table 24
Flotation Factor of Safety
Loading Flotation Required Factor of Safety
Usual Load Cases 1.3
Unusual Load Cases 1.2
Extreme Load Cases 1.1
17
FS,=
wt + we+s
uwg
Equation 22
Where:
Ws = Weight of the structure, including weight of the fixed equipment
and soil above the top of the surface of the structure. The moist
or saturated unit weight should be used for soil above the ground
table and the submerged unit weight should be used for soil
below the ground water table.
Wc
S
U
Weight of the water contained within the structure.
Surcharge loads.
uplift force acting on the base of the structure.
Wg = Weight of the water above top surface of the structure.
18
3. Design Earthquakes
This section describes the design earthquake used for the evaluations of Bloomfield
Dam and Los Alamos Dam intake tower structures. These two towers are located in
seismic zone 1 and 2, respectively [6]. The seismic zone map for the United States
which is adopted from 1994 Uniform Building Code is shown on Figure 3.1. Since no
site specific earthquake was developed for these two particular towers, standard
design response spectra for rock site were used [7,17],
National Seismic Hazard Map, adopted from the United States Geological Survey
(USGS) was used to develop the design response spectrum for earthquakes with
reoccurrence period of 5000year satisfying the current New Mexico Dam Safety
Regulation and standard [8,19],
The recent Java application which includes Seismic Hazard Curves and Uniform
Hazard Response Spectra (2002) was used for Conterminous 48 States based Latitude
and Longitude degrees for both structures. Earthquake response spectrum used for
evaluation for Bloomfield and Los Alamos intake towers are shown on Figure 3.2 and
3.3, respectively.
19
Figure A7. 1994 Uniform Building Code zone map. Zones are identified by the numbers from 0 tojt. Seismic
zone factors are assignedtoeaohzone; Zone 0 = 0, Zonel = 0.075, Zone 2A = 0. tij Zone 2Bwi
O.^yKbn** = t'lfj andE2E23SB Eaoh zone also has speoifio structural detailing requirements.
After ICBO, 1994 (This map was redrawn from the original source, if differences occur, the original
source should be used).
Figure 3.1 Seismic Zone Map for United States
20
Figure 3.2 Bloomfield Dam Earthquake Response Spectrum for
Latitude = 34.9738 and Longitude = 104.7025
21
Figure 3.3 Los Alamos Dam and Reservoir Earthquake Response Spectrum
Latitude = 35.8784 and Longitude = 106.3123
22
4. SAP2000 Structural Software
SAP2000 is a commercial software package for structural analysis and design from
computers and Structures, Inc. It has a complete capacity for modeling, analyzing and
designing. This chapter describes how SAP2000 used to create a model and perform
response spectrum analysis for Bloomfield and Los Alamos Dams intake towers, built
from steel plate and concrete, respectively. Regular towers with symmetrical
distribution of mass and stiffness about their principal axis of symmetry along the full
height can be modeled with a beam element model [17].
The following key procedures were used:
Defining
Drawing
Assigning
Analyzing
4.1 Defining
Defining includes material properties, frame sections, response spectrum functions
and analysis case. Selection of material properties consists of section properties, such
as steel minimum yield stress and compressive strength of concrete. A frame section
23
includes selection of property types, shapes and dimensions based on material type
selected previously. Design response spectrum function was defined using user
spectrum option based on the data obtained from USGS for 5000year earthquake
recurrence period for each site location. Analysis case type was set to the response
spectrum with modal combination and directional combination of SRSS (Square Root
of the sum of the Square).
4.2 Material Properties
The material properties assumed for Bloomfield and Los Alamos intake tower
structures used to create model in SAP2000 are summarized in Table 3.1 and 3.2,
respectively.
24
Table 31
Assumed Material Properties for Bloomfield Dam Intake Tower
Parameter Steel Plate Material Properties
Modulus of Elasticity, E 29,000 ksi
Minimum Yield Stress, Fy 36 ksi
Poisons Ratio, U 0.3
Minimum Tensile Stress, Fu 58 ksi
Effective Yielding Stress, Fye 54 ksi
Effective Tensile Stress, Fue 63.8 ksi
Shear Modulus, G 11,153.8 ksi
25
Table 32
Assumed Material Properties for Bloomfield Dam Intake Tower
Parameter Material Properties
Modulus of Elasticity Concrete 3605. ksi
Compressive strength of Concrete, Fc 4 ksi
Poisons Ratio of Concrete, U 0.2
Shear Modulus Concrete, G 1,502.1 ksi
Modulus of Elasticity of Rebar, E 29,000 ksi
Nominal Yielding Strength of Rebar Fy 40 ksi
Minimum Tensile Stress for Rebar, Fu 60 ksi
Effective Yielding Stress for Rebar, Fye 44 ksi
Effective Tensile Stress for Rebar, Fue 66 ksi
Shear Modulus for Rebar, G 0
26
4.3 Drawing
Draw command was used to draw a two dimensional beam (stick) element model
with location inserted as special joint to account for lumped masses effect from
structure weight, water surrounding the structure and water inside the tower.
4.4 Assigning
Assignment operations are selected from the assign menu, which includes assigning
material properties, restraints, section properties, lumped mass load to the elements
defined and drawn. This method completes the structural model creation.
4.5 Analyzing
Subsequent to creating structural model using the above methods, the option from the
analyze menu, available degree of freedom of plane frame was selected and set the
analysis case to run. These models created to perform response spectrum analysis
provide an output file. The critical results considered for evaluation of stability and
strength of tower structure are displacement, reactions, period and frequency of
structure and participating mass ratio.
27
5. Bloomfield Dam
5.1 Overview
Bloomfield Dam was constructed in 1956 and is located north of the US Highway 64
at approximately Milepost 68, east of the City of Bloomfield, New Mexico as shown
in the location map Figure 31. The City owns and operates the Bloomfield Dam,
which serves as a water storage reservoir for the Citys municipal water supply. The
storage capacity of the dam is 130 acrefeet. Dam Safety Bureau of New Mexico
State classifies the dam as a high hazard structure.
The existing dam consists of a homogeneous earth embankment with a crest elevation
of 5580 feet, a crest length of about 300 feet, and a normal reservoir pool level at
elevation of 5574.6 feet. The dam structural height at the maximum section is 45
feet. The asbuilt drawing for the dam profile is shown on Figure 5.2 Dam Profile. A
concrete spillway is located along the left abutment of the dam, consisting of a broad
crested weir section and a chute that merges into a natural cut channel located just
downstream of the dam toe.
Intake tower consists of five multilevel circular steel gates with an 18inch diameter
steel outlet pipe that discharges water into a downstream gate control structure. The
downstream gate structure controls the flow to the Citys municipal water supply. As
28
built drawing presented in Appendix A shows the general arrangement of gates,
platform and concrete foundation.
Figure 5.1 Bloomfield Dam Location Map
29
r^
Figure 5.2 Bloomfield Dam Profile
5.2 Description of the Intake Tower
The intake structure was made out of 3/8inch thick circular steel plate with 4foot
inside diameter. The steel plate has deteriorated and experienced a loss of section,
and the gates are inoperable. According to the AsBuilt Drawings, dated October
1956, the structure was founded on an 11.5foot square concrete foundation that
extends 8feet down from the top of concrete foundation slab at elevation 5533.0 ft.
The top of the existing intake tower deck is at elevation 5573.0 ft. The tower has five
manually operated 24inch diameter gates with 18inch outside diameter outlet pipe at
invert elevation of 5534.0 ft. Photo 5.1 shows the reservoir and intake tower during
the normal reservoir pool and Photo 5.2 shows the intake tower after the reservoir is
drained.
30
31
Photo 5.2 Bloomfield Dam Intake Tower
32
5.3 Earthquake History at Bloomfield Dam
The most seismic areas in the State of New Mexico is located northeast part of the
state between Socorro and Albuquerque, where major earthquakes occurred between
1868 and 1973, as reported by the USGS. The earthquake intensity (Modifies
Mercalli intensity) was VI or higher [26], The Mercalli intensity VI was designated as
strong and equivalent to Richter magnitude of 5.0. The closest recorded faults to
Bloomfield Dam are Gallina and Nacimiento at fault length of 39 and 36 kilometers,
respectively. See Figure 52 for location of these and other faults [19].
33
Aztec 1 x 2 Sheet
Home > US Map
> New Mexicc^
BLOOMFIELD DAM
(LAT.36.8015 & LONG.
107.8057)
Number Name
2001 Gallina fault
2002a Nacimiento fault, northern section
2003 Canones fault
2004 Lobato Mesa fault zone
2005 La Canada del Amagre fault zone
2006 Black Mesa fault zone
2007b Embudo fault, Hernandez section
2008 Pajarito fault zone
2009 Puye fault
2010 Pojoaque fault zone
2020 Las Tablas fault
2143b Unnamed faults of Jemez Mountains, Toledo caldera section
2143c Unnamed faults of Jemez Mountains, caldera margin section
Last modified December 15, 2005
URL http://earthquake.usgs.gov/regional/qfaults/nm/azt.html
Figure 5.2 Earthquake Faults around Bloomfield Dam
34
5.4 Intake Tower Structural Strength and Stability Analysis
The following analyses were performed for evaluation of strength and stability of
tower with the calculations information provided in Appendix A.
Added mass
Dynamic analysis of intake tower
Allowable strength of intake tower
5.4.1 Added Mass
Added mass includes the mass of the tower and hydrodynamic mass which consist of
water pressure acting on the outside (surrounding) and inside surface of tower wall.
Hydrodynamic masses for Bloomfield dam were represented by circular cylindrical
towers surrounded by water [1],
Total added mass distributed to the elements to estimate the actual effect of water.
The locations and values of added mass are shown in Table 5.1. Structural
idealization for excitation of intake tower used for Bloomfield tower is shown on
Figure 5.4.
35
Figure 5.4 Structural Idealization of Bloomfield Dam Intake Tower
36
Table 51
Total Added Mass and Locations
Distance from the Base (ft) Outside Hydrodynamic Added Mass (k s2 /ft) Inside Hydrodynamic Added Mass (k s2 /ft) Actual Mass of Tower (k s2 /ft) Total Added Mass (k s2 /ft)
40 0.000 0.000 0.006 0.006
38 0.000 0.000 0.018 0.018
34 0.043 0.015 0.024 0.082
30 0.090 0.030 0.024 0.145
26 0.096 0.031 0.024 0.151
22 0.098 0.031 0.024 0.153
18 0.099 0.031 0.024 0.154
14 0.099 0.031 0.024 0.176
10 0.112 0.035 0.029 0.176
5 0.245 0.079 0.043 0.367
0 0.183 0.059 0.026 0.268
37
5.4.3 Dynamic Analysis of Intake Tower
Dynamic analysis for Bloomfield tower was performed by creating a model of beam
element with total added masses using SAP2000. Response spectrum for an
earthquake of 5000year reoccurrence period for Bloomfield Dam was used for
analysis. Figure 3.2 shows Response spectrum used for analyses.
The first 10 modes of vibration and 5 percent damping were used to compute the
dynamic response of tower [5]. Dynamic analysis result obtained from SAP2000
includes natural frequencies and shape of vibration modes, deformed shapes,
moment and shear.
The first and second vibration contribution modes were significantly higher than the
rest of fundamental modes; the modal participating mass ratios were 92.861 and
5.728 percent, respectively, with a total of 98.589 percent. The deformed shapes for
mode 1 and 2 are shown on Figure 5.5 and 5.6, respectively.
38
Figure 5.5 Bloomfield Intake Tower Deformed Shape Mode 1 Period
0.1748 second
39
40
Diagrams of moment and shear forces obtained from the analysis are shown on
Figure 5.7 and 5.8, respectively.
0.22
1.79
7.27
17.83
32.32
49.78
Â£9.31
90.23
103.93
117.63
131.38
145.92
Figure 5.7 Bloomfield Intake Tower Moment Diagram form Dynamic
Analysis in kip ft unit
41
0.11
0.39
2.67
3.70
4.47
5.03
5.41
5.67
5.89
 5.89
Figure 5.8 Bloomfield Intake Tower Shear Diagram form Dynamic
Analysis in kip unit
42
5.4.3 Allowable Strength of Intake Tower
Allowable flexural and shear strength were computed based on current the American
Institute of Steel Construction (AISC) design guidelines for Bloomfield dam intake
tower built from 3/8 inch thick plate. Since the dynamic analysis of tower is done
based on the assumption of cantilever beam, the maximum loads were occurred at the
base of structure [4,11],
The following are the material properties and dimensions assumed for calculations of
allowable strength of tower:
Elastic modulus of steel, E = 29,000 ksi
Yield strength of steel, Fy = 36 ksi
Steel plate thickness, t = 3/8 in
Inside diameter of the tower base, Dj = 10 ft
Height of tower, h = 10 ft
Radius of gyration, r = 3.55 ft
Section modulus, S = 4,254.5 in3
Reduction factor used for shear and flexure strength 1.67
43
The allowable structural strength obtained from the above analysis are 166 kip and
5792 ft kip for shear and flexure strength, respectively.
5.5 Summary and Conclusion
5.5.1 Summary
The allowable structural strength of tower for shear and flexure, made out of 3/8 inch
thick steel plate were evaluated against the result obtained from dynamic analysis
using SAP2000 structural software. The allowable flexural and shear strength values
based on the current AISC design guidelines without a reduction for aging and
corrosion are used for comparisons purpose.
The analyses for flexural strength of tower indicate that the strength is
governed by local buckling rather than the flexure due to slender walls. The
allowable flexural strength however significantly exceed the loads obtained
from SAP2000.
The analysis for shear strength indicates that the allowable shear strength of
tower significantly exceeds the loads obtained from SAP2000.
Allowable Connection Strength could not be performed due to lack of design
information.
44
5.5.2 Conclusion
Bloomfield Dam tower structural design and assumption were not available for this
study. However, from the analysis and structural strength evaluation performed for
this study, the steel plate thickness for the tower wall were not governed by flexure
and shear requirement for seismic loading conditions. Other possible reasons to
increase steel thickness may come from construction and handling requirements or
additional factor of safety might have been added to account for future corrosion.
Even though the structural strength result indicates the structure has the required
strength for an earthquake of a 5000year recurrence period, the structure has many
complex issues. The field inspection has revealed that the structure has experienced
extensive corrosion in spite of the coating on the surface to resist corrosion. It
resulted degrading the steel properties. The loss of cross section reduces the member
strength and stiffness that increased stress levels and deformations. The other
potential structural deficiency is the weld and bolted connections. The welding done
prior to current practice has incomplete joint penetration and incomplete fusion; this
defect could result in cracks which decrease strength and toughness in the weld
material [18].
According to the US ACE, many of the existing hydraulic steel structures designed
and build in the early mid1900s have design deficiencies, and Bloomfield Dam
intake tower is not an exception in terms of meeting the current design and
construction criteria [13].
45
6. Los Alamos Dam
6.1 Overview
Los Alamos Dam was constructed by the United States Corps of Engineers (US ACE)
for the United States War Development (USWD) and was completed in 1943. The
dam is located in on Los Alamos Creek as shown on Figure 6.1. The Los Alamos
County owns and operates the Dam and it serves as a water supply storage reservoir
and for recreational uses. The water storage capacity of dam is 1400 acrefeet. The
dam is classified as an intermediate size, high hazard structure by the State of New
Mexico Dam safety and Regulation office.
46
Figure 6.1 Los Alamos Dam Location Map
6.2 Intake Tower Description
The tower is a freestanding reinforced concrete structure, which maintains a square
shape uniform with inside area of twentyfive square feet. The surrounding wall at the
bottom of the tower is one foot and four inches thick for approximately half of the
structure height and it changes to a foot thick wall for the remaining height. The
foundation slab is three feet thick and has sixteen feet square shape area.
According to the construction drawing, dated, July 1948, the tower has a total of four
one feet square area sluice gate that can be operated from the eightinch thick
47
concrete platform. As built drawing, presented in Appendix B, shows the general
arrangement of gates, platform and concrete foundation.
Photo 6.1 Los Alamos Dam Intake Tower
48
6.3 Earthquake History at Los Alamos Dam
Los Alamos Dam is located around 60 miles north of the well known seismic areas of
New Mexico State, between Socorro and Albuquerque where intensive earthquake
occurred between 1868 and 1973, as reported by USGS. The Los Alamos Dam area
has been enclosed on all sides by faults at a distance of about 20 miles from the dam
site as shown in Figure 4.2.
49
ZHJSGS
Quaternary Fault and Fold Database for the United States
Albuquerque 1 x 2 Sheet L0S ALAM0S dam, NM
0 tt 20 43 SO 0 S 10 30 30 43
Number Name
2002a Nacimiento fault, northern section 2037
2002b Nacimiento fault, southern section 2038
2008 Pajarito fault zone 2039
2009 Puye fault 2040
2010 Pojoaque fault zone 2041
2026 Rendija Canyon fault 2042
2027 Guaje Mountain fault 2043
2028 Sawyer Canyon fault 2044
2029a JemezSan Ysidro fault, Jemez section 2045
2029b JemezSan Ysidro fault, San Ysidro section 2046
2030a San Felipe fault zone, Santo Ana section 2047
2030b San Felipe fault, Algodones section 2048
2031 San Francisco fault 2049
2032 Lo Bajada fault 2121
2033a TijerasCafioncito fault system, Galisteo section 2128
2033b TijerasCafioncito fault system. Canyon section 2142
2034 Bernalillo fault 143a
2035 Calabacillas fault 2143b
2036 Rincon fault 2143c
2143d
Sandia fault
County Dump fault
Sand Hill fault zone
East Paradise fault zone
Unnamed faults near Picuda Peak
West Paradise fault zone
Faults north of Placitas
Four Hills Ranch fault
Unnamed faults near lama Borbon
Zia fault
Unnamed faults near Loma Colorado de Abojo
Unnamed faults near Star Heights
Unnamed faults near Albuquerque Volcanoes
Intrabasin faults on the Uano de Albuquerque
Coyote fault
Faults near Cochiti Pueblo
Unnamed faults of Jemez Mountains, Valles caldera section
Unnamed faults of Jemez Mountains, Toledo caldera section
Unnomed faults of Jemez Mountains, caldera margin section
Unnamed faults of Jemez Mountains, intracaldera section
Last modified March 16, 2006
URL http://earthquake.usgs.gov/regional/qfautts/nm/alb.html
Figure 6.2 Earthquake Faults around Los Alamos Location
50
6.4 Strength and Stability of Intake Tower Structure
The following analyses are performed for the evaluation of strength and stability of
tower. Calculations are provided in Appendix B.
Twomode model
Dynamic analysis of tower
Comparison of analysis results
Global moment capacity of intake tower structure.
Structural strength of intake tower wall
Stability
Reinforcement anchorage and splice failure mode
51
6.4.1 TwoMode Model
Approximate twomode model is a manual procedure developed by the US ACE for a
response spectrum analysis of an intake tower [17]. This method provides sufficient
accuracy for a regular shape intake towers and supported on rigid foundation [1]. A
minimum of six lumped masses are recommended and added mass can be calculated
using a spreadsheet. Los Alamos intake tower is a free standing stepped tower and the
following procedures were applied:
Added mass
Stiffness and mode shape
Natural period, frequency and spectral acceleration
Lateral displacement, shear force and moment
6.4.1 1 Added Mass
Added mass includes the effect of mass of concrete and the lateral hydrodynamic
from the water outside and inside of tower per unit length of the element [17]. The
total mass per element used for Los Alamos intake tower is shown on Table 6.1.
Structural idealization for excitation of intake tower used for Los Alamos tower is
shown on Figure 6.3.
52
18.00'___________________,_______________________i.oy
Geometry of Existing Intake Tower
o;
Â£
a n
8
8 Y
8 i
8
8 n
8 *
o
g n
8
8 7 zo 8 *$
GROUMO U01XM
STRUCTURAL IDEALIZATION FOR EXCITAION
ALONG EACH DIRECTION (SYMMETRICAL TOWER)
LOS ALAMOS DAM INTAKE TOWER
Figure 6.3 Los Alamos Intake Tower Structural Idealization
53
Table 61
Total Added Mass and Locations
Distance from the Base (ft) Outside Hydrodynamic Added Mass. (ks2 /ft) Inside Hydrodynamic Added Mass (ks2/ft) Structure Mass (ks2/ft) Total Added Mass (ks2/ft)
40 0.000 0.000 0.088 0.09
39.33 0.000 0.000 0.240 0.24
36.6 0.000 0.000 0.320 0.32
33.6 0.000 0.000 0.341 0.34
30.5 0.000 0.000 0.369 0.37
27 0.111 0.028 0.363 0.50
24 0.247 0.094 0.335 0.68
21 0.313 0.138 0.404 0.86
18 0.360 0.144 0.473 0.98
15 0.371 0.145 0.473 0.99
12 0.379 0.145 0.473 1.00
9 0.384 0.145 0.473 1.00
6 0.387 0.145 0.473 1.01
3 0.389 0.07 2.025 2.49
0 0.195 0.00 1.789 1.98
54
6.4.1 2 Stiffness and Shape Functions
Calculated stiffness of tower per unit length was 4,465 kips/ft for the first mode shape
and 136,192 kips/ft for the second mode shape. The shape functions for the first and
the second mode are shown on Table 6.4.
Figure 6.4 Shape Functions
55
6.4.1 3 Natural Period, Frequency and Spectral Accelerations
Natural period of vibration T in second and the natural frequency in radians per
second were calculated for the first two shape functions. The corresponding pseudo
spectral acceleration were calculated from the response spectrum. The results are
shown in Table 6.2.
Table 62
Natural Period, Frequency and Spectral Acceleration
Mode Shape Natural Period (second) Natural Frequency (Radians/Second) Spectral Acceleration (g)
First 0.112 56.076 0.609
Second 0.0243 257.962 0.360
56
6.4.1 4 Lateral displacement, Shear Force and Moment
Lateral displacement, shear force and moment were calculated for each element. The
final results from simplified twomode model method using spreadsheet are shown in
Table 6.3.
Table 63
Lateral Displacement, Shear Force and Moment
Distance from the Base (ft) Displacement (in) Shear Force (Kip) Moment (kip. Ft)
40 0.140 3 0
39.33 0.136 13 2
36.6 0.124 23 34
33.6 0.108 33 101
30.5 0.093 42 203
27 0.078 51 344
24 0.063 62 496
21 0.051 74 682
18 0.038 85 902
15 0.028 94 1152
12 0.018 101 1424
9 0.011 105 1710
6 0.005 107 2005
3 0.002 110 2304
57
6.4.2 Dynamic Analysis of Tower
Dynamic analysis for Los Alamos tower was performed by creating a model of
beam element with total added masses using SAP2000. Response spectrum for an
earthquake of 5000year reoccurrence period for Los Alamos Dam was used for
analysis [8,19]. Figure 3.3 shows Response spectrum used in the analyses.
The first 10 modes of vibration and 5 percent damping were used to compute the
dynamic response of tower. Dynamic analysis output from beam modal analysis
from SAP2000 includes natural frequencies and shape of vibration modes, deformed
shapes, moment and shear.
The first and second vibration contribution modes were significantly higher than
other eight modes; the modal participating mass ratios were 97.176 and 2.609
percent for the first and second modes, respectively. The deformed shapes for mode
1 and 2 are shown on Figure 6.5 and 6.6, respectively.
58
Figure 6.5 Los Alamos Intake Tower Deformed Shape Mode 1 Period
0.10359 second
59
Figure 6.6 Los Alamos Intake Tower Deformed Shape Mode 2 Period
0.02337 second
60
Diagrams of moment and shear forces obtained from the analysis are shown on
Figure 6.7 and 6.8, respectively.
o
2.29
35.92.
104.69
202.99
354.09
510.31
700.55
925.75
1182.76
1465.27
1767.43
2083.70
2409.29
2743.90
Figure 6.7 Los Alamos Intake Tower Moment Diagram form Dynamic
Analysis in kip ft unit
61
3.42
12.32
22.93
32.78
42.03
t
52.27
63.98
76.37
87.91
97.19
104.30
109.34
112.53
116.74
116.74
Figure 6.8 Los Alamos Intake Tower Shear Diagram form Dynamic
Analysis in kip ft unit
62
6.4.3 Comparison of Analysis Results
Out put from dynamic beam modal analysis using SAP2000 and twomode model
calculated on spreadsheet for Los Alamos Dam intake tower were compared for
displacement, shear and moment evaluation for reinforced concrete section. Figures
6.9, 6.10 and 6.11 shows the comparison of results, respectively.
63
Tower Elevation,
Figure 6.9 Los Alamos Intake Tower Displacement Values Comparison
64
Tower Elevation,
Shear, Kip
Figure 6.10 Los Alamos Intake Tower Shear Values Comparison
65
Tower Elevation,
0.00 500.00 1000.00 1500.00 2000.00 2500.00 3000.00
Moment, ftkip
Figure 6.11 Los Alamos Intake Tower Moment Values Comparison
66
6.4.4 Global Moment Capacity of Intake Tower Structure
Global moment capacity of Los Alamos Dam intake tower is computed similar to
cantilever column with distributed reinforcement applying ACI 318 appropriate
reduction factor. Since column is a compression member, the buoyant weight of
tower walls were considered above the section where capacity was calculated [16,17].
A spreadsheet was developed with the following assumptions:
Strain in reinforcement or concrete is directly proportional to distance from
the neutral axis.
Tensile strength of concrete was neglected.
Moment reduction factor equal to 1.
Compressive force of concrete lets to be equal to axial load of structure at
section.
Since the maximum moment demand was at the base of tower, the following input
data were used to perform complete cracked section analysis of concrete.
Compressive strength of concrete, fc = 3000 psi
Modulus elasticity of concrete, Ec= 3.122xl06 psi
Yield strength of steel, fy = 40,000 psi
Modulus elasticity of steel, Es= 3.122xl06 psi
67
Modular ratio, n = 9.3
MDE moment factor =1.1
Moment reduction factor = 1
Axial load, P =172,000 lb
Uncracked moment of inertia, I = 4,843,441 in
Maximum strain at extreme tension face = 0.00138 in/in
The following results were obtained:
Uncracked moment capacity = 2977 ft kip
Factored Moment (from dynamic analysis) = 3020 ft kip
Nominal moment capacity = 1792 ft kip (less than the factored moment)
Ratio of Nominal moment capacity to uncracked moment capacity = 0.60, which is
less than 1.2 required according to US ACE EM 111022400 to prevent a brittle
failure.
68
6.4.5 Stability of Intake Tower Structure
The external global stability, for dynamic analysis of 5000year earthquake
recurrence at Los Alamos Dam intake tower structure is evaluated for sliding,
dynamic rotational and overturning stabilities [15].
The stability factor of safety for sliding was 1.5, which is greater than the minimum
required factor of safety for critical structures [15].
The dynamic rotational stability result indicates that rocking can occur during
earthquake event and the tower is rotationally unstable [17].
Computation results for overturning stability of tower indicate that the overturning
moment from the dynamic analysis was higher than the resisting moment from the
tower. Therefore, the structure fails to satisfy the minimum requirement for
overturning.
6.4.6 Reinforcement Anchorage and Splice Failure
Minimum anchorage and splice length for longitudinal reinforcement were calculated
to evaluate if the existing tower has adequate length of bar to protect detonation of
structure during earthquakes [2,17]. Minimum required lengths calculated were 11.9
inch and 14 inches for minimum required length of anchorage and splice,
respectively. Vertical reinforcement used for existing tower satisfies the above
requirements.
69
6.S Summary and Conclusion
6.5.1 Summary
Comparison of results from twomode procedure with a dynamic analysis using
SAP2000 commercial software were performed for response spectrum of 5000year
earthquake reoccurrence period. The first 10 modes of vibrations and 5 percent modal
damping were used to compute dynamic response. The results obtained from these
methods for moment, shear and displacement of tower were graphed for comparison.
The graphs indicate that the twomode procedures are in general agreement with the
results from SAP2000.
6.5.2 Conclusion
The results of the dynamic analysis for Los Alamos Dam intake tower for an
earthquake of a 5000year reoccurrence indicate that:
The nominal moment capacity of the intake tower is lower than the
factored design moment obtained. The moment reduction factor of 2
can not be applied to the design moment in this case because the
nominal moment capacity does not exceed the uncracked moment
capacity by at least 20 percent. This indicates the existence of potential
for a brittle failure of structure [17].
The tower satisfies the minimum sliding factor of safety. However it
fails to satisfy the overturning and rotational stability.
70
The tower has the required length of vertical reinforcement to prevent
anchorage and splice failure.
6.5.3 Future Study
The following additional studies need to be done to reach a final conclusion on the
stability and strength of Los Alamos intake tower after lowering of reservoir for the
following structural condition assessment; concrete material properties, foundation
investigations and evaluation of the connection between the tower, and outlet works
conduit.
71
7. Recommendations for Further Studies
The study adopted by Corps Design and Regulation for intake tower has been applied
to structural strength and stability evaluation for Bloomfield and Los Alamos Dam
intake towers. Great effort has been implied to study the response of tower, built of
steel plate and reinforced concrete. The level of study performed is preliminary in
comparison with the nature of earthquake and the possible structural responses. Since
many intake towers are located or will be built in high seismic areas, significant effort
should be placed to research and develop reliable design procedure and practice.
Recommendation is forwarded for further studies in the following areas:
Foundation anchor bar can be used for maintaining stability of intake tower
structures. It provides an additional factor of safety for sliding, overturning
and increase the foundation material bearing strength. It is recommended to do
further study to consider the placement of foundation anchors with a cement
grout underneath the existing foundation slab to increases the stability of
structure.
Approximate twomode model analysis procedure does not consider the
towerfoundation material interaction for obtaining design loads; shear
moment and deflection. This method provides conservative values for the
72
purpose of structural design. Attention should be given that these loads are
generated for MDE load cases. It is recommended to investigate tower
foundation material interaction to come up with an appropriate load reduction
factor to make the structures more economical.
There are no clear guidelines for application of uplift force underneath the
foundation slab for tower seismic stability analysis during earthquake,
especially for overturning. It is, therefore, recommended to perform a further
study if full uplift force is needed to be considered due to the cyclic nature of
earthquake load.
Structural designs for freestanding intake tower perform on the assumption of
cantilever structure. The base of structure is the critical location where plastic
hinge could occur and it is also an area for outlet pipe connection and low
level gates installed. It is recommended to study further to understand the
impact on hydraulic gates and outlet works pipe to avoid a possible
malfunction after the occurrence of earthquake.
73
REFRENCES
E Alok Goyal & Anil K. Chopra, (1989), Earthquake Analysis and Response of
Intake Towers, University of Berkeley, California
2. American Concrete Institute, (2008), Building Code Requirements for
Structural Concrete, ACI 31808, Farmington Hills, MI.
3. American Concrete Institute, (2006), Code Requirements for Environmental
Engineering Concrete Structures, ACI 35006, Farmington Hills, MI.
4. American Institute of Steel Construction, (2005), Steel Construction
Manual, 13th Edition, United States of America
5. Anil K. Chopra, (2007), Dynamic of Structures, University of Berkeley,
California
6. American Society of Civil Engineers, (2005), Minimum Design Loads for
Buildings and Other Structures, SEI/ASCE 705, Reston, VA.
7. Enrique E. Matheu, Don E Yule and Raju V. Kala, (2005), Determination
of Standard Response Spectral and Effective Peak Ground Acceleration for
Seismic Design and Evaluation, U.S. Army Corps of Engineers, Washington,
D.C
74
8. New Mexico Office of the State Engineer, Dam Safety Bureau, (2005).
Rules and Regulations Governing Dam Design, Construction and Dam
Safety. March 31.
9. Steven OBrien, (2007), Current International State of Practice fro the
Seismic Design and Analysis of Appurtenant Structures at Dams, ANCOLD
Young Professional Scholarship, Australia
10. U.S. Army Corps of Engineers, (1995), Earthquake Design and Evaluation
for Civil Works Projects, EM 11101806. Washington, D.C.
11. U.S. Army Corps of Engineers, (1993), Design of Hydraulic Steel Structures,
EM 111022105, EM 111022105. Washington, D.C.
12. U.S. Army Corps of Engineers, (2007), Earthquake Design and Evaluation of
Concrete Hydraulic Structures, EM 111026053. Washington, D.C.
13. U.S. Army Corps of Engineers, (2001), Inspection, Evaluation, and Repair of
Hydraulic Steel Structures, EM 111026054. Washington, D.C.
14. U.S. Army Corps of Engineers, (1999), Response Spectra and Seismic
Analysis for Concrete Hydraulic Structures, EM 111026050, Washington,
D.C.
15. U.S. Army Corps of Engineers, (2005), Stability Analysis of Concrete
Structures, EM 111022100, Washington, D.C.
16. U.S. Army Corps of Engineers, (2003), Strength Design for Reinforced
Concrete Hydraulic Structures, EM 111022104. Washington, D.C.
75
17. U.S. Army Corps of Engineers, (2003), Structural Design and Evaluation of
Outlet Works, EM 111022400, Washington, D.C.
18. U.S. Army Corps of Engineers, (2003), TimeHistory Dynamic Analysis of
Concrete Hydraulic Structures, EM 111026051, Washington, D.C.
19. U.S. Geological Survey, (2002), Seismic Design Values for Buildings,
National Seismic Hazard Maps,
http ://earthq uake. us gs. go v/hazards/desi gnmaps/buildinqs. php
20. United States Department of Interior, Bureau of ReclamationUSBR, (1987).
Design of Small Dams. Third Edition, 1987. United States Government
Printing Office, Washington D.C.
76
APPENDIX A
Bloomfield Dam
(
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OUTLET STRUCTURE
&UMFl<=Lb TOILER.
EL PASO
ASSiMLT peUHINS
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NATURAL GAS COMPANY
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Reference: U.Sl Array Corpe of Engineers, Structural Deetgs and HvaluaSu ofOuSetWcrfre,
EM 11 10M404 1 Juno 2Q09
Material Propertlee Units (Imperial) Units
Weight Oensfty of Water P 82.4 pef
Weight Danatty of Concrete isao prf
weight Density of Stoat 4900 pcf
Acceleration Dun to Gravity a 312 ftfar*
Coneraia
Coraraselva Stranalh ro 3000.0 09)
Modulus of Elasticity Eg 3122 kal
Reinforcement
Yield Strenoth . (y 300
Modulus of Elsstldtv EM 29000 k3
Geometrical Pro partita of tin Tower
Too Bovation of Tower ssn It
Bottom Bavalfon of Tower 3533 ft
Roservtor Normal Water Surface Bovalon 5971 ft
Rnrv(or Minimum Retervlor water Surfaca Elevation It
Inside* Water Surfaca Elevation under Normal Ooerallon (for Selamlo Condnl 5371 It
Ttilctnees of Bass Stab of the Tower 8.0 It
Total Number of Nodae IS
MetoM of Tower 40.0 ft
Heed of Oulslda Water at Base of Tower, H, 38 ft
Httd of memo wotor stout wo Floor of rowa/,n (tor ssttmio conan) 38.0 It
Too Shalt (Inside 0101001411 4 It
Too Shaft fOutsfdo Dlamtcri J2i3 It
Â§ 1 0 it 1 5 ft
Mid Shaft (Outside Olametrt 8.0628 ft
Bottom Shaft i Intkia Dtameter) 10 II
Bottom Shall Outside Dlamleri 10.&44 ft
Node Distance Item tho ban Z 00 Bain. # faction Aral (ft1) Dlstrfcuted MisaOualo SaKWaight Distance botwaen node* a (ft) Masam, Due to Self WWjM (ks^ft)
11 40.0 10 0.396 0.01 2.000 0.006
10 30 9 0.396 0.01 4.000 0.018
9 34 8 0.396 0.01 4.000 0.024
a 30 7 0.396 0.01 4.000 0.024
7 20 0.396 0.01 4.000 0.024
a 22 S 0.390 0.01 4.000 Â£53
s 18 1 0.396 0.01 4.000 0.024
4 14 3 0.396 0.01 4.000 0.024
3 10 2 0.448 0.01 6.000 0.020
2 s 1 0.590 0.01 6.000 0.043
1 0 0 0.739 0.00 0.000 0.028
0.07
40.004 0i41
Olamttar 2* m VWdlh 2ft. m a r.(fi) VH. Bam. Cantar Z/Ho Ngnraltod AddadMau
4.0625 4.0625 1 1031 0.063 0.00 0.000 aooo 1.0 0.000 1000 0.000 0.000
4.0825 4.0625 1 2.031 0.053 1.00 aooo 0025 1.0 0025 4.000 04)00 0.000
4.0625 441825 1 2.031 0.053 089 0.653 ao2S 1.0 0.025 4.000 ao2i 0043
4.0625 4.0625 1 2.031 0.053 0.79 0.941 0026 1.0 0.025 4.000 0.024 amo
4.0625 4.0625 1 2.031 0.053 0.59 0.971 0026 14) 0025 4.000 0.024 aooo
4.0629 4.0625 1 2.031 0063 0.56 0.982 0.023 1.0 0.025 4.000 0.025 aom
4.0625 4.0825 1 1031 0.053 0.47 0.989 0.025 1.0 0.025 4.000 0.025 0099
4.0625 40628 1 1031 0.093 0.37 0.992 a.025 1.0 0.029 4.000 0.025 a 099
4.0825 4.0625 1 1031 0.063 089 0.993 0.025 1.0 0025 5.000 0.025 0.112
7.0625 7.0625 1 3.631 0.003 0.13 0.963 0075 1.0 0.076 6.000 0.073 0846
10.0625 10.0625 1 5.031 0.132 0.00 0.963 0.196 1.0 0.196 0.000 0.189 0183
40.000
1.066
m Depth 2b W WMth2* m MW r/H, ntoroA AoA p ud 0#. Distance between nodes e () Distributed Hydrodynamic Added Man m. cmVi Unpedintidt Wydredyntmla MM Mata. N1, Total Lumped (kVl)
1.0S 4.00 4.00 1.00 zoo aooo 0.00 0.01 0.00 2.00 0.00 aoo 0.000
1.00 4.00 4.00 1.00 2.00 0093 0.00 0.01 0.00 4.00 0.00 0.000 0.01S
0.89 4.00 4.aa 1.00 2.00 0.053 0.90 aoi 0.01 4.00 0.01 0.013 0.082
0.79 4.00 4.00 i.ao 2.00 0.053 0.99 aoi aot 4.00 aot 0.030 at43
0.68 4.00 4.00 1.00 zoo 0.083 1.00 0.01 aoi 4.00 aoi 0.031 aisi
0.88 4.00 4.00 1.00 zoo 0.083 1.00 aoi 0.01 4.00 0.01 0.031 0.183
0,47 4.00 4.00 1.00 zoo 0083 i.ao 0.01 0.01 4.00 aoi 0.031 ai54
0.37 4.00 4.00 1.00 zoo 0.053 1.00 aoi 0.01 4.00 531 0.031 0.153
0.28 4.00 4.00 1.00 zoo 0.053 1.00 0.01 0.01 5.00 0.01 0.035 ans
0.13 7 7 1.00 3.80 5355 Too 0.02 532 3.00 535 5355 0.387
aoo 10 10 1.00 8.00 0.132 1 0.05 0.05 0 0 0,059 0288
1.8TB
