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RECENT DEVELOPMENTS IN SEISMIC ANALYSIS OF BRIDGE SUBSTRUCTURES. Mohiuddin A. Khan Ph.D., P.E. Manager Bridge Department, STV Inc., Trenton, NJ . What is an earthquake?

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recent developments in seismic analysis of bridge substructures

RECENT DEVELOPMENTS IN SEISMIC ANALYSIS OF BRIDGE SUBSTRUCTURES

Mohiuddin A. Khan Ph.D., P.E.

Manager Bridge Department,

STV Inc., Trenton, NJ

slide4

What is an earthquake?

  • In New Jersey earthquakes usually occur when slowly accumulated strain within the Earth's crust is suddenly released along a fault.
  • Energy from this movement travels as seismic waves along ground surface and within the crust but away from the core. The arrival of this released energy is felt as an earthquake.
  • It’s intensity is determined by observing its effects at a particular place on Earth's surface. Intensity depends on
      • the earthquake's magnitude,
      • the distance from the epicenter, and
      • local geology.
  • The most commonly used scales in the United States are
      • Modified Mercalli Intensity Scale (usually reported in Roman numerals to distinguish them from magnitudes).
      • Magnitude - It is determined by using an instrument called a seismograph.
  • The best known magnitude scale is one designed by C.F. Richter in 1935.
slide5

Comparison of MM

and Richter Scales

  • From Draft NJDOT LRFD Bridge Design Manual (Under Publication)
slide7

The Largest Earthquakes, Eastern States vs. CA

The time periods varies from about three centuries in eastern states to slightly more than a century in some western states.

Date ----- Magnitude --- Intensity

Year Other Moment

California 1857 7.6MS 7.92 IX

1906 7.80MS 7.68 XI

Delaware 1871 VII

Georgia 1914 4.50Mfa V

Maine 1904 5.10Mfa VII

Maryland 1990 2.5Mn V

Massachusetts 1755 VIII

New Hampshire 1940 5.50Mn 5.25 VII

1940 5.50Mn 5.60 VII

New Jersey 1783 5.30Mfa VI

New York 1944 5.80Mn 5.52 VIII

North Carolina 1916 5.20Mfa VII

Pennsylvania 1998 5.2Mn VI

Rhode Island 1976 3.50Mn 2.07 VI

South Carolina 1886 6.70Mfa 7.02 X

Vermont 1962 4.2Mn V

Virginia 1897 5.60Mfa VIII

slide8

Earthquake History of Pennsylvania

  • Limited information on effects in Pennsylvania until 1737.
  • A very severe earthquake centered in the St. Lawrence River region in 1663
  • at Newbury, Massachusetts, in 1727 affected towns in Pennsylvania.
  • A strong earthquake on December 18, 1737 reported felt at Philadelphiaa.
  • Shocks with origins outside the State were felt in 1758, 1783, and 1791.
  • In 1800, two earthquakes (March 17 and November 29) were reported as "severe" at Philadelphia.
  • On November 11 and 14, 1840, earthquakes were felt at Philadelphia..
  • Intensity V felt at Allentown by a strong shock on May 31, 1884.
  • West Chester, Pennsylvania reported shocks on August 10, 1884.
  • A tremor, on March 8, 1889, at Harrisburg, Philadelphia, Reading, York of intensity V.
  • An earthquake on May 31, 1908 at Allentown shook down a few chimneys (VI).
  • On October 29, 1934, a shock of intensity V was felt at Erie.
  • Southern Blair County on July 15, 1938. (VI) were reported at Clover Creek and Henrietta.
  • West of Reading experienced minor damage from an earthquake on January 7, 1954..
  • On September 14, 1961, a moderate earthquake was centered in the Lehigh Valley.
  • On December 27, 1961, in the northeast portion and suburbs of Philadelphia. .
  • A strong local shock measured at magnitude 4.5, (VI) at Cornwall on May 12, 1964.
  • Intensity V effects at Darby, and Philadelphia on December 10, 1968.
  • On December 7, 1972, damage (V) was reported at New Holland.
  • Abridged from Earthquake Information Bulletin, Volume 8, Number 4, May - June 1973, by Carl A. von Hake.
slide9

10 Largest Earthquake in the World

LocationDateMagnitude

1. Chile 1960 05 22 9.5

2. Prince William

Sound, Alaska 1964 03 28 9.2

3. Aleutian Islands 1957 03 09 9.1

4. Kamchatka 1952 11 04 9.0

5. Off the Coast of

Ecuador 1906 01 31 8.8

6. Aleutian Islands 1965 02 04 8.7

7. India-China Border 1950 08 15 8.6

8. Kamchatka 1923 02 03 8.5

9. Banda Sea, Indonesia 1938 02 01 8.0

10. Kuril Islands 1963 10 13 8.5

slide10

The 11 Largest Earthquakes in the Contiguous United States

Location Date Time Magnitude

1. New Madrid, Missouri 1811 12 16 8:15 8.1

2. New Madrid, Missouri 1812 02 07 9:45 ˜8

3. Fort Tejon, California 1857 01 09 16:24 7.9

4. New Madrid, Missouri 1812 01 23 15:00 7.8

5. Imperial Valley, California 1892 02 24 7:20 7.8

6. San Francisco, California 1906 04 18 13:12 7.8

7. Owens Valley, California 1872 03 26 10:30 7.6

8. Gorda Plate, California 1980 11 08 10:27 7.4

9. N Cascades Washington 1872 12 15 5:40 7.3

10. CA - Oregon Coast 1873 11 23 5:00 7.3

11. Charleston South Carolina1886 09 01 2:51 7.3

slide11

Earthquake History of New York

Strong earthquakes in 1638, 1661, 1663, and 1732 in the St. Lawrence Valley

First notable tremor centered within the State was recorded on December 18, 1737 (intensity VII).

Walls vibrated, bells rang, and objects fell from shelves (intensity VI) at Buffalo from a shock on

October 23, 1857.

A rather severe earthquake centered in northeastern New York area in 1877 (Intensity VII).

On August 10, 1884, an earthquake caused large cracks in walls at Amityville and Jamaica

(intensity VII).

A shock reported as severe (intensity VI), occurred in northeastern New York on May 27, 1897.

A very large area of the northeastern United States was shaken by a magnitude 7 earthquake on

February 28, 1925.

A maximum intensity of VIII was reached in the epicentral region, near La Malbaie, Quebec, Canada.

Extensive damage occurred in the Attica area from a strong shock on August 12, 1929.

On April 20, 1931, an earthquake centering near Lake George (intensity VII).

On September 4, 1944, an earthquake centered about midway between Massena, New York, and

Cornwall, Ontario, Canada, caused damage in the two cities. The shock was of Intensity VIII).

slide12

Quaternary

Earthquakes are linked to geologic conditions.

Precambrian Period is shown

slide17

Case Studies:

Skew bridge

Bridge curved in plan

Bridge curved in elevation

Railroad bridge

Bridge with Integral Abutment

slide18

INTRODUCTION

    • 1. A number of bridges located in New Jersey, New York and
    • Pennsylvania were analyzed for acceleration factors between
    • 0.10 and 0.18, except for Long Island Railroad Bridge, which are . higher. The case studies of analytical procedures of five
    • bridges are presented here. Selection of suitable analytical, computational methods,bearing types, their locations and modeling techniques are discussed.
    • 2. Experience gained with analysis procedures contributed towards developing a chapter ‘ Seismic analysis and design’ in the proposed New Jersey LRFD Bridge Design Manual.
  • 3. Since the 1971 San Fernando earthquake FHWA published
  • “Seismic Design and Retrofit Manual for Highway Bridges” in
  • 1987. Further knowledge of seismic phenomenon was gained
  • from the 1989 Lorna Prieta and 1994 Northridge earthquakes.
slide19

Case Study 1

SKEW BRIDGE

Route U.S. 322/N.J. 50

PRECAST CONCRETE PIERS

slide20

Case Study 1

Precast Pier

slide21

Case Study 2 - Curved Girders in Plan

Route U.S. 23/ U.S. 80 (NJ)

slide23

Case Study 4 - Replacement of

Long Island R.R. Bridge (NY)

slide27

State of Art and Overview of Seismic Engineering

Seismic Study/ Literacy

Support Disciplines

A. Seismology B. Geotech Eng. C. Disaster Managment

D. Info. Links

Seismic Disciplines

E. Education F. Archives

G. US Coord. Agencies

H. International Coord. Agencies

I. Earthquake Engineering

J. Key Issues in Structural Engineering

K. Structural Engineers Role

a. Flow diagram for seismic study /seismic literacy

slide28

A. SEISMOLOGY

    • Geodesy
    • Earth’s Interior
    • Plate Tectonics / Location of Faults in NJ
    • Fault Maps
    • Wave Propagation
    • Seismometry, Seismographs
    • Intensity of Shaking/Strong Ground Motions
    • Acceleration, Velocity and Displacement Histories
    • Frequency and duration
    • Seismic Maps
  • B. GEOTECHNICAL ENGINEERING
    • Engineering Geology
    • Liquefaction
    • Site Effects
    • Soil Structure Interaction
    • Landslides
    • Micro-zoning
slide29

C. DISASTER MANAGEMENT

    • Vulnerable Populations
    • Risk Assessment
    • Loss Estimation (loss of life, property and commerce is the root of this problem)
    • Disaster Planning
    • Implementation
    • Mitigation (maintain Lifelines)
    • Recovery
    • Hazard Insurance
  • D. DISASTER MANAGEMENT LINKS/INFORMATION
    • Earthquake Information Network
    • Information Centers/Sites for Natural Hazards -Multidisciplinary Center for Earthquake Engineering Research, at SUNY Buffalo (MCEER)
    • National Information Service for Earthquake Engineering, at UCB (NISEE)
    • National Earthquake Information Center at USGS (NEIC)
    • Natural Hazards Research and Applications Inform. Center, CO (NHRAIC)
    • Databases
slide30

E. SEISMIC EDUCATION

    • College Curriculum
    • Laboratory Facilities/Shake Table Studies - 4 full size shake tables in USA and
  • smaller sizes in Ankara, Turkey, Imperial College London and Roorkee, India
    • Research Programs and Funding
    • Continuing Education
    • Seminar/Conference Funding
  • F. RELATED ARCHIVES ON SEISMIC LITERACY
    • News Archives of past earthquakes -Turkey, Iran, India, Mexico, California, Japan, Eastern Europe
    • Photos/Slides/ Videotapes
    • Library/Information Services
    • Natural Hazards Data Resources Directory
    • Abstracts/Journal Articles/Fact Sheets/Disaster Maps
slide31

G. US COORDINATING ENGINEERING AGENCIES/ASSOCIATIONS

    • Federal Emergency Management Agency (FEMA)
    • Government Agencies (Federal/State/Local)
    • National Agencies (USGS/FHWA/ACI/AISC/ASCE/PCA/SEI)
    • Civil/Structural Engineering Associations (SEAOC/NEHRP)
    • US Army Corps of Engineers/Red Cross
  • H. INTERNATIONAL SOURCES AND AGENCIES
    • ITALY (Universita degli Studi di Trieste)
    • JAPAN (Earthquake and Volcanic Disaster Prevention Laboratory, Tsukuba-shi)
    • MEXICO (Ciudad Universitaria, Coyoacan, Mexico/Seismology Lab. UNR, NV)
    • NEW ZEALAND (Institute of Geological and Nuclear Sciences, Lower Hutt, NZ)
    • RUSSIA (Center of Geophysical Data Studies and Telematic Applications, Moscow)
    • SWITZERLAND (Swiss Seismological Service)
slide32

I.EARTHQUAKE ENGINEERING

    • Basic Terminology / Glossary
    • Definition of Seismic Hazards
    • Deterministic and Probabilistic Seismic Hazard Analysis
    • Selection of Seismic Zone
    • Pseudo Velocity and Acceleration
    • Concepts of Quasi-static, Response Spectrum and Time History Analysis
    • Deformation Response Spectra
    • Importance of Ductility
    • Importance of Proportions / Sizing / Mass Distribution
    • Performance Characteristics of Lateral Load Resisting Systems
    • Condition Assessment of Damaged and Deteriorated Structures
slide33

J.KEY ISSUES IN STRUCTURAL ENGINEERING

    • Codes/Standards - Purpose of Seismic Code Lifelines
    • Critical Facilities
    • Structural Analysis - Quasi-static/Dynamic, D’Alembert’s Principle , Newton’s 2nd Law of motion, Non-linear matrices, Eigenvalues and Eigenvectors
    • Structural Design & Detailing
    • Theoretical Model, Multi-degree-of-freedom systems
    • Selection and application of Computer software for dynamic analysis
    • Computer Hardware requirements
    • Structural Repairs
    • Seismic Retrofit Concepts and Measures
    • Bearings, Restrainers, Shear Keys
    • Case Studies of past structural design
    • Earthquake Engineering Research and Development-Universities, Research Departments, Engineering Organizations
slide34

K.ROLE OF STRUCTURAL ENGINEER IN SEISMIC DESIGN

    • Advising the client on seismic criteria and costs
    • Structural planning, including locations and types of bearings (Rotational / Translational)
    • Compliance with relevant seismic codes
    • Selecting computer software for seismic analysis
    • Preparing construction drawings and guidelines for the contractor
    • Seismic detailing
    • Training design engineers in seismic design procedures
    • Following QA/QC procedures for the project
    • Keeping seismic design costs to a minimum
    • Solving any constructibility problems in the field
    • Maintain professional license and compliance with ethical requirements
    • Purchase liability insurance
slide35

Regular

Draft LRFD Bridge Manual

Selection of method of analysis for bridges

slide37

For any supplied response spectrum (either acceleration vs. period or displacement vs. period), joint displacements, member forces, and support reactions may be calculated.

Time-History Analysis– This is an analysis of the dynamic responseof a structure when the base is subjected to a specific ground motion time history.

This analysis is performed using the modal superposition method. Hence, all the active masses should be modeled as loads in order to facilitate determination of the mode shapes and frequencies. In the mode superposition analysis, it is assumed that the structural response can be obtained from the "p" lowest modes.

The equilibrium equations are written as

slide39

Numerical Methods for solving differential equations

  • Solution of the Eigenproblem
  • The eigenproblem is solved for structure frequencies and mode
  • shapes considering a lumped mass matrix, with masses at active
  • d.o.f. included.
  • Two solution methods are available:
  • determinant search method, and
  • the subspace iteration method,
  • with solution selection based on problem size.
  • Modal responses may be combined using either the
  • Square root of the sum of squares (SRSS) or the
  • Complete quadratic combination (CQC) method to obtain the resultant responses.
  • Review Phase: Interpret and gain confidence with the analysis results.
  • Design Phase: Accomplish the AASHTO LRFD Code required design tasks.
  • Detailing Phase: Develop AASHTO LRFD required details.
slide52

NCHRP/MCEER PROJECT 12-49, 2001

3.10.3.8 CAPACITY DESIGN

Capacity design principles require that those elements not participating as part of the primary energy

dissipating system (flexural hinging in columns), such as column shear, joints and cap beams, spread

footings, pile caps and foundations be “capacity protected”. This is achieved by ensuring the maximum

moment and shear from plastic hinges in the columns (overstrength) can be dependably resisted by

adjoining elements.

3.10.3.4 SDAP C – CAPACITY SPECTRUM DESIGN METHOD

3.10.3.4.1 Capacity Spectrum Design Approach

SDAP C combines a demand and capacity analysis, including the effect of inelastic behavior of ductile

earthquake resisting elements. The procedure applies only to bridges that behave essentially as a single

degree-of-freedom system. SDAP C is restricted to bridges with a very regular configuration

and with the recommended earthquake resisting systems (ERS) as described in Section 2.

Similar to the Caltrans procedures.

Refer to

Table 3.10.1-1 Design Earthquakes and Seismic Performance Objectives

Table 3.10.3-2 - Seismic Design and Analysis Procedures (SDAP) and Seismic Detailing

Requirements (SDR)

slide58

SEISMIC CODESThe purpose of a seismic code is to

  • protect life and property,
  • develop a quality structure and
  • implement uniformity in construction.

Flow Diagram for seismic Codes

Seismic Codes

Bridge Seismic

Codes

Building Seismic

Codes.

Railway Bridges

Highway Bridges

State/City

Int./Other

FHWA/

AASHTO

slide59

US Codes and Publications Related to Seismic Design

    • AASHTO LRFD Specifications for the Design of Bridges (AASHTO), Washington
    • D.C. 1998
    • International Building Code (IBC), 2000
    • International Handbook of Earthquake Engineering: Codes, Programs and Examples
    • (IHEE) – Kluwer Academic Publishers, 1995
    • Practice of Earthquake Hazard Assessment (PEHA), Robin McGuire, International
    • Association of Seismology, IASPEI, 1993
    • Regulations for Seismic Design: A world List (RSD)- International Association for
    • Earthquake Engineering (IAEE), Tokyo 1996
    • Supplement 2000 to RSD (SUP), IAEE, Tokyo 2000
    • Seismic Design of Buildings (SDB), Dept. of Army, Navy and Air Force, Washington
    • D.C., 1992
    • Uniform Building Code (UBC) – ICBO, Whittier, CA, 1997
slide61

ABBREVIATIONS

  • RSD: Regulations for Seismic Design: A World List 1996*
  • SUP: Regulations for Seismic Design: Supplement 2000*
  • PEHA: Practice of Earthquake Hazard Assessment
  • IHEE: International Handbook of Earthquake Engineering
  • SDB: Seismic Design for Buildings
  • UBC: Uniform Building Code, 1997
  • AASHTO: American Association of State Highway and Transportation Officials
  • NCHRP: National Cooperative Highway Research Program
  • MCEER: Multidisciplinary Center For Earthquake Engineering Research
  • ATC Applied Technology Council
  • TRB Transportation Research Board
  • *Includes Eurocode 8
slide62

BUILDING SEISMIC CODES VS. BRIDGE SEISMIC CODES

a. There are significant physical differences, such as multi-bay, multi-frame action in a building, compared to the use of Continuous Beam, with alternate fixed and free bearing supports, in a bridge.

b. Magnitude of live load is considerably higher in a bridge, compared to building live load. AASHTO Code requires a tributary live load to be added to vertical dead load reaction, depending on the seismic zone, when calculating horizontal design connection force.

c. International Building Code (IBC) 2000 is composed of 3 model code organizations ICBO, BOCA and SBC. Erstwhile, NEHRP and ASCE 7 Codes were based on USD unlike UBC & SEAOC.

d. IBC uses “Seismic Design Category”, which is based on contours representing mapped spectral response acceleration, at short periods of 0.2 seconds. It is a function of three parameters: probable ground motion defined by spectral response accelerations (Site Class A to F), soil class, and building occupancy (Seismic Use Groups I, II, III).

e. AASHTO LRFD Code uses “Seismic Zones” 1 to 4 (Acceleration Coefficient 0.09 to 0.29) to establish design earthquake ground motion.

f. AASHTO uses Importance category (Critical bridge, Essential bridge or Other categories) and Soil Profiles I to IV. In addition, AASHTO uses separate Response Modification Factor (R Factor varies between 0.8 to 5.0) depending on Substructure type and Connection detail.

g. The method of analysis in AASHTO code (Uniform Load, Single Load, Multimode elastic methods or Time History method) is directly related to Importance Category.

h. The scale and magnitude of each of the parameters is different in the two codes. Hence, it is not easy to correlate each parameter for building and bridge seismic analysis.

slide63

1.8 m

MAXIMUM TRUCK LOADS DURING AN EARTHQUAKE.

slide65

From Draft NJDOT LRFD Bridge Design Manual

LENGTH OF NJ PERMIT VEHICLE =

1/2 OF COOPER TRAIN LENGTH

MAXIMUM TRAIN LOADS DURING AN EARTHQUAKE

slide68

AASHTO (1998) VS. AREMA (1999) SEISMIC CODES

1. Railway bridges have more conservative design approach

2. Have historically performed well in seismic events with little or no damage.

3. Railway Bridges are traversed by track structure that functions very effectively as a restraint against longitudinal and transverse movement during earthquakes.

4. Spans are smaller.

5. Heavy concrete decks are not present and dead load inertia forces are smaller.

6. Types of damage that are permissible are very limited compared to highway bridges.

7. Post-seismic event operation guidelines put restrictions on train traffic and speeds of train, depending on the intensity of earthquake, until proper inspection has been carried out.

8. Ground Motion Level for level 1 has smaller earthquake return period for railway bridges 100 years as compared to 450 years for highway bridges. Acceleration coefficients are expressed as % of gravity, for 50, 100, 250, 475 and 2400 year return periods.

slide69

9. Risk factor parameter is included as an integral part of seismic design for railway bridges.

  • 10. For ground motions levels 2 and 3, the risk criteria is based on a high passenger train
  • occupancy rate.
  • 11. Seismic Response Coefficient (Cm) is multiplied by a Damping Adjustment Factor,
  • usually > 1, resulting in higher seismic force.
  • 12. Analysis is based on Serviceability, Ultimate and Survivability Limit States,
  • unlike LRFD method for highway bridges.
  • 13. Substructure Response Modification Factor (R) are smaller for ground motion level 3,
  • resulting in higher seismic design moments for the railway bridges.
  • 14. Train live load is combined with the dead load to give higher horizontal forces.
slide71

Case Studies 1 to 5 - Applications of Computer Software

1. Modeling of superstructure

2. Modeling of substructure

3. Modeling of bearings- Guided, Unguided and Fixed

Placing fixed bearing on shorter abutment or on shorter pier to minimize seismic moments

4. Selection of methods of analysis

5. Selection of numerical method- SRSS, CQC

6. Sub-models for connections- Alternates

7. Determine pile-bent height from pile programs,such as COM624P or L-pile.

slide72

Case Study 1

Route U.S. 322/N.J. 50

slide73

Case Study 1

Precast Pier

slide74

Applications of Seismic Analysis Methods

  • 1. Equivalent Static Analysis Method (UniformLoad Method or Single-mode SpectralMethod):Both these methods are applicable to regular bridges only.
  • Application to case study 1
  • Regular Skew bridge with two spans: Uniform Load Method was applied to low seismicity regular bridge with acceleration coefficient of 0.10.

Elastic seismic response coefficient Cs = (1.2AS)/(Tm^23) was computed, by hand calculations. A limiting value of 2.5 A was used.

The simplified equation Tm = 2 (W/gK)^ 0.5 from AASHTO Sec. 4.7.4 was used.

For single mode analysis, using Deck weight Vs and , ,  factors,

from Section 5.3 of AASHTO Code

Equivalent static earthquake loading pe (x) = Cs/ w(x) Vs (x) was then computed and modified by the response modification factor R.

Seismic force = pe (x). L/R Comparison was made with the results obtained from the two methods. Results were found to be within 5 % of the hand computed values.

Acceleration Coefficient A was modified due to energy dissipation.

2. Elastic Dynamic Analysis Method (Multi-mode Spectral method):

Multimode method of analysis is selected based on irregular geometry.

Acceleration factor = 0.18 was used for all cases.

slide76

Case Study 2

Route U.S. 23/ U.S. 80 (NJ)

slide77

Application to case study 2

  • Multispan horizontally curved girders, supported on transverse bents:
  • The bridge had 4 curved spans and required a new design.
  • SEISAB Program was used for seismic analysis.
  • Guided expansion bearings were located on abutments.
  • Non-guided expansion bearings were located on piers, with fixed bearings on the
  • middle transverse bent. Centrifugal forces were computed for 45 mph speed.
  • Seismic Load combinations of (DL + 100% longitudinal + 30% transverse) and
  • (DL + 30% longitudinal + 100% transverse) were used.
  • Seismic pile capacities in the lateral and vertical directions were computed using
  • COM624P.
slide78

Case Study 3

Route U.S. 80 (NJ)

slide79

Application to case study 3

Case study 3-Multispan vertically curved girders, supported on transverse frames:Existing bridge was evaluated for seismic adequacy in order to provide rehab. to the existing bridge structure. Three-dimensional modeling was carried out using STAAD-PRO program.

Two sub-models were tried to simulate the highly eccentric connection between the 3.5 feet deep longitudinal girders and the 4.0 feet deep pier cap.

The first was a Tee shaped rigid link in the plane of longitudinal girders.

The alternate was a Vee shaped rigid link and the vertical member was replaced by two inclined members. The longitudinal girders were connected to horizontal members of the rigid link.

The second model gave an improved transfer of forces from superstructure to substructure.

Due to discontinuity between adjacent transverse frames SEISAB could not be applied.

slide80

Complete Quadratic Combination (CQC) method was used for elastic dynamic analysis

with 5 % damping.

Seismic Response Coefficients Cs were computed for various periods T.

For a range of values of T = 0.05, 0.45, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 the computed values were

Cs = 1.59, 0.37, 0.34, 0.22, 0.17, 0.14, 0.10, 0.09. using the Equation for period T.

Site Coefficient was taken as unity.

BSDI program (Bridge Software Development International Ltd.) modeled vertically curved

girders for computing deflections and dead load camber.

Centerlines of beams were curved upwards by maximum 8 inches at the midspan.

A comparison of dead load analysis with alternate STAAD-PRO analysis showed a difference of

up to 5 % in vertical deflection values.

Seismic isolation bearings were not required.

slide81

Case Study 4

Long Island R.R. Bridge (NY)

slide82

Application to case study 4

Multispan girders supported on column bentsfor railroad bridge

Three continuous spans were planned for the Railroad bridge.

Steel girders were supported on transverse bents.

The geometry of Continuous transverse column bents was simplified by using repeated single bent with cantilever beams.

Spectral acceleration values with 5% damping for OBE level design were for

T =0, 0.04, 0.1, 0.2,0.5, 1.0, 2.0 and 5.0,

the computed values for site class D were 0.12, 0.29, 0.29, 0.29, 0.20, 0.10, 0.05, 0.02.

slide83

Spectral acceleration values for MDE level design were

T = 0, 0.04, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0

the computed values were 0.37, 0.87, 0.87, 0.87, 0.59, 0.30, 0.15 and 0.06.

Group Load = 1.0 (D + LL + PS + B + SF + E + EQM);

LL = Cooper E-80 train loads Live load acting on one track only;

EQM = Elastic seismic force (MDE and ODE) modified by dividing by the appropriate

R-factor. For foundations and for ODE R = 1.

For connections design R = 0.8.

For MDE multiple column bent R =4.

Although AREMA does not require live loads to be combined with D + EQ, project

design criteria required seismic forces resulting from both dead and live loads,

due to a higher importance classification factor for the railroad bridge.

slide84

Discussions

1. Equivalent Static Analysis method is suitable only for preliminary design. For final design moments and forces should be checked by Spectral methods.

2. Computer models: 3 standard softwares, SEISAB, STAAD-PRO, and BSDI were used for the 4 case studies. For discontinuous frames within the same bridge, STAAD-PRO was found to be more suitable than SEISAB. Within the STAAD program, either self weight analysis or equivalent density method for composite beam can be used. The two methods gave identical results.

3. Comparisons of BSDI line girder dead load analysis with STAAD analysis, to account for camber, showed a difference of about 7%. BSDI program appears to model vertical curvature more accurately than STAAD.

4. Idealization of longitudinal and deep beam connections in computer model is sensitive to magnitudes of lateral forces and moments. Vee shaped rigid link was found more accurate.

5. To comply with all aspects of bridge analysis procedures, additional to those prescribed in AASHTO and AREMA seismic codes, are considered necessary. Familiarity with numerous computer programs, modeling techniques, selection and locations of guided, unguided and fixed bearings is necessary.

slide85

Discussions

6. Use of supporting software, such as for dead load camber analysis or for computation of seismic pile capacities, will lead to an accurate analysis, and improved connections design.

7. Laboratory models of important (Essential) bridges with complex geometry need to be tested by simulating earthquakes, using shake-tables displacements and forces.

8. AASHTO LRFD Code specifies R Factors which are rounded off figures 1 to 4 which appear to be approximate. Since this factor scales down the design moments future research should focus on arriving at a more accurate assessment of the R Factors.

9. An independent Code Committee should also verify the suitability/accuracy of each of the standard software. Such a comparative study is beyond the capacity of most consulting firms.

10. Application of 3 state seismic codes from NJ, PA and NY has shown refinements of AASHTO code with emphasis on different aspects.

NJ addresses seismic retrofit aspects, PA on seismic detailing and NY on multimode analysis and soil behavior.

References:

AASHTO, “Standard Specifications for Highway Bridges,” Sixteenth Edition, 1996

AREMA, “Manual for Railway Engineering,” 1999

NJDOT, “Bridges and Structures Design Manual,” Third Edition, 1998

slide97

Earthquake Image Glossary

A

acceleration

accelerogram

accelerograph

active fault

amplitude

B

basement

bedrock

body wave

C

core

crust

D

displacement

E

earthquake

epicenter

F

foreshocks

frequency

G

geodesy

geodetic

ground motion

H

hazard

Hertz (Hz)

I

intensity

L

lifelines

liquefaction

M

magnitude

mantle

N

natural frequency

P

P wave

period

plate tectonics

Q

Quaternary

R

recurrence interval

regression analysis

return period

Richter scale

S

S wave

seismicity

seismic zone

seismograph

seismology

seismometer

slip

spectral acceleration (SA)

spectrum

strain

strain rate

T

tectonic

tectonic plates

thrust fault

V

velocity

W

wavelength