seismic design of bridge, lrfd

Seismic Designof Bridges

Lucero E. Mesa, P.E.

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AASHTO - Division IA Draft Specifications, 1996 SCDOT 2001 Seismic Design Specifications Comparison Between LRFD & SCDOT Specs. SCDOT Seismic Hazard Maps Training and Implementation Conclusions

SCDOT Seismic Design Of Bridges Overview

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USGS 1988 Seismic Hazard Maps Force based design Soil Classification I-IV No explicit Performance Criteria Classification based only on acceleration

coefficient http://www.tekniksipil.org/civil-

engineering

AASHTO Div IA

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CHARLESTON, SOUTH CAROLINAAugust 31, 1886 (Intensity IX-X)

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Earthquake of August 31, 1886 Charleston, South Carolina

Magnitude=7.3M, Intensity = X

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1996 USGS Seismic Hazard Maps

Difference in spectral acceleration between South Carolina and California

Normal Bridges : 2/3 of the 2% in 50 yr. Event

Essential Bridges: Two-Level Analysis

Draft Specifications

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Force based specifications N (seat width) Soil classification: I IV Draft Specifications Version of

1999

Draft Specifications

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Maybank Bridge over the Stono River

Carolina Bays Parkway Broad and Chechessee River

Bridges New Cooper River Bridge Bobby Jones Expressway

Site Specific Studies

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SC-38 over I-95 - Dillon County

Maybank Highway Bridge over the Stono River - Charleston County

SEISMIC DESIGN TRIAL EXAMPLES

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SC-38 over I-95Description of Project

Conventional bridge structure

Two 106.5 ft. spans with a composite reinforced concrete deck, supported by 13 steel plate girders and integral abutments

The abutments and the interior bents rest on deep foundations

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Original Seismic Design

SCDOT version of Div-IA AASHTO (Draft)

2/3 of 2% in 50 yr 1996 USGS maps used PGA of 0.15g, low potential

for liquefaction Response Spectrum

Analysis

Trial Design Example

Proposed LRFD Seismic Guidelines

MCE 3% PE in 75 yr. Expected Earthquake 50%

PE in 75 yr. 2000 USGS maps PGA of 0.33g, at MCE,

further evaluation for liquefaction is needed.

Response Spectrum Analysis

SC-38 over I-95

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Maybank Highway Bridgeover the Stono River

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118 spans 1-62 flat slab deck supported by PCP 63-104 /33 -meter girder spans and 2 columns per bent supported by shafts. The main span over the river channel consists of a 3 span steel girder frame w/ 70 meter center span. 105-118 flat slab deck supported by PCP

Maybank Highway over Stono RiverDescription of project

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Original Seismic Design SCDOT version of AASHTO

Div. I-A (Draft) Site Specific Seismic Hazard Bridge classified as essential Project specific seismic

performance criteria Two level Analysis:

FEE 10% in 50 yr. event SEE - 2% in 50 yr. event

Trial Design Example Proposed LRFD Guidelines -

2002 Two Level Analysis: Expected Earthquake - 50%

in 75 yr. MCE 3% in 75 yr.

Maybank Highway over Stono River

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Table C-1. LRFD Spectral Accelerations and Site Coefficients

Earthquake Spectral Accelerations Site Coefficients SS S1 SDS SD1 Fa Fv Maximum Considered 1.43 0.407 1.43 0.651 1.00 1.60 Expected 0.0503 0.0104 0.0503 0.0167 1.00 1.60

SEE - Compare LRFD to Original Design Curve

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0Period, T (sec)

Spec

tral

Acc

eler

atio

n, S

a (g

)

LRFD CurveSite Specific Original CurveSCDOT Curve, soil type IISCDOT Curve, soil type III

* The cumulative mass participation for mode shapes at periods indicated and higher, is approximately 70%.

* Transverse

* Longitudinal

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Original Seismic Design

Soil Classification: Type II

Trial Design Example

Stiff Marl classified as Site Class D

Maybank Highway over Stono River

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The SCDOT 's new specifications adopted the NCHRP soil site classification and the Design Spectra described on LRFD 3.4.1

If this structure were designed using the new SCDOT Seismic Design Specifications, October 2001, the demand forces would be closer if not the same to those found using the Proposed LRFD Guideline -2002 .

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Cooper River BridgeCharleston Co.

Seismic Design Criteria- Seismic Panel

Synthetic TH PGA - 0.65g Sa 1.85 at T=0.2

sec Sa 0.65 at T=1 sec Liquefaction

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0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

Period, sec

Sp

ect

ral

Acc

ele

rati

on

, g

Cooper River Bridge2500 Yr - SEE for Main Piers

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New Specifications South Carolina Seismic

Hazard Maps

Need for:

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The new SCDOT specifications establish design and construction provisions for bridges in South Carolina to minimize their susceptibility to damage from large earthquakes.

SCDOT Seismic Design Specifications October 2001

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PURPOSE & PHILOSOPHY (1.1) SCDOT Seismic Design Specifications

replace AASHTO Division I-A SCDOT Draft Principles used for the developmentSmall to moderate earthquakes, FEE, resisted

within the essentially elastic range.State-of-Practice ground motion intensities are

used.Large earthquakes, SEE, should not cause

collapse. Four Seismic Performance Categories (SPC)

are defined to cover the variation in seismic hazard of very small to high within the State of South Carolina. 26

New Design Level Earthquakes New Performance Objectives New Soil Factors Displacement Based Design Expanded Design Criteria for Bridges

New Concepts and Enhancements

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New USGS Probabilistic Seismic Hazard Maps

New Design Level Earthquakes

New Performance Objectives

A706 Reinf. Steel

New Soil Factors Displacement Based

Design Caltrans (SDC) new

provisions included

SCDOT Seismic Design Specifications Background (1.2)

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New Provisions meet current code objectives for large earthquakes.

Life Safety Serviceability

Design Levels Single Level 2% / 50 years

Normal Bridges Essential Bridges

Two Level : 2% / 50 years and 10% / 50 years Critical Bridges

Upgraded Seismic Design Requirement (1.3)

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SCDOT Seismic Design Specifications Seismic Performance Criteria

III II I

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VALUES OF Fa AS A FUNCTION OF SITE CLASS AND MAPPED SHORT-PERIOD SPECTRAL RESPONSE ACCELERATION SS (TABLE 3.3.3A)

SiteClass

Design Spectral Acceleration at Short Periods

SS 0.25 SS=0.50 SS=0.75 SS=1.00 SS1.25

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.2 1.2 1.1 1.0 1.0

D 1.6 1.4 1.2 1.1 1.0

E 2.5 1.7 1.2 0.9 a

F a a a a a

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DESIGN SPECTRA FOR SITE CLASS A, B, C, D AND E, 5% DAMPING (3.4.5E)

Ss=1.00g, SEE(2%/50years)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 1 2 3 4

SD_4ASD_4BSD_4CSD_4DSD_4E

Periods T (sec)

Site Class A B C D E

SDI-SEE

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APPLICABILITY (3.1)

New Bridges Bridge TypesSlabBeam GirderBox Girder

Spans less than 500 feet Minimum Requirements Additional Provisions are needed to achieve

higher performance for essential or critical bridges

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DESIGN PHILOSOPHY AND STRATEGIES

Specifications can be used in conjunction with rehabilitation, widening, or retrofit

SPC B demands are compared implicitly against capacities

Criteria is focused on member/component deformability as well as global ductility

Inherent member capacities are used to resist higher earthquake intensities

Using this approach required performance levels can be achieved in the Eastern US

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Design Approaches (4.7.1)

May require closure or removal

Not warrantedMay be higher

Significant Plastic Action

May require closure of

limited usage

May be UsedLimitedModerate Plastic Action

Not required to Maintain

May be UsedLimitedMinimal Plastic Action

ReparabilityProtection Systems

Ductility Demand

Design Approach

2D

seismic design of bridge, lrfd

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