seismic design of bridges - .xyzlibvolume3.xyz/.../seismicdesignphilosophypresentation2.pdf · 2...

1

Seismic Design

of Bridges

Lucero E. Mesa, P.E.

2

• 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

3

• USGS 1988 Seismic Hazard Maps

• Force based design

• Soil Classification I-IV

• No explicit Performance Criteria

• Classification based only on acceleration

coefficient

AASHTO Div IA

4

CHARLESTON, SOUTH CAROLINA

August 31, 1886 (Intensity IX-X)

5

Earthquake of August 31, 1886 Charleston, South Carolina

Magnitude=7.3M, Intensity = X

6

Sandblow in Charleston

7

• 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

8

• Force based specifications

• N (seat width)

• Soil classification: I – IV

• Draft Specifications Version of

1999

Draft Specifications

9

• Maybank Bridge over the Stono River

• Carolina Bays Parkway

• Broad and Chechessee River Bridges

• New Cooper River Bridge

• Bobby Jones Expressway

Site Specific Studies

10

• SC-38 over I-95 - Dillon County

• Maybank Highway Bridge over the

Stono River - Charleston County

SEISMIC DESIGN TRIAL

EXAMPLES

11

SC-38 over I-95 Description 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

12

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

13

Maybank Highway Bridge

over the Stono River

14

Highest Hazard

Lowest Hazard

Highest Hazard

Lowest Hazard

Seismicity of South Carolina 1977 to 1996

15

• 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 River Description of project

16


• 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


• Proposed LRFD Guidelines -

2002

• Two Level Analysis:

• Expected Earthquake - 50%

in 75 yr.

• MCE – 3% in 75 yr.

Maybank Highway over Stono River

17

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)

Spectral Acceleration, Sa (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

18


• Soil Classification: Type II


• Stiff Marl classified as Site Class D

Maybank Highway over Stono River

19

• 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 .

20

Cooper River Bridge

Charleston 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

22

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

Period, sec

Spectral

Acceleration, g

Cooper River Bridge

2500 Yr - SEE for Main Piers

23

• New Specifications

• South Carolina Seismic

Hazard Maps

Need for:

25

• 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

26

PURPOSE & PHILOSOPHY

(1.1)

• SCDOT Seismic Design Specifications replace AASHTO Division I-A SCDOT Draft

• Principles used for the development

�Small 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.

27

• New Design Level Earthquakes

• New Performance Objectives

• New Soil Factors

• Displacement Based Design

• Expanded Design Criteria for Bridges

New Concepts and

Enhancements

28

• Small to Moderate Earthquakes

�Essentially Elastic

�No Significant Damage

�Functional Evaluation Earthquake

(FEE) or 10% in 50 yr. event


29

• Large Earthquakes

�Life Safety

�No Collapse

�Serviceability

�Detectable and Accessible Damage

�Safety Evaluation Earthquake

(SEE) or 2% in 50 yr. event


30

• 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)

31

• 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)

32

SCDOT Seismic Design Specifications Seismic Performance Criteria

III II I

33


34

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

Site

Class

Design Spectral Acceleration at Short Periods

SS≤≤≤≤ 0.25

SS=0.50

SS=0.75

SS=1.00

SS≥≥≥≥1.25

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

35

SCDOT – Pilot WorkshopImbsen & Associates, Inc. –

I IA

1-6

Increasing

performanceIncreasing earthquake

hazard

Recent

Technology

bc

de

f

ih

gCollapse

Prevention

Limited

Damage

Essent ially

Elast ic

2% in 50 Yrs.

2/3 (2% in 50 Yrs.)

10% in 50 Yrs.

Proposed Design or Retrofit Objective

a

f, h, ia, b, c, d,

e, g

Secondary

System

Primary

SystemDesign or

Retrofit

Objective

36


37

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_4A

SD_4B

SD_4C

SD_4D

SD_4E

Periods T (sec)

Site Class

A

B

C

D

E

SDI-SEE

38

APPLICABILITY (3.1)

• New Bridges

• Bridge Types

�Slab

�Beam Girder

�Box Girder

• Spans less than 500 feet

• Minimum Requirements

• Additional Provisions are needed to achieve higher performance for essential or critical bridges

39

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

40

Design Approaches

(4.7.1)

May require

closure or

removal

Not

warranted

May be higher

Significant

Plastic Action

May require

closure of

limited usage

May be Used Limited Moderate

Plastic Action

Not required to

Maintain

May be Used Limited Minimal

Plastic Action

Reparability Protection

Systems

Ductility

Demand

Design

Approach

2D

µ <

4D

µ <

41

• Plastic Hinge Region Lpr (4.7.7)

• Plastic Hinge Length (4.7.7)

• Seat Width SPC A and B, C, D (4.8.2)

• Detailing Restrainers (4.9.3)

• Butt Welded Hoops

• Superstructrure Shear Keys (4.10)

Other New Concepts and

Improvements

42

Seismic Design

of Bridges

Lucero E. Mesa, P.E.

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