Seismic Design of Bridges. Lucero E. Mesa, P.E.

Transcription

1 Seismic Design of Bridges Lucero E. Mesa, P.E. 1

2 SCDOT Seismic Design Of Bridges Overview AASHTO - Division IA Draft Specifications, 1996 SCDOT 2001 Seismic Design Specifications Comparison Between LRFD & SCDOT Specs. SCDOT Seismic Hazard Maps Training and Implementation Conclusions 2

3 AASHTO Div IA USGS 1988 Seismic Hazard Maps Force based design Soil Classification I-IV No explicit Performance Criteria Classification based only on acceleration coefficient 3

4 CHARLESTON, SOUTH CAROLINA August 31, 1886 (Intensity IX-X) 4

5 Earthquake of August 31, 1886 Charleston, South Carolina Magnitude=7.3M, Intensity = X 5

6 Sandblow in Charleston 6

7 Draft Specifications 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 7

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

9 Site Specific Studies Maybank Bridge over the Stono River Carolina Bays Parkway Broad and Chechessee River Bridges New Cooper River Bridge Bobby Jones Expressway 9

10 SEISMIC DESIGN TRIAL EXAMPLES SC-38 over I-95 - Dillon County Maybank Highway Bridge over the Stono River - Charleston County 10

11 SC-38 over I-95 Description of Project Conventional bridge structure Two 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 11

12 SC-38 over I-95 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 USGS maps PGA of 0.33g, at MCE, further evaluation for liquefaction is needed. Response Spectrum Analysis 12

13 Maybank Highway Bridge over the Stono River 13

14 Highest Hazard Seismicity of South Carolina 1977 to 1996 Lowest Hazard 14

15 Maybank Highway over Stono River Description of project 118 spans 1-62 flat slab deck supported by PCP /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 flat slab deck supported by PCP 15

16 Maybank Highway over Stono River 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 Two Level Analysis: Expected Earthquake - 50% in 75 yr. MCE 3% in 75 yr. 16

17 Table C-1. LRFD Spectral Accelerations and Site Coefficients Earthquake Spectral Accelerations Site Coefficients S S S 1 S DS S D1 F a F v Maximum Considered Expected SEE - Compare LRFD to Original Design Curve Spectral Acceleration, Sa (g) LRFD Curve Site Specific Original Curve SCDOT Curve, soil type II SCDOT Curve, soil type III * The cumulative mass participation for mode shapes at periods indicated and higher, is approximately 70%. * Transverse * Longitudinal Period, T (sec) 17

18 Maybank Highway over Stono River Original Seismic Design Trial Design Example Soil Classification: Type II Stiff Marl classified as Site Class D 18

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

20 Cooper River Bridge Charleston Co. Seismic Design Criteria- Seismic Panel Synthetic TH PGA g Sa 1.85 at T=0.2 sec Sa 0.65 at T=1 sec Liquefaction 20

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22 Cooper River Bridge 2500 Yr - SEE for Main Piers 2.5 Spectral Acceleration, g Period, sec 22

23 Need for: New Specifications South Carolina Seismic Hazard Maps 23

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25 SCDOT Seismic Design Specifications October 2001 The new SCDOT specifications establish design and construction provisions for bridges in South Carolina to minimize their susceptibility to damage from large earthquakes. 25

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. 26

27 New Concepts and Enhancements New Design Level Earthquakes New Performance Objectives New Soil Factors Displacement Based Design Expanded Design Criteria for Bridges 27

28 SCDOT Seismic Design Specifications October 2001 Small to Moderate Earthquakes Essentially Elastic No Significant Damage Functional Evaluation Earthquake (FEE) or 10% in 50 yr. event 28

29 SCDOT Seismic Design Specifications October 2001 Large Earthquakes Life Safety No Collapse Serviceability Detectable and Accessible Damage Safety Evaluation Earthquake (SEE) or 2% in 50 yr. event 29

30 SCDOT Seismic Design Specifications Background (1.2) 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 30

31 Upgraded Seismic Design Requirement (1.3) 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 31

32 SCDOT Seismic Design Specifications Seismic Performance Criteria III II I 32

33 SCDOT Seismic Design Specifications October

34 VALUES OF F a AS A FUNCTION OF SITE CLASS AND MAPPED SHORT- PERIOD SPECTRAL RESPONSE ACCELERATION S S (TABLE 3.3.3A) Site Class Design Spectral Acceleration at Short Periods S S 0.25 S S =0.50 S S =0.75 S S =1.00 S S 1.25 A B C D E a F a a a a a 34

35 Design or Retrofit Objective Collapse Prevention Primary System a, b, c, d, e, g i Secondary System f, h, i f h c e g b d a Recent Techn ology Limit ed Damage 2% in 50 Yrs. Increasin g perfor mance Essent ially Elast ic 10% in 50 Yrs. 2/3 (2% in 50 Yrs.) Incre as ing earthqua ke ha zard Proposed Design or Retrofit Objective I A I SCDOT Pilot Workshop Imbsen & Associates, Inc

36 SCDOT Seismic Design Specifications October

37 DESIGN SPECTRA FOR SITE CLASS A, B, C, D AND E, 5% DAMPING (3.4.5E) S s =1.00g, SEE(2%/50years) S DI-SEE Site Class SD_4A A SD_4B B SD_4C C SD_4D D SD_4E E Periods T (sec) 37

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 38

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 39

40 Design Approach Minimal Plastic Action Moderate Plastic Action Significant Plastic Action Design Approaches Ductility Demand Limited µ < D Limited µ < D 2 May be higher (4.7.1) 4 Protection Systems May be Used May be Used Not warranted Reparability Not required to Maintain May require closure of limited usage May require closure or removal 40

41 Other New Concepts and Improvements Plastic Hinge Region L pr (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) 41

42 Thanks Seismic Design of Bridges Lucero E. Mesa, P.E. 42