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

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Presentation on theme: "1 Seismic Design of Bridges Lucero E. Mesa, P.E.."— Presentation transcript:

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

2 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 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 4 CHARLESTON, SOUTH CAROLINA August 31, 1886 (Intensity IX-X)

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

6 6

7 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 8 Force based specifications N (seat width) Soil classification: I – IV Draft Specifications Version of 1999 Draft Specifications

9 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 10 SC-38 over I-95 - Dillon County Maybank Highway Bridge over the Stono River - Charleston County SEISMIC DESIGN TRIAL EXAMPLES

11 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

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

13 13 Maybank Highway Bridge over the Stono River

14 14

15 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 Maybank Highway over Stono River Description of project

16 16 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. Maybank Highway over Stono River

17 17

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

19 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 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

21 21

22 22 Cooper River Bridge 2500 Yr - SEE for Main Piers

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

24 24

25 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 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 27 New Design Level Earthquakes New Performance Objectives New Soil Factors Displacement Based Design Expanded Design Criteria for Bridges New Concepts and Enhancements

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

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

30 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 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 32 SCDOT Seismic Design Specifications Seismic Performance Criteria IIIII I

33 33 SCDOT Seismic Design Specifications October 2001

34 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 =0.50S S =0.75S S =1.00 S S 1.25 A0.8 B1.0 C D E a Faaaaa

35 35

36 36 SCDOT Seismic Design Specifications October 2001

37 37 DESIGN SPECTRA FOR SITE CLASS A, B, C, D AND E, 5% DAMPING (3.4.5E) S DI-SEE

38 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 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 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 UsedLimitedModerate Plastic Action Not required to Maintain May be UsedLimitedMinimal Plastic Action ReparabilityProtection Systems Ductility Demand Design Approach

41 41 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) Other New Concepts and Improvements

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


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