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Seismic Design of Bridges

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

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Seismic Design of Bridges

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  1. Lucero E. Mesa, P.E. Seismic Designof Bridges

  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

  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

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

  5. Earthquake of August 31, 1886 Charleston, South CarolinaMagnitude=7.3M, Intensity = X

  6. 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. Draft Specifications • Force based specifications • N (seat width) • Soil classification: I – IV • Draft Specifications Version of 1999

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

  10. 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

  11. 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

  12. Maybank Highway Bridgeover the Stono River

  13. 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

  14. 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

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

  16. 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 .

  17. 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

  18. Cooper River Bridge2500 Yr - SEE for Main Piers

  19. Need for: • New Specifications • South Carolina Seismic Hazard Maps

  20. 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.

  21. 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.

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

  23. 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)

  24. 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

  25. SCDOT Seismic Design Specifications Seismic Performance Criteria III II I

  26. SCDOT Seismic Design Specifications October 2001

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

  28. SCDOT Seismic Design Specifications October 2001

  29. DESIGN SPECTRA FOR SITE CLASS A, B, C, D AND E, 5% DAMPING (3.4.5E) SDI-SEE

  30. 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

  31. 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

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

  33. Other New Concepts and Improvements • 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)

  34. Lucero E. Mesa, P.E. Seismic Designof Bridges Thanks

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