SEISMIC VULNERABILITY ASSESSMENTS OF BRIDGES IN AREAS OF LOW TO MODERATE SEISMIC ACTIVITY

1. SEISMIC VULNERABILITY ASSESSMENTS OF BRIDGES IN AREAS OF LOW TO MODERATE SEISMIC ACTIVITY Presented by: José A. Santos, PE - NYSDOT Artur Kasperski, PE - Earth Tech

2. First project of its type undertaken by NYSDOT • New York City – first study area (2100 bridges) • First Phase – Assess 450 Bridges on and over Primary Emergency Access Routes • Study to update NYSDOT Seismic Design Manual to current State of the Art practice • Electronic spreadsheet developed by Earth Tech to prepare the assessments • Tri-venture: Imbsen Consulting Engineers, Earth Tech and B&H Engineering • http://www.dot.state.ny.us/structures/manuals/man_bsa_seismic.html

3. Canada Clinton Lake Champlain Franklin St. Lawrence Essex Jefferson Lewis Lake Ontario Hamilton Warren Herkimer Oswego Washington Oneida Orleans Niagara Monroe Wayne Saratoga Fulton Onondaga Genesee Montgomery Seneca Madison Ontario Schenectady Cayuga Livingston Erie Rensselaer Yates Lake Erie Wyoming Otsego Cortland Schoharie Albany Chenango Tompkins Schuyler Greene Steuben Allegany Cattaraugus Delaware Columbia Chautauqua Tioga Chemung Broome Ulster Dutchess Sullivan Putnam Orange Westchester Rockland SEISMIC PERFORMANCE CATEGORY 'A' Bronx Suffolk SEISMIC PERFORMANCE CATEGORY 'B' Nassau New York Queens SEISMIC PERFORMANCE CATEGORY 'C' Kings Richmond MAGNITUDE 4 EVENT, 1985 Atlantic Seismic Performance Categories

5. Bridge Inventory Bridge and Inspection System (BIIS) SCREEN Susceptibility Groups 4 3 2 1 Special Bridges Available Bridge Information Classify Seismic Vulnerability Classes L M H Rating 1 2 3 4 5 6

6. Susceptibility Grouping • All bridges are evaluated based on their configuration and are assigned to one of the four susceptibility groups: • Group 1: High seismic vulnerability • Group 2: Moderate-high vulnerability • Group 3: Moderate-low vulnerability • Group 4: Low seismic vulnerability • Factors: • Superstructure type and continuity • Bearing type • Skew angle and/or alignment

7. Classifying • Classification Score, CS = V x E • High, Medium, or Low Vulnerability Class is assigned • The manual provides guidance with assigning an appropriate class • Vulnerability Score, V = Max (V1, V2) • V1 - Vulnerability score for connections • Maximum value of the vulnerability to structure collapse or loss of the support in transverse and longitudinal directions • V2 - Vulnerability score for substructure • V2 is the sum of pier (PV), abutment (AV) and liquefaction (LV) vulnerability scores

8. Vulnerability Score For Connections, V1 • Satisfactory bearing details, V1 = 0 • Continuous superstructure • Rocker bearings not used • Adequate bridge seat • Small skew • Transverse behavior, VT = 0, 5, or 10 • Bearings without transverse restraints, VT = 5 (minimum) • Rocker bearings, VT = 5 or 10 • Longitudinal behavior , VL = 0, 5, or 10 • N > Bridge Support > 0.5N, VL = 5 (10 for rocker bearings) • Bridge Support < 0.5N, VL = 10

9. Bridge Support Length • New York City: N = (305+2.5L+10H)x(1+0.000125xSkew2) (mm) • Seismic Performance Categories A and B: N = (203+1.67L+6.66H)x(1+0.000125xSkew2) (mm)

10. Vulnerability Rating Procedure STEP 1 Yes Are bearing details satisfactory? No STEP 2 Check transverse behavior No Restraint fails? Yes Yes 2 or 3 Girder bridge with outside girder on seat edge? No No High Pedestals (> 305 mm avg. height)? Yes Yes Rocker Bearings? No No Overturning of Bearings Possible? Yes Yes Bridge Collapse Likely? No VT = 5 VT = 0 VT = 10 B A

11. Vulnerability Rating Procedure, Cont. A B STEP 3 Check longitudinal behavior Yes N < L No No N/2 < L < N Yes No Rocker Bearings Yes Yes Overturning of bearings possible No VL = 0 VL = 10 VL = 5 STEP 4 VL = 0 V1 = Maximum of VT, VL

12. Pier Vulnerability Score • Masonry or Stone Piers - the most vulnerable, PV = 10 • Gravity Piers, PV = 10 • Potential for excessive cracking • Unreinforced or lightly reinforced construction • Cantilever type piers are assigned a maximum score of 6 • Multi-column Concrete Piers - prone to failure due to: • Transverse reinforcement • Splice location • Anchoring of longitudinal reinforcement into footings • Keeper bars or anchor bolts are expected to fail, PV = 0 • Piers and Footing with adequate steel reinforcement per NYSDOT Specifications, PV = 0

13. Multi-column Piers • Column Vulnerability due to shear failure CV = Q – R • Empirically derived formula • based on studies of column shear failures in bridges during the San Fernando earthquake in 1971 • checked against column failures in the Northridge earthquake (1994). • Reliable for short and medium height columns • May not apply to tall and/or slender column • Column Vulnerability due to flexural failure • Unknown reinforcement details, PV = 10 for piers taller than 7 m, 7 otherwise • Single column bents superstructure longer than 90 m, PV = 7 • Reinforcement spliced at the potential plastic hinge zone, PV = 7

14. Abutment Vulnerability, AV • AV = 5 for bridges crossing water and the expected fill settlement greater than 150 mm • AV = 0 all other bridges • Abutment failures typically don’t result in a bridge failure. • Liquefaction Vulnerability Score, LV • Bridges assumed to have moderate potential for liquefaction related damage are assigned a minimum LV = 5 • LV increased to 10 when the vulnerability rating for the bearings, V1, is 5 or more

15. Failure Type • Catastrophic – sudden and complete collapse of a superstructure span or spans • Partial Collapse – excessive displacement or discontinuities of a span, loss of service • Structural Damage – localized failures not noticeable to the traveling public • Parameters influencing the failure type: • Bearing type, bearing restraints • Available seat, pedestal dimensions • Bridge redundancy (2- or 3-girder bridges, single column pier)

16. Modes of Failure • Shear or flexural failure of pier columns • Substructure settlement • Tilting of piers • Large displacements resulting in a loss of support • Soil liquefaction • Insufficient seat width • The tri-venture recommended that a catastrophic failure be assigned when V1 = Max (VT, VL) = 10

17. 1 2 Simply Supported Spans • Prone to catastrophic failures • Large movements at expansion joints • Loss of support • Insufficient seat width • Tilting of piers

18. 1 2 Continuous Spans • Less susceptible to catastrophic failures • Prone to substructure damage due to higher seismic loads

19. Bridge Classification Exposure Score Failure Type Score Traffic Volume Functional Classification Likelihood Score Consequence Score Vulnerability Rating Vulnerability Rating Procedure

20. Alternative Method For Critical Bridges • Provide approximate Demand/Capacity ratios • Superstructure Unseating (Longitudinal and transverse) • Seat/Pedestal Adequacy (shear demand to capacity) • Bearing Adequacy vs. Demand • Displacement capacity of substructures • Established column foundation D/C ratio • Establish Liquefaction induced pile D/C ratio • Advantages of the proposed procedure • Checks are performance based using a defined hazard level • Prioritizing bridges with different remaining life spans

21. Limitations of the current procedure • Geared toward bridges supported by concrete substructures • Steel substructures not included • Rigid frame and multi-level bridges not included • The tri-venture developed specific procedures and guidelines • Uniform rating system for the various types of bridges • Vulnerability score for steel column bents

22. Conclusions • 50% of the structures require some form of capital improvement • The major seismic vulnerabilities governing the assessments: • Steel rocker bearings • Soft soil • Moderate potential for liquefaction related damage • Bridge Support length • Adequate support length critical for bridges with rocker bearings • High risk bridges: • Bridges with rocker bearings and founded on soft soil • Supported on soil susceptible to liquefaction

23. Seismic Vulnerability Assessment Results Bridge Screening Bridge Classification

24. 30% 20% 10% 0% 1 2 3 4 5 6 Vulnerability Rating Vulnerability Rating Results

25. Questions / Comments • Dr. Roy Imbsen, PE • Contact Information: • Artur Kasperski, PE Earth Tech, Inc 212-798-8569 artur.kasperski@earthtech.com • José A. Santos, PE NYSDOT 718-482-7885 jsantos@dot.state.ny.us