Performance-Based Seismic Bridge Design (2013)

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64 CHAPTER NINE PROJECT-SPECIFIC CRITERIA This chapter reviews example seismic design performance criteria for several major projects that have been completed or are under way (as of 2012) in the United States. The sum- maries focus on the stated performance and seismic hazard aspects of the project, not on the details of the analysis and design to achieve the stated performance goals. Compari- sons of various project-specific criteria can be found in chap- ter ten. CALTRANS—WEST APPROACH SEISMIC RETROFIT OF SAN FRANCISCO–OAKLAND BAY BRIDGE PROJECT The west approach of the San Francisco–Oakland Bay Bridge (SFOBB) is located in San Francisco on the extreme west end of the bridge that crosses from San Francisco to Oakland. The final seismic retrofit criteria were issued in 2002 (Caltrans 2002) as a result of criteria suggested by the Governor’s Board of Inquiry (following the Loma Prieta earthquake), the Caltrans Seismic Advisory Board, and Cal- trans itself. The general design philosophy behind the criteria allows for controlled inelastic action, provided such action is consis- tent with the performance objectives. Inelastic action could be limited to predetermined locations that can be made duc- tile or that can have their displacements controlled by spe- cial components or devices, such as isolators or dampers. Such locations should be accessible for inspection, repair, or replacement without limiting the functionality of the bridge. In-ground damage is permissible provided the excavation for inspection and repair is compatible with the functional- ity requirements. To ensure capacity protection, upper-bound forces are cal- culated for the yielding or ductile elements, and these forces are used to check the strength of adjacent non-ductile or force-controlled elements. The capacities of force-controlled elements must be based on nominal material properties. The performance criteria are as follows: • Two-level seismic hazard criteria – Safety evaluation earthquake (SEE)—approxi- mately 1,000- to 2,000-year return period – Functional evaluation earthquake (FEE)—300-year return period • Performance criteria – Following an SEE earthquake there should be immediate access for emergency vehicles and full access to normal traffic within 72 hours. ¥ There may be repairable damage, such that: • Inelastic response limits the damage limited such that lateral displacement capacity is maintained following the maximum credible event. • Lateral strength may be reduced immediately following the earthquake due to failure of shear keys, wing walls, and other nonductile sacrificial members. • Damage can be repaired such that the lateral load carrying capacity can be returned to its original strength. • There is no reduction in the vertical load car- rying capacity. ¥ Acceptable damage may be described as: • Cover concrete may crack and spall. • The core of well-confined concrete columns may crack, but repairs will be limited to epoxy injection of these cracks. • Main column reinforcing steel may yield, but will not buckle or rupture. • Joints may crack, but repairs shall be limited to epoxy injection and patching of cracks. • Footing, superstructure, and bent cap mem- bers shall remain essentially elastic. – Following an FEE earthquake there should be immediate access to all normal traffic. ¥ There should only be minimal damage, consist- ing of minor inelastic response. ¥ Acceptable damage may be: • Minor cracking or spalling of column cover concrete may occur but should be avoided if economically possible. • Narrow cracking of cast-in-drilled-hole pile shaft cover concrete may occur, but the cover concrete will not spall. • Main column reinforcing steel may yield. • Original geometry is essentially maintained, with columns nearly plumb.

65 CALTRANS—ANTIOCH TOLL BRIDGE SEISMIC RETROFIT PROJECT Antioch Toll Bridge Seismic Retrofit Project—Final Design Report was issued in March 2011 (Caltrans 2011). The bridge is located east of the San Francisco Bay Area and carries SR-160 over the San Joaquin River. The main 8,650-ft-long structure comprises two-column piers supported on driven piles, and this substructure arrangement supports two steel plate girders that are continuous over the piers. The bridge has five main span frames. The structure has a relatively low average daily traffic (ADT) count of 15,000 vehicles per day, compared with typical values of 100,000 to 200,000 closer to major population centers in the Bay Area. The bridge was built in 1978 and evaluated for retrofit following the Loma Prieta earthquake in 1989, and owing to its recent construction date, the bridge did warrant seismic retrofit by the evaluation process in place at that time. In 2004 the bridge was reevaluated and found sufficiently defi- cient to warrant seismic retrofit. Retrofit designs were com- pleted, and in 2010 a construction contract was awarded for the retrofit work. The project was substantially completed in 2011. The seismic retrofit criteria were based on the bridge’s low ADT and, accordingly, only an SEE was selected for the retrofit. This earthquake had a return period of 1,000 years, and the single-level performance criteria included a no-collapse damage state, with limited damage in the sup- porting piles, deck joints, abutment shear keys, and abut- ment backwalls. Although design of new structures uses a philosophy of restricting damage to inspectable and acces- sible areas for repair, for retrofit such a philosophy may not be economical. Thus, the objective was to permit damage in some piles of the main span (typically the outermost piles), but retain enough undamaged piles in the core of the pile groups to ensure that the gravity capacity of the structure is maintained. This design approach is intended to prevent col- lapse. Such a strategy of permitting foundation damage was judged appropriate because of the low ADT of the bridge. CALTRANS—VINCENT THOMAS TOLL ROAD SEISMIC RETROFIT PROJECT The Vincent Thomas Bridge on Route 47 over the Los Angeles River in Los Angeles County is a cable suspension structure (Caltrans 1996). The seismic retrofit design crite- ria were developed in 1996 and were based on the remain- ing useful life of the bridge being 150 years, which is twice the life assumed in the AASHTO specifications for a typical bridge. The ADT of the bridge in 1993 was 38,000, and the projected 2015 ADT was 38,700. The performance criteria used to design the seismic ret- rofit are as follows: • Two-level seismic hazard criteria – SEE—84% probability of not being exceeded dur- ing the remaining 150-year service life (return period of approximately 950 years) – FEE—60% probability of not being exceeded dur- ing the remaining 150-year service life (return period of approximately 285 years) • Performance criteria – After an SEE event, limited service is acceptable. Limited access (reduced lanes, light emergency traffic) is to be available within days. Normal traffic access is to be available within months. No collapse is the limiting damage state that must be provided. Suspension span stiffening trusses were permitted to experience small (25% over yield) inelastic duc- tility demands. – After an FEE event, full access to normal traffic is available almost immediately. Repairable damage is acceptable. Repairable damage is defined as that which can be repaired with a minimum risk of los- ing functionality. The bridge crosses the Palos Verdes fault, which under- lies the Los Angeles River channel. Specific consideration of near-fault effects and fault rupture horizontal and verti- cal displacements were considered, and included maximum fault displacements in relation to the return period. SOUTH CAROLINA DEPARTMENT OF TRANSPORTATION—COOPER RIVER BRIDGE (RAVENEL BRIDGE) PROJECT The Cooper River Bridge, also known as the Arthur Rav- enel Jr. Bridge, is a cable-stayed bridge completed in 2005 that carries US-17 into Charleston, South Carolina. The overall performance objective for the bridge was to obtain a 100-year service life. The bridge is highly important to the communities on either end of the structure, Charleston and Mount Pleasant. The bridge is classified as a critical bridge under the seismic criteria, which corresponds to OC I in South Carolina’s seismic design criteria (SCDOT 2008). The bridge is required to provide secondary life safety. The time that it would be closed after a large earthquake, should it be damaged, would produce a severe economic impact on the region, and the bridge is formally part of the local emergency response plan (SCDOT 2006). For these reasons, the bridge is designated with the most restrictive OC. To functionally address the need to keep the bridge open after a major earthquake, but recognizing that not all structures within the entire bridge needed to be functional, SCDOT designated some structures as critical access path (CAP). Examples of CAP structures are the main spans, the high- and low-level approach structures, and one ramp enter-

66 ing and one ramp exiting the bridge on each end. The CAP structures have higher performance requirements than non- CAP structures. Table 22 shows the performance criteria for the bridge. TABLE 22 SEISMIC PERFORMANCE CRITERIA FOR COOPER RIVER BRIDGE (SCDOT 2002) Seismic Hazard Critical Access Path (CAP) Structures Non-CAP Structures (Ordinary Bridge) Functional evalua- tion earthquake (FEE) 500-year earthquake Service Level—Imme- diate access Damage Level—Essen- tially elastic response (minimal damage) Service Level—Limited access Damage Level—Lim- ited, repairable damage Safety evaluation earthquake (SEE) 2,500-year earthquake Service Level—Func- tional (open to emer- gency vehicles follow- ing inspection) Damage Level—Repairable Service Level—No col- lapse Damage Level—Signif- icant (major) damage. Damage may not be repairable Additional clarifying performance requirements were established for the bridge. For instance, in the FEE for CAP structures, the bridge should be opened immediately after inspection, which could occur within hours, and no reduc- tion should occur in the number of lanes available. For the SEE and CAP structures, the structures were designed to incur only limited ductility demands, although detailing to produce full ductility capacity was provided. This provides a displacement margin of safety, and the fac- tor was set at 2/3 (i.e., the usable displacement capacity was set at 2/3 of the full ductility capacity). The FEE and non-CAP structures should open after inspection, although lane restrictions may be necessary, meaning that not all lanes may be open. However, the load- carrying capacity of the open lanes should not be reduced below the normal carrying capacity. For the SEE and non-CAP structures, there shall be no collapse, while significant damage is permitted. These structures would be designed as full-ductility structures, and the full-ductility capacity that they could provide may be used in the earthquake. WASHINGTON STATE DEPARTMENT OF TRANSPORTATION—STATE ROUTE 520 BRIDGE PROJECT The State Route 520 (SR-520) Evergreen Point Floating Bridge and Landings project between Seattle and Medina, Washington, uses project-specific essential bridge design criteria developed by WSDOT (WSDOT 2010). The proj- ect comprises a new floating bridge across Lake Washing- ton with fixed approach structures on the west and east ends. SR-520 is one of two major corridors across Lake Washington between Seattle and Bellevue and other east- side communities and carries approximately 100,000 vehi- cles per day. The SR-520 floating bridge project uses an enhancement to the AASHTO SGS that seeks to achieve seismic perfor- mance whereby the facility would be open to emergency vehicles immediately following the design seismic event. The criteria also state that the bridge shall be open for secu- rity, defense, and economic purposes immediately after the design earthquake and shall be open to all traffic within days after a design event. The performance criteria are as follows: • Single-level seismic hazard criteria—7% chance of exceedance in 75 years ground motion, which is the same as AASHTO, approximately a 1,000-year seis- mic hazard • Performance criteria – Immediately following the design earthquake, the bridge shall be open to emergency traffic, and open to all traffic within days. – “The extent and amount of damage should be suf- ficiently limited that the structure can be restored to essentially its pre-earthquake condition with- out replacement of reinforcement or structural members. Repair should not require complete clo- sure. Replacement of secondary members may be allowed if it can be done under traffic. Secondary members are those which are not a part of the grav- ity load resisting system” (WSDOT 2010). – Displacement capacity of the lateral load-resisting system is assessed with strain limits that are reduced below those given in the AASHTO SGS. These limits are 2/3 of the concrete strain that would be permitted by the SGS and steel strains for A706 of 0.060 and 0.050 for #4 to #10 bars and #11 to #18 bars, respectively. These strains reflect a permis- sible strain that is 50% of the minimum elongation permitted for A706 by ASTM. OREGON AND WASHINGTON DEPARTMENTS OF TRANSPORTATION—COLUMBIA RIVER CROSSING PROJECT The Columbia River Crossing (CRC) is a joint project between WSDOT and ODOT to replace the aging and under- capacity twin bridges that I-5 uses to cross the Columbia River and join Portland, Oregon, and Vancouver, Washing- ton. The planning and design are under way, with construc- tion scheduled to begin in 2013 and finish in 2020. Approach structures and landside bridges on both the Washington and

67 Oregon sides of the Columbia River are designed accord- ing to ODOT and WSDOT bridge design manuals and AASHTO SGS. Project-specific design criteria for the main bridge have been developed (ODOT/WSDOT 2008). The performance criteria are as follows: • Two-level seismic hazard – SEE—approximately 2,500-year return period – FEE—approximately 500-year return period • SEE – The structure must not collapse and component damage is restricted as listed here: ¥ Piers/columns—repairable damage ¥ Superstructure and pier caps—no damage ¥ Piles/drilled shafts—minimal damage ¥ Pile/shaft caps—minimal damage ¥ Bearing and shear keys—repairable damage ¥ Expansion joints—significant damage • FEE – The structure should perform such that minimal damage is incurred with no permanent offsets. The final design of the main CRC bridge is required to be verified by nonlinear response history analyses. The input ground motions must account for spatial variation (multiple support excitation) of the ground motion along the length of the bridge structure based on wave passage, wave scattering/ incoherency, and local site response effects. A minimum of three 3-component ground motions are to be used, with the maximum response in each orthogonal direction defining the design actions on structural components. Performance acceptance criteria are based on material strain limits and minimum component curvature ductility capacities. TENNESSEE DEPARTMENT OF TRANSPORTATION— HERNANDO DE SOTO BRIDGE—INTERSTATE 40 BRIDGE PROJECT The Hernando de Soto Bridge carries I-40 across the Mis- sissippi River at Memphis, Tennessee. The bridge was built in the 1960s with little seismic protection. Because of the bridge’s importance to the regional economy, the Tennes- see Department of Transportation and Arkansas State Highway and Transportation Department, with assistance from FHWA, performed a seismic assessment, which led to seismic retrofit in 1992. The performance objective for the seismic retrofit was to keep the bridge serviceable following a maximum probable “contingency-level” earthquake (2% chance of exceedance in 50 years—approximately 2,500- year return period) (Jaramilla 2004).