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12 Project-Specific Seismic Design Criteria Respondents were able to provide specific criteria documents for new designs. The collection of project-specific design criteria was the most definitive source for discerning the current stan- dard of practice for seismic design of non-conventional bridges. A total of 11 project criteria are included (see Table 5). Most of 11 projects are located in areas of high seismicity, with one (Project #6) in a region of low seismicity for comparison (see Figure 3). The following is a brief summary of the 11 project-specific design criteria included in this synthesis (refer to Appendix A for complete seismic criteria). Project 1. Sixth Street Bridge Replacement Project (California) (2016) Bridge Type: Arch. Seismic design based on nonlinear time-history analysis. Design in accordance with Caltrans Memos to Designers, ATC-32, NCHRP Report 472, Guide Spec, AASHTO BDS. Two-level performance criteria: SEE (1,000-year) and FEE (100-year). Demand/capacity ratios based on strain/rotation/ductility limits. Capacity protected components based on Caltrans Seismic Design Criteria with demand/ capacity of 1.0 or less. No specifics in terms of componentsâ strain limits in reference criteria. Project 2. New Benicia Martinez Bridge (California) (2000) Bridge Type: Haunched Girder. Seismic design based on nonlinear time-history analysis. Design in accordance with AASHTO BDS, ATC-32. Two-level performance criteria: SEE and FEE. Demand/capacity ratios based on strain limits as listed in reference criteria. Project 3. Tacoma Narrows Parallel Suspension Bridge (Washington) (2000) Bridge Type: Suspension. Seismic design based on nonlinear time-history analysis (10). Design in accordance with Washington State DOT (WSDOT) BDM and project-specific criteria. C H A P T E R 3 State of Practice Based on Project Criteria
State of Practice Based on Project Criteria 13 Two-level performance criteria: SEE (2,500-year) and FEE (100-year). Demand/capacity ratios based on strain limits as listed in reference criteria. Detailed description in criteria for concrete and steel strain limits for towers and pile rein- forcement and concrete, as well as for steel pile casing. In-elastic sections designed to achieve curvature ductility above 4.0. Maximum plastic hinge length defined. Limitations on permanent lateral drift from SEE inelastic deformations for foundations and towers. ID Project Name State Bridge Type 1. California Arch 2. California Haunched Girder 3. Washington Suspension 4. California Cable Stayed 5. Nevada/Arizona Concrete Arch 6. Iowa Network Arch 7. British Columbia (Canada) Cable Stayed 8. California Suspension 9. California Concrete Segmental 10. New York Cable Stayed 11. Oregon Cable Stayed Sixth Street Bridge Replacement Project New Benicia Martinez Bridge Tacoma Narrows Parallel Suspension Bridge Gerald Desmond Bridge Replacement Project Hoover Dam Bypass Colorado River Bridge I-74 Bridge Port Mann Bridge Highway 1 Project San Francisco-Oakland Bay Bridge Self-Anchored Suspension Bridge San Francisco-Oakland Bay Bridge Skyway Structures Tappan Zee Hudson River Crossing Project Willamette River Transit Bridge (Tilikum Crossing Bridge) Table 5. Project-specific seismic design criteria. Figure 3. Geographical location of bridges in NCHRP Synthesis 532 with project-specific criteria.
14 Seismic Design of Non-Conventional Bridges Project 4. Gerald Desmond Bridge Replacement Project (California) (2012) Bridge Type: Cable-Stayed. Seismic design based on nonlinear time-history analysis. Design in accordance with Caltrans Seismic Design Criteria, Caltrans Guide Specs for Seismic Design of Steel Bridges, ATC-32, NCHRP 12-49, AASHTO BDS (refer to criteria for further details). Two-level performance criteria: SEE (1,000-year) and FEE (100-year). Demand/capacity ratios based on strain limits as listed in reference criteria. Detailed description in criteria for concrete and steel strain limits for towers and pile rein- forcement and concrete, as well as for steel pile casing. Detailed definition of Rayleigh damping for various components for dynamic analysis. Project 5. Hoover Dam Bypass Colorado River Bridge (Nevada/Arizona) (2003) Bridge Type: Concrete Arch. Seismic design based on nonlinear time-history analysis. Design in accordance with: No references in provided criteria. One-level performance criteria (1,000-year). Demand/capacity ratios based on strain limits as listed in reference criteria. Detailed description in criteria for concrete and steel strain limits. Structural steel component displacement demand/capacity ratios defined. In-elastic sections to have curvature ductility above 4.0. Project 6. I-74 Bridge (Iowa) (Low Seismic Region) (2010) Bridge Type: Network Arch. Design based on elastic response-spectrum analyses. Design in accordance with AASHTO BDS (2007). One-level performance criteria (1,000-year). Supplemental pushover analyses to understand behavior. Project 7. Port Mann Bridge Highway 1 Project (Vancouver, BC) (2009) Bridge Type: Cable-Stayed. Seismic design based on nonlinear time-history analysis. Design in accordance with Canadian code CAN/CSA-S6-06 with British Columbia Ministry of Transportation supplements. Three-level performance criteria: 10% in 50-year (1/475), 5% in 50-year (1/975), 2% in 50-year (1/2475), as well as subduction earthquake (deterministic). Demand/capacity ratios based on strain limits as listed in reference criteria (ATC-32 and ATC-49 listed as reference). Detailed description in criteria for concrete, reinforcement, and steel casing strain limits. Project 8. San Francisco-Oakland Bay Bridge Self-Anchored Suspension Bridge (California) (2002) Bridge Type: Suspension Bridge. Seismic design based on nonlinear time-history analysis. Design in accordance with AASHTO BDS augmented with ATC-32.
State of Practice Based on Project Criteria 15 Two-level performance criteria: SEE (â¼1,500-year return) and FEE (â¼450-year return). Demand/capacity ratios based on strain limits as listed in provided criteria (ATC-32 referenced). Detailed description in criteria for concrete, reinforcement, and steel casing strain limits. Tower design utilizing energy-dissipating ductile steel shear links with rotation ductility limit defined (designed in accordance with AISC Seismic Provisions for Structural Steel Buildings). Capacity protected design requirements specified. Permanent deck displacements due to inelastic behavior specified at deck level for both SEE and FEE events (300 mm and 50 mm, respectively). Deck hinge beams to be proportioned for forces and displacements calculated by the time- history analyses. Project 9. San Francisco-Oakland Bay Bridge Skyway Structures (California) (2002) Bridge Type: Concrete Segmental Skyway. Seismic design based on nonlinear time-history analysis. Design in accordance with AASHTO BDS augmented with ATC-32. Two-level performance criteria: SEE and FEE [see Project 8 and reference (8)] Demand/capacity ratios based on strain limits as listed in provided criteria (ATC-32 referenced). Detailed description in criteria for concrete, reinforcement, and steel casing strain limits. Capacity protected design requirements specified. Permanent deck displacements due to inelastic behavior specified at deck level for both SEE and FEE events (300 mm and 50 mm, respectively). Deck hinge beams to be proportioned for forces and displacements calculated by the time- history analyses. Project 10. Tappan Zee Hudson River Crossing Project (New York) (2015) Bridge Type: Cable-Stayed. Seismic design based on nonlinear time-history analysis. Design in accordance with the AASHTO BDS. New York State DOT (NYSDOT) LRFD mentioned. Two-level performance criteria: SEE (2,500-year) and FEE (1,000-year). Definition of limiting damage levels. No detailed description in provided criteria for component strain limits. Project 11. Willamette River Transit Bridge (Tilikum Crossing Bridge) (Oregon) (2011) Bridge Type: Cable-Stayed. Seismic design based on nonlinear time-history analysis. Design in accordance with AASHTO BDS augmented with ATC-32. Two-level performance criteria: âServiceable Earthquake Evaluation (500-year)â and âNo Collapse Earthquake (1000-year).â* Demand/capacity ratios based on strain limits as listed in provided criteria. *The SEE term is defined as the service evaluation event, with the typical SEE event then termed the NCE for no-collapse event. However, this is not standard terminology.
16 Seismic Design of Non-Conventional Bridges Review of Criteria Documents A review of the entire set of criteria documents provides the common elements of the state of practice in the seismic design of non-conventional bridges. While there are a few variations on the basic practices, the criteria identify the following basic elements of current engineering practice for non-conventional bridges: 1. All project criteria provide for a limited ductility design basis. 2. All projects in moderate and high seismic zones provide for repairable damage at the safety- level seismic event. 3. Most criteria call for design for both a functional and a safety-level event. 4. Most criteria establish allowable strain levels to establish acceptance for each limit state. 5. All project criteria for bridges in moderate and high seismic zones require design based on a nonlinear time-history analysis. 6. Most (U.S.) project criteria reference either the AASHTO BDS, Guide Spec, or ATC-32. One criteria document was obtained for a moderate seismic region (see Project 10). One cri- teria document was also obtained for a low seismic region (see Project 6), which illustrates how the AASHTO BDS is applied for seismic design where seismic does not control lateral forces. The use of elastic seismic forces in a force-based design is used without applying capacity design provisions; however, pushover analysis was performed to determine the limiting lateral force mechanism for the structural system. For these elastic seismic design cases, lateral force demands such as vessel impact and wind are similar to or greater than the level of elastic seismic forces based on the AASHTO BDS criteria. A full ductility design is not applied in these cases, since it would result in significant increase in foundation and other related costs without any com- mensurate benefit in life safety. Most criteria documents have a direct reference to the AASHTO BDS or the Guide Spec, and those that do not are covered by the broader application of the AASHTO BDS to general design. The project-specific criteria address the basis for limited ductility design, which is a local and global bending design basis. Detailing for confinement, shear design, and minimum reinforcing requirements all are drawn from the AASHTO BDS and Guide Spec, and as such follow the design basis for shear and confinement as applied to conventional bridges. This point is described in more detail as it relates to the approach to capacity protection for non-conventional bridges. Table 6 summarizes the project-specific criteria for non-conventional bridges drawn from the criteria documents in this section. There are variations within the current engineering practice as to how some of these criteria are implemented. The major items are described in the following sections. Strain-Based Criteria in Current Practice Table 7 illustrates typical strain-based performance limits taken from the criteria for a non- conventional bridge in California [see Appendix A for San Francisco-Oakland Bay Bridges (SFOBBs)]. Other criteria documents obtained from high seismic zone locations have simi- lar performance measures. The criteria reference the limit-state capacity protection concept through differential strain limitations for capacity protected elements based on a demand dis- placement check rather than a singular plastic column criterion (the bridge in Table 7 included a shear-linked tower) (11). This approach to capacity protection is employed to assess strain- based damage levels associated with limited ductility design. In the high seismic regions of California, the dominance of ground motion for lateral forces and foundation design gener- ally removes the issue of competing demands that can occur in regions of lower seismicity. In regions with moderate and lower seismic demand, competing lateral force demands can factor
State of Practice Based on Project Criteria 17 into the performance measures, and may control design requirements. However, application of a demand displacement-based capacity check eliminates the conflict that otherwise can occur with the single limit-state criterion. Means by Which Nonlinearity Is Considered The details of nonlinear analysis vary with the specific project criteria, the characteristics of the structure type, and the differing standards of practice among engineers. The procedures for nonlinear analysis vary according to the degree of detail needed to address the performance requirements established for a project. In those cases where performance requirements are descriptive damage states, the nonlinear material model is sometimes limited to iterative defi- nitions of effective stiffness or local moment-curvature definitions at assigned inelastic zones within the structural model. In those projects with defined allowable strain limits for perfor- mance limit states, material models will be based on full moment-curvature-axial interaction definitions for nonlinear section response, either over the full extent of major members or at assigned hinge locations. In both cases, the nonlinear analyses incorporate geometric nonlinear- ity, which for some non-conventional bridges influence review for drift criteria of towers and long-span arches. For non-conventional bridges on deep foundations, particularly those in riparian and marine environments, foundation modeling for nonlinear analysis represents a significant effort and source of variability in the design process. In several of the bridges in the criteria review, a full dis- cretized foundation model of piles was included for soil structure interaction. Sufficient models exist for liquefied and non-liquefied soil conditions. However, the triggering mechanisms to Item Typical Project-Specific Criteria Seismicity Characterization In regions of high seismicity, the typical project-specific ground motions are based on a probabilistic site hazard analysis, with a suite of motions to address multiple sources for each level of ground motion (functional and safety events) (14). Typical Design Standard Design is based on damage limit states that apply to both ductile and protected elements within the structure, based on either a general description of the target damage levels (moderate, repairable, or significant) or on quantitative strain limits that are assigned to these same damage limit states. Nonlinear, inelastic dynamic analysis is used to determine compliance with strain-based performance standards. Member Protection Protocol The separation of ductile element design from protected element design is achieved through a separation of allowable strains at each damage limit state so that at the given performance limit state, the ductility level associated with the damage state is limited to the members and elements selected for ductile behavior, and the protected elements experience low or essentially elastic strain demands. Ground Motion Definition Spectrally matched ground motion time histories are typically developed for both functional- and safety- level events. Site Class Definition (geotechnical characterization) Project-specific site characterization does not follow conventional bridge standards, since the specific soil characteristics are modeled for the soil-structure interaction analysis. Analysis Type Nonlinear time history for force and displacement. Deep Foundation Modeling Discrete foundation modeling with nonlinear soil springs [or as determined from finite element analysis (FEA models)], ground motion input with depth, and radiational damping. Table 6. Summary of criteria for non-conventional bridge seismic design.
18 Seismic Design of Non-Conventional Bridges define the transition between the two states and the post-liquefied strength state remain a subject for continued research. The general practice today is to bracket the liquefied and non-liquefied conditions when assessing compliance with performance criteria. The discretization and data management needed for tracking strain time history in a non linear regime is considerably more involved than for the more general structural analysis. The distribu- tion of inelastic response is a product of the analysis, and the concurrent forces associated with member response are tracked in the time domain and post-processed back through the associated moment-curvature section definitions to report element strains. This level of analysis has been 7.9.1 ALLOWABLE STRAINS Normal weight concrete Allowable strains in normal weight concrete shall: Piers (Average extreme fiber strains in plastic hinge): Functional evaluation earthquake Safety evaluation earthquake where Îµcu is the ultimate concrete strain according to the Mander model Piles (Maximum extreme fiber strains in potential plastic hinge): Functional evaluation earthquake Safety evaluation earthquake 7.9.2 Reinforcing Steel Allowable strains in reinforcing steel shall be: Piers (Average extreme fiber strains in plastic hinge): Functional evaluation earthquake Safety evaluation earthquake Where Îµsu is the steel strain at ultimate stress. For ASTMA 706M Gr. 415 reinforcement, Îµsu may be taken as: Confinement bars No. 10-25 (No. 3 -8) Main bars No. 29-57 (No. 9-18) Piles (Maximum extreme fiber strains in potential plastic hinge): Functional evaluation earthquake Safety evaluation earthquake Where Îµsu is the steel strain at ultimate stress. For ASTMA 706 MGr. 415 reinforcement, Îµsu may be taken as: Confinement bars No. 10-25 (No. 3 -8) Main bars No. 29-57 (No. 9 -18) Hardening strains may be taken as: Bars No. 10-25 (No. 3 -8) Bars No. 29-36 (No. 9 -11) Bar No. 43 (No. 14) Bar No. 57 (No. 18) 7.9.3 Structural Steel Pile Shells (Casings) Functional evaluation earthquake Safety evaluation earthquake ÎµcFEE = 0.004 ÎµcSEE = 2/3 Îµcu Îµsu = 0.12 Îµsu = 0.09 ÎµsFEE = 0.015 Îµsu = 0.12 Îµsu = 0.09 Îµsh = 0.015 Îµsh = 0.010 Îµsh = 0.0075 Îµsh = 0.005 ÎµsFEE = 0.015 ÎµsSEE = 0.02 ÎµsSEE = 0.02 ÎµsSEE = 2/3 Îµsu ÎµsFEE = 0.015 ÎµcFEE = 0.004 ÎµcSEE = 0.01 Table 7. Sample of performance-based seismic design criteria.
State of Practice Based on Project Criteria 19 performed on a number of the major non-conventional bridges covered in the criteria reviewed in this section. Accounting for this level of behavior is needed for those criteria that are based on explicit strain limits. In the case of descriptive performance limit states, classical section force analysis can be used to review damage levels, similar to the methods used for conventional design. Table 8 contains a summary of features currently employed for nonlinear dynamic analysis of bridges reviewed for this synthesis. Limited Ductility and Capacity Protection In regions of moderate and high seismicity, the principle of capacity protection is addressed differently for non-conventional bridges than for conventional bridges. The singular no- collapse limit state for conventional bridges provides a two-step procedure for reviewing ductil- ity demands at the level of demand displacement for those members selected as fully ductile and applying the resulting demands to review the limited ductility demands in capacity protected elements. There is no assessment of structural behavior for conventional bridges during the progression of the seismic event or across a series of events, and there is no means by which to assess any performance measures other than the no-collapse limit state. Capacity protection is managed in a different way for non-conventional bridges. Since limited ductility is required to satisfy the moderate and repairable limit states associated with non- conventional bridges, the simple threshold between fully inelastic and pseudo-elastic (reinforced concrete under strength-level loads is never truly elastic) is not used for design. As evidenced in the criteria documents reviewed, the method of providing capacity protection for non-conventional bridges is to prescribe appropriate levels of allowable strains for the structural elements selected Parameter Scope of Practice Structure Non-Conventional Bridges Kinematic model Full geometric stiffness Structural Elementsâ Concrete Modeling approach varies from providing full moment-curvature profiles for axial demand across all elements in the nonlinear model to assigning specific plastic hinge zones within the model definition that focus inelastic demand by prescription. Structure ElementsâSteel Inelastic material definitions are applied to steel materials. Practices vary as to whether this material definition is bilinear (elasto-plastic) or includes material strain hardening. Structure ElementsâCables Modeling of cables varies from the use of spars (truss member with adjusted stiffness) to full beam-column (geometric stiffness) discretized cable models. The choice is often influenced by the level of ground motion and the degree of geometric nonlinearity in the global bridge response. Damping The use of Rayleigh damping values along with material (hysteretic) modeling is generally applied for the nonlinear dynamic analysis. Soils and Foundations Soil Model Finite difference (FLAC) with liquefaction triggering models to analyze the soil continuum response to free field firm ground motions are often used to compute a discretized array of nonlinear springs and dashpots for soil structure interaction analysis of the structural model. Finite element modeling of the continuum with a simplified structural model is also used by some practitioners. Substructure (Piles and Shafts) In cases where soil liquefaction is considered, the structural model generally includes full discretization of substructure elements as structural members coupled to the soil models noted above. Ground Motions The standard method for full foundation modeling is to develop depth varying discretized time history based on firm ground free field time history of the continuum. Table 8. Standard practice for nonlinear analysis of non-conventional bridges.
20 Seismic Design of Non-Conventional Bridges as inelastic or pseudo-elastic elements of the structural system to satisfy the repairable damage (or other) limit state assigned to the structure. This graduation of allowable strains is verified through the details of nonlinear analysis. The nonlinear analysis can be extended beyond the demand displacement derived for the design-level seismic event to review system behavior by analyzing higher ground motions (nonlinear analysis does not admit scaling resultsâdirect analysis for a scaled-up ground motion is required). However, the design of elements is based on the specified limiting criteria for the damage (strain) levels assigned throughout the structural system for the design event. Other Elements of Practice Based on the literature review and procedures provided through the survey, California pres- ents the most complete protocol for addressing seismic design of non-conventional bridges. The criteria documents summarized include a number of California bridges, and those criteria are consistent with the summary of practice. However, California has a number of practices for non-conventional bridges that are not typical of other states in the survey (8). Caltrans Memo to Designers (CMD) 20-1 provides instructions that reflect the principles of the Guide Spec for conventional bridges (12). Caltrans CMD 20-16 provides for a Seismic Safety Peer Review for non-conventional bridges (13). The terms of CMD 20-16 extend the tenets of CMD 20-1 and the Guide Spec into a more rigorous performance-based design approach for non-conventional bridges that requires full nonlinear analysis and review of strain demands that goes well beyond simple limit-state capacity design principles. Table 9 contains a summary of these current standards of practice and damage limit states that are typically addressed in the seismic design for non-conventional bridges in the United States. Performance Limit State Typical Qualitative Criteria Typical Quantitative Criteria Comments Minimal Damage Minor cracking or spalling of concrete and limited yielding of steel that does not impair bridge function. The expectation is that full service can continue through any repairs. ec ~ .004; eps~.008; es~.01 for rebar and es~.002 for structural steel Strain levels are consistent with strength limit-state design parameters. Repairable Damage Cracking or spalling of concrete and yielding of steel to a level that may require out-of-service repairs to restore full function. Residual drift and permanent deformations are limited to maintain future serviceability. ec~.006 (confined per Mander); eps~.015; es~.02 for rebar and .01 for structural steel Strain levels are consistent with nominal design strength for confined concrete and ductile behavior for steel. Stability maintained for main members, with minor buckling or extensive yielding of replaceable members. Prestress must be reviewed in the damaged state to confirm no loss of function. Significant Damage Significant cracking and spalling of concrete and yielding of steel, with local buckling and significant permanent deformations allowed provided overall stability and structural integrity is maintained. ec~0.75*ecu (confined per Mander); eps~.045; es~.75*esu for rebar and .025 for structural steel Strain levels provide a margin against element failure. Prestressing ineffective as prestress and must be reviewed for strength. Table 9. Seismic design practice for non-conventional bridges.