Performance-Based Seismic Bridge Design (2013)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

76 CHAPTER ELEVEN SUMMARY OF QUESTIONNAIRE RESULTS OVERVIEW A questionnaire was sent to all 50 states regarding their seis- mic design practice. Questions covered conventional and large signature structures for both new design and retrofit practice. The questions were grouped into the following categories: seismic classification, seismic design, seismic research sponsored by the organization, and decision mak- ing for seismic design. These categories form the following subsections. There were 27 questions in the survey. The question number is used to present the summary data here. Appendix A is a copy of the questionnaire, and Appendix B is a summary of the responses. SEISMIC CLASSIFICATION QUESTION Question 1. Is your state or region a seismic state? Forty-one states responded to the survey, and most of the respondents were states that include areas of the higher seismic zones or categories. In the two AASHTO seismic design specifications, the higher categories include Seismic Zones 2, 3, or 4, or Seismic Design Categories B, C, or D for the force-based and displacement-based specifications, respectively. There are 34 states that include these three higher categories, known here as “seismic states,” with all but three seismic states represented in the survey. Figure 27 shows the states that fall under the seismic state designa- tion for the 48 contiguous states. Because the categories are functions of both seismic hazard and site classification, the total number of states in the higher categories corresponds to those states that fall into the higher categories when Site Class E soils are present. These are poorest soils that are standardized in the AASHTO specifications. There may be cases where sites requiring special studies (Site Class F) could place other states beyond the basic 34 states into the higher categories. It is of note that 27 states (about 66% of responding states) recognized that they are seismic states, which means that they have some territory that falls into the upper three design categories. Some states still may not fully recognize that the categories are now dependent on site classification, and therefore, some poor soil sites (Site Class E, for exam- ple) could put the state nominally in the higher categories. Many of these states will fall into SDC A only when Site Class A, B, C, or D soils are present. FIGURE 27 Seismic Design Categories (SDCs) for Site Class E. SEISMIC DESIGN QUESTIONS Question 2. What seismic design provisions does your state or agency use? Approximately half of the 27 states that acknowledge being seismic states use the force-based procedure of the LRFD specification, whereas roughly the other half use the dis- placement-based procedure of the SGS or some other dis- placement-based criteria. As outlined earlier in the report, two states, California and South Carolina, use their own custom-developed seismic design specifications. Two states responded that they permit both the LRFD and the SGS methods to be used, while another state responded that it prefers the SGS displacement-based method, but the LRFD method better addresses critical and important bridges. A single state still uses the seismic provisions from the 17th edition of the AASHTO Standard Specifications. Question 3. If you are using the AASHTO LRFD force-based procedure currently, do you have any plans to change over to the AASHTO Seismic Guide Specifica- tion displacement-based procedure in the near future? Only three states that are still using the force-based procedure have any plans to change over to the newer specifications (i.e., the displacement-based provisions).

77 Interestingly, nine states or 35% of the respondents indi- cated that they have no plans to change over to the SGS displacement-based methodologies. This response might be expected primarily of states that are generally in the lower seismic categories. However, five of these states actually have the potential, depending on site classifica- tion for bridges, to be in the highest seismic categories, meaning Seismic Zone 4 or Seismic Design Category D. Question 4. If you are using “Other” seismic design provisions currently, do you have any plans to change over to the AASHTO Seismic Guide Specification dis- placement-based procedure in the near future? Of the two states that use custom-developed seismic criteria, neither plans to change over to the AASHTO SGS provisions in the near future. Included beyond the fully cus- tomized specifications are a number of states that modify their seismic criteria into more rigorous requirements, such as Oregon, and that also have no plans to modify their requirements down to the AASHTO minimum levels. Question 5. Does your state or agency include addi- tions or modifications to the AASHTO seismic provisions in your bridge design manual? Eleven of 26 states that responded to this question (42%) make additions or modifications in their bridge design man- uals to the seismic requirements that AASHTO provides. Modifications include the following: • Two-level seismic design requirements • Cold weather effects as they affect seismic design • Additional requirements for single-span bridges • Modifications to the seat length provisions and R-factors • More rigorous requirements for high-population areas of the state • Minimum ground acceleration and spectral accelera- tion levels, and so on. Question 6. Does your state or agency use either sin- gle- or multi-level, performance-based seismic design for new bridges, other than the “no collapse” performance level in AASHTO LRFD and the Seismic Guide Specifi- cations? If yes, how were these used? Of 27 respondents to this question, 10 (37%) answered that they used requirements other than the no collapse requirements in AASHTO. Of course, how these additional requirements are used is important in assessing the signifi- cance of this response. Figure 28 illustrates the distribu- tion of the types of projects that use criteria other than no collapse. Sixteen respondents contributed answers to the “check all that apply if yes” portion of the question, and eight of these cited project-specific criteria. FIGURE 28 Types of projects on which criteria other than “no collapse” were used. Question 7. If you use an alternate or more specific definition of Performance-Based Design than the work- ing definition provided in this survey, please summarize your definition below. The survey expressed the working definition of PBSD as “criteria and methodology that links post-earthquake opera- tion or specific other behavioral outcome (e.g., damage level) to typical engineering design parameters (strain levels, dis- placements, forces, etc.).” Respondents summarized alternative definitions, which included the following: • Operational; minimal damage • No collapse in large earthquakes and serviceability in smaller, more frequent earthquakes. These specific definitions could be interpreted as subsets of the more general survey definition. Question 8. If your state or agency uses multi-level seismic performance criteria (multiple return periods with different performance objectives), please describe the reason(s) for doing so. There were a number of responses to this question, but the overarching theme was that such multilevel criteria were primarily used for major or critical bridges. However, several states responded that they apply multilevel seismic criteria to ordinary bridges, and in at least one case the location within the state, in addition to importance, determined whether multi- or single-level criteria were to be used. In this case, the more populous area of the state had the higher criteria requirements. Question 9. If performance-based design criteria have been used, would you be willing to share the criteria with the NCHRP 43-07 project? About 52% of the 27 respondents to this question were willing to share information with this project. Many pro- vided either sample criteria or links to such criteria. In the

78 cases where there was reluctance to share data, it is pre- sumed that the time and effort to compile and send data was the reason for not participating. Question 10. Have you ever used performance-enhanc- ing measures, such as isolation or damping systems? Again, 27 states responded to this question, and of those states 56%, or 15 states, have used performance-enhancing measures. Question 11. If yes, what was the desired performance level? Figure 29 provides the responses of the states that had used performance-enhancing measures. Fourteen states responded, the distribution of response is well beyond the no collapse or life safety objective. Most states were looking to achieve at least repairable damage, although no attempt was made to quantify “repairable” in the survey. Five states responded that they had used such measures or devices to achieve operational performance, either full operations or operations for emergency vehicles only. Question 12. Does your state or agency have a seismic retrofit program? Of 26 responses to this question, nine states or 35% of respondents have a seismic retrofit program. When que- ried whether seismic improvements are made outside of a formal retrofit program, for instance when other bridge improvements are made, many states responded that they do implement seismic retrofits at that time. A common improvement is to replace or improve bearings when main- tenance projects are completed. One state indicated that its seismic program has been phased out, but that seismic improvements are still recommended when other substruc- ture work is required. FIGURE 29 Performance objective, given that performance- enhancing measures were used. Question 13. If you have a seismic retrofit program, do you use performance-based criteria beyond new bridge provisions intended to meet the “No Collapse” damage state to design retrofits? Of the states that have a retrofit program, about half use the same performance requirements that are used for new design (e.g., no collapse in a single-level rare earthquake— 1,000-year return period). The other half tend to use FHWA’s Seismic Retrofitting Manual for Highway Structures: Part 1, Bridges (FHWA 2006). However, several states use a higher return period for the lower-level or serviceability check, and this is taken at a 500-year level rather than a 100-year level for two states. Additionally, retrofits of major or critical structures often have project-specific criteria that are differ- ent and more rigorous than the FHWA basic requirements. SPONSORED SEISMIC RESEARCH QUESTIONS Question 14. Does your state or agency sponsor seismic design related research? Two-thirds of the states responding sponsor seismic design research. Most of the states provided links to websites where such research could be found. These links can be found in Appendix A, which includes the full listing of the survey ques- tions and responses. Sample projects include the following: • Development of pile-bent seismic response data (South Carolina) • ABC for high seismic areas • State-specific seismic hazard mapping (Arizona) or state-specific site effects (Tennessee) • Design guidelines for highly populated areas of New York state • Risk analysis for regional transportation networks within a state (Oregon) • Instrumentation of major bridges in the New Madrid seismic zone. Some of this research may be funded through pooled studies, such as the work being done in the Pacific North- west on decision making for ABC. Thus, not all work is being sponsored solely by one organization, and FHWA also contributes to this work. Question 15. What is the purpose of the research (check all that apply format)? Roughly two-thirds of the research is done to develop design criteria that are not adequately covered by the AASHTO speci- fications (see Figure 30). Such criteria often include types of structures that are not covered in depth in the AASHTO seis- mic specifications, such as pile bents, or the emerging area of ABC for use in seismic regions. Almost equal in response numbers is work to improve the seismic performance of stan- dard bridges (61.1% of responses). By contrast, less than 30%

79 of the research in the seismic arena is being done to develop performance-based design criteria and methods. FIGURE 30 Purpose of seismic-related research. Question 16. Has your state or agency sponsored research correlating damage to engineering design parameters (strain levels, displacements, forces, etc.)? Only three of 25 states (12%) responded in the affirma- tive. An implication of this is that many states may be relying on others to develop data for use in the PBSD area, and this may reflect a lack of data on types of bridges that may be unique to certain areas of the country. Question 17. What are the most important areas for which research is needed to deploy Performance-Based Seismic Design? Table 30 lists the most important areas that states believe need to be addressed before PBSD can be deployed on a routine basis. States were asked to rank a preset list of areas, and the table provides the results of that ranking. Areas that are not listed in the table may also be important, but are not represented. TABLE 30 MOST IMPORTANT TOPIC AREAS THAT NEED TO BE ADDRESSED, LISTED IN RANK ORDER Areas that need to addressed Total Score1 Overall Rank Correlation between performance level and damage states 140 1 Structural displacement limits for operability 127 2 Strain or rotation limits for given dam- age states 105 3 Improved structural analysis techniques 93 4 Construction cost data for higher performance levels 90 5 Nonstructural displacement limits for operability 88 6 Probabilistic data for damage states 75 7 Total Respondents: 26 1 Score is a weighted calculation. Items ranked first are valued higher than the following rankings; the score is the sum of all weighted rank counts. Correlation between performance level and damages states is clearly the most important area, whereas probabilis- tic data comes in last. Both areas must be addressed before full PBSD can be achieved, and the disparity in rank likely bespeaks the preference of designers to use deterministic data rather than probabilistic data. For example, it is likely that designers prefer a single level of strain limit for a given damage state, rather than a fragility curve. Although the lat- ter is more complete, the single level is more understandable at the level of most practicing engineers. Between the top and lowest rank are items that also must be developed, such as displacement limits for operability, which is not entirely independent from the top-ranking item. The close order of these two items shows consistency in the responses. Improved limits, construction data, and non- structural effects are also important. QUESTIONS RELATED TO DECISION MAKING FOR SEISMIC DESIGN Question 18. Does your state or agency have a policy for establishing bridge operational classification (for exam- ple: Critical, Essential, or Other bridges as defined in AASHTO LFRD Bridge Design Specifications)? The responses to this question were split evenly between yes and no. As the profession moves forward with PBSD, this answer must shift to 100% yes. This is an area that perhaps could use some research effort. Question 19. Does your state or agency use criteria for the design of Critical or Essential bridges beyond that given in the AASHTO LRFD Bridge Design Specifications? Again, there is a definite split between positive and nega- tive responses: 27% yes, 65% no, and 8% varies on a case- by-case basis. The negative responses to this question were almost all contributed by the same responders who indicated no for the previous question. The case-by-case responses provided some insight into the types of additional criteria in use: • Limits on ductility and strains, which in a way cor- respond directly to the approach in the SGS (thus, the question may not have been as clearly worded as it could have been.) • Operational performance • Repairable (essential) and operational (critical) performance • Essentially elastic and repairable damage targets for large projects • Multilevel seismic performance (all two-level hazard with corresponding performance)

80 Question 20. If you implemented Performance-Based Design, would you prefer if the procedures were optional or mandatory? This question sought to determine the users’ preference for PBSD as an optional extension of the current specification approach or as a mandatory element. As indicated in Figure 31, 65% prefer to keep PBSD as an optional methodology. This could either be as an extension of the existing methodologies or a replacement approach. Only three of 26 (12% of respon- dents) preferred to have PBSD be mandatory. The results for this question represent an important preference for the bridge design community, that optional PBSD is the way forward. Question 21. If you implemented Performance Based Design, would you be more likely to apply it to major bridges or conventional bridges? The most common application for PBSD is for major, critical, or essential bridges, as indicated in Table 31, where 54% prefer to apply this design approach only to larger, more important structures. This parallels actual practice for major projects where project-specific seismic criteria have been used. For such projects, some form of PBSD is typically used, as was shown in the “Project-Specific Criteria” sum- mary in chapter nine. FIGURE 31 Preference for manner of potential PBSD implementation. TABLE 31 PREFERENCE FOR PBSD BY BRIDGE OPERATIONAL CLASSIFICATION Value Count Percentage Major/Critical/Essential Bridges 13 54.2 Both 7 29.2 Not Sure 4 16.7 Conventional Bridges 0 0 Total Respondents: 24 Skipped: 2 Seven of the 24 responders also indicated that they would use PBSD for conventional bridges and major bridges. Sev- eral DOTs already are using this as policy, particularly for corridors where there is heavy traffic, high population, and high risk to the economy. Question 22. Would you consider Performance Based Design of conventional bridges if design or construction costs increased, but bridge performance was signifi- cantly improved? Check the box corresponding to the cost increase range you would be willing to bear for each performance level listed on the left. The purpose of this question was to determine the tol- erance for cost increases relative to potential benefits of improved performance for conventional bridges. Obviously, obtaining increased performance at little or no cost increase is a highly desirable objective. Such is the case in the aggre- gate of the responses. Table 32 shows that most respondents would be willing to spend up to 10% more for their con- ventional bridges to obtain increased seismic performance. However, one can see the reluctance to spend money for improved seismic performance when the costs reached the 25% and over range. Also of note are the percentages of respondents who would not consider PBSD for conven- tional bridges, even if fully operational performance could be bought with only a 10% increase in cost. This is curious, TABLE 32 RESPONDENTS’ TOLERANCE FOR COST INCREASE RELATIVE TO PERFORMANCE FOR CONVENTIONAL BRIDGES Performance Level Cost Increases Would Not Consider PSBD for This Category Total Responses 0–10% 10–25% 25–50% >50% Immediate Access for Emergency Vehicles in Frequent Earthquake 16 (61.5%) 5 (19.2%) 0 (0.0%) 1 (3.8%) 4 (15.4%) 26 (100%) Immediate Access for All Vehicles in Frequent Earthquake 15 (57.7%) 4 (15.4%) 1 (3.8%) 0 (0.0%) 6 (23.1%) 26 Immediate Access for Emergency Vehicles in Rare Earthquake 10 (38.5%) 7 (26.9%) 1 (3.8%) 0 (0.0%) 8 (30.8%) 26 Immediate Access for All Vehicles in Rare Earthquake 9 (34.6%) 3 (11.5%) 0 (0.0%) 0 (0.0%) 14 (53.8%) 26

81 because it would appear a bargain to achieve operational performance at less than a 10% cost increase. Question 23. Would you consider Performance Based Design of major/critical/essential bridges if design or construction costs increased, but bridge performance was significantly improved? Check the box correspond- ing to the cost increase range you would be willing to bear for each performance level listed on the left. The purpose of this question was to determine the tol- erance for cost increases relative to potential benefits of improved performance for major/critical/essential bridges, which would also typically be the signature or important structures to a region. For these bridges, there is a much higher tolerance of cost increase to achieve improved performance. This can be seen in the broader spread of responses over the higher cost ranges in Table 33. Addi- tionally, fewer respondents indicated that they would not consider PBSD for such bridges, where the numbers dropped by about 50% relative to the same answer for con- ventional bridges in Table 32. This is consistent with prac- tice, where many major, critical, or essential bridges have been designed using project-specific criteria in which some type of performance-enhancing criteria has been used. Increasingly, such criteria are based on performance-based design principles. Question 24. Please rank the following impediments to the use of Performance Based Design. Respondents were asked to rank impediments to the use of PBSD from a list of seven items. This list (Table 34) may not be complete, but major impediments are included. The most severe impediment, by its ranking at the top of the list is the lack of proven methodologies for PBSD and the lack of appropriate design standards. However, not far back in the ranking are the lack of decision-making tools and the potential for increased construction costs of bridges designed using PBSD. The respondents generally believed that they had sufficiently trained staff and could control the work, or at least these issues fell further down the list. The perception of legal issues with applying PBSD was not one of the higher concerns. TABLE 34 RANKING OF COMMON IMPEDIMENTS TO THE IMPLEMENTATION OF PERFORMANCE-BASED SEISMIC DESIGN Item Total Score1 Overall Rank Lack of proven methodologies and design standards 145 1 Lack of decision-making tools for estab- lishing performance levels 132 2 Increased construction costs 122 3 Increased design costs 95 4 Lack of staff with adequate technical skills and expertise 87 5 Legal risks of not meeting targeted performance goals 76 6 Difficulty in quality control, in-house or contracted work 52 7 Total respondents: 26 1 Score is a weighted calculation. Items ranked first are valued higher than the following rankings; the score is the sum of all weighted rank counts. Question 25. Do you have the necessary tools/data to frame PBSD or operational criteria decisions for upper management, policy makers, and/or the public? Regarding the necessary tools or data to frame meaning- ful decisions for upper management or policy makers within an organization, 81% said they either did not have the tools or were not sure. Only 19% of respondents indicated that they had sufficient tools and data to help decision makers. The respondents were also asked to check entries from a list of information types that would be needed. This por- tion of the question was in a check-all-that-apply format, and the results are shown in Figure 32. The most checked TABLE 33 TOLERANCE FOR COST INCREASE RELATIVE TO PERFORMANCE FOR MAJOR/CRITICAL/ESSENTIAL BRIDGES Performance Level Cost Increases Would Not Consider PBSD for This Category Total Responses 0–10% 10–25% 25–50% >50% Immediate Access for Emergency Vehicles in Frequent Earthquake 9 (34.6%) 7 (26.9%) 4 (15.4%) 3 (11.5%) 3 (11.5%) 26 (100%) Immediate Access for All Vehicles in Frequent Earthquake 11 (42.3%) 7 (26.9%) 4 (15.4%) 1 (3.8%) 3 (11.5%) 26 Immediate Access for Emergency Vehicles in Rare Earthquake 7 (26.9%) 10 (38.5%) 4 (15.4%) 2 (7.7%) 3 (11.5%) 26 Immediate Access for All Vehicles in Rare Earthquake 8 (30.8%) 6 (23.1%) 2 (7.7%) 3 (11.5%) 7 (26.9%) 26

82 data types were construction costs and repair costs asso- ciated with the potential seismic performance improve- ments. User delay costs and correlation between EDPs and damage states were also high on the list, followed closely by design costs. With all responses being contributed by 50% to 75% of respondents, these are clearly areas where information is to be developed to assist the decision-mak- ing process. Question 26. Is adequate information available for your agency to formulate a statement for the public explaining what to expect in terms of availability and recovery of bridges following a design earthquake? Regarding adequate information and data to formulate a meaningful statement to the public about what to expect in terms of seismic response, most (62%) indicated that they had enough information. This may reflect that since the 1989 Loma Prieta and 1994 Northridge earthquakes, there has been some effort to make sure stakeholders understand that the design methodologies are intended to permit some dam- age and that bridges may not be immediately available fol- lowing a seismic event. This is an area where the tools that are being developed for PBSD will ultimately help designers articulate what the public should expect following a large earthquake. Currently, as one respondent pointed out, the engineering community may understand what to expect, but the public may not fully understand that bridges are expected to suffer damage, even if those bridges have been seismically retrofitted. Respondents who indicated that they did not have enough information cited the following issues. • More studies need to be done to assist in this area. • Some states have not prepared statements of what to expect, especially the lower seismic states. • Screening for vulnerabilities needs to be done. • Some departments are still trying to catch up to other requirements, such as load ratings. • What to expect in terms of recovery is not well understood. • There is such a short history with the current design methods that the actual performance in earthquakes remains to be understood, particularly in those areas of the country where earthquakes occur infrequently. Data development and dissemination to help the public understand what to expect probably are areas where contin- ual efforts need to be made, in addition to keeping engineers and owners up-to-date with new design developments. FIGURE 32 Information needed to assist decision makers in deciding whether to allocate resources to improve earthquake performance.

83 SUMMARY QUESTION Question 27. Do you have any comments you would like us to consider? This final query was intended to let the respondents add comments that may not have been addressed in the other questions. There were eight additional comments, and they paraphrased here. • PBSD needs to be pursued, especially for critical and essential bridges. • Where only the lower two seismic zones are present seismic design is not critical, seismic issues have a minimal effect on bridges. • Until FHWA comes out with design examples show- ing how to use the displacement-based method and PBSD, the LRFD force-based method will continue to be used. A series of examples, such as the 1996 FHWA series, would be helpful. (Fortunately, several similar examples have recently been developed for the displacement-based approach, and these are being dis- tributed with NHI Course 130093, which covers seis- mic design by both methods.) • Additional links to state-specific design procedures in lower-seismic, but high-population states were provided. • In some states and areas, higher performance may need to be considered when alternative routes are not readily available should bridges be damaged by earthquakes. • Another respondent indicated that in high-population centers, even though seismic hazard is low, the risk to the economy if bridges are damaged could be high. Therefore, higher performance objectives need to be considered. • Design for frequent earthquakes continues to be a con- cern for some and an area for which design criteria do not appear to be available for new bridge design. Overall, the final comments suggest that continual devel- opment of earthquake design criteria is needed in the areas of performance, decision making, low seismic hazard require- ments, and policy for assigning critical and essential bridges. These comments tend to suggest that PBSD is worthwhile and necessary to pursue. SURVEY SUMMARY The questionnaire indicates that there is support for a two- level seismic hazard in a PBSD format (guide specification or other), provided the application of the design guidelines is optional. The responses show that states are using PBSD when it makes sense to them, which typically is on large and important projects. There also is tolerance for limited project cost increases when those costs can provide improved seismic performance. However, the states mostly indicated that there is a need for development of PBSD specifications or guide- lines and training on the use of the methodology. Additionally, methods and data to help decision makers choose perfor- mance objectives are sorely needed. Such methodology and data should be widely available for all public agencies respon- sible for bridge design, maintenance, and improvements.