Performance-Based Seismic Bridge Design (2013)

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86 CHAPTER THIRTEEN CONCLUSIONS AND SUGGESTED RESEARCH PERFORMANCE-BASED SEISMIC DESIGN IMPLEMENTATION Performance-based seismic design (PBSD) comprises four primary activities or steps: hazard analysis, structural analy- sis, damage analysis, and loss analysis. The literature survey and state-of-the-practice assessment indicate that the earth- quake engineering community is most highly skilled and its practices most developed in the seismic hazard and struc- tural analysis areas. As one moves further along the PBSD activity list, engineers are less capable in terms of their abil- ity to predict damage, and then even less capable in terms of calculating loss or quantifying risk of loss of service. There- fore, efforts to develop effective tools in these areas would help the engineering community deliver true PBSD. The seismic hazard calculation procedures in use today routinely use probabilistic methods, although determin- istic limits are often used to limit the maximum predicted ground motions. However, our structural analysis method- ologies are, by contrast, mainly rooted and applied using deterministic methods. The usual approach is to start with a probabilistic expression of seismic input (e.g., spectral accel- eration that has a 7% chance of being exceeded in 75 years), and use this input in a single analysis to determine whether permissible forces or displacements (actually strains) have been exceeded. The resulting judgment then is binary—is the demand strain less than the capacity strain? This method is simple. It is not fully probabilistically based, and should not be misconstrued as a precise prediction. However, such methods for design will likely remain the tool of choice in the near future. Thus, adaptation of PBSD features into this format is likely the logical near-term first step. Given the uncertainties in the seismic input, properties of the structure, quality of construction, and accuracy of analy- sis, to name a few, a probabilistic approach may ultimately be preferred, and such a direction would need to be taken to complete all four PBSD steps and have a probabilistically based risk of loss (e.g., ASCE 7-10’s 1% chance of structural collapse in 50 years). Thus, strategically the profession may move toward this more complete PBSD approach, but this will take time. An observation based on the numerous knowledge gaps to be closed in developing full PBSD is that a multiphased approach might be the most likely way to implement PBSD, assuming this is a goal of the bridge industry. The prefer- ences cited in the survey performed for this synthesis sug- gest that implementation of PBSD follow a nonmandatory path, where the method is an accepted alternative or addi- tion to the current design approaches, and may be restricted primarily to use with more important structures. This is the trend in applications of PBSD to date. A strategic development panel or steering group may help provide consistency in making progress toward all the goals—some of which are applied research, some of which are policy and philosophically based. This panel might be a facilitated subgroup of the AASHTO T-3 committee or a panel formed within TRB/NCHRP. Such a panel would probably be more effective if it is not as large as the TRB AFF50 seismic committee, so a subgroup of that committee might be an appropriate avenue. The panel could keep track of efforts by individual states, research centers, and related industries, such as the building industry. One of the first activities of such a panel might be to coordinate and oversee a road-mapping session. There are parallels to this approach in the buildings industry Such a session might consist of a range of bridge industry stake- holders who would participate in a 1- or 2-day facilitated workshop. This session might be coordinated by a strategic development panel, as outlined previously. The goal would be to produce a document outlining different approaches to PBSD implementation and potential impediments that might require additional research and development. Prioritized and coordinated research needs statements might also be gener- ated from this session or by the panel following the session. The phases of implementation based on the results of this synthesis might comprise both near- and long-term goals, and suggestions for research to support implementation and close the knowledge gaps described earlier are provided here: Near-term research (next 5 years)—the work products might include additional sections or appendices to the AAS- HTO Guide Specifications for LRFD Seismic Bridge Design. 1. Develop prescriptive deformation limits (e.g., strain) that reflect clearly identifiable damage states that can

87 be linked to user- or owner-selected performance levels and then matched with user-selected seismic hazard levels. Envisioned are multiple performance- based deformation limit states, such as no collapse, minimal damage, and no damage. The method would build on the databases and damage catalogs reviewed herein and would also leverage department of trans- portation (DOT) experience from actual damaging earthquakes. This research would produce an alterna- tive design method for use on major/critical/essential structures. 2. Develop a national consensus document describing deformation limits, damage states, and performance objectives and levels. The beginning point would be the damage catalog data, current database informa- tion, and compiled project- and agency-specific crite- ria, similar to that used for Item 1. This effort would help produce consistent use of the PBSD concept and lead to more uniform application of the methods. Or, if it is too difficult to develop a single document that everyone agrees to use, then develop protocols for reporting laboratory and field test data and for using analytical data for design. For example, it would be useful if everyone related data on a pushover curve in the same fashion using the same performance terminology. 3. Develop guidance for use of the prescriptive deforma- tion limits, similar to those in Item 1, which could be matched with multilevel seismic hazard levels. This could be used, for example, to assess bridges for load- ing at a lower-level event and at the current 1,000-year event, or at other user-selected hazard levels. 4. The research efforts outlined in Items 1, 2, and 3 might be made in conjunction with each other. The commonality between the requirements for different practice areas and different structure types is evident in the criteria reviewed for this synthesis. The impli- cation is that consensus of standards for different per- formance levels and for different earthquake levels would be possible. Assuming that the application of the near-term results would be major/critical/essential structures, the combination of multiple performance levels with multiple hazards (multiple return periods) outlined in this item is perhaps the most logical for a research project to undertake. 5. Develop clear guidelines for establishment of desired performance, based on facility importance to emer- gency response, postearthquake recovery, regional transportation network, and regional economy. This effort would develop more clearly defined guidelines than those included in the current AASHTO docu- ments. This work could include case studies of DOT experience and approaches. For example, Caltrans’ approach to selection of performance objectives for bridges in the Bay Area would be useful in inform- ing the development of the proposed guidelines. DOTs from around the country must have a stake in this effort, because seismic hazard and highway net- work use vary significantly across the country. Thus such guidelines should provide flexibility to address regional differences. 6. Development of cost data, estimated downtime, and reparability for different structural alternatives and associated levels of damage (e.g., spalling repair ver- sus partial column replacement in the plastic hinge region). Some DOTs, such as Caltrans should have detailed information on repair costs and time to repair different types and levels of earthquake damage. A database could be created that could be geared for use by decision makers and senior design management during the establishment of performance objectives early in a project. 7. Use of local, state, or regional-level applied research projects, which use risk assessment tools such as REDARS, to help assess seismic risk to regional transportation networks. The objective would be to focus resources on specific economically important, but seismically vulnerable regions, as Oregon has done with its transportation network. Longer-term research (5 years and beyond)—work products to fill the larger knowledge gaps might include completely new guide specifications with software tools for application of the PBSD methodology, but additional prod- ucts might also accompany this guide. 8. Develop improved, enforceable performance objec- tive requirements to support a PBSD specification that might be a stand-alone document similar in philoso- phy to the International Code Council-Performance Code (ICC-PC), but including more detail. This document might also include guidance on, or refer- ence to, minimum prescriptive measures that would be applied where sufficient performance information is not available to provide a link between structural demands and risk of failure. An example of such a prescriptive measure might be the minimum ductility capacity of a column. 9. Compile databases of fragility information (proba- bilistic data for potential damage states) similar to those under development by the buildings industry. These would include data for all types of bridge struc- tures and substructure commonly used in the United States. As part of this effort, or possibly a separate intermediate effort, the damage state descriptions

88 developed under Items 1, 2, and 3 in the short-term category would be used with fragility relationships. Such damage states may include primary structural systems of bridges and nonstructural systems, such as the roadway alignment. These should be standardized for consistency across the country. 10. Seismic risk should be considered relative to risk of loss by other hazards that are considered by the AAS- HTO specifications. Research is under way in the multiple-hazard area, and perhaps PBSD methodolo- gies could be incorporated or extended to that work. Consideration of risk would combine hazard (load- ing) with response (capacity) to define outcomes (risk of nonperformance). 11. Development of facilitation tools to assist decision makers in making risk-informed or risk-based choices regarding seismic performance. Engineers could develop methods to clearly and succinctly explain the choices, impacts, and risks of earthquake damage to the public. 12. Education of bridge designers and stakeholders should be focal areas. Explaining and framing the questions related to PBSD will be challenging, and tools and training could be developed to improve the effectiveness of PBSD. a. For example, use of scenario-type approaches might be more effective with the general pub- lic than probabilistic data alone. ATC-58 (2002) encountered this in the building design area. b. Meaningful metrics, such as downtime, costs of repair, and loss of life, are more relevant to the public than EDPs. Drawing from the trajectory toward PBSD in the buildings practice area, a few observations extrapolated to the bridge practice area are provided here. A regulatory framework will likely need to be developed to support PBSD. The framework should be robust enough to permit innovation and unique solu- tions that may not have been used before, but realistic enough to ensure that minimum standards are met. This means that simple statements of desired performance are likely not going to be sufficient. It is important that some means of account- ability be incorporated to ensure that reasonable solutions are achieved. Such methods may include peer review, limited prescriptive requirements, demonstration projects, laboratory testing, and other controls. The ICC-PC provides some of the framework for buildings, but more detail is likely to be added as the practice of PBSD evolves. Some of the previously listed research recommendations can be handled in the traditional research arena, primarily at universities or research centers. However, some of this work should be completed, or at least augmented, by teams of prac- ticing bridge engineers, and, therefore, might be completed outside of the traditional research avenues. The code develop- ment work will likely require this second type of team. Work in the buildings area is following a similar approach. Performance-based design is not unique to seismic design, and there have been pressures to develop performance-based specifications for other areas of engineering. The ICC-PC is an example, where earthquake loading is but one of many topics covered. Elsewhere, there have been efforts to put such things as concrete mix design into a fully performance- based framework. These efforts have had their champions and detractors, their successes and failures. It would be pru- dent to keep abreast of such activities, their progress, their success, and their stumbles. An overall steering panel may be able to provide continuity in these areas. CONCLUSIONS Performance-based seismic design of bridges will likely be achieved in incremental steps, with small extensions of existing practice occurring first. More of the reliability- based probabilistic design features may be added, but it will take time to develop the necessary methodology, databases, technical expertise, and willingness on the part of the public and of owners to use PBSD in a meaningful way. Moves toward PBSD have already occurred in project- specific criteria that have been developed out of the desire on the part of informed owners to protect the public’s investment in new major/critical/essential bridges or ret- rofit of existing important bridges. The knowledge devel- oped for such projects provides an ideal departure point for implementing nationally applicable consistent guidelines for such major/critical/essential bridges. As ambitious as this sounds, such a guide would be a first step on the road to full PBSD, where decisions are made by an informed public/owner and stakeholders using metrics that are not engineering based, but rather based on impacts and risks to the public and stakeholders that include direct losses, indirect losses, costs of construction, cost of repair, and estimates of downtime—more meaningful metrics than engineering design parameters such as strains, rotations, and displacements. It is likely that PBSD is a tool that will be used for larger, more important projects and perhaps not for conventional or ordinary bridges without good justification. The experience of the building industry is that PBSD requires significant technical skills beyond those of the ordinary engineer. Such skills typically do not come cheaply, and the analysis and design effort are proportionately more sophisticated. That said, this does not mean that some aspects of PBSD cannot be used on conventional bridges, such as checks of opera- tional performance under smaller earthquake shaking.

89 The bottom line is that PBSD may be useful to the profes- sion in several ways, and there likely is a short-term strategy that will serve the engineering community well and a long- term strategy that will serve the public with more meaning- ful data and decision points regarding the design for natural hazards in a way that heretofore has not been possible. This will be a technical and an educational challenge for the bridge engineering community in the coming years.