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261 9.1 Summary From a survey of owners and a literature review that included other national and international bridge design specifica- tions, a set of possible service limit states (SLSs) was devel- oped. Those SLSs were reviewed to determine which could be calibrated using reliability theory. These are identified in Table 2.21 in Chapter 2. Calibrated, reliability-based load factors or resistance factors, or both, were developed for each of the following SLSs: ⢠Foundation deformations; ⢠Cracking of reinforced concrete components; ⢠Live load deflections; ⢠Permanent deformations; ⢠Tensile stresses in prestressed concrete components; and ⢠Fatigue of steel and reinforced concrete components. Although the same general process was followed in calibrat- ing each of the SLSs, customization was needed in most cases. The calibration process produced target reliability levels that were much different from those used for the strength cali- bration. This was expected. Although the strength calibration, with some exceptions, was based on a target reliability index of about 3.5 for a 75-year life, most of the SLS calibration was done with a target reliability index of around 1.0 to 1.5 based on an annual probability. Once the target reliabilities were deter- mined, changes to load factors, resistance factors, or other design parameters were developed. Proposed modifications to AASHTO LRFD design provisions were drafted for consider- ation by the appropriate AASHTO Highway Subcommittee on Bridges and Structures Technical Committees. Several of the more important underlying changes needed to develop the proposed specification changes included the following: ⢠Use of WIM (weigh in motion) data to develop multipliers for the HL-93 live load model to be used in the calibration processes for the various SLSs. These statistical parameters were tabulated for various average daily truck traffic val- ues from 250 to 10,000, time periods ranging from 1 day to 100 years, and spans ranging from 30 to 200 ft. ⢠Analysis for fatigue that included treating the WIM data for each site as a continuous stream of traffic for rainflow counting and estimation of cumulative damage using the PalmgrenâMiner model. This calibration also involved a reassessment of a comprehensive international database of fatigue tests. ⢠Verification of the Service II load factor for permanent defor- mations based on a WIM-based multiplier on HL-93 and a set of bridges from the NCHRP Project 12-78 database. This verification was further informed by considering the average number of times per year that bending moment for various spans exceeded various percentages of the HL-93 bending moment. ⢠A statistically calibrated load factor for live load for use in the Service III load combination for prestressed concrete in tension. ⢠A reliability-based comparison of pre- and post-2005 loss calculations for prestressed concrete components. ⢠A calibration process for foundation deformations, which was demonstrated by calibration of both the Schmertmann et al. (1978) and Hough (1959) methods of predicting set- tlement of footings on the basis of measured field data and the presentation of a methodology to allow for calibration of locally preferred settlement models. The same processes can be applied to horizontal movements, as well as other foundation types (e.g., deep foundations). The foundation deformation calibration processes are formulated to allow adaptation to different levels of reliability as expressed by different values of probability of exceedance (or reliability index). ⢠Use of frequencyâdeflectionâperceptionâbased criteria to calibrated bridge movement. These criteria have been used by Ontario and Canadian bridge codes for over two decades. C h a p t e r 9 Summary and Recommendations
262 The objectives of this project were to provide calibrated SLSs to provide 100-year life and to develop a framework for further development of calibrated SLSs. Generally, it has been assumed that maintenance activities will be sufficient to prevent sig- nificant loss of the strength and stiffness that would result in unsatisfactory performance. No well-accepted direct link between the National Bridge Inventory (NBI) condition data and the types of unsatisfactory performance related to the SLSs calibrated in this study has been found. Several locally devel- oped predictors of changes in the NBI condition number over time have been presented to provide guidance to owners on possible changes to the resistance side of the limit states used here within the context of the direct-link caveat above. There are three sources of significant future improvements to the reported work: ⢠Improved limit state functions; ⢠Improved knowledge of structural behavior; and ⢠Improved knowledge of the change in strength and stiffness with time (i.e., response deterioration). As discussed in Chapter 2, the limit state function related to permanent deformations is based on six data points from the AASHO Road Tests (1962) and the anecdotal evidence that there appears to be virtually no record of permanent deformation in the superstructures of modern steel bridges that has been tied to premature yielding or bolt slip. It is rea- sonable to ask whether any systematic assessment of this type of response is being made. The same question about systematic assessment applies to live load response, settlement, and cracking. Certainly very exceptional cases are probably noted by individual owners, but detailed investigations are few in number and not cen- trally archived. Fatigue cracking of steel components is prob- ably the most identified and best recorded of the SLS responses, as well as being the most directly tied to truck traffic density and time in the design process. The ongoing Long-Term Bridge Performance Program (LTBPP) is probably the best source of improved future data on loads, response, and deterioration (both response deterio- ration and routine maintenance needs as a function of time and environment). The initial focus, based on stake-holder input, is on bridge decks, but other aspects of bridge perfor- mance will also be considered. The project objective to cap- ture research-quality performance data on a systematically selected national bridge set through visual inspection and nondestructive evaluation techniques should provide the information needed to advance calibrated SLSs further. 9.2 recommendations The following recommendations are made: ⢠Submit the specification proposals in Chapter 7 to the AASHTO Highway Subcommittee on Bridges and Struc- tures for possible implementation. ⢠Evaluate the feasibility of establishing a clearinghouse for observed SLS issues, such as unsatisfactory user perception, permanent distortion, unanticipated cracking of concrete components, and settlement. ⢠Continue to interact with LTBPP to recommend SLS- oriented data collection, as well as to take advantage of evolving data and knowledge on behavior and deteriora- tion to improve SLS calibration and design provisions. ⢠Assess the impact of any of the specification proposals that are adopted by working directly with the states involved or by engaging LTBPP to include data gathering in those proj- ects. In either event, a set of expectations, associated met- rics, and assessment protocols should be developed for consistent evaluation of impacts. ⢠Initiate a research project to identify possible ways to more directly link NBI data to changes in resistance, as they may affect both service and strength behavior. ⢠Initiate a research project to identify failure modes in decks, potentially differentiating among shear and bending fail- ure modes, failure modes in composite and noncomposite systems, end diaphragm and flange width and fillet effects, and possible fatigue effects. Finally, there is much interest nationally and internation- ally on the improved implementation of SLSs that should be considered in any continued development of AASHTO LRFD. Some late developments are described in Walraven and Bicaj (2011), Balázs et al. (2013), and Helland (2013), all of which address provisions in the 2010 fib Model Code (Fédération Internationale du Béton 2010). 9.3 Implementation If the recommendations in Chapter 7 are accepted by the AASHTO Highway Subcommittee on Bridges and Structures, a webinar is probably the most effective way to explain changes in design to state engineers and consultants. A separate webinar or short National Highway Institute course would be valuable to owners who want to modify the databases in Appendix F and rerun calibration calculations.
