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46 4.1 Conclusions NCHRP 12-87A had the objective of developing system analysis procedures for the evaluation of redundancy in steel bridges with members traditionally designated as FCMs. The original intent of the FCP with regard to FCMs was to require more stringent fabrication and inspection requirements, based on the risk of collapse associated with the failure of an FCM. However, the lack of analytical guidelines for the evaluation of redundancy led to designation of FCMs based on the bridge configuration without the need for any supporting calculations. The research has shown that (1) there is a need for redun- dancy evaluation procedures, as most bridges classified as having FCMs actually possess considerable system redun- dancy; (2) FEA can be used to develop redundancy evalu- ation procedures while achieving a target reliability; and (3) an FEA methodology can be prescribed and applied to evaluate steel bridges with the objective of designating mem- bers as FCMs or SRMs. The resulting FEA methodology is a comprehensive analytical tool that provides a computational framework to characterize the capacity of steel bridges in the faulted state, a load model in which the loading conditions during and after the failure of a primary steel tension mem- ber are considered, and a set of requisites in the faulted state used to classify members as FCMs or SRMs. Evaluating the capacity of a bridge after the failure of a primary member requires complex analysis. In typical design and evaluation practice, a single load path and linear elastic behavior are assumed, which have been shown to be insufficient to capture system-level redundancy. Nonlinear behavior of the structure and multiple load paths need to be considered simultaneously when evaluating system-level redundancy; hence, FEA was employed as a basis for the development of system analysis procedures. A set of ana- lytical procedures, techniques, and inputs were employed to create models of steel bridges. These were used to benchmark the methodology with available field data and to characterize the dynamic amplification of load after the failure of a primary steel tension member. To consider uncertainty of load and resistance in system analysis, the reliability principles employed in current and valid LRFD specifications were used to create a load model appli- cable to FEA. Since the results of FEA do not directly translate to member capacities used in current LRFD specifications, a minimum performance criteria based on stress, strain, and displacements was developed to evaluate the performance of a steel bridge in the faulted state. Additionally, the applicabil- ity of system analysis was evaluated to guarantee that bridges with problematic features do not see their inspection require- ments lessened, and a set of design guidelines for new bridges was assembled to assure adequate strength and resistance to fatigue and fracture. The final product of NCHRP 12-87A is a stand-alone document that can be used by AASHTO as a guide specifica- tion. This guide specification is envisioned as a complement to other current governing design and evaluation specifica- tions. Additional materials, such as application examples and extended background of the evaluation requirements and system analysis screening criteria, were developed to ease application of the guide specifications. 4.2 Suggested Research The methodology presented is new ground for the U.S. steel bridge industry. It is likely that additions and simplifica- tions, which will improve and facilitate its application, could be realized with additional research. Primarily, two features of the FEA methodology could be simplified: (1) modeling of the reinforced concrete slab and (2) modeling of the interac- tion between the bottom of the slab and the top of the steel members. Modeling the slab with truss elements embedded in solid elements to represent reinforcing steel is the most accurate procedure, but it requires a large mesh. The number of elements could be largely reduced if a valid approach using C H A P T E R 4 Conclusions and Suggested Research
47 shell elements is developed. If the contact interaction algo- rithms could be replaced with an element approach (e.g., cohesive elements), convergence issues that require the use of explicit dynamic analysis could be avoided. Additionally, it might be possible to implement the shear stud forceâ displacement relations to those elements; hence, eliminating the necessity for multiple connector elements that typically need to be explicitly input one by one. Finally, as the methodology is applied to a larger num- ber of cases, bridges could be classified into families shar- ing particular critical members or locations to be inspected to avoid FCM hands-on inspection of the entire structure or member. Ideally, as the understanding and modeling of system-level redundancy improves, simplified analysis pro- cedures may be developed in similar fashion to those used in fitness-for-service analysis and analysis of orthotropic decks. In other words, the methodology could be expanded to include a basic level of analysis in which computational analysis is not required; an intermediate level of analysis based on two-dimensional elastic models; and advanced three-dimensional nonlinear FEA, such as the one presented in the current document.
