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Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2018. Fracture-Critical System Analysis for Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25230.
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Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2018. Fracture-Critical System Analysis for Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25230.
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Page 2
Page 3
Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2018. Fracture-Critical System Analysis for Steel Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25230.
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1 S U M M A R Y This report summarizes the research and findings of NCHRP Project 12-87A, “Fracture- Critical System Analysis for Steel Bridges,” which focused on the development of system analysis procedures to evaluate the redundancy of bridges with members traditionally des- ignated as fracture-critical members (FCMs). Since about 1980, FCMs must meet stringent requirements for design, material requirements, and fabrication, according to the AASHTO LRFD Bridge Design Specifications (AASHTO LRFD BDS) and AASHTO/American Weld- ing Society (AWS) D1.5M/D1.5 Bridge Welding Code (AWS D1.5). By law, bridges with FCMs must undergo FCM hands-on inspections at least every 24 months. These inspec- tions give rise to high lifetime costs that are justified by the perceived risk of bridge collapse associated with failure of an FCM. Most bridge engineers and owners presume that the extra inspection efforts will detect a critical flaw prior to resulting in complete member fracture. NCHRP Synthesis 354: Inspection and Management of Bridges with Fracture-Critical Details concluded that the overwhelming majority of fractures observed in steel bridges could not have been prevented by hands-on inspection since the causes were not related to observable fatigue crack growth. A primary steel member is designated as an FCM solely depending on the bridge configu- ration without any required analytical requirements or recommendations being met. It is presumed that failure of an FCM will likely lead to collapse of the entire bridge or a portion of the bridge. However, there are more documented cases where failure of a member tradi- tionally designated as an FCM did not result in bridge collapse. In fact, failure of an FCM is rare and, according to NCHRP Synthesis 354, there are no more than two documented cases where failure of an FCM resulted in the bridge being immediately unserviceable. Since the historical performance of bridges suggests there is significant reserve capacity when an FCM fails, engineers and owners can clearly benefit from analysis methods to evaluate the actual system redundancy of bridges with members traditionally designated as FCMs. Although there are codified fabrication and inspection requirements for FCMs, there are no codified provisions to evaluate whether the failure of a primary steel tension member results in collapse or loss of serviceability of the bridge. In practice, the designation of a primary steel tension member as an FCM is done in accordance with the bridge configura- tion; for example, the girders of a two-girder bridge are considered as FCMs, but no analysis needs to be performed to support the designation of a primary steel tension member as an FCM. Hence, there are structures that are not unanimously considered to contain FCMs, (e.g., three-girder bridges). In 2012, FHWA published a memorandum in which the definition, fabrication, and inspection requirements of FCMs were reinforced and recognized the use of advanced struc- tural analysis methods to evaluate system-level redundancy of steel bridges (Lwin 2012). In that memorandum, a system-redundant member (SRM) was defined as a new type of Fracture-Critical System Analysis for Steel Bridges

2 primary steel tension member. An SRM is a primary steel tension member—initially consid- ered an FCM—in which redundancy is proven by advanced analysis methods. However, the requirements to perform redundancy evaluations of primary steel tension members were not defined, nor were special inspection requirements, if any, for SRMs. Given the high costs of hands-on FCM inspections that are carried out because of the assumed risks associated with the failure of an FCM and that steel bridges with FCMs have been shown to be capable of operating after the failure of such members, bridge engineers and owners can benefit from codified guidance to evaluate the redundancy of primary steel tension members, as stated above. The objectives of NCHRP 12-87A are to (1) develop a methodology to establish whether a member is an FCM or an SRM, based on loads, existing conditions, material properties, and bridge configurations; and (2) recommend specifica- tions to AASHTO, based on the developed methodology for design of new bridges and evaluation of existing bridges. To perform system analysis of steel bridges in which redundancy is questioned, it is neces- sary to simultaneously consider all possible load paths within the bridge and characterize nonlinear structural response. The complexity of such analysis prevents the use of conven- tional structural analysis methods (e.g., line girder analysis) and requires the use of refined computational methods. Hence, a methodology based on the finite element analysis (FEA) method was developed to perform redundancy evaluations of steel bridges. The method- ology is composed of three main aspects: (1) a set of analysis procedures, techniques, and inputs to develop sufficiently detailed finite element models of steel bridges; (2) reliability- based load combinations to characterize the loading conditions during and after the failure of a primary steel tension member; and (3) a set of minimum performance requirements to guarantee bridge safety and serviceability. The set of analysis procedures, techniques, and inputs was developed to model quasi- static and dynamic response of a steel bridge after the failure of a primary steel tension member. Further, the procedures take into account the effects of bridge slab construction, inelasticity of steel and concrete members, interaction between the slab and the structural steel (including partial composite action and shear stud behavior), flexibility of the sub- structure, and connection failure. Finite element models of bridges that underwent failure of members considered as FCMs were created in accordance with the researched analysis procedures, techniques, and inputs. When available, field data was used to benchmark these finite element models, and instantaneous primary steel tension member failures were simu- lated to characterize dynamic amplification of load associated with sudden member failure (e.g., brittle fracture). The reliability principles used in current design and evaluation specifications were used to develop load combinations that capture uncertainty in load and resistance. Two new load combinations, referred to as Redundancy I and Redundancy II, were developed. Redun- dancy I characterizes the instant when the failure of a primary steel tension member occurs, in which the dynamic amplification of load is considered; and Redundancy II characterizes an extended period of service between the occurrence of the failure and the discovery of the failure. A set of strength and serviceability requirements were also developed. If the bridge is able to satisfy these requirements after the failure of a member previously designated as an FCM, such member can be redesignated as an SRM. The applicability of system analysis per the FEA methodology developed during the research was evaluated to prevent the use of system analysis to lessen the inspection require- ments of bridges with known problematic details, or to prevent the inclusion of nonbench- marked features that cannot be reliably implemented in FEA. Screening criteria based on research by Parr et al. (2010) and NCHRP Report 782: Proposed Guideline for Reliability- Based Bridge Inspection Practices was specified to disqualify steel bridges with the afore- mentioned detrimental attributes from system analysis. Additionally, detailing requirements for new bridges were researched to guarantee adequate strength and resilience against fatigue

3 and fracture. These include detailing requirements for the design of structural steel compo- nents and reinforced concrete slabs, as well as general design guidelines. Finally, the results of the research conducted for NCHRP 12-87A were used to create a comprehensive set of requirements to evaluate the redundancy of steel bridges via finite element analysis with the objective of classifying primary steel tension members as FCMs or SRMs, along with a set of design and detailing guidelines for FCMs and SRMs. These requirements and guidelines were written as a stand-alone document that is compatible with the AASHTO LRFD BDS so that it can be adopted by AASHTO as a guide specification. The proposed guide specification includes complementary background information and application examples. The proposed guide specification also provides owners and engineers with a solid benchmarked analysis methodology that can be used to increase the effective- ness and efficiency of inspection in steel bridges.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 883: Fracture-Critical System Analysis for Steel Bridges presents a proposed specification for the analysis and identification of fracture-critical members and system-redundant members. The report describes the analysis methodology and provides application examples. The analysis methodology is based on comprehensive 3-D finite element analyses (FEA) and case studies to evaluate the redundancy of new and existing steel bridges with members traditionally designated as fracture-critical members (FCMs), including simple- and continuous-span I-girder and tub-girder, through-girder, truss, and tied-arch steel bridges.

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