Steel Truss Retrofits to Provide Alternate Load Paths for Cut, Damaged, or Destroyed Members
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Steel Truss Retrofits to Provide Alternate Load Paths for Cut, Damaged, or Destroyed Members

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    Final Report
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    Continued stability and performance of long-span truss bridges after loss of a critical member is broadly attributed to “redundancy” because of alternate load paths (ALPs). This report presents an extensive investigation on the load-path redundancy of long-span truss bridges, including quantification of ALP, defined as the spectra of surrounding members undergoing load redistribution to prevent bridge collapse after sudden damage to a member or members. This research developed an integrated framework to quantify ALP of long-span truss bridges in terms of demand-to-capacity ratio (DCR) for linear elastic analysis and strain ratio for nonlinear dynamic (NLD) analysis. ALP of long-span truss bridges was investigated through finite-element simulations of two example long-span truss bridges. Results of the member removal analysis showed that the three-dimensionality of truss bridges, stemming from upper and lower braces, side trusses, floor beam trusses, and the deck, plays a primary role in protecting the bridge from collapse after removal of a member or members. Simulation results showed that the stress contribution to DCR changes from primarily axial to predominantly moment (both in-plane and out-of-plane) for truss members affected by sudden removal of another truss member. This change occurs because the superstructure tends to undergo torsional motion about its longitudinal axis due to the asymmetrical geometry created after removal of a member. Upper and lower braces and floor truss systems resist this torsional motion, thereby redistributing the load among truss members. Various retrofit approaches were investigated to improve ALP of two example bridges. The typical member strengthening approach used during seismic retrofit had limited effectiveness in improving ALP of long-span truss bridges; however, retrofits that involved member strengthening as well as adding new members as braces or parts of floor trusses (i.e., members that enhance the three-dimensionality of the bridge) were the most effective and added the least amount of additional weight. NLD analysis using LS-DYNA software (Hallquist 2014) resulted in more cost-effective retrofit than linear dynamic analysis using SAP2000 software (Computers and Structures, Inc.). Performance levels are presented for practicing engineers to use for the retrofit of long-span bridges to protect against progressive collapse. Experimental and theoretical needs for investigating ALP of long-span truss bridges are also discussed
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