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Publication Number:  FHWA-HRT-20-055    Date:  November 2020
Publication Number: FHWA-HRT-20-055
Date: November 2020


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

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This study was part of the Federal Highway Administration Hazard Mitigation Research Program addressing bridge vulnerabilities to single or multiple hazards, developing countermeasures for mitigation and adaptation, and developing analysis and design tools and methodologies. This report presents the results of a study on load-path redundancy and its importance in redistributing loads and maintaining stability, which may become necessary to prevent collapse at the onset of critical member failure. It focuses on long-span steel truss bridges of interest to the bridge engineering community and design and practicing engineers.

This research extensively investigated load-path redundancy, including quantification of alternate load path (ALP), which is defined as the spectra of the surrounding members undergoing load redistribution after sudden damage to bridge members. Bridge members were compared before and after potential retrofit to judge the effectiveness of retrofit schemes in improving the ALP. An integrated framework to quantify the ALP of long-span truss bridges in terms of demand-to-capacity ratio (DCR) for the linear elastic analysis and strain ratio (SR) for the nonlinear dynamic analysis was developed. Different performance levels in terms of DCR and SR are also presented for practicing engineers to use for the retrofit of long-span bridges to protect against progressive collapse.

Cheryl Allen Richter, P.E., Ph.D.
Director, Office of Infrastructure
Research and Development


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