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| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-20-055 Date: November 2020 |
Publication Number: FHWA-HRT-20-055 Date: November 2020 |
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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
Notice
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Technical Report Documentation Page
| 1. Report No.
FHWA-HRT-20-055 |
2. Government Accession No. | 3. Recipient's Catalog No. | ||
|---|---|---|---|---|
| 4. Title and Subtitle
Steel Truss Retrofits to Provide Alternate Load Paths for Cut, Damaged, or Destroyed Members |
5. Report Date
November 2020 |
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| 6. Performing Organization Code | ||||
| 7. Author(s)
A.K. Agrawal (ORCID: 0000-0001-6660-2299), M. Ettouney, X. Chen, H. Li (ORCID: 000-0002-0858-2258), and H. Wang |
8. Performing Organization Report No. | |||
| 9. Performing Organization Name and Address
The City College of New York |
10. Work Unit No. (TRAIS) | |||
| 11. Contract or Grant No.
DTFH61-14-D-00010/0004 |
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| 12. Sponsoring Agency Name and Address
Office of Infrastructure Research and Development |
13. Type of Report and Period Covered
Final Report; August 2014–June 2018 |
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| 14. Sponsoring Agency Code
HRDI-20 |
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| 15. Supplementary Notes
The Federal Highway Administration (FHWA) Study Managers were Eric Munley (FHWA, retired) and Sheila Rimal Duwadi, P.E. (HRDI-20). |
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| 16. Abstract
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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| 17. Key Words
Alternate load path, redundancy, long-span truss bridges, progressive collapse |
18. Distribution Statement
No restrictions. This document is available through the National Technical Information Service, Springfield, VA 22161. |
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| 19. Security Classification (of this report) Unclassified |
20. Security Classification (of this page) Unclassified |
21. No. of Pages
216 |
22. Price
N/A |
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| Form DOT F 1700.7 (8-72) | Reproduction of completed page authorized |
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