FHWA has initiated a number of studies focused on load path redundancy to ensure that a bridge component failure does not translate to complete collapse of the bridge.  These studies focused on large trusses, cable stays, tied arches, and suspension bridges and used high-end computational programs and simulations to provide reasonable conclusions on bridge survivability to both natural and man-made hazards based on load path redundancy.^{(1, 2)} However, these efforts were not validated experimentally.  The outcome of this study will be validated computational models that can be utilized by bridge owners to demonstrate load path redundancy, specifically for trusses. 

1) Agrawal, A.K., El-Tawil, S., Chen, Q., and Wang, H. (2021). "Redundancy in Long-Span Bridges for Risk Mitigation in a Multi-Hazard Environment." Report Number FHWA-RC-22-0002.

2)Agrawal, A.K., Ettouney, M., Chen, X., Li, H., and Wang, H. (2020). "Steel Truss Retrofits to Provide Alternate Load Paths for Cut, Damaged, or Destroyed Members.” Report Number FHWA-HRT-20-055.

The objective is to validate modeling processes for truss bridge redundancy during a sudden loss of a member.

Within a prior Interagency Agreement between FHWA and the U.S. Army Engineer Research and Development Center (ERDC), a partial experimental work plan was developed to address the project objective, focusing strictly on steel truss bridges. After reviewing numerous major truss designs and working within the constraints of ERDC’s structural engineering testing facilities, it was decided to use a 1:4 scale ratio resulting in a 10-panel, subdivided Warren truss for the experimental testing. The experimental truss would have a 60-foot, simply-supported span, with panels being 6-foot wide and 8-feet high. The main truss members were designed to be hot-rolled sections; W6x20 for chords, W6x20 or W6x9 for diagonals, W6x9 for verticals, W8x10 for floor beams, and M6x4.4 for stringers. Member loss scenarios would be investigated via use of a hydraulic toggle release mechanism built into certain members and as pictured below in Figure 1.

Figure 1. Scaled truss depicting hydraulic release mechanism in bottom tension chord member.

Planning thus far has been around the sudden loss of a bottom tension chord in configurations with and without a composite deck. Further details regarding the truss specimen, planned loading scenarios and instrumentation can be seen in the file entitled “26.07.31 LPRS Test Matrix and Instrumentation Plan.pdf” in the Documents section. The planned instrumentation will be able to track the redistribution of load when a member is removed, and this can be compared to the predictions from a 3D beam element representation of the truss with nonlinear material properties. The testing and instrumentation plan provided is just a draft and contributors to the study will be able to guide refinements to it through their participation in the Technical Advisory Committee.

The project will develop a final report with the experimental methods, experimental results, results of analytical modeling, and conclusions. The goal is bridge owners can use the validated modeling approach with any steel truss bridges in their inventory and prove load path redundancy for certain member loss scenarios. This would allow them to ascertain which members need hardening/strengthening to maintain load path redundancy, which could be applicable to scenarios such as:

• Proving a truss bridge is tolerant to vehicular strikes if truss members are within a zone of intrusion, and
• Proving a truss bridge is tolerant to intentional blast threats on, or near certain members. 

Funding: $750,000 is needed to begin the work and deliver the planned tests without the bridge deck and $1,300,000 is needed to do all planned tests with the bridge deck. Funding received in-excess of $1,300,000 can be used to add scope at the direction of the Technical Advisory Committee. Entities who are interested in continuing FHWA’s effort should consider contributing funds to fabricate the specimen truss, conduct the planned testing, and report out the findings, conclusions, and recommendations.

In-Person Meetings and Peer Exchanges: Funds may be used to cover travel expenses for staff from contributing agencies to attend in-person meetings or events.