With the National Transportation Safety Board (NTSB) starting to talk about the possible causes of the failure of the Minnesota bridge, the following is some information about how those issues relate to Washington State's bridges and construction practices. The two issues; gusset plates and weights and loads on the bridges during construction.
The focus of the federal review is narrowing to the gusset plates that join steel truss members and the weight of construction equipment and materials on the bridge during construction.
Gusset plates are used to connect two or more structural members at a joint in a steel truss. These plates are primary load carrying members in steel truss bridges. Plates can be as large as 6 ft. x 6 ft. and need to be strong enough to accommodate the loads and stresses that the bridge will experience throughout its lifetime.
WSDOT has 26 steel deck truss bridges. Typical steel truss bridges like the Aurora Avenue Bridge or the I-5 Lake Washington Ship Canal bridges have hundreds of these plates.
WSDOT has never had a gusset plate failure in its history. Our inspections have not revealed any potential problems with any of these plates.
With preliminary information about the cause of the Minnesota Bridge collapse, and the request by FHWA to review reports or re-inspect bridges, WSDOT’s bridge inspection team is reviewing reports and looking at gusset plates in particular.
WSDOT is complying with FHWA’s request to take a look at our steel deck truss bridges. We’re close to finishing the process of reviewing and going back through inspection reports looking for any deficiencies or flaws like cracking, or corrosion. The team will dispatch inspection crews to bridges that may have reported suspect gusset plates. The team will produce a report when the review process is complete.
Bridge Construction Projects
During construction or rehab work, if a contractor wants to take a load on the bridge that is heavier than the legal load limit, the construction contract allows him to temporarily exceed that limit an additional 35 percent. This equipment might be a crane or an industrial sized water truck. Any equipment exceeding that level requires prior approval by the WSDOT Bridge design office.
Contractors submit their plans in advance for how they will stage equipment like cranes or concrete trucks. The WSDOT bridge design office reviews recent inspection reports to find out the current bridge rating and condition and assesses how much weight and stress the bridge is able to currently handle.
Bridge plans set weight limits to accommodate any structural change to the bridge during construction phases. Construction teams are required to place cranes on adjacent spans or under bridges when those loads would exceed safe levels. The contractor is also required to remove debris, rocks, and dirt from the structure to avoid unnecessary stresses on the structure.