Investigation of Bridge Floor-Truss Behavior in Tied-Arch Span

Floor trusses are often used as primary load-carrying members in a bridge superstructure. Load is typically transferred to the floor truss from the concrete deck through the stringers. The floor truss can be supported by two or more plate girders or a pair of tied arches. Several assumptions are often made about the structural behavior of floor trusses in bridge superstructures to simplify the design process. This paper analyzes the validity of several of these assumptions based on field measurements. The measurements were made on three floor trusses in a tied-arch span over the Mobile River on I-65. The measurements were made as part of a larger research project conducted for the Alabama Department of Transportation. Floor trusses were instrumented with strain gauges and strain was recorded as test trucks of known weight either stopped over the floor trusses (static tests) or were driven across the span (dynamic tests) as well as when trucks in the normal traffic flow crossed the span (random tests). In addition, the results from a simple planar computer model of a floor truss were compared to the field measurements. The assumptions were found to be conservative and reasonable with the exception of one.