Cracking failures of hubs of trunnion-hub-girder (THG) assemblies have occurred during assembly of bascule leafs for three bridges in Florida since 1998. Economic losses due to these failures are in the order of many hundreds of thousands of dollars for the Florida Department of Transportation (FDOT).

Headed by Professor Besterfield of Mechanical Engineering and four other faculty members in the College of Engineering, University of South Florida received the following two grants from FDOT to study the trunnion-hub-girder assembly design.

Grant 1:  Parametric Finite Element Modeling and Full-Scale Testing of Trunnion-Hub-Girder Assemblies for Bascule Bridges (Year: 1998-2001) 

We developed Finite Element Analysis (FEA) and experimental models to explain these assembly failures and then use the results to develop specifications for the assembly procedure. 

Using Ansys, the FEA modeling includes studying transient thermal and mechanical stresses during assembly.  It is then followed by full-scale testing of a THG assembly.

Specifications and testing requirements for these assemblies are being developed for possible inclusion in American Association of State Highway and Transportation Officials (AASHTO) Specifications.

Grant 2: Hub-girder bolt assembly without an interference fit in bascule bridges (Year: 2001-2003)

From Grant 1, we found a high probability of cracks developing in the hub exists when a trunnion-hub assembly is cooled in liquid nitrogen for shrink-fitting into the girder during one of the assembly procedures; namely, when the trunnion is shrink fitted into the hub and then the trunnion-hub assembly is shrink fitted into the girder.

To do away with cooling a trunnion-hub assembly, elimination of shrink-fit at the hub-girder is proposed.  By eliminating the interference fit at the hub-girder, new problems arise because the bolts now transmit most of the loads from the bridge through the girder, then to the hub (and backing ring), then to the trunnion and finally, to the bearings.  For the hub and bolts to now be able to transmit the load, numerous parameters need to be studied and defined or specified.  Some of these parameters include: girder thickness, type of bolts (that is, grade and material), bolt sizes, number of bolts, number of bolt circles, bolt material, hub flange thickness, and hub flange diameter, to name a few.