PARAMETRIC FINITE ELEMENT MODELING AND FULL-SCALE TESTING OF TRUNNION-HUB-GIRDER ASSEMBLIES FOR BASCULE BRIDGES

Abstract

Failures during fabrication of trunnion-hub-girder (THG) assemblies of bascule bridges can result in losses of hundreds of thousands of dollars. Crack formation in the hub of the Miami Avenue Bridge, Christa McAulliffe Bridge and Bricknell Avenue Bridge during assembly led the Florida Department of Transportation (FDOT) to commission a project to perform a complete numerical and experimental study to investigate why the assemblies failed. A preliminary study of the steady state stresses using the Bascule Bridge Design Tools revealed that the stresses are well below the yield strength of the material and could not have caused failure. This called for a further investigation of the transient thermal and structural stresses developed during the assembly process. In the second study, a numerical modeling of the different assembly procedures of the THG assembly is developed. A parametric finite element model is designed in ANSYS called the Trunnion-Hub-Girder Testing model (THGTM) to analyze the stresses, temperatures and critical crack lengths in the THG assembly during the two assembly procedures. The THGTM is used to compare the hoop stress and critical crack lengths during the two assembly procedures for different bridges. The critical points in the assembly and the critical stages during the two assembly processes are identified based on the critical crack length. A third study presents results from experiments used to determine the thermal and structural response of the trunnion-hub girder assembly during the two assembly procedures. Quarter-scale and full-scale trunnion-hub-girders were instrumented with cryogenic strain gages and thermocouples to determine stresses and temperatures at critical points. It was found that large tensile hoop stresses develop in the hub at the trunnion-hub interface in AP1 (an assembly procedure involving cooling the trunnion and shrink fitting it into the hub, and subsequently cooling the trunnion-hub assemble to shrink fit it into the girder) when the trunnion-hub assembly is cooled for insertion into the girder. These stresses occur at low temperatures, and result in low values of critical crack length. Peak stresses during AP2 (an assembly procedure in which the hub is shrink fitted into the girder first, followed by cooling the trunnion and shrink fitting it into the hub-girder assembly) occur when the hub is cooled for insertion into the girder. The critical crack length allowed under AP1 is less than half that could be allowed under AP2.