Analysis of heating and cooling methods for assembly of steel fulcra in bascule bridges

Abstract

Assembly procedures for assembling steel fulcra of simple-trunnion bascule bridges by heating the girder with heating coils instead of dipping an already stressed trunnion-hub assembly in liquid nitrogen are studied for decreasing the likelihood of failure by cracking and yielding. In the assembly procedure called AP3-A, only the girder is heated to shrink-fit the trunnion-hub assembly in the girder. This assembly procedure AP3-A is found to be infeasible because the girder fails by yielding if heating is expected to be completed in a reasonable amount of time. An alternative assembly procedure called AP3-B is suggested for shrink-fitting where the heating of the girder is combined with cooling the trunnion-hub assembly in dry-ice/alcohol mixture. This assembly procedure AP3-B is found to be feasible. A complete design of experiments study is conducted on AP3-B to find the influence of parameters like hub radial thickness, and radial interference at trunnion-hub interface on the design parameter of critical crack length – a measure of likelihood of fracture. Critical crack lengths during the assembly procedure AP3-B are quantitatively compared with currently used assembly procedures. The results of this work suggest that increasing the hub radial thickness decreases the likelihood of fracture significantly. For hubs with large radial thickness, heating the girder combined with cooling the trunnion-hub in dry-ice/alcohol mixture (AP3-B) is recommended but for hubs with high radial thickness, multistage cooling of the trunnion-hub assembly in dry-ice/alcohol mixture followed by dipping in liquid nitrogen (AP1- multistage cooling) is recommended.