To assemble the fulcrum of bascule bridges, a trunnion is immersed into liquid nitrogen so that it can be shrunk fit into the hub. This is followed by immersing the resulting trunnion-hub assembly into liquid nitrogen so that it can be then shrunk fit into the girder. On one occasion in Florida, when the trunnion-hub assembly was put into liquid nitrogen, development of cracks on the hub was observed. Experimental and numerical studies conducted since 1998 at University of South Florida show that the cracking took place due to combination of high interference stresses in the trunnion-hub assembly, low fracture toughness of steel at cryogenic temperatures, and steep temperature gradients due to sudden cooling. In this study, we are studying the benefit of staged cooling to avoid cracking in the trunnion-hub assembly when it is cooled down for shrink fitting. We looked at three cooling processes - 1) Direct immersion into liquid nitrogen 2) Immersion into a refrigerated chamber, then liquid nitrogen 3) Immersion into a refrigerated chamber, then a dry-ice/alcohol bath, and finally liquid nitrogen. The geometry of the trunnion-hub assembly was approximated by a composite made of two infinitely long hollows cylinders. The transient problem of temperature distribution and the resulting stresses was solved using finite difference method. Using critical crack lengths and Von-Mises stress as failure criteria, the three cooling processes were compared. The study showed that the minimum critical crack length and stress ratio is increased by as much as 200% when cooling first in refrigerated air followed by liquid nitrogen. However, there is little benefit from adding dry-ice/alcohol as an intermediate step in the cooling process.

The current assembly procedure to install a trunnion and hub into a bascule bridge girder involves cooling the trunnion in liquid nitrogen and shrink fitting it into the hub. The resulting trunnion-hub assembly is then allowed to warm to room temperature. Next, the trunnion-hub assembly is cooled in the liquid nitrogen and shrunk fit into the girder. The cooling of the trunnion does not cause any problems, however, when the trunnion-hub assembly is cooled in the liquid nitrogen, the hub experiences a large thermal shock. These thermal shocks induce large stresses into the hub, which has been known to cause it to crack. This study investigates an innovative assembly procedure to install the trunnion-hub assembly into a bascule bridge girder. To avoid cooling the trunnion-hub assembly, the girder was heated instead. Laboratory testing and finite element analysis were used to determine if the girder could reasonably be heated to install the trunnion-hub assembly. An experiment was conducted to analyze the heating process that will be used. A rectangular steel plate ( 60"x60x"0.75") was used to model the girder in the lab. Inductance-heating coils were used to heat the steel plate to 350°F. The heating process was recorded using a data acquisition system with thermocouples and strain gages. ANSYS was the finite element analysis (FEA) program that was used to model the heating process of the plate. The FEA results from ANSYS were compared with the experimental results. This confirmed the parameters of the finite element analysis were correct. Those parameters were then used to model a full-scale girder. The feasibility of heating the girder was determined from the finite element analysis results. It was determined that heating the girder with 2250 BTU over min for 90 minutes, was sufficient energy for the assembly procedure to work. The girder was heated to a maximum temperature of 350°F and a 0.015" clearance was created for the assembly of the trunnion-hub. The finite element analysis of the girder showed that the placement of the heating coils on the girder was critical. Therefore this innovative assembly procedure can easily be accomplished, however, each girder must first be analyzed to determine the optimal heating configuration.