Graduation Year

2005

Document Type

Thesis

Degree

M.S.M.E.

Degree Granting Department

Mechanical Engineering

Major Professor

Autar K. Kaw, Ph.D.

Committee Member

Glen Besterfield, Ph.D.

Committee Member

Muhammad M. Rahman, Ph.D.

Keywords

Thermal stresses, Design of experiments, Non-linear material properties, Interferences, Ansys

Abstract

Hundreds of thousands of dollars could be lost due to failures during the fabrication of Trunnion-Hub-Girder (THG) assemblies of bascule bridges. Two different procedures are currently utilized for the THG assembly. Crack formations in the hubs of various bridges during assembly led the Florida Department of Transportation (FDOT) to commission a project to investigate why the assemblies failed.

Consequently, a research contract was granted to the Mechanical Engineering department at USF in 1998 to conduct theoretical, numerical and experimental studies. It was found that the steady state stresses were well below the yield strength of the material and could not have caused failure. A parametric finite element model was designed in ANSYS to analyze the transient stresses, temperatures and critical crack lengths in the THG assembly during the two assembly procedures. The critical points and the critical stages in the assembly were identified based on the critical crack length. Furthermore, experiments with cryogenic strain gauges and thermocouples were developed to determine the stresses and temperatures at critical points of the THG assembly during the two assembly procedures.

One result revealed by the studies was that large tensile hoop stresses develop in the hub at the trunnion-hub interface in AP1 when the trunnion-hub assembly is cooled for insertion into the girder. These stresses occurred at low temperatures, and resulted in low values of critical crack length. A suggestion to solve this was to study the effect of thickness of the hub and to understand its influence on critical stresses and crack lengths.

In addition, American Association of State Highway and Transportation Officials (AASHTO) standards call for a hub radial thickness of 0.4 times the inner diameter while currently a thickness of 0.1 to 0.2 times the inner diameter is used.

In this thesis, the geometrical dimensions are changed according to the design of experiments standards to find the sensitivity of these parameters on critical stresses and critical crack lengths during the assembly. Parameters changed are hub radial thickness to trunnion outer diameter ratio, trunnion outer diameter to trunnion bore diameter ratio and variations of the interference. The radial thickness of the hub was found to be the most influential parameter on critical stresses and critical crack lengths.

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