How does geometry affect successful (no cracking) trunnion-hub-girder assembly procedure?

To make the fulcrum, also called trunnion-hub-girder (THG) assembly, of bascule bridges, a trunnion is shrunk fit into the hub, followed by cooling of the trunnion-hub assembly to shrink fit it into the girder of the bridge.  Development of cracks on the hub was observed in one THG assembly when the trunnion-hub assembly was cooled for insertion into the girder.  A suggestion to solve this problem was to study the effect of radial thickness of the hub and to understand its influence on critical stresses and crack lengths.  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 paper, to quantitatively find the sensitivity of these parameters on critical stresses and critical crack lengths, the geometrical dimensions of the trunnion-hub assembly are changed following Design of Experiments’ standards.  Parameters changed are the radial thickness of the hub, the inner radius of the trunnion, and the radial interference at the trunnion-hub interface.  The radial thickness of the hub was found to be the most influential parameter (90% effect) on increasing resistance to fracture.

a) Trunnion, Hub and Girder

b) Trunnion fitted into the hub

c) Trunnion-Hub Fit into the Girder

d) Completed Trunnion Hub Assembly

Related Papers

C. Nguyen, A.K. Kaw, J. Paul, “Sensitivity Analysis of Cooling Methods and Geometric Parameters in Assembly Procedure of Bascule Bridge Fulcrums”, The Journal of Strain Analysis for Engineering Design, pp.  337-349, Vol. 42, 2007.