Ultrasonic Stress Measurement for Evaluating the Adequacy of Gusset Plates

The I-35W bridge in Minneapolis, MN collapsed on August 1, 2007, as a result of an overstressed gusset plate. Following the collapse, state departments of transportation sought to confirm the adequacy of gusset plates in other bridges to ensure overstressed gusset plates were not present. However, the analytical tools to assess the adequacy of gusset plates depend on assumed loading based on the intended design configuration of the bridge. Actual stresses and loads in a gusset plate, which may be affected by unintended or unexpected load distributions, cannot be determined using conventional tools such as strain gages. This paper will report on research intended to develop a methodology for nondestructive assessment of actual stress levels in gusset plates, to support safety analysis. This research utilizes the acoustoelastic effect to evaluate actual stress levels by assessing the acoustic birefringence in the plate. The birefringence measurement evaluates normalized variations of polarized shear waves propagating through the plate thickness, which vary proportionally as a function of stress. This paper discusses exploratory testing to evaluate the potential of the approach as a tool for the condition assessment of gusset plates.

References
1. Board, N.T.S. Collapse of I-35W Highway Bridge Minneapolis, Minnesota August 1, 2007, National Transportation Safety Board: Washington, D.C. p. 178, 2008 2. Washer, G.A. "The Acoustoelastic Effect in Prestressing Tendons", in Department of Materials Science and Engineering. Johns Hopkins University: Baltimore, MD. p. 163. 2001. 3. Washer, G.A., R.E. Green and R.B. Pond. "Velocity Constants for Ultrasonic Stress Measurement in Prestressing Tendons," Research in Nondestructive Evaluation, 2002. 14(2). 4. Lozev, M.G., J. Alfred, V. Clark and P.A. Fuchs. Application of Electromagnetic-Acoustic Transducers for Nondestructive Evaluation of Stresses in Steel Bridge Structures, Virginia Transportation Research Council: Charlottesville, VA. p. 37. 1996.
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