A Failure Analysis Case Study of the Fluid-Structural Interactions on Trashracks

Abstract

A failure analysis investigation was completed on the trashracks at Powerhouse One of Rock Island Dam. The investigation was part of the overall condition assessment and rehabilitation work being performed at the dam. The trashracks were deemed a faulty system in 2005 when MWH, a contractor for Chelan County PUD, initially inspected the racks. Building on the original work completed by MWH, this project compiled background information and conducted field inspections to evaluate the future reliability of the trashracks. The field inspections identified consistent failures of the diagonal tie rods, discovered cracks in the rackbar welds, and highlighted the noticeable presence of corrosion damage. Though localized failures were identified, a static analysis concluded that the overall structural design was adequate for continued operations. Field tests and a finite element model were compared in a dynamic analysis to ensure the vibration induced stresses did not reach the allowable limit. Ultimately, the trashracks received new diagonal braces constructed from angle iron to replace the failing tie rods. Damaged welds, caused by the propagation of cracks from welding defects, were repaired and the racks were sandblasted to remove corrosion debris and repainted to prevent future degradation.