Field modal testing and evaluation of modifications for bridge crane structures designed for sensitive equipment

Large overhead bridge cranes in particular facilities at the Department of Energy (DOE) uranium enrichment plants are required to exhibit severly restricted vibration levels over a limited frequency range to prevent damage to payloads of sensitive equipment. The large span and unique payload grapple/rotation mechanisms of the crane presented complex challenges in the analysis and solution of excessive payload vibration levels. The problems characterized and solved included (1) testing of heavily damped structural components and massive in-situ structures with high modal density carrying dynamically sensitive payloads; (2) work in poorly accessible locations on in-use equipment within a limited time frame; (3) assessing test results and proposing structural modifications; and (4) qualifications of modifications. Methodology for the test consisted of taking frequency response function (FRF) data (using two techniques) for the crane components and payload, analyzing the data to characterize the components and identify component problems, assessing probable payload damage versus the rigorous crane performance specification, recommend structural modifications, and verifying the fitness of the modifications