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469 Failure of load test setup
Report ID: 469
Published: Newsletter No 36 - October 2014
Report Overview
A composite steel girder was designed to support the platform for a Metro Rail station and it was decided to do a working load test on it at the fabrication yard before erection.
Report Content
A composite steel girder was designed to support the platform for a Metro Rail station and it was decided to do a working load test on it at the fabrication yard before erection. Unfortunately the load test setup failed due to over-estimation of the capacity of the ISMB (Indian standard steel beam) provided for supporting the girder.
An investigation into the failure found three contributory causes.
1) A detailed 3D analysis simulated the design bearing load over the ISMB-300 member and it was found that the stresses in the member were more than 3 times greater than yield strength.
2) The design size of the Elastomeric bearing pad was greater than the flange width of the ISMB300 so the bearing area was insufficient.
3) The main cause of the failure of the load test was the lack of proper stiffening plates.
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Assuming that it was an overestimation of the strength of the supporting member then the lesson is that test rigs should be designed to the same level of detail as the piece being tested. SWL need to be defined for the test rig, as often the tested piece may be significantly stronger/stiffer than expected and loading to deflection/failure criteria of the test piece can fail the rig. As is so often the case attention to detailing is required. All elements on the load path need to be looked at - there is always a weakest point.