When gusts of wind huffed and puffed they were able to blow down the support structure used in the construction of the Grayston Drive pedestrian bridge‚ because couplers holding it together had failed. This was the testimony of Murray & Roberts expert witness Professor Roelf Mostert at the Department of Labour's inquiry into the M1 bridge collapse on Thursday.Mostert is the head of the University of Pretoria's Materials Science and Metallurgical Engineering department.The temporary structure supporting the construction of the pedestrian and cycling bridge on Grayston Drive collapsed onto the M1 highway on October 14 last year‚ killing two people and injuring at least 19. Mostert's expert report found that parts‚ called swivel couplers‚ used in the support structure's scaffolding had failed.As a result the structure was not able to resist collapsing when it swayed as it was hit by repeated gusts of wind.Mostert said that two types of swivel couplers were used in the scaffolding - new couplers and old‚ used couplers.Mostert said seven of the eight couplers that failed were new. They had failed due to a pull-out action which caused their two halves to snap at the central rivet‚ he said.A coupler is used to connect two tubes by clamping them together so that they do not slip.Mostert said that the use of two types of couplers on the structure could be linked to the collapse.The old and new couplers were provided by Form-Scaff‚ a company sub-contracted by Murray & Roberts to assist with the project.Murray & Roberts‚ the main contractor on the bridge's construction‚ had previously told the inquiry it was responsible for erecting the support structure‚ including the scaffolding and super beams.It said‚ however‚ that Form-Scaff‚ was responsible for the structure’s design. Form-Scaff denied this.An expert report commissioned by Form-Scaff suggested that the couplers had failed due to poor workmanship‚ saying they were not tightened adequately‚ but Mostert said he found no evidence to back that theory.The inquiry continues...