An expert witness has been called by construction firm Murray & Roberts to explain the material failure that led to the fatal M1 bridge collapse in Sandton last year. This came as the Labour Department's inquiry into the deadly incident resumed on Thursday in Pretoria.Delays frustrate M1 bridge victimsThe family of one of those killed when a pedestrian bridge over the M1 collapsed last year are frustrated with the slow progress of the inquiry after it was postponed for two months. The temporary structure supporting the construction of the pedestrian and bicycle bridge on Grayston Drive collapsed on October 14 last year‚ killing two people and injuring at least 19.M1 bridge collapse revisitedTwo bolts were missing from the structure that collapsed on the M1 in Sandton, killing two people and injuring 19 others in October. Commissioner Lennie Samuel‚ a departmental forensic investigator and co-commissioner in the Tongaat Mall Structural Collapse Inquiry‚ began hearing testimony from main contractor Murray & Roberts.The inquiry must make a finding on what went wrong and who is responsible.Witness Professor Roelf Mostert‚ head of the University of Pretoria's Materials Science and Metallurgical Engineering Department‚ said security video footage of the collapse showed the girders‚ which held the structure together‚ separate.“The structure didn't show any noticeable movement until the time of collapse [on that day]‚” Mostert said.Mostert‚ who was on the scene on the day of the incident‚ talked the inquiry through the photographs he had taken of the wreckage.Murray & Roberts’ advocate‚ Sias Reyneke SC‚ said that the firm had carefully preserved the remnants of the support structure‚ used together with computer modelling‚ to investigate the cause of the collapse.Previously‚ Murray & Roberts had told the inquiry it was responsible for erecting the support structure‚ including scaffolding and super beams.It said‚ however‚ that Form-Scaff‚ a company it had subcontracted to assist with the project‚ was responsible for the structure’s design. Form-Scaff denied this.The support structure was to be removed from the site after the bridge’s completion.Mostert's testimony continues...