More than two years after the roof of WIN Stadium's western grandstand collapsed, we now know why it happened.
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The roof of the newly constructed western grandstand buckled in strong winds on the afternoon of September 20, 2011.
A NSW government report into the incident and subsequent repairs was obtained by the Illawarra Mercury.
According to the report, by the NSW Public Works department, the fault centred on one of the trusses at the southern end of the roof.
There are a series of trusses in the roof, each of which features members that form the shape of a triangle.
The connected pieces forming the top of a truss are called the upper chord, and those forming the bottom of a truss are called the lower chord.
The report states that the cause of the failure were the two connections - each held with three M16 bolts - that attached a member (identified as TC4) at the upper chord.
Police cordon off Harbour Street after the incident."Investigations concluded that member TC4 failed because the capacity of the bolted connection at the upper end had insufficient capacity to withstand the forces in member TC4 caused by 75-90km/h winds," the report said.
This force was greater than the bolts could handle, causing them to shear.
The report did not assign blame to any organisation but did state that under the contract signed between contractor Lipman Pty Ltd and the NSW government that Lipman carried "responsibility for both design and construction of the works - and as such have responsibility for the design of both any temporary repair works and any long-term solutions".
Lipman appointed engineering firm Taylor Thomas Whitting to review the original design and provide advice on the redesign.
The government also appointed its own expert to oversee the development of the redesign - Professor Nicholas Trahair, from the University of Sydney's School of Civil Engineering.
As part of the redesign, wind tunnel testing was undertaken and "was intended to cover all potential extreme wind scenarios".
The report said Lipman submitted plans for the redesign on December 16, 2011.
The plans included replacing the damaged trusses, opting for fully welded connections over using bolts in the end sections and "additional stiffening plates" in other connections, and increasing the size of a number of members.
The report noted that a "thorough process of validating the redesign" had taken place, leading to the redesign being given the thumbs up.
