The 230m long section of Interstate 580 collapsed following a massive lorry fire early last Sunday.
A petrol truck travelling along Interstate 880 on the MacArthur Maze interchange near the Bay Bridge crashed into a column, creating an explosive fireball that rapidly heated and demolished the steel superstructure of the section motorway passing above it.
Independent structural expert Jolyon Gill explained that the mode of failure clearly showed the steel beams supporting the road deck were not tied together at the supporting piers. It is something which should have been done to provide resistance to earthquake loading following recommendations made after the 1989 quake, he said.
The interchange consisted of three tiers of highway of which the truck was travelling along the middle road. The deck of the I580 was supported on longitudinal steel girders that rested on steel cross beams at approximately 115m intervals. These steel cross beams were supported by a single large circular concrete pier, except at the point of the fire where the single pier was replaced by two circular concrete columns carrying the road over the I880 below.
In the intense heat of the explosion the girders on the north side of the I580 appear to have failed first, explained Gill, forcing girders on the south side to bear a greater load, these rapidly failed and twisted, kicking off the second section of deck which then collapsed partly onto the I880 and partly onto the ground below that.