PITTSBURGH — A design flaw and the use of the wrong type of steel caused a section of concrete floor to collapse at the city's convention center last year, according to a report.

A large slab of concrete gave way under the weight of a tractor-trailer in the loading area of the David L. Lawrence Convention Center on Feb. 5, 2007. No one was injured, but the center remained closed for more than a month.

An engineering firm hired to investigate the collapse, Wiss, Janney, Elstner Associates, said a slotted bolt connection used to attach a support beam to an expansion joint was inappropriate given the friction that would develop with temperature changes.

The steel used to connect the beam to the expansion joint was not the type called for in the design, which specified a more flexible steel.