A prominent engineering firm that has investigated everything from Boston's Big Dig to the collapse of a walkway at a Kansas City hotel began a top-to-bottom review yesterday to determine if the David L. Lawrence Convention Center is safe.

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Experts with Wiss, Janney, Elstner Associates of Cleveland are expected to be in Pittsburgh throughout the weekend assessing the structural integrity of the building after the collapse of a 20-foot-by-60-foot section of flooring in the second floor loading dock Monday.

"They're checking everything," said Mary Conturo, executive director of the city-Allegheny County Sports & Exhibition Authority.

The engineering firm, headquartered in Illinois, is one of two companies hired by the authority to conduct the independent investigation ordered by Mayor Luke Ravenstahl and county Chief Executive Dan Onorato in the wake of the collapse, which forced the postponement of the Pittsburgh International Auto Show scheduled this weekend and next.

The other, Leslie E. Robertson Associates of New York, will review the engineering calculations used in the design and construction of the convention center to determine if any mistakes were made. That firm is expected to begin that work this weekend.

Those independent reviews will be in addition to the investigation being done by the center's architect and builders. Both groups are trying to determine what caused the concrete flooring, a heavy support beam and a cherry picker to tumble 30 feet to a walkway and water feature below, while a 48-foot tractor-trailer delivered equipment.

Mr. Ravenstahl said yesterday city and county leaders will not reopen the building, which has been closed since the incident, until they know "exactly what happened, why it happened and will it happen elsewhere in that facility."

"We don't want to expose anybody to harm, and we want to make sure that that thorough analysis is done before we open that up to the public," he said.

Ms. Conturo said the independent reviews may not be completed until the end of next week, meaning that the convention center could stay closed until at least then. And if structural defects are discovered, the closure could end up running far beyond that.

As the independent examinations geared up, representatives for the architect and builders have been in town since Tuesday trying to figure out what caused the collapse.

One initial focus has been on an expansion joint at the site of the collapse. Ron Graziano, chief of the city's Bureau of Building Inspection, has said he was told investigators have been "looking at connections along the expansion joint" that runs the length of the building.

Ms. Conturo said, however, that those conducting the review have drawn no conclusions about a possible cause. She refused to speculate further.

Officials decided to postpone the auto show after learning that the independent inspections would not be completed until at least early next week. It has tentatively been rescheduled for April 26-30.

Wiss, Janney, Elstner Associates, the company that will review the safety of the building, has expertise in a variety of fields, including structural evaluation, failure investigation, earthquake engineering, repair design and construction, and structural testing.

Among its work, the firm investigated the walkway collapse at the Kansas City Hyatt Regency Hotel in 1981 and was hired to do a safety audit of the Big Dig project in Boston. It also helped to evaluate the safety of buildings surrounding the World Trade Center in New York after the collapse of the twin towers on Sept, 11, 2001, and was involved in the reconfiguring of Boeing 747 TWA aircraft that crashed in July 1996.

Ms. Conturo said the cost of the independent investigation would either be covered through insurance or by billing the party or parties found to be responsible for the floor collapse.

What that investigation uncovers could go a long way toward determining how long the Downtown convention center will remain closed.

Ms. Conturo and others have said that preliminary assessments seem to suggest that whatever caused the collapse appears to be isolated to one section of the building.

Meanwhile, the 48-foot tractor-trailer that apparently triggered the floor collapse was successfully removed yesterday from its position straddling the hole.

Ellen Beckert, director for marketing with Dallas-based Freeman Co., the production firm responsible for setting up the auto show, said the truck was being hauled to a company facility in Maryland.

The damaged truck was just part of the Freeman Co. fleet on its way out of Pittsburgh last night. Ten to 15 other trucks hauling auto show displays and equipment also were headed to Maryland.

Some equipment, however, remains in the closed convention center and will likely stay there until officials declare the facility safe.