PITTSBURGH — A contractor didn’t obtain a key safety permit before welding caused plastic pipe and a construction tarp to burn on the Liberty Bridge last month, resulting in a 24-day shutdown of a primary artery into downtown Pittsburgh, the city’s fire chief said.

Officials with Joseph B. Fay Co. acknowledge they didn’t have the permit for “hot” work — like welding — but said that’s because city officials never told the company it needed one during a meeting on the bridge reconstruction project months before the fire Sept. 2 fire. The company also said Fire Chief Darryl Jones wasn’t at the meeting, but sent a representative.

Jones said that the $16 permit would have required a fire inspection, and for a “fire watcher” to be present with an extinguisher near any welding or other hazardous work.

But Fay said in an emailed statement Friday that it did have a fire watch on the bridge equipped with hoses and extinguishers when the fire occurred.

Jones contends hot welding slag dripped onto some PVC pipe, causing the highly flammable plastic to burn along with the tarp.

Company officials have declined to discuss other specifics about the fire’s cause, citing an ongoing investigation by the federal Occupational Safety and Health Administration, which probes workplace accidents.

The fire’s heat bent a 30-foot steel chord — a key support beam — which put the bridge in danger of collapse. Fay consulted with the Pennsylvania Department of Transportation and other experts before deciding to straighten the chord and using hydraulic jacks to move a bridge a critical 1½ inches. That repair took longer than using temporary braces to buttress the bridge, but it will enable a permanent repair to be completed in the future without closing the bridge again, company officials said.