The federal occupational safety agency has fined a Falls Church construction company $46,800 for what it calls "serious" and "willful" safety violations in connection with last September's collapse of a gymnasium under construction at Coolidge High School in Northwest Washington.

The company plans to contest the fines.

One worker was killed and two suffered serious injuries when the steel frame of the building collapsed Sept. 25. Two workers were thrown from atop the structure, and they and the third were pinned under a pile of twisted steel.

Before the structure fell, an inspector for the D.C. Department of Public Works had twice tried to stop work on the project -- once just moments before the collapse -- alleging unsafe working conditions.

According to Larry Liberatore, head of the U.S. Occupational Safety and Health Administration's Baltimore office, the fines levied in the case this month were the largest imposed "in recent years" by the office, which covers Maryland, Virginia and the District of Columbia.

Williams Enterprises Inc., the company cited by OSHA, plans to contest the validity of the agency's findings, according to James Brandt Clarke Jr., an attorney for the firm, a subcontractor that erected the steel frame of the building. "I have serious questions on whether or not the violations have any basis in fact and whether the alleged facts caused the collapse," Clarke said.

The company has until Saturday to appeal to OSHA's review commission.

There are five $9,000 fines for each of five alleged "willful violations," including inadequate fall protection, poor bolting, two instances of insufficient safeguards to provide lateral stability to the structure, and placing a load on top of the structure before safety requirements were met, Liberatore said.

He said the company was fined a total of $1,800 for two alleged "serious violations" relating to a job safety program, employe training and safety records on one of the cranes at the construction site.

Liberatore said a "willful violation" is one that an employer "knows constitutes a violation or when he is aware a hazardous situation exists and makes no reasonable effort to correct it." The maximum fine for such a violation is $10,000, he said.

A spokeswoman for the D.C. Department of Public Works said the collapse of the gymnasium caused the building project to fall about four months behind schedule. She said it is 43 percent complete and should be finished in June 1986.