Large-scale thefts of material by workmen renovating a former FBI Building for use as a congressional officer building has led to the firings of 36 skilled craftsmen and implementation of rigid security measures on the $15 million project.

George M. White, the Capitol architect whose office is in charge of the project, confirmed yesterday that an eight-month-long investigation by the FBI left little doubt that the thefts were occurring, but he said investigators were not able to get enough evidence to justify indictments.

Because investigators could not prove the allegations, no names were released, and the workers suspected of being the ringleaders were dismissed in a "reduction-in-force" move that included employes not suspected of criminal activity.

Among items stolen from the six-story brick building at Third Street and New Jersey Avenue SW, according to the FBI, were 20-year-old window air conditioners that were cannibalized for parts and pipes that were cut up and sold for scrap.

Roy A. Tait, general engineer for the architect's office, said yesterday ensure that he fired the dozen or so principal culprits.

"It was a good time to clean house," said Tait, adding that the cutback came at a time when was a lull in the renovation.

The building, now known as House Annex No. 2, formerly housed the FBI's fingerprint files. It is being converted to offices, and part of the building is being used as transition space for newly elected congressmen, although only one, Rep.-elect Michael D. Barnes (D-Md.), has thus far moved in.

A spokesman for the FBI's Washington field office said yesterday that while investigators were convinced that widespread thefts had occurred, they couldn't be proved.

"Sure it went on," FBI spokesman said. "But most of the workers said they heard about it from someone else, or didn't remember who was carrying the material."

"It was a shock to us," White said. "We had no idea it was going on," White said "there is no construction job ever built that workers didn't take home some two-by-fours in the trunks of their car, but wer're talking about more than that here."

The investigation began eight months ago when a carpentry foreman complained to Capitol police about the thefts. The worker, who has since retired, said in an interview this week that he personally helped build consoles for oyster boats in one of the shops of the architect. He said he is satisfied that major offenders have been dismissed.

None of the dismissed workers was among the 2,300 regular employes of the architect's office, many of whom are charworkers and maintenance employes.

Renovation of the building is the largest project ever undertaken by the architect's office, which usually confines its work to minor renovations and repairs. Large projects, such as construction of the new Madison annex of the Library of Congress and the Hart Senate Office Building, are put out to bid and built by private contractors under supervision of the architect.

White said the decision to renovate the FBI building in-house, with workers employed temporarily from the hiring halls of the respective trade unions, was necessitated by the desire of Congress to use part of the building during the renovation.

"If we had put it out to bid, it would have been impossible to keep up with the changing demands of Congress," White said yesterday in an interview in his office in the Capitol.

According to workers who complained about the thefts, some of the stolen material was sold to fences, while some was used by the thieves for such diverse projects as building a flag pole for a volunteer fire department, and constructing additions to houses, garages, marinas and shops of the workers and their friends.

Part of the problem in trying to nail down the allegations, according to both White and Tait, is that it is common practice for Hill employes to hire fellow workers on their own time to perform private work for them.

"When you find a good carpenter, lots of people want him to do odd jobs on weekends." said the pipe-smoking Tait, who acknowledged that he hired two carpenters to build cabinets in the kitchen of his McLean home two years ago.

Although one of those men was among those he fired this month, Tait said the dismissal was unrelated to the work performed at his house.

Tait, a 21-year employe of the archietect's office, said the FBI asked him about the project, but when he offered to show them canceled checks for the work, they declined, saying they believed him.

White said he, too, had employed workmen from his office at his home, but he said he was not asked about it by the FBI. White said shortly after he moved here from Cleveland about five years ago, following his appointment by President Nixon as the nation's seventh architect of the Capitol, he hired carpenters and painters to work on his newly purchased Georgetown home on weekends. "I remember I had to pay them time and one-half," White said with a grin.

To prevent more thefts, Tait said, new procedures require workers who move material from one building to another to have a receipt, and new inventory controls have been instituted.