The Labor Department assessed a record $5.11 million in fines yesterday against contractors for a half-built Bridgeport, Conn., apartment building whose collapse killed 28 workers in April. A small bracket that bent under heavy pressure triggered the disaster, investigators said.

"We found a serious disregard for basic, fundamental engineering practices," said Assistant Labor Secretary John A. Pendergrass, chief of the Occupational Safety and Health Administration.

Fines for the April 23 collapse of L'Ambiance Plaza, the worst U.S. construction accident of the decade, are nearly double the previous OSHA record fine. OSHA and the National Bureau of Standards said builders failed to conduct stress-analysis tests to discover whether the lifting brackets, a steel piece about 4 inches by 12 inches, could hold up a stack of concrete floors being inched into position.

Pendergrass said the "unacceptable design deficiencies" could have been easily detected with "rudimentary engineering analysis."

OSHA cited Texstar Construction Corp. of San Antonio with 238 instances of alleged willful violations for using lifting brackets that did not meet federal standards. The bureau requires that they be able to hold 2 1/2 times the anticipated load. The company was fined $2.52 million.

TPMI-Macomber, the primary contractor and project manager, was cited for identical willful violations because it was contractually responsible for overall health and safety at the site, OSHA said. The enterprise, a joint venture of TPM International of Darien, Conn., and B.H. Macomber of Boston, was fined $2.48 million.

The balance of the penalties was assessed against Lift Frame Builders of Elmsford, N.Y., $104,000; Fairfield Testing Labs of Stamford, Conn., $10,000; and Preforce Corp. of New York City, $1,000.

Spokesmen for the two major contractors declined comment, saying they have not had time to receive and review the documents.

Arnold Bai, an attorney for Lift Frame, vowed to appeal the fine, saying the company disagrees with much of the report.

Texstar, responsible for lifting the floors, had problems with the brackets twice before at L'Ambiance and at a Stamford, Conn., project, Pendergrass said. But he said company officials failed to act.

The plate, part of a steel collar embedded in the concrete slabs, holds metal rods attached to hydraulic jacks atop steel columns. Under the lift-slab method, the jacks hoist stacks of 320-ton concrete floors into position, with the plates holding the rods in place.

The day of the accident, workers had lifted floors nine, 10 and 11 and were trying temporarily to secure the ninth floor to steel girders when one of the lifting plates bent slightly upward, according to Charles Culver, the standards bureau's chief investigator for L'Ambiance.

An adjustment of less than a half-inch of the ninth floor could have shifted enough added weight to the bracket to cause the bending, he said.

Culver said the jacking rod slipped out of the bent U-shaped plate, releasing the three floors and setting off a chain reaction that tumbled the building within seconds.

Culver said inadequate engineering throughout the structure rendered other parts unable to hold the extra weight. "It's like a woman's nylon stocking," said Culver. "You tweak it here and it runs all along."