An unfinished two-story town house in Franconia collapsed yesterday, injuring and trapping two carpenters who were freed by fellow workers and rescue personnel. Fairfax County officials said the structure apparently was blown over by a wind gust.

The carpenters were working on the first floor, Fairfax fire officials said, when the wooden second story and roof collapsed on top of them. The collapse occurred about 1:30 p.m., well before last night's storm and strong winds.

Other carpenters working at the large Manchester Lakes subdivision site rushed over after what several described as "a sound like an explosion," braced the collapsed structure and cut away walls in an effort to free the men.

Fairfax fire spokeswoman Pamela Weiger said that by the time rescue units arrived, enough debris had been cut away to permit rescuers access to the men, who were extricated after the structure was braced to prevent further collapse.

One of the men, John Robert Herion, 30, of 5410 Brixham Ct., Burke, suffered two broken vertebrae; the other, Christopher Brennen, 20, of 3909 Tasico Place, Fairfax, suffered back abrasions. Both were listed in fair condition in Fairfax Hospital.

The $80,000 town house unit, an interior unit in a row of four, will be located at 6119 Bristol Way. It is part of the 192-acre subdivision just east of Beulah Road, which will eventually have 1,394 units. The town house unit that collapsed is being built by Pulte Homes Co.

The accident is being investigated by the Fairfax County building inspector's office and the Virginia Department of Occupational Safety and Health.

A National Weather Service forecaster said that about 1:30 p.m., winds at National Airport, about six miles away, were gusting to about 26 miles an hour. no

"The question is . . . one of whether adequate bracing had been installed," said Rick Lawson, the county's director of inspections. "And investigating that . . . is a matter of sorting through broken members and debris to see if we can identify the braces."

Lawson said it appeared there were no violations of the county building code, and said investigation of the proper bracing was the responsibility of state officials, who declined to comment yesterday pending completion of their investigation.

Robert Crosby, project manager for Pulte Homes at the site, said the row of units containing the collapsed structure was erected Tuesday and was not scheduled to be inspected by the county until next week.

Fire officials said the two carpenters were independents hired by a subcontractor, Steve Miller Construction.