An iron worker was seriously injured yesterday when a heavy steel beam fell on him, toppling him two stories to the muddy ground at a school gymnasium under construction in Annandale.

"It was the muddy floor that kept him from being injured even more severely," said Warren Carmichael, spokesman for the Fairfax County Police Department. "The force of the fall pushed him deeper into the mud and kept the beam from crushing him."

George Salmon, 52, of 208 Taylor Ave., Rockville, suffered chest and foot injuries in the fall at Braddock Elementary School, according to a Fairfax Hospital official. Salmon was admitted to the hospital.

The accident occurred shortly before noon when a crane operator attempted to place decking material atop a metal structural beam on which Salmon was sitting, said Gary Mesaris, a spokesman for the county fire department. The weight of the decking forced the beam, which apparently was not welded at either end, to roll over and fall atop Salmon as it crashed to the floor below, Mesaris said.

Annandale Supervisor Audrey Moore, a longtime advocate of tougher safety regulations for construction work, asked local and federal building inspectors to investigate the incident after she inspected the site where the Tycon Corp. is adding the gym to the school.