Washington ? Mangled steel beams that once formed the skeleton of the World Trade Center will undergo rigorous fire and impact tests as the government develops ways to make skyscrapers stronger in the aftermath of Sept. 11.

The National Institutes of Standards and Technology on Wednesday began a two-year study of the 110-story twin towers to analyze why they fell and what can be learned from their collapse. The project, expected to cost about $23 million, could produce improved building codes.

“I am confident that we will come up with recommendations for code reform that will lead eventually and even immediately to safer buildings,” agency director Arden Bement said.

The agency, which has more than 100 pieces of steel from the towers, put on view several pieces twisted and rust-colored at a warehouse at the agency’s headquarters in Gaithersburg, Md. One hulking beam from the north tower was sliced by a hijacked jetliner when it slammed into the building between the 94th and 99th floors.

Survivors and relatives of those killed will be interviewed in an effort to study the evacuation and emergency response, said Shyam Sunder, the lead investigator.

A study by the American Society of Civil Engineers and funded by the Federal Emergency Management Agency was criticized by some for not looking at evacuation procedures.

That study, which concluded in May, determined the buildings could have survived being struck by the hijacked airliners but succumbed to the ensuing fire. That blaze ignited office furniture and paper, and the heat caused the buildings’ steel columns to soften and buckle.

The relationship between fire and structural collapses will be an important part of the investigation, Bement said. Fluffy fireproofing sprayed onto the trade center’s steel beams was jarred loose when the jetliners hit, the FEMA study found. Institute investigators will look at ways to keep the fireproofing intact.

In addition to the trade center, investigators will study 7 World Trade Center, which is believed to be one of the first fireproofed steel structures to collapse due to fire damage alone. That 47-story building collapsed later in the day on Sept. 11.