WASHINGTON, Jan. 23 (UPI) -- The Pentagon's billion-dollar renovation program got the lion's share of credit for the relatively low number of casualties on Sept. 11, 2001, when terrorists crashed a plane into the southwest wall of the building. But a newly released study of the Pentagon's original structure reveals many of those who escaped the ruins that day have the original builders to thank.
Inside the building, 125 people perished from the impact, fire and subsequent collapse. Another 64 people on board the plane also died, including the four terrorists.
Recent improvements to the windows and walls kept them from shattering, and newly installed sprinklers helped keep the fire under control while those that could escaped. However, engineers who just completed a study of the building's underlying structure say the low death toll is attributable to the Pentagon's original architectural plans, which can offer builders guidance on protecting structures from collapse.
"The Pentagon survived this extraordinary event better than would be expected," said Paul Mlakar, technical director of the Army Corps of Engineers.
The Boeing 757 that slammed into the Pentagon at 530 miles per hour knocked down or significantly impaired 50 concrete columns supporting the five floors on that side of the building. However, because they were reinforced with spiral steel, they were larger in diameter than was standard at the time, and because the slab floor they supported was reinforced and distributed its load more evenly across the remaining columns, only a small part of the building -- 50 feet by 60 feet of the 6 million square feet in the building -- collapsed from the fire 20 minutes after impact. The entire section was later torn down to be rebuilt.
It was the ensuing fire rather than the impact of the plane that caused the section to collapse, the study found.
The study was conducted by the American Society of Civil Engineers and the U.S. Army Corps of Engineers, which conducted similar studies on the Alfred P. Murrah building in Oklahoma City, and the World Trade Center.
The team recommends "that the features of the Pentagon's design that contributed to its resiliency in the crash -- that is, continuity, redundancy, and energy-absorbing capacity -- be incorporated in the future into the designs of buildings and other structures in which resistance to progressive collapse is deemed important," the report states.
