Quote from ratboy88:
funny how the atf bldg had the chit blown out of its side yet half remained standing.
Some of it remained standing for the same reason the Pentagon did not collapse when Flight 77 hit it.
Steel-reinforced concrete was used extensively in both buildings, the wtc buildings did not.
WTC 7 had a large number of perimeter support columns destroyed. The buildings were not designed for that.
The most noticeable change in the modern high-rise construction is a trend to using more steel and shaping lightweight steel into tubes, curves, and angles to increase its load bearing capability. The WTC has tubular steel bearing walls, fluted corrugated steel flooring and bent bar steel truss floor supports. To a modern high rise building designer steel framing is economical and concrete is a costly material.
Years ago before the computer, builders were not sure of a structural elements load bearing capability, so they over built by using a so called âsafety factorâ. This built in safety factor could result in a structure with twice the required load bearing strength. Because of computer calculation this no longer occurs. The older buildings use to have built in a so called âsafety factorâ of two-to-one. Not so today, if the building code requires a load bearing factor of 40 pounds per square foot that is exactly what you get. There is no margin for error.
http://vincentdunn.com/wtc.html
Vulnerabilities in the design of New York's World Trade Center (WTC) are likely to have contributed to the collapse of its two main towers and adjacent buildings, according to Ronald O. Hamburger, a structural engineer currently investigating the Sept. 11 disaster.
"I believe that the hijackers flew the aircraft into the lowest part of the buildings they had access to," Hamburger commented. "If there had been no nearby structures, they would have hit the towers lower."
One member of the audience asked Hamburger if he believed that the pilots intentionally banked the planes at an angle to take out as many floors as possible.
"Yes," he replied.
According to Hamburger's preliminary analysis, the impact of the jetliners shattered and fractured two-thirds of the support columns on one face of each tower, causing the partial collapse of several floors. Debris penetrated each building's core and may have damaged the core columns located in the center of the 110-story structures.
http://news-service.stanford.edu/news/2001/december5/wtc-125.html