NIST says that debris from the collapse of the North Tower ignited fires on at least 10 floors of World Trade Center 7, some of which burned out of control because the automatic sprinkler system for some floors had failed partly due to city water lines damaged by the collapse of the Twin Towers. Heat from the fires then caused steel support beams to expand which led to several floors collapsing.
This then triggered a number of other structural failures, which eventually resulted in the whole building collapsing. This article is part of our work fact checking potentially false pictures, videos and stories on Facebook. You can read more about this—and find out how to report Facebook content— here. Building on that information are researchers and practicing structural engineers striving to make our structures safe and economical for everyday use and also survivable when damaged.
Following the bombing of the Alfred P. But engineers and other partners to the construction industry are not complacent when there are failures. We study, we learn, and we improve. This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American. Already a subscriber? Sign in. Thanks for reading Scientific American.
Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science. Get smart. Sign up for our email newsletter. Sign Up. In addition, the flame would be white hot, like a giant sparkler. There was no evidence of such aluminum ignition, which would have been visible even through the dense soot.
It was noted above that the wind load controlled the design allowables. The WTC, on this low-wind day, was likely not stressed more than a third of the design allowable, which is roughly one-fifth of the yield strength of the steel. The additional problem was distortion of the steel in the fire. The temperature of the fire was not uniform everywhere, and the temperature on the outside of the box columns was clearly lower than on the side facing the fire.
The temperature along the 18 m long joists was certainly not uniform. This produced distortions in the slender structural steel, which resulted in buckling failures. Thus, the failure of the steel was due to two factors: loss of strength due to the temperature of the fire, and loss of structural integrity due to distortion of the steel from the non-uniform temperatures in the fire. Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column.
However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down. The perimeter tube design of the WTC was highly redundant.
It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. With a Pa floor design allowable, each floor should have been able to support approximately 1, t beyond its own weight.
The total weight of each tower was about , t. As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell.
The floor below with its 1, t design capacity could not support the roughly 45, t of ten floors or more above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of km per hour.
There are several points that should be made. First, the building is not solid; it is 95 percent air and, hence, can implode onto itself. Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the side such that it is not within the base footprint of the structure. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a , t structure has too much inertia to fall in any direction other than nearly straight down.
The World Trade Center was not defectively designed. No designer of the WTC anticipated, nor should have anticipated, a 90, L Molotov cocktail on one of the building floors. Skyscrapers are designed to support themselves for three hours in a fire even if the sprinkler system fails to operate. This time should be long enough to evacuate the occupants. No normal office fires would fill 4, square meters of floor space in the seconds in which the WTC fire developed.
Usually, the fire would take up to an hour to spread so uniformly across the width and breadth of the building. This was a very large and rapidly progressing fire very high heat but not unusually high temperature.
After all, 1,, t of rubble will require 20, to 30, truckloads to haul away the material. Mark Shwartz, News Service ; e-mail: mshwartz stanford. Hamburger, a structural engineer currently investigating the Sept. He pointed out that four buildings were immediately destroyed in the WTC assault, and three others suffered irreparable damage and are in the process of being razed.
Another half-dozen buildings were harmed structurally but can be repaired, and more than 50 others were damaged by the enormous debris cloud and the burning material that followed the collapse of the twin towers. Hijackers' plan. Using photographs of the WTC taken before, during and after the Sept. He noted that the first jetliner banked into the north tower at a degree angle, damaging floors 92 to About 40 minutes later, the second jet crashed into the south tower, hitting floors 78 to 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.
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