Posted by Nardblomp Klashelfloogie on February 04, 2002 at 09:45:23:
In Reply to: Question about the Empire State Building posted by Jared Hicks on February 03, 2002 at 06:35:55:
You have asked a most excellent question. It is true that the basic construction of the Empire State Building is superior to that of the former WTC structurally. Lets start with the frame. The Empire State Building frame is made up of many closely spaced columns and column clusters. This results in an exceptionally sturdy frame which has much structural redundancy, that is, there are many alternate paths for loads to be transmitted from the building to the ground. If the frame is damaged at one point there are other ways for the static and live loads to be supported. Many engineers estimate the Empire State Building is several times stronger than the WTC was. Few doubt that the Empire State Building would have withstood the disaster that collapsed the WTC towers. The disadvantage of this type of construction is that with all those columns the layout of the offices has to be done around the grid formed by the columns. The WTC was designed to have as open a floor plan as possible, and so each tower had light floors supported on the inside with a central core, and on the outside by a tube of closely spaced square columns. There were no intervening columns. Technically, the WTC towers were bearing wall structures, in a way. This resulted in a very flexible layout, but unfortunately concentrated the load bearing structure at the core and the outside perimeter of the towers, making them quite vulnerable to progressive collapse.
Another big difference between the Empire State Building and the former WTC is fire resistance. The fire "proofing" in the Empire State Building is accomplished with eight inches (two courses) of brick, which is built up around columns and beams. Brick is an excellent fire resistive material because it is refractory, that is, it comes from fire. Brick is baked at temperatures of 1000 degrees or so when it is made. Since brick comes from fire, it is not severely compromised by high temperatures. The fire "proofing" in the WTC was a spray on coating of a couple of inches thickness. This coating was of various compositions, according to when it was placed. The earlier coating was a gunite with asbestos fibers in it, the later coating had the asbestos fibers replaced by a combination of ceramic material and fiberglass. THe material that held all this in place was the gunite, a highly liquid form of concrete widely used in modern construction. The problem with this stuff is that it is NOT refractory. When subject to fire, it will hold out for a while, then it spalls and sometimes actually pops or explodes, depending on how much moisture it contains. In addition, it undergoes a chemical reaction which turns it to powder. Now you may ask why anybody would use such a thing as fire "proofing". It is widely used because it does have some fire resistance and is much cheaper that brick to install.
Most of New Yorks older skyscrapers are built like the Empire State Building and posess many of its excellent structural qualities. Newer buildings do not, for the most part. It is ironic that in these modern dangerous times, a person is safer in one of the older buildings from another era than in one built in our own time.
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