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September 20, 2001
Mr. Kenneth Chang
NY Times Re: NY Times Article: Defending Skyscrapers
229 W. 43rd St. Against terror
New York, NY 10036 September 18, 2001, Tuesday
Defending Skyscrapers Against Terror
By KENNETH CHANG
Dear Mr. Chang,
Your article is very informative and interesting and we agree with the comments made by Dr’s Thornton, Williamson, and Astaneh-Asi.
We do however want to point out to Dr. Mir that reinforced concrete buildings have collapsed or have been extensively structurally damaged when they were exposed to fires of much less severity.
Sao Paulo Brazil has the unenviable record of having 3 of the most severe fires in the western hemisphere. These were all reinforced concrete buildings. 1/3rd of the floor area of the 21 story CESP 2 building suffered a total collapse from the 21st floor to the ground in 1987. The Andraus Building in 1972 and the Joelma Building in 1974, suffered extensive structural damage. The post tensioned concrete roof of the Falabela department store in the Plaza Vesuvias Mall in Santiago Chile collapsed onto the 2nd floor. All of these fires were of normal combustibles i.e., furniture, carpets, paper, etc, that are found in any office building, and none of these buildings experienced the tremendous impact or the added massive weight of a jet plane and passengers as the WTC did. News videos of the Andraus and Joelma fires show massive portions of the exterior concrete walls literally exploding apart during the fire.
The WTC was exposed to a jet fuel fire. Jet fuel is not a normal combustible. It is a “hydrocarbon”, a source of much higher temperatures that will negatively affect both steel and concrete much faster than normal combustibles would.
Dr. Mir states that, “An all concrete structure would have lasted longer”. We question the basis for that statement. Our experience with fire testing over the past 38 years tells us that concrete exposed to the temperatures of a hydrocarbon fire will cause the moisture in the concrete to turn into steam so rapidly that the concrete will literally and explosively break apart.
Dr. Thornton correctly points out that concrete is not impervious to heat, and the process that he describes, that some types of concrete can flake apart under intense heat, exposing the steel inside is very true. However this is what happens to concrete exposed to the temperatures of a normal fire, not when it is exposed to the temperature of a hydrocarbon fire, which causes the action explained above.
To give you an example:
Fire resistive products used to protect commercial hi-rise buildings are tested to ASTM E119. This standard requires that the fire resistive product protects the steel from reaching a limiting temperature during the time period required by the building codes. For the structural frame (columns and primary beams) the time period is 3 hours and the limiting temperature is 1200 F at any measured point on the column. For the primary beams the limiting temperature is 1300 F at any measured point on the beam. Floor beams require the same limiting temperature, but the time period is reduced to 2 hours.
The test furnace follows a temperature curve so that the test specimen is exposed to a temperature of 1550 F at 1/2 hour, 1700 F at 1 hour 1850 F at 2 hours, 1960 F at 3 hours, and 2000 F at 4 hours. In other words under this test a fire resistive product with a 3 hour column rating, the highest temperature it was exposed to was 1960 F and only towards the end of the test.
On the other hand, fire resistive products used to protect refineries, petrochemical plants, etc, against hydrocarbon fires are tested to UL 1709. This standard requires that the test furnace reaches 2000 F in 5 minutes. Under this test then, the fire resistive product with a 3 hour column rating has been exposed to a temperature of 2000 F for 2 hours 55 minutes.
You can see that the severity and speed of a hydrocarbon fire is much greater than a standard fire. The WTC towers were exposed to the severity of a hydrocarbon fire.
We should also comment that when concrete is exposed to this type of fire it will perform much differently than when it is exposed to a normal fire. Under a normal fire, moisture in the concrete has a better chance of coming out of the concrete without explosively spalling and more as Dr.Thornton describes. Under a hydrocarbon fire the moisture in the concrete is heated so rapidly that it immediately turns to steam and this literally causes the concrete to explode apart. Once this happens not only is the reinforcing steel exposed, but the concrete that holds the steel rigidly in place is gone as is it’s stability and load carrying capacity.
Lastly, quoting your own article:
“Each World Trade Center absorbed the impact of a jet with a shudder as they were designed to do, and stood.
Inside, though, 2000-degree infernos started burning, fed by thousands of gallons of Jet Fuel”.
“The south tower collapsed 56 minutes after impact. The north tower lasted an hour and 40 minutes”.
We hope this explains our disagreement with some of the comments made by Dr. Mir. We would be happy to show you the videos of the Sao Paulo fires, articles and test reports comparing ASTM E119 and UL 1709 to back-up what we are saying.
If you have any questions or comments or need additional information please let us know.
Fire Protection Consultant