was change headers: The Fate of VM was: Baby MVS 4410

There is no life limit to structural steel. There are relatively few structural steel buildings over 100 years old because steel was too expensive to use as a building material until about 1880, and old buildings are taken down to be replaced with ones more suited to modern requirements. One of the first "fireproof" office buildings, The Equitable Life butturance office building in Manhattan, was built in 1870 and burned and collapsed in 1912. That fire was one of the events that helped to push the development of modern fire resistive construction. In the old days, the structural steel in high rise buildings was protected by from fire by layers of brick, hollow tile or concrete, which was heavy and expensive to apply. Lighter materials were developed in the 1950s and sprayed on fireproofing began appearing in the 1960s. In the first building with sprayed on fireproofing that I was aware of, the insurance man was on the site daily to check the thickness of the fireproofing.

The fire resistance of materials is measured in the standard fire test, ASTM E-119. You build a section of the struture inside a test furnace, load the structure with weights and expose it to a controlled natural gas fire. The temperature of the fire and of the structural steel is monitored with thermocouples connected to chart recorders. The temperature of the furnace follows the standard time-temperature curve where the temperature is 1,000 degrees F at 5 minutes, 1,700 degrees F at one hour, 1,850 degrees F at two hours and 2,000 degrees at four hours. Structural steel will fail at a temperature of about 1,100 degrees, making the life expectancy of unprotected steel in a fire very short. The New York building code required beams and columns supporting more than one floor to have a rating of 3 hours and floors to have a fire resistance rating of 2 hours. Last year, NIST did a fire test of a model floor built to plans for the World Trade Center and developed a fire resistance rating for the floor of 2 hours. The original specifications would have produced a lower rating (3-4 hours), but the final installation exceeded specifications and the fireproffing was upgraded over the years. The buildings did not survive that long partly because the aircraft damaged the structure and partly because the real fire was more severe than the standard test fire. Real fires frequently are more severe than the standard test fire. Office buildings contain a surprising amount of fuel. Normal fires also don't start in multiple locations on multiple floors. You have a single point of ignition and the fire grows from there. There was a severe fire in one of the towers in 1975. After that they retrofitted the buildings with automatic sprinkers. Modern, quick response sprinklers are expected to control fires in office occupancies with four or fewer sprinklers operating. The sprinkler system was not a factor in the 2001 fire because it was destroyed by the aircraft.

Bud