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Question:

Can fire resistant building materials burn hot enough to melt steel columns?

I mean 4 inch thick steel like was used in the WTC buildings, especially WTC 7 which did not get hit by a plane. Can carpeting, desk chairs, water coolers and dry erase boards burn hot enough and long enough to melt steel that is normally made in a blast furnace?

Answer:

Steel loses strength as it is heated and will fail long before it melts. A mild steel that starts out at 100 degs F with an allowable stress level of 20,000 psi will be down to only about 2000 psi at 1000 degrees. As to fireproofing for steel. It doesn't have an unlimited life. In fact most has only a 2 to 4 hour rating and that is determined in ideal conditions in a laboratory. The idea is to give firefighters time to get everyone out of the buidling.
they shouldn't, provided that they are properly fireproofed. in the case of the wtc the explosion of the plane instantaneously removed the spray on fireproofing in the members. the unprotected members then melted. had the fireproofing remained on the members the office equipment etc most likely would have burnt itself out before the members became comprimised.
A fire doesn't have to melt steel to do damage. Merely softening the steel makes it weaker and easier to bend. Structural collapse can then occur at temperatures far lower than melting temps. Ask yourself why does a blacksmith heat steel to work and bend it?
Gavin, the danger posed by steel columns and girders during a fire in a building, is not the danger of the steel melting but of the steel expanding and snapping the bolts that hold all of the steel together. Very high temperatures can be achieved in a building fire because the up-draft caused by the rising hot air, delivers a massive amount of oxygen to the burning carpet, paper and furniture. When a steel frame office tower burns, the heat expands the steel and snaps the bolts. This causes the building to fall-down and not necessarily, burn-down. I am too lazy to look-up the melting point of steel but it is not very high. That is one reason for the columns and girders in steel framed buildings, to be covered in asbestos (which is a fatally toxic material) or magnesium di-oxide.

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