Monday, February 12, 2007
Since Wandering Author's question about fireproofing in the World Trade Center, I've been thinking about this detail. I found this one in an office building built by our office a few years ago. Those C-shaped things in the walls are metal studs. If you do your own remodeling projects at home, you'll use wood studs, which would be drawn in this detail as a rectangle with an "x" through it. While wood studs can be treated with fire-retardant chemicals, metal studs are used more often than wood studs in commercial and large multifamily residential construction projects that require a high degree of noncombustability (is that even a word?).
Building codes dictate so many aspects of a building, including the materials used to make them. First, building codes allow an architect to use wood frame construction if the building is under a certain area (in square feet) and under a certain number of stories. How many square feet and how many stories depends on what kind of building it is: a wood-frame office building can be larger than a wood frame nursing home. Some types of buildings, such as structures that store hazardous materials, are not allowed to be of wood construction at all. Also, putting a sprinkler system in your building allows the architect to greatly increase the size of the building.
Structural systems for large buildings and buildings that require a great deal of fire resistance are either steel or concrete. Concrete columns and beams have steel reinforcing bar in them (called "rebar" in da biz) that allows them to stretch when the beams get really warm, and concrete is especially handy for long spans (this is one of several reasons concrete beams are used to build bridges--it can span 60 to 100 feet). Concrete is also naturally fire resistant, but it's often more expensive that steel. Steel is usually an economical way to support normal spans in a building, such as 25 to 35 feet.
However, steel columns, like the one shown above, are not inherently fire resistant.; when exposed to high temperatures, the steel deforms and fails. Sometimes, steel beams and columns are painted with a paint called intumescent paint, which creates water when it comes in contact with heat and drips on the fire, defending the steel from the intense heat and buying some more time for the firefighters to arrive and put out the flames. The more typical manner in which steel is protected and given a "fire rating" is the sprayed-on fireproofing that is shown above. I'm not sure what it's made of exactly, but it protects and insulates the steel from fire. It's lumpy, whitish-grey stuff, so if the column can be seen, the column is covered with framing and gypsum board (drywall). The framing-and-drywall covering also serves to protect the firespray from being damaged, causing it to flake off and reduce the fire protection.
That is all for now. Tell your friends at the water cooler tomorrow....