Protecting The Structure

The structure of a building is protected to prevent collapse of the building within the time the fire runs its course or to delay the collapse of low buildings until all occupants have escaped and firefighters have had a reasonable chance to save the building. The building may survive to be salvaged rather than being demolished after the fire. Protecting the structure maintains the value of the building. It protects the occupants, firefighters, and neighboring buildings. Tall buildings present a significant danger if all or part of the building falls.

The most important elements of the structure to be protected are the columns (Fig. 42-1). Next in importance are the girders and the beams, and lastly the floor slabs. Most large buildings are constructed of either reinforced concrete or protected steel. Steel doesn't burn but loses much of its structural strength in a fire and will sag or collapse at the sustained temperatures frequently reached by ordinary building fires. Steel reinforcing bars in concrete beams and columns are buried a specific distance within the mass of the concrete and are protected by its thermal mass and natural fire-resistant properties.

Brick, tile, and mineral fibers that are unaffected by fire can be used to protect the building structure. Brick and tile are made using the intense heat of a kiln, and are not weakened by fire. However, mortar joints may disintegrate, and the construction can fail.

Concrete is more resistant to fire than steel. Because concrete and plaster are largely made up of hydrated crystals, they will absorb very large quantities of heat during a fire before the water of crystallization is evaporated. Consequently, concrete and plaster provide a barrier to the fire as they slowly disintegrate. If the fire lasts long enough, though, concrete or plaster will not be able to prevent serious structural damage.

Both solid brick and poured concrete are expensive and add to the structural load and the cost of the building's structure. Today, steel beams and columns are encased in lath and plaster and surrounded with multiple layers of gypsum wallboard (drywall). They are also sometimes sprayed with lightweight mineral insulations in cementitious binders or have preformed slabs of mineral insulation attached to them.

Spray-on insulation

Figure 42-1 Protecting columns from fire.

Spray-on insulation

Figure 42-1 Protecting columns from fire.

In the past, asbestos was used to insulate steel columns. We now know that loose asbestos fibers can get into our lungs. Other forms of spray-on materials or slabs of mineral insulation are now available. Intumescent coatings, in the form of paint or a thick coating that is put on with a trowel, soften when exposed to heat and release bubbles of a gas that expands the coating to create an insulating layer.

Low industrial and commercial buildings of unprotected steel are considered to be incombustible, but there is an unlikely possibility that they may collapse rapidly in a hot fire before occupants have time to escape. Buildings constructed of heavy timber are considered to be slow burning buildings, and are permitted to be one to two stories higher than unprotected steel buildings. Plaster or plasterboard walls and ceilings offer one-half hour of protection for smaller wooden buildings.

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