Building Þ res are not hot enough to melt steel, but are often able to weaken it sufÞ ciently to cause structural failure (Figures 11.68 and 11.69). For this reason, building codes generally limit the use of exposed steel framing to buildings of one to Þ ve stories, where escape in case of Þ re is rapid. For taller buildings, it is necessary to protect the steel frame from heat long enough for the building to be fully evacuated and the Þ re extinguished or allowed to burn out on its own.
Fireproofing (Ò Þ re protectionÓ might be a more accurate term) of steel framing was originally done by encasing steel beams and columns in brick masonr y or poured concrete (Figures 11.70 and 11.71). These heavy encasements were effective, absorbing heat into their great mass and dissipating some of it through dehydration of the mortar and concrete, but their weight added considerably to the load that the steel frame had to bear. This added, in turn, to
the weight and cost of the frame. The search for lighter-weight Þ reprooÞ ng led Þ rst to thin enclosures of metal lath and plaster around the steel members (Figures 11.70Ð 11.72). These derive their effectiveness from the large amounts of heat needed to dehydrate the water of cr ystallization from the gypsum plaster. Plasters based on lightweight aggregates such as vermiculite instead of sand have come into use to further reduce the weight and increase the thermal insulating properties of the plaster.
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Figure 11.68 An exposed steel structure following a prolonged fire in the highly combustible contents of a warehouse. (Courtesy of National Fire Protection Association, Quincy, Massachusetts) |
TodayÕs designers can also choose from a group of Þ reproofing techniques that are lighter still. Plaster Þ reprooÞ ng has largely been replaced by beam and column enclosures made of boards or slabs of gypsum or other Þ re-resistive materials (Figures 11.70Ð 11.75). These are fastened mechanically around the steel shapes, and in the case of gypsum board Þ reprooÞ ng, they can also ser ve as the Þ nished surface on the interior of the building.
Where the Þ reprooÞ ng material need not ser ve as a Þ nished
Figure 11.70
Some methods for fireproofing steel columns. (a) Encasement in reinforced concrete. (b) Enclosure in metal lath and plaster. (c) Enclosure in multiple layers of gypsum board. (d) Spray-on fireproofing. (e) Loose insulating fill inside a sheet metal enclosure. (f) Water-filled box column made of a wide-flange shape with added steel plates.
surface, spray-applied fire-resistive materials (SFRM), commonly referred to as Ò spray-applied Þ reprooÞ ng,Ó have become the most prevalent type. These generally consist of either a Þ ber and a binder or a cementitious mixture, and are sprayed over the steel
to the required thickness (Figure
11.76). These products are available in weights of about 12 to 40 pounds per cubic foot (190Ð 640 kg/m3). The lighter materials are fragile and must be covered with Þ nish materials. The denser materials are generally more durable. All spray-applied materials act primarily by insulating the steel from high temperatures for long periods of time. They are usually the least expensive form of steel Þ reprooÞ ng. Spray-applied Þ reproofing is most commonly applied in the
Figure 11.71
Some methods for fireproofing steel beams and girders. (a) Encasement in reinforced concrete. (b) Enclosure in metal lath and plaster. (c) Rigid slab fireproofing. (d) Sprayon fireproofing. (e) Suspended plaster ceiling. (f) Flame-shielded exterior spandrel girder with spray-on fireproofing inside.
Þ eld after the steel has been erected and the connections between members are completed. It can also be applied in the fabrication shop, where controlled environmental conditions and easier access to the steel members can result in faster and more consistent-quality application.