Structured steel may be protected with any of many materials—brick, stone, concrete, gypsumboard, gypsum block, sprayed-on mineral fibers, and various fireresistant plasters.
Concrete insulation serves well for column protection, in that it gives additional stability to the steel section. Also, it is useful where abrasion resistance is needed.
Concrete, however, is not an efficient insulating medium compared with fireresistant plasters. Normally, it is placed completely around the columns, beams, or girders, with all reentrant spaces filled solid (Fig. 7.64a). Although this procedure contributes to the stability of columns and effects composite action in beams and slabs, it has the disadvantage of imposing great weight on the steel frame and foundations. For instance, full protection of a W12 column with stone concrete weighs about 355 psf, whereas plaster protection weighs about 40 psf, and lightweight concretes made with such aggregates as perlite, vermiculite, expanded shale, expanded slag, pumice, pumicite and sintered flyash weigh less than 100 psf.
Considerable progress has been made in the use of lightweight plasters with aggregates possessing good insulating properties. Two aggregates used extensively are perlite and vermiculite. They replace sand in the sanded-gypsum plaster mix.
A 1-in thickness weighs about 4 psf, whereas the same thickness of sanded-gypsum plaster weighs about 10 psf.
Typical details of lightweight plaster protection for columns are shown in Fig. 7.64b and c. Generally, vermiculite and perlite plastic thicknesses of 1 to 13⁄4 in afford protection of 3 and 4 h, depending on construction details. Good alternatives include gypsum board (Fig. 7.64d and e) or gypsum block (Fig. 7.64ƒ).
For buildings where rough usage is expected, a hard, dense insulating material such as concrete, brick, or tile would be the logical selection for fire protection.
For many buildings, finished ceilings are mandatory. It is therefore logical to employ the ceiling for protecting roof and floor framing. All types of gypsum plasters are used extensively for this dual purpose. Figure 7.65 illustrates typical installations. For 2-h floors, ordinary sand-gypsum plaster 3⁄4 in thick is sufficient.
Three- and four-hour floors may be obtained with perlite gypsum and vermiculite gypsum in the thickness range of 3⁄4 to 1 in.
Instead of plastered ceilings, use may be made of fire-rated dry ceilings, acoustic tiles, or drop (lay-in) panels (Fig. 7.65d and e).
Another alternative is to spray the structural steel mechanically (where it is not protected with concrete) with plasters of gypsum, perlite, or vermiculite, proprietary cementitious mixtures, or mineral fibers not deemed a health hazard during spraying (Fig. 7.66). In such cases, the fire-resistance rating of the structural system is independent of the ceiling. Therefore, the ceiling need not be of fire-rated construction.
Drop panels, if used, need not be secured to their suspended supports.
Still another sprayed-on material is the intumescent fire-retardant coating, essentially a paint. Tested in conformance with ASTM Specification E119, a 3⁄16-inthick coat applied to a steel column has been rated 1 h, a 1⁄2-in-thick coating 2 h.
As applied, the coating has a hard, durable finish, but at high temperatures, it puffs to many times its original thickness, thus forming an effective insulating blanket.
Thus, it serves the dual need for excellent appearance and fire protection.
Aside from dual functioning of ceiling materials, the partitions, walls, etc., being of incombustible material, also protect the structural steel, often with no additional assistance. Fireproofing costs, therefore, may be made a relatively minor expense in the overall costs of a building through dual use of materials.