Access to a smokeproof tower should be provided in each story through vestibules open to the outside on an exterior wall, or from balconies on an exterior wall, neither exposed to severe fire hazards. Doors should be at least 40 in wide, selfclosing, and provided with a viewing window of clear, wired glass not exceeding 720 in2 in area. It also is wise to incorporate some means of opening the top of the shaft, either with a thermally operated device or with a skylight, to let escape any heat that might enter the tower from a fire. Exits at the bottom of smokeproof towers should be directly to the outdoors, where people can remove themselves quickly to a safe distance from the building. FIGURE 16.3 Typical construction for wood stairs: (a) typical stair components; (b) carriages cut to receive steps; (c) wall stringer cut to receive steps; (d ) junction of steps with a closed stringer. Stairs outside a building are acceptable as a required fire exit instead of inside stairs, if they satisfy all the requirements of inside stairs. Where enclosure of inside stairs is required, however, outside stairs should be separated from the building interior by fire-resistant walls with fire doors or fixed wire glass windows protecting openings. Some building codes limit the height of outside stairs to a maximum of six stories or 75 ft. Fire escapes, outside metal-grating stairs, and landings attached to exterior walls with unprotected openings were acceptable at one time as required exits, but are generally unacceptable for new construction. See also Art. 3.5.10. (Life Safety Code, National Fire Protection Association, Batterymarch Park,

Good to excellent None to very slight GM Silty gravels, gravel-sand-silt mixtures Good to excellent Slight to medium GC Clayey gravels, gravel-sand-clay mixtures Good Slight to medium Sand and sandy soils SW Well-graded sands or gravelly sands, litttle or no fines Good None to very slight SP Poorly graded sands or gravelly sands, little or no fines Fair to good None to very slight SM Silty sands, sand-silt mixtures Fair to good Slight to high SC Clayey sands, sand-clay mixtures Poor to fair Slight to high Fine-grained soils Silts and clays with liquid limit less than 50 ML Inorganic silts, rock flour, silts of low plasticity Poor to fair Medium to very high CL Inorganic clays of low plasticity, gravelly clays, sandy clays, etc. Poor to fair Medium to high OL Organic silts and organic clays of low plasticity Poor Medium to high Silts and clays with liquid limit greater than 50 MH Inorganic silts, micaceous silts, silts of high plasticity Poor Medium to very

Recessed light fixtures, 2 by 4 ft, set in protective boxes, occupy no more than 25% of the gross ceiling area. Air-duct openings, 30 in maximum in any direction, are spaced so as not to occupy more than 576 in2 of each 100 ft2 of gross ceiling area. They must be protected with fusible-link dampers against spread of smoke and heat. These conclusions are not always applicable. Reports of fire tests of specific floor systems should be consulted. FIGURE 7.65 Fire protection of floor framing with incombustible floor construction: (a) section showing suspended plaster ceiling; (b) attachd plaster ceiling; (c) furred plaster ceiling; (d ) suspended ceiling with lay-in, fire-rated acoustic panels; (e) detail of panel support in (d ); ( ) detail showing fire protection around recessed lighting; (g) detail showing fire protection around air-conditioning duct and grille. FIGURE 7.66 Typical fire protection with sprayed material. A serious infringement of the fire rating of a floor system could occur when pipes, conduit, or other items pierce the floor slab, a practice called poke-through. Failure to calk the openings with insulating material results in a lowering of fire ratings from hours to a few minutes. Most standard fire tests on structural-steel members and assemblies have been conducted at one of two placesthe National Institute of Standards and Technology, Washington, D.C., or the Underwriters Laboratories, Northbrook, Ill. Fire-testing laboratories also are available at Ohio State University, Columbus, Ohio, and the University of California, Berkeley, Calif. Laboratory test reports form the basis for establishing ratings. Summaries of these tests, together with tabulation of recognized ratings, are published by a number of organizations listed below. The trade associations, for the most part, limit their ratings to those constructions employing the material they represent. The American Insurance Association (formerly The National Board of Fire Underwriters), 1130 Connecticut Ave., N.W., Suite 100, Washington, DC 20036 The National Institute of Standards and Technology, 100 Bureau Drive, Administration Bldg. #101, Mailstop 4701, Gaithersburg, MD 20899 Gypsum Association, 810 First St., N.E., #510, Washington, DC 20002 Metal Lath/ Steel Framing Association, 600 Federal St., Chicago, IL 60605 Perlite Institute, 88 New Dorp Plaza, Staten Island, NY 10306-2994 American Iron and Steel Institute, 1000 16th St., N.W., Washington, DC 20036 American Institute of Steel Construction, One E. Wacker Dr., Chicago, IL Designing Fire Protection for Steel Columns; Designing Steel Protection for Steel Trusses; Fire-Safe Structural Steel, American Iron and Steel Institute, 1101 17th Street, N.W., Washington, Design GuideIron and Steel Buildings, 18731952; Guide to Shop Painting of Structural Steel; Structural Steel Detailing, American Institute of Steel Construction, OneWacker Drive,