Various types of steel roof deck are available and may be classified in accordance with recommendations of the Steel Deck Institute (SDI). All types consist of long, narrow sections with longitudinal ribs at least 11⁄2 in deep and spaced about 6 in on centers (Fig. 8.14). Other rib dimensions are shown in Fig. 8.14a to c for some standard styles.
Types of Steel Roof Deck
Steel roof deck is commonly available in 24- and 30-in covering widths, but sometimes in 18- and 36-in widths, depending on the manufacturer. Thickness of steel commonly used is 0.048 or 0.036 in, but most building codes permit 0.030-in-thick steel to be used. Figure 8.14d and e shows full-width decking in cross section.
Usual spans, which may be simple, two-span continuous, or three-span continuous, range from 4 to 10 ft. The SDI ‘‘Design Manual for Composite Decks, Form Decks, Roof Decks and Cellular Deck Floor Systems with Electrical Distribution’’ gives allowable total uniform loading (dead and live), lb / ft^2, for various steel thicknesses, spans, and rib widths.
Some manufacturers make special long-span roof-deck sections, such as the 3- in-deep, Type N roof deck shown in Fig. 8.15, in 24- to 16-ga black and galvanized.
Load-Carrying Capacity of Steel Roof Deck
The Steel Deck Institute has adopted a set of basic design specifications, with limits on rib dimensions, as shown in Fig. 8.14a to c, and publishes allowable uniform loading tables for narrow-, intermediate-, and wide-rib steel roof deck (Table 8.13, for example). These tables are based on section moduli and moments of inertia computed with effective-width procedures stipulated in the AISI ‘‘Specification for the Design of Cold-Formed Steel Structural Members’’ (Art. 8.8). SDI has banned compression flange widths otherwise assumed to be effective. Moreover, SDI ‘‘Basic Design Specifications’’ recommends the following:
Moment and Deflection Coefficients. Where steel roof decks are welded to supports, a moment coefficient of 1⁄10 (applied to WL) should be used for three or more spans. Deflection coefficients of 0.0054 and 0.0069 (applied to WL3 /EI) should be used for two span and three span, respectively. All other steel roof-deck installations should be designed as simple spans, with moment and deflection coefficients 1⁄8 and 5⁄384, respectively. (W total uniform load, L span, E modulus of elasticity, I moment of inertia.)
Maximum Deflections. The deflection under live load should not exceed 1⁄240 of the clear span, center to center of supports. (Suspended ceiling, lighting fixtures, ducts or other utilities should not be supported by the roof deck.)
Anchorage. Steel roof deck should be anchored to the supporting framework to resist the following uplifts:
45 lb / ft^2 for eave overhang
30 lb / ft^2 for all other roof areas
The dead load of the roof-deck construction may be deducted from the above uplife forces.
Diaphragm Action of Decks
In addition to their normal function as roof panels under gravity loading, steel roof deck assemblies can be used as shear diaphragms under lateral loads, such as wind and seismic forces. When steel roof deck is used for these purposes, special attention should be paid to connections between panels and attachments of panels to building frames. For design purposes, see SDI ‘‘Diaphragm Design Manual.’’
Details and Accessories of Steel Roof Deck
In addition to the use of nesting or upstanding seams, most roof-deck sections are designed so that ends can be lapped shingle fashion.
Special ridge, valley, eave, and cant strips are provided by roof-deck manufacturers (Fig. 8.16).
Roof decks are commonly arc welded to structural steel supports with puddle welds at least 1⁄4 in in diameter or with elongated welds of equal perimeter. Electrodes should be selected for amperage adjusted to fuse all layers of steel roof decking to supporting members without creating blowholes around the welds.Welding washers are recommended for thicknesses less than 0.028 in.
Fillet welds at least 1 in long should be used to connect lapped edges of roof deck.
Tapping screws are an alternative means of attaching steel roof deck to structural support members, which should be at least 1⁄16 in thick. All edge ribs and a sufficient number of interior ribs should be connected to supporting members at intervals not exceeding 18 in. When standard steel roof deck spans 5 ft or more, adjacent sheets should be fastened together at midspan with either welds or screws.
Roof Deck Insulation and Fire Resistance
Although insulation is not ordinarily supplied by the roof-deck manufacturer, it is standard practice to install 3⁄4- or 1-in-thick mineral fiberboard between roof deck and roofing. SDI further recommends that all steel decks be covered with a material of sufficient insulating value to prevent condensation under normal occupancy conditions.
Insulation should be adequately attached to the steel deck by means of adhesives or mechanical fasteners. Insulation materials should be protected from the elements at all times during storage and installation.
The UL ‘‘Fire Resistance Directory,’’ Underwriter’s Laboratories, Inc., 333 Pfingsten Rd., Northbrook, IL 60062, lists fire-resistance ratings for steel roof-deck construction. Some systems with fire ratings up to 2 h are listed in Table 8.14.