Steel deck

Steel deck panels have been used as part of roof and floor systems for many years, functioning structurally as supports for vertical gravity and wind loads and as diaphragms resisting horizontal wind and earthquake loads. The building codes, including the International Code Council’s (ICC) 2006 International Building Code (IBC), do not present the panels’ structural capacities, but provide references used to derive load resistance. Steel deck panels are produced from thin steel sheets, described as cold-formed steel in the codes. Section 2209 of the 2006 IBC refers to the AISI/COS/NASPEC 2001, North American Specification for the Design of Cold-Formed Steel Structural Members, including 2004 Supplement (AISI-NAS) for carbon or low-alloy steel and ASCE 8-02, Specifications for Design of Cold-Formed Stainless Steel Structural Members for stainless steel. Most steel deck panels are produced from carbon steel. AISI-NAS encompasses allowable stress design and load and resistance factor design methods for determining the nominal and design capacities of cold-formed steel.

Steel deck panels have a "corrugated" profile and are produced in a number of types, depending on the application: roof, composite floor, non-composite floor, and cellular deck. Roof decks are formed with alternating top and bottom flutes in various configurations and finishes. The top surface is usually flat to support roofing materials. The vertical webs may be perforated for improved acoustical characteristics. Composite floor decks are formed with alternating top and bottom flutes in various configurations, with embossments in the vertical and zinc-coated finishes. The decks are filled with structural concrete and resisting loads compositely, with the embossments interlocking to the concrete. Non-composite floor decks are formed from high-strength steel and are shallower than roof or composite deck. The decks function as a form for normal-weight, structural lightweight or insulating concrete. Cellular decks consist of formed panels resistance-welded to flat sheets underneath and may be used for either floor or roof applications. Available for installations where a flat ceiling surface is desired, the space between provides passageways for cables, piping, etc.

In order to design with a steel deck, one needs to know the structural properties. As cold-formed steel products possess relatively high length-to-thickness ratios for each element, such as the vertical web and horizontal flange or flute in steel deck, the deck may be subject not only to global buckling in compression, but localized buckling within the individual web or flute. AISI-NAS limits the effectiveness of cold-formed steel by imposing length-to-thickness limits. These limits result in a reduction in the element length permitted for design, which in turn produce net section properties, such as cross-sectional area, section modulus, and moment of inertia. AISI NAS also requires design for computing web-crippling effects, where the vertical web may buckle due to the high length-to-width ratio of the web and the bearing length of the support for the deck. Greater bearing lengths distribute the bearing stresses and reduce the likelihood for buckling. AISI-NAS also describes fastening methods for cold-formed steel, which include bolts, welds, and screws.

In accordance with IBC Section 2209.1, composite decks need to comply with ANSI/ASCE 3-91, Standard for the Structural Design of Composite Slabs (ASCE 3). ASCE 3 describes analytical and testing procedures to be implemented to qualify concrete-filled steel deck assemblies for vertical load-carrying capacities. In some cases, the deck needs to be shored to support plastic concrete until curing and strength development occur. The shoring may reduce the composite capacity that otherwise would be derived. Another reference is the Steel Deck Institute’s Composite Steel Deck Design Manual No. CDD2, which reports design loads for commonly available deck profiles.

Diaphragm design incorporating steel deck, with or without concrete fill, is not shown in the codes. Diaphragm strength and stiffness are influenced not only by steel deck panels, but by the connections of panels to supports and between panels. Two resources provide information on commonly available deck types: TM 5-809-10, Seismic Design for Buildings, published by the Departments of the Army, Navy, and Air Force (aka Tri-Services Manual) and the Steel Deck Institute’s Diaphragm Design Manual No. DDM03. For unique deck configurations or proprietary fastenings outside the scope of these documents, special testing is needed to derive the structural characteristics of such assemblages.

The means to derive structural data for steel deck floor and roof systems is fairly complex, detailed, and lengthy. Structural engineers would not only need to fully comprehend the reference material, they would also need to develop proper analytical procedures, including computer-abased analysis, and testing programs. Steel deck roof and floor systems are evaluated by ICC Evaluation Service, Inc. (ICC-ES), a subsidiary of the ICC. Results of the ICC-ES evaluation are published in an Evaluation Service Report (ESR). ICC-ES bases these evaluations on the ICC-ES Acceptance Criteria for Steel Deck Roof and Floor Systems (AC43). AC43 outlines the data requirements for utilizing steel deck structurally, including structural analysis, test procedures and conditions of acceptance, manufacturing specifications, and quality control during manufacture. The results of an evaluation according to AC43 are used to develop an ICC-ES ESR, which offers building officials an independent resource for accepting the systems; structural engineers also can use the report as a resource for preparing a design.

ICC-ES has issued the following evaluation reports on anchors in accordance with AC43: ESR-1116 to Wheeling Corrugating Company on roof decks, high-strength roof decks, cellular decks, composite decks, and high-strength form decks; and ESR- 1414 to ASC Profiles on roof decks, cellular decks, deep cellular decks, composite decks, and high-strength form decks. The evaluation reports include structural properties such as base steel thicknesses, coating descriptions, yield and tensile strengths, and section properties. For floor deck, composite vertical loads are included. For both roof deck and composite floor deck, diaphragm capacities are summarized. For diaphragms, the evaluation reports also include guidelines for diaphragm size based on stiffness, and also indicate limits relative to concrete or masonry wall supported by diaphragms, as shown in Table 1.

The evaluation reports also may describe fire-resistance-rated floor and roof assemblies incorporating steel deck. Quality control during manufacture is administered with oversight by ICC-ES. Special inspection guidelines during installation, which applies to welding and concrete placement, are also described in the evaluation reports.

Brian Gerber, S.E., is the principal structural engineer for ICC Evaluation Service, Inc., in Whittier, Calif. He may be contacted at bgerber@icc-es.org. Additional information regarding ICC-ES and its product evaluation program is available at www.icc-es.org.