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The New SDI Diaphragm Design Manual
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Dong Li, P.E.
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Expanding on its 1st and 2nd Editions, the Steel Deck Institute (SDI) published the 3rd edition of the Diaphragm Design Manual (DDM03) in 2004. This edition continues the design approaches presented in the previous editions, adapted to include both Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) methods and added several design examples to cover a wider variety of diaphragm applications.

DDM03 explains the method developed for calculating the design properties for diaphragms made with bare steel decks and concrete filled steel decks, provides examples on how to calculate the shear in a diaphragm, and demonstrates how to use the diaphragm load tables that have been developed as design aids.

Summary of Major Changes

DDM03 is modified for the ease of the user who is familiar either with LRFD or ASD, which represents a departure from the previous editions. The adoption of both design methods is reflected in the diaphragm design consideration, design examples and load tables. US customary and SI unit systems are used for calculations, some formulas are presented in unitless format first, then in each unit system. Design verification and examples are added for combined shear and tension on fasteners. Nominal strength values are presented in the diaphragm load tables, and must be adjusted by resistance or safety factors before comparing with the loads calculated with the corresponding design approach.

Increased Application Range

Diaphragm design tables for decks up to and including 3 inches (76mm) deep are included in Appendix V in DDM03. It is indicated in an SDI published white paper Deeper Steel Deck and Cellular Diaphragm, that diaphragm values for the strength and stiffness of cellular decks and regular decks up to and including 7.5 inches (190mm) deep can be derived using the procedure illustrated in DDM03 for regular decks of 9/16 to 3 inches deep and 0.014 to 0.064 inches thick. Diaphragm values for steel deck attached to wood structural members may be calculated using approaches similar to those in DDM03. The Metal Construction Association published A Primer on Diaphragm Design that addresses these cases.

Use of ASD and LRFD Methods

With LRFD, factored nominal strength (or design strength) is the nominal strength of applicable limit state multiplied by the corresponding resistance factor (F). Required strength is the effect of applied factored loads. With ASD, allowable strength (or design strength) is the nominal strength of applicable limit state divided by the corresponding safety factor (W). Required strength is the effect of applied service loads. For both design methods, the design strength must be greater than the required strength. The applicable limit state that controls the diaphragm strength is typically the connector failure. Plate-like shear buckling (or panel buckling) can control for steel deck diaphragm having a shallow depth, relatively long span and closely spaced fasteners. Resistance and safety factors vary with the type of load (cyclic or quasi static), type of fastener (welds or screws) and limit state (connector failure or panel buckling). The selection of the resistance and safety factor is per Table 2.1, reproduced herein, which is based on the Supplement 2004 to the North American Specification for the Design of Cold-Formed Steel Structural Members, 2001 Edition.

Filled Diaphragm

Steel diaphragms may be reinforced by an overlay of insulating concrete, structural concrete, or by directly attaching flat panels used to produce a flat surface. Such devices eliminate the panel end warping and local corner buckling so to increase diaphragm strength and stiffness; they also present additional load paths for the shear forces that are developed in the diaphragm. It must be remembered that it may be necessary to increase the number or strength of perimeter connectors in order to develop the required diaphragm strength. The combination of steel deck and the covering material can lead to increased variability in strength. DDM03 Table 5.1 lists the resistance and safety factors for a filled diaphragm under different load cases. Intermediate side-lap fasteners in a concrete filled diaphragm add little to the diaphragm shear strength once the concrete is cured. Reasonable side-lap spacing is stipulated to limit the differential deflection between adjacent panels, which may result in concrete leakage.

More Fasteners Options Available

New to DDM03, Section 4 presents the nominal shear resistance for power driven fasteners, in addition to arc spot welds and screws. The power driven fastener strength is provided for Buildex BX-14 or BX-12; Hilti ENP2 and ENPH2, ENP2K, X-EDN19, X-EDNK22; Pneutek SDK61-series, SDK63 series, K64-series. The addition of Appendix VI allows the inclusion of the new Hilti fastener X ENP-19 L15. Appendix IV and VI have the nominal shear strength and flexibility listed for these fasteners. The nominal resistance for both weld and screw shear connectors is calculated according to equations that were previously presented in the second edition of DDM.

Diaphragm Load Tables in Nominal Values with F and W

DDM03 Appendix V&VI present the load tables for the nominal values for roof, composite and form deck for common design thicknesses having specific combination of support and side-lap fastening. The upper part of the sheet shows the type of deck, fastening method, resistance and safety factors for different load cases. The limiting nominal shear strength is listed for the different fastener failure modes. The shaded values do not comply with the minimum spacing requirement for the side-lap connections and shall not be used except with properly spaced side-lap connections. Also tabulated is the nominal shear strength based on panel buckling as well as the appropriate resistance and safety factors associated with such failure.

Fasteners in Tension and Shear-Tension Interaction

Generally as a result of wind uplift pressure, roof diaphragm fasteners are subjected to tension. DDM03 Section 4 addresses the nominal tensile strength of arc-spot weld, screw and some power driven fasteners. Different resistance and safety factors are applied to the tension strength in Section 4.9.4. Appendix IV presents tables for the nominal tensile strength for commonly used fasteners. Research at West Virginia University has concluded that there is an interaction between shear force and tension force on the diaphragm fasteners. DDM03 provides a solution for shear-tension interaction for arc-spot weld, screw, Hilti power driven fastener and Pneutek fastener. Example 7 in Appendix III demonstrates the design check for shear-tension interaction, and Example 7A shows how the diaphragm shear is affected when tension is introduced. As an example, the following is the shear-tension interaction equation for arc-spot welds.

The interaction between shear and tension for arc-spot welds is described by:

Conclusion

Modifications and enhancements in the DDM03 were made to meet the designer’s needs using either ASD or LRFD design methods. DDM03 can be implemented for any consistent set of units, for example either Imperial or SI units. For more information regarding DDM03 and the design of cold-formed steel deck, see www.sdi.org.▪

Dong Li, P.E., is an engineer of Cold-Formed Products for Canam Steel Corporation at Point of Rocks, Maryland. She can be reached at dong.li@canam.ws.
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