Learn all about steel decking designed as a diaphragm at the 2024 NASCC: The Steel Conference.

Steel deck diaphragms can be an efficient and cost-effective way of resisting lateral loading on a structure because the deck can do so while supporting gravity and uplift loading at the same time. Furthermore, the Steel Deck Institute (SDI) continues to provide resources that make designing with steel deck easier. Many of those will be discussed during our presentation at 2024 NASCC: The Steel Conference.

The presentation will provide an overview of the design methodology outlined in the SDI Diaphragm Design Manual (DDM4) and review common conditions designers often encounter when using steel decking. It will start with an overview of the design standards that are considered for steel deck diaphragms and SDI’s work to provide designers with additional resources. Various limit states ultimately determine the diaphragm capacity of a steel deck system.  

Roof diaphragm limit states fall under two categories: the strength of the connections and buckling capacity of the deck panel itself. The connection limit state is the minimum strength of the interior panel connection, the corner connection, the parallel edge connection, and the exterior support connection. The panel buckling limit state is the minimum of the buckling capacity of the deck sheet and the local web buckling of the exterior support. The connection limit state and panel buckling limit state are subject to different resistance and safety factors in both wind and seismic design. Each of these limit states will be explained in further detail.

For steel deck with concrete, the concrete offers significant contribution to the lateral resisting capacity, and the presentation walks through how to consider the concrete in the design phase. The fasteners are an essential component of the floor diaphragm system, so a basic overview of fastener strengths and the corresponding design equations are also covered, including the interaction of shear and uplift. Moreover, developing steel deck diaphragms over light gage framing has been a common topic for designers and will be discussed. The presentation will also introduce the SDI Steel Deck on Cold Formed Steel Framing Design Manual (CFDDM).

The design and usage of steel deck on cold formed steel framing shares similarities with deck supported by open web steel joists or rolled beams, although notable differences do exist. CFDDM highlights these differences and offers essential design guidance for proper utilization of steel deck on cold formed steel framing applications. More on CFDDM will be addressed during the presentation.

The balance of the presentation addresses common questions designers run into when designing diaphragms with steel deck. Gage metal accessories are often an essential part of a complete steel deck installation but must be considered in the lateral load path. Acoustical deck offers a great solution for reducing echo in spaces, but additional design considerations for lateral capacity are required due to the perforations in the webs of the steel deck. SDI’s Perforated Metal Deck Design with Commentary (Tech Note 15) and Perforated Metal Deck Diaphragm Design (Tech Note 16) go hand in hand for determining lateral capacities for perforated steel deck panels. Provisions for this application are found in AISI S310.

Openings are commonplace in roof and floor deck installations, so reviewing how the load is transferred around the openings is essential. SDI’s Manual of Construction (MOC3) includes guidance on how to correctly address openings in roof deck and floor deck applications. Per MOC3, for most 1.5 in. roof decks the loss of one rib per sheet, either by severe denting or penetration, may be tolerated. No reinforcing may be required for an opening of 6 in. as long as not more than two webs are removed. For holes or dents larger than 6 in., the use of a plate as reinforcement is required. Per MOC3, for openings in floor deck, concrete stops are required at all openings. Steel deck should be inspected for adequate attachment at supports and at sidelaps. Sidelaps must be tightly connected to prevent opening during concrete pouring. More details will be discussed on openings in steel deck during the presentation.  

Additionally, not every structure is a perfect rectangle which results in design considerations such as sloped roofs, hip roofs, or skewed areas. SDI’s Diaphragm Analysis with Skew Walls (Tech Note 18) is another resource available to engineers. The majority of diaphragm design examples are presented on rectangular buildings, however, many building contain skewed walls with or without vertical appendages. “No vertical appendages” include buildings with angles other than 90 degrees between the shear wall and diaphragm chords. The presentation will provide methods to analyze such buildings. These are just some of the design challenges addressed by SDI that will be discussed.

Lastly, it is not only the strength of the diaphragm that needs to be considered, but also the lateral deflection of the structure. The presentation concludes with several topics around computing shear deflection, including both balanced and unbalanced diaphragms. It is important to mention that while balanced diaphragms are often preferred for their predictable and symmetrical behavior, certain structural considerations may lead to the intentional design of unbalanced diaphragms. Designers must analyze and address the consequences of such designs to ensure the stability and performance of the structure.  

Additional insight on how the SDI continues to expand on its efforts to provide resources to designers and users of steel deck will be shared. In fact, SDI launched a new website (www.sdi.org) in September 2023 that allows for easier navigation through available Codes & Standards, Design Manuals, and Technical Notes. We also provide updates on SDI news and activities, include blog posts about common questions on steel deck, and past and current research projects. An expanded focus on research and collaboration with other trade organizations has allowed SDI to continue exploring design challenges and to enhance the understanding and ease of steel deck design.

Want to learn more? Attend SDI’s presentation at NASCC 2024: The Steel Conference.

This article is a preview of the 2024 NASCC: The Steel Conference session “Everything You Wanted to Know About Diaphragms (But Were Afraid to Ask).” To learn more about this session and others, and to register for the conference, visit www.aisc.org/nascc. The conference takes place March 20-22 in San Antonio, Texas.

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