How Steel Deck Is Shaking Up Cold-Formed Steel Framing Design
All too often, we get stuck in the mindset that the way we currently do things is the way we must continue to do them. An example that hits close to home is the way steel deck is underutilized by designers in the low- and mid-rise construction market. Current practice favors the use of plywood or oriented strand board (OSB) sheathing on cold-formed steel (CFS) floor and roof trusses, because that’s how things have been done for years. But why not shake things up and consider using steel deck on cold-formed steel trusses?
New, user-friendly technical resources now are available for architects, engineers and designers interested in using steel deck for their projects. The “SDI Steel Deck on Cold-Formed Steel Framing Design Manual” provides comprehensive information on how to use steel deck as sheathing on CFS trusses, an ideal solution in multi-family construction. The manual includes applications for steel floor and roof deck, and it provides design examples, diaphragm design tables for common applications, fastener properties and useful commentary. It also takes into account common proprietary fasteners and CFS members, and it’s compatible with the 2018 International Building Code. The manual is available for free download at www.sdi.org or a minimal cost for a printed copy.
The Steel Deck Institute (SDI) also developed the SDI Diaphragm Interaction Calculator available as a free web-based tool (also at www.sdi.org; see accompanying graphics for examples) that provides diaphragm design tables which include wind uplift and diaphragm shear for steel roof decks. This tool saves significant time for designers and allows them to utilize framing as thin as 20 gauge (33 mil).
Steel deck has many advantages over plywood or OSB on CFS trusses, including the following:
• Less Material, Lower Cost. Designers can double or even triple the distance between trusses. Instead of the maximum 2-foot-on-center model for plywood or OSB sheathing, steel deck can support truss spacing of 4- or even 6-foot-on-center. The resulting reduction in materials cost makes steel deck assemblies highly cost-competitive with plywood and OSB sheathing.
• Noncombustibility. Steel deck is inherently noncombustible, a clear advantage over wood sheathing products. In fact, its inherent noncombustibility often leads to lower builder’s risk insurance costs because of dramatically lower fire risk during construction.
• Reduced Construction Costs. At the jobsite, steel deck on CFS framing is faster to install than plywood and OSB sheathing. In addition, there’s less need for onsite labor, because steel decking is available in a variety of dimensions (up to 3- by 40-foot) vs. standard plywood and OSB 4- by 8-foot board. Workers can load collated screw packages into screw guns and use them from a standing position, dramatically reducing installation time and associated labor costs. Both of these attributes work to minimize the construction cost component in the total project cost.
• Virtually No Construction Waste. Steel products are typically fabricated to precise dimensions, reducing waste at the job site. Any steel scrap created has substantial recycling value, which keeps it out of a landfill, unlike wood construction waste.
• More-Resilient Buildings. Use of galvanized steel deck on CFS dramatically reduces maintenance and repair costs for situations such as storm damage or roof leaks. Steel is durable—even the screw fasteners for steel deck provide more strength in wind uplift situations than the straight nails used in wood applications, leading to the use of less fasteners to carry the same load.
So with the benefits of steel deck established and the tools in place to make steel deck design easier for architects and engineers, what’s the next step for this versatile material?
Seismic Testing and Research
The next step for advancing the use of steel deck in mid-rise building construction is to evaluate its seismic performance. SDI, the American Iron and Steel Institute (AISI), and the Steel Framing Industry Association are working with Dr. Kara Peterman, assistant professor at the University of Massachusetts Amherst, on a multi-year project to conduct seismic testing of steel deck on cold-formed steel framing.
The research still is in the planning phase, with Dr. Peterman currently conducting analytical modeling to determine how test specimens should be put together. Feedback from the framing industry is being solicited to implement the best possible framing system for the models.
Initially, the focus will be on standard cantilever testing for floors (and potentially roofs), followed by testing of a small prototype building on the shake table at the University of California at San Diego. This research will examine the performance of a “dry” floor system using fiber cement board on top of steel deck in lieu of the current practice of using “wet” concrete cast on top of steel deck. Further testing will evaluate if the ”dry” floor system would benefit from a thin layer of leveling material on top.
The test will attempt to measure if 1) steel deck with screw fastening to steel framing provides a stronger and more-ductile response than wood sheathing under seismic loading, and 2) steel deck on cold-formed steel framing, when covered with fiber cement board, leads to superior stiffness for vertical deflection and vibration.
A major advantage of this construction is the ability to easily panelize the “dry” floor system. Although the industry has been panelizing walls for years, it’s now seeking to do the same with floors. In fact, a system of steel deck on cold-formed steel framing could be panelized into 8-foot-wide modules where the diaphragm connections are made onsite. The panelized system would work with both platform and ledger framing. The validation of this system would significantly advance the use of cold-formed steel for this application.
Consider, for instance, that a panelized floor system with floor framing at 4-foot-on-center could be married with a wall system that also has load-bearing members at 4-foot-on-center. The 4-foot wall system would greatly reduce the number of headers needed in the walls, resulting in cost savings on wall systems. Preliminary estimates indicate a potential reduction of 30 percent in steel framing weight when using a 4-foot module system, as compared to the typical 2-foot module system. In addition, using steel deck with a “dry” floor system would speed up construction, further reducing overall construction costs.
Additional Steel Deck Testing
The results of seismic research conducted by Dr. Kara Peterman will benefit a companion project undertaken by AISI to compare the performance of steel deck to fiber cement board and OSB in terms of seismic and fire resistance. The project’s objective is to explore in sharper focus the interaction between the diaphragm in cold-formed steel buildings and other variables currently not accounted for in design: shear wall location, presence or absence of an opening, and the structural system itself. Although there has been a large swath of monotonic and cyclic cantilever diaphragm tests (in which diaphragms are fixed on one end and loaded on the other), there’s a need to examine these isolated systems under seismic loading.
Additional steel deck research projects are being conducted under the Steel Diaphragm Innovation Initiative (SDII) being managed by the Cold-Formed Steel Research Consortium (CFSRC). SDII is a multi-year industry-academic partnership to advance the seismic performance of steel deck floor and roof diaphragms utilized in steel-framed buildings through better understanding of diaphragm-structure interaction, new design approaches, and new 3D modeling tools that provide enhanced capabilities to designers utilizing steel deck diaphragms in their building systems. More information is available at www.steeli.org.
For technical information and educational tools for architects, engineers and designers interested in incorporating steel deck into their projects, contact SDI at www.sdi.org.
Steel Deck Solutions
With increasing demands on their time, architects, engineers and designers are seeking more durable, creative and cost-effective solutions for their projects. In the low- and mid-rise construction markets, the benefits of using steel deck are well established, including ease of construction, durability, noncombustibility, recyclability and resiliency. New, easy-to-use tools such as the SDI Design Manual and Diaphragm Interaction Calculator now are available as time-saving resources. And the performance of steel deck in seismic situations soon will be explored, expanding the body of knowledge on this versatile material. As design professionals seek innovative solutions for their projects, steel deck on cold-formed steel trusses is a viable and cost-effective option.