by Don Allen, PE, LEED AP, and Robert Wills, PE
Cold-Formed Steel (Cfs) Has Been Available In Construction For More Than A Century, But Some Specifiers, Architects, Engineers, And Other Building Professionals Do Not Yet Understand The Full Range Of The Material'S Benefits And Applications. Lightweight, Strong, Noncombustible, And Easy To Install, It Is Versatile Enough To Address Many Common Obstacles (Such As Code, Time, And Cost Restrictions) Faced By Specifiers And Other Building Professionals.
When using CFS in a construction project, the material will have the same properties throughout the entire application. Steel is an isotropic material, therefore offering widespread consistency across construction products. Unlike alternative, bio-based materials like wood, cold-formed steel will not crack, warp, or change dimensions with moisture content, and does not have knots or other inconsistencies. This regularity allows manufacturers to create products fitting very tight tolerances, as they do not have to worry about products bowing or varying from other pieces.
In addition to material consistency, the logistical benefits of cold-formed steel extend from the manufacturing process to installation on the jobsite. Many CFS products are manufactured to suit specifications ensuring they fit seamlessly within a building. Manufacturers can pre-cut members to exact lengths-frequently within 1.6 mm (l/16 in.). This reduces waste both during production and on the construction site.
Once materials arrive at the jobsite, this combination of consistency and tight manufacturing tolerances allows builders to safely and easily construct building modules before installing them. The strength and stiffness of the material enables the construction of whole sections or systems (e.g. roof assemblies) on the ground. Such systems can include bracing, mechanical, plumbing, and sprinkler components, decreasing the amount of work required at high elevation, which in turn increases efficiency and reduces fall hazards. Offsite panelization of coldformed steel wall and floor systems, when deemed appropriate for a project by the design team, can also reduce onsite labor costs and improve overall project cycle times.'
The versatility of cold-formed steel allows specifiers and architects to develop mid-rise, multi-story buildings safely and efficiently, taking advantage of members with different thicknesses and yield strengths to distribute load effectively. For example, ledger framing can be implemented by stacking cold-formed steel-framed walls directly atop one another, eliminating the need to transfer roof loads through floor framing.
Cold-formed steel framing allows building professionals to change spacing, thickness, and yield strength of studs between floors for greater flexibility using load distribution members. As cold-formed steel is stronger than alternative materials, members can be placed farther apart within walls-they can be installed on 609-mm (24-in.) centers rather than the traditional 406 mm (16 in.). Beginning with thinner, more widely spaced members with lower yield strength in upper floors and then transitioning to thicker, more tightly spaced members in lower floors allows a more economical structure to be built, optimizing the steel for the applied loads. Having many light columns and beams rather than widely spaced heavy ones also leads to greater building resiliency, as members can pick up loads if adjacent ones are damaged. In the case of an emergency or natural disaster, these alternate load paths improve occupant safety and enhance resistance to disproportionate collapse.
In building design, shear walls are composed of braced panels that counter the effects of wind and seismic loads acting on a structure. For shear-wall applications, cold-formed steel can be used with diagonal-strap bracing or thin solid-steel sheathing to economically support the building in earthquakes and windstorms. When improperly detailed or specified, issues arise in these walls-high-profile screws used with thick straps and gusset plates can lead to uneven finishes. This problem can be prevented by using sheet-steel sheathing with low profile head screws, or by using thinner, wider straps of steel with welds to eliminate gusset plates.
Some specifications require all exterior wall cavities to be filled with insulation, which can prove difficult when studs are tightly spaced within the wall, especially at jambs and posts supporting concentrated loads. The strength and versatility of CFS studs allows them to be more widely spaced, and using larger, thicker members at posts and jambs reduces the number of studs. With fewer studs at concentrated loads and greater gaps between existing studs, the number of thermal bridges are reduced, and spaces in walls are easier to insulate. Prepanelized, cold-formed steel walls enables manufacturers to tightly control fabrication quality, and can equip members to help support adjacent studs laterally, leaving more room for insulation.
Building Information Modeling's Proper Implementation
As the construction industry moves toward building information modeling (BIM) for project planning and management, building professionals are looking for products and materials that can be efficiently integrated into BIM. Since cold-formed steel panels and prefabricated components are typically created as computer models before they are manufactured, they make this process easy. Further, the material's inherent uniformity allows building professionals to understand, with a great deal of accuracy, the attributes of cold-formed steel products and how those products will interact with other building systems.
Many manufacturers have developed BIM libraries incorporating attributes such as yield strength and load capacity into the model. Panel and component manufacturers have developed various framing methodologies that seamlessly dovetail into the building model, allowing layouts that consider the needs of other trades using the same wall, roof, and floor spaces both in the model and in the building. Given the experience level of most CFS component detailers, many systems for wall, roof, and floor assemblies have designed-in areas such as wall chases, large openings in floor joists, and header configurations that allow room for other systems such as HVAC, electrical, plumbing, and sprinklers. This leads to less time spent during the clash-detection phases of building modeling, and more on optimizing the materials and systems.
Specifying for Current Standards
When using cold-formed steel products, specifiers need to ensure they work within the most up-to-date specifications for all applications. If professionals employ past projects to inform current decisions, they run the risk of including coldformed steel standards no longer referenced in the latest building codes.
Outdated standards such as ASTM A525, Specification for General Requirements for Steel Sheet, Zinc-coated (Galvanized) by the Hot-dip Process, or ASTM A446, Specification for Steel Sheet, Zinc-coated (Galvanized) by the Hot-dip Process, Structural
(Physical) Quality, should be avoided. Instead, specifiers should reference ASTM Al003/Al003M, Standard Specification for Steel Sheet, Carbon, Metallic- and Nonmetallic-coated for Coldformed Framing Members, and ASTM A653/A653M, Standard Specification for Steel Sheet, Zinc-coated (Galvanized) or Zinciron Alloy-coated (Galveannealed) by the Hot-dip Process. Guidance on the proper referencing of these ASTM standards is in Cold-formed Steel Engineers Institute (CFSEI) Technical Notes GS00-12 and G801-13.
Durability & Sustainability
In structural applications, the minimum galvanized coating required for steel is 2180 (i.e. G60, or 180 g of zinc per m2 of steel [0.6 oz per sf]). In framing applications, industry studies have shown coatings used for steel are predicted to last for more than 100 years.2 Applications where there are concerns about moisture wetting the wall or floor cavity, or more corrosive environments such as the windward shore in coastal areas, call for heavier coatings such as Zl75 (G90 or CP-90). Technical notes on corrosion protection for fasteners and cold-formed steel framing in both coastal and more temperate climates are available from CFSEI.
As the significant impact that buildings and their construction have on the environment becomes dearer, consumers, businesses, and building owners around the world have mandated more sustainable and efficient products and systems from the construction industry. To meet this demand, the North American steel industry has made significant progress in increasing the efficiency and lessening the environmental impact of steel production over the years. With cold-formed steel, designers can develop more creative buildings while using less material. The increased strength, durability, and resilience of this material give architects and engineers the freedom to create structures that use fewer resources and have less environmental impact while maintaining building integrity. The versatility of CFS systems allows for a wide variety of shapes and configurations, including curved and flowing walls, ceilings, and roofs.
Steel's complete recyclability also allows steel construction products and systems to be sustainably managed at the end of a building's life cycle. Steel construction products contain between 25 and 100 percent recycled content and are fully recyclable at end of life. Where materials such as wood or plastics are often disposed of in landfills or downcycled for fuel or mulch, cold-formed steel in buildings can be collected and recycled as another steel construction product or any of a range of other products. Specifiers should require a minimum recycled content of 25 percent for steel framing products and accessories to help achieve overall building recycled content targets.
Project teams pursuing certification in green building rating systems like the U.S. Green Building Council's (USGBC's) Leadership in Energy and Environmental Design (LEED) find additional benefits in using cold-formed steel. ln the latest edition, LEED v4, building material credits are available in areas including life cycle assessment (LCA), environmental product declarations (EPDs), and improved product transparency. Steel-intensive building designs can be significant contributors to earning points in these categories because of the availability of data on supply chain, raw materials, and energy use for all steel products.
Available Cold-Formed Steel Resources
There are abundant resources available to building professionals interested in learning more about how to use cold-formed steel (CFS). covering a wide range of topics from the material's general benefits and properties to assistance with very specific project applications.
- BuildSteel (www.buildsteel.org) is a newly launched collaboration of steel and construction industry organizations, which offers original and curated content from around the construction industry as well as complimentary project assistance for applications of coldformed steel framing in the United States and Canada.
- The American Iron and Steel Institute (AISI) offers a suite of design and test standards covering the testing of individual connections/members and complete systems, as well as design manuals and guides illustrating the practical application of standards at www.aisistandards.org.
- The Cold-formed Steel Engineers Institute (CFSEI) offers technical notes, case studies, webinars, seminars, and other technology transfer resources on topics such as fire, acoustic, and thermal performance. Visit www.cfsei.org.
- The Steel Framing Alliance (SFA) operates the Steel Hotline (1-800-79-STEEL), enabling engineers, architects, and builders working with cold-formed steel framing to find answers to specific project-related questions. For more information, visit www.steelframing.org/hotline.html).
- Associations of cold-formed steel product manufacturers, including the Steel Framing Industry Association (SFIA), Steel Stud Manufacturers Association (SSMA), and Certified Steel Stud Association (CSSA) can also be invaluable sources of information.
Cold-formed steel has many uses in both structural and non- structural applications in mid-rise and multi-family construction. This article focuses on some of the more technical aspects of such assemblies-for example, achieving shear transfer mechanisms in between floors-and delves into some of the related codes that need to be considered, as well as some of the resources available to help design professionals who are working with the material.
05 40 00-Cold-formed Metal Framing
B1010.10-Floor Structural Frame
B1020.10-Roof Structural Frame
B2010.20-Exterior Wall Construction
Architecture Specifications for Cold-Formed Metal Framing Division 5 and Division 9 can be found here: http://www.buysuperstud.com/architectural-specifications
Division 5: Structural Metal Framing(Specification section 05 40 00)
Super Stud is your full service source for structural cold-formed steel (CFS) products and accessories, including studs, tracks, clip angles, connectors, bridging, slip connectors, and other accessories.
Division 9: Nonstructural Metal Framing(Specification section 09 22 16)
Nonstructural products include The EDGE™ interior studs, shaft wall studs, ceiling joists and tracks, clips, connectors, and a full line of both steel and vinyl accessories for gypsum construction. Using Super Stud specifications will ensure quality products and accessories for your structure: with true Galvanized metallic coating for corrosion protection (no “EQ” coatings), and 3rd party code compliance certification through Home Innovation Research Labs, Inc.