Specifying cold-formed steel for resilient buildings

by maz_atta | April 30, 2021 12:00 pm

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by Greg Ralph

With the number of weather-and climate-related disasters in the United States on the rise, the engineering and construction industries have rightly prioritized a focus on adopting methods for designing resilient buildings able to withstand hurricanes, wildfires, flooding, and other natural events. According to the National Oceanic and Atmospheric Administration (NOAA)[2], there were 22 such events in 2020 to affect the country, with losses exceeding $1 billion each. This marks a new annual record, shattering the previous record of 16 events that occurred in 2011 and 2017.

Among the numerous strategies for designing resilient buildings, material selection is of particular importance—putting the specification community in a uniquely impactful position to help mitigate the risks associated with extreme events. A recent report from the National Institute of Building Sciences[3] (NIBS) reveals an 11-to-1 payback for pre-disaster investments that include compliance with modern building codes versus the prior generation of codes and requirements.

When it comes to framing, there is no more resilient option than cold-formed steel (CFS). By analyzing resiliency in the context of the built environment, this article will explore the various attributes of a resilient structure, and make the case for why CFS performs best in each scenario.

‘Resiliency’ defined

Put simply, resilience refers to a building’s ability to withstand, respond to, and recover rapidly from extreme events in a cost-effective manner. Of course, fortifying a structure against hurricane force winds and minimizing loss from fire present entirely unique sets of design challenges. With so many potential forces at work against a building, it is important to take a holistic approach to resilient design.

A joint report from the U.S. Department of Homeland Security (DHS) and NIBS[4] lays out five key attributes comprising resiliency: safety, security, durability, environment, and energy conservation. These attributes are presented as basic requirements for addressing natural and man-made hazards, as well as the environmental conditions of the building’s location. The building’s resilience corresponds directly to how well these attributes enable the structure to meet these demands.

Let us take a closer look at these attributes and how CFS is uniquely suited to addressing the design challenges of each.

Safety

The first attribute of a resilient building is whether it can protect occupants during a life-threatening event such as a hurricane or earthquake. CFS has a number of inherent properties that help ensure a building remains intact during and following an extreme event.

1. It is highly ductile. CFS can easily bend or stretch without breaking when force is applied to it, and later return to its original shape without losing its material properties. This gives it a higher degree of resistance to lateral loads, uplift, and gravity loading, such as those imposed on a structure by seismic or high wind events.
2. It has the highest strength-to-weight ratio of all commonly used framing materials. When CFS is formed into a C-shape, like a stud, the bends act as stiffeners and increase the strength of the steel dramatically, providing a significantly greater strength-to-weight ratio than that of dimensional lumber. This inherent strength, plus the fact cold-formed steel is such a relatively light material, also makes CFS-framed structures less susceptible to the forces of inertia that wreak havoc on buildings during seismic events.
3. CFS is non-combustible. Since it will not burn, the material is eligible for use in Type 1 buildings (constructed with concrete and protected steel) where fire-resistance standards are most stringent. According to the Steel Framing Industry Association (SFIA), both load-bearing and non-loadbearing CFS-framed assemblies are fireproof up to four hours when subjected to tests conforming to ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials[5]. Cold-formed steel has also displayed resilience against fire exposure in tests that follow rigorous National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components, protocols.
4. It is consistent. Wood and concrete have a number of variables that can affect their performance, but once a steel stud has been formed it will remain straight with no change to the thickness, width, or other dimensions, as well as strength and stiffness. Building professionals can be assured CFS framing members, produced under a third-party certification program, will arrive at the jobsite certified to comply with all standards.

Durability

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The second measure of a building material’s resilience is its durability, or how long it can be exposed to outside elements with minimal wear or damage. A key component of a resilient building is one that is designed to have a long life. Particularly in areas where atmospheric moisture, flooding, or any other inadvertent exposure to water is a threat, CFS is well-suited to help buildings stand the test of time.

Cold-formed steel is corrosion resistant, does not retain moisture, and will not harbor mold growth. When materials are underwater for any length of time, many are not salvageable when flood waters recede. CFS utilizes zinc or similar coatings to boost durability and will last hundreds of years before its corrosion resistance deteriorates. For areas prone to frequent flooding, CFS framing can be the difference between salvaging a structure or needing to completely gut and rebuild.

Of course, any level of moisture within a wall cavity can quickly become a structural and a health problem. Cladding failure or plumbing leaks can also create significant problems within a structure. The Environmental Protection Agency (EPA)[7] states there is a 24- to 48-hour window to effectively reduce the potential for mold propagation following exposure to water. That time window can be even shorter if the materials used to build the wall absorb and hold water. This is not a problem with cold-formed steel, which is inorganic and does not function as a food source for mold. Also, CFS is dimensionally stable in a moist environment and will not warp like lumber does when it gets wet, so walls and floors remain plumb and level.

Finally, cold-formed steel is one of the few building materials that is completely impervious to termites and other pests in any climate or building type. According to Orkin[8], termite damage in the United States results in more than $5 billion annually. Not only are these infestations costly, but they can also compromise the integrity of the structure and limit its ability to respond to some of the more immediate events discussed previously in this article.

Security

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Weather-related events are not the only potential threats buildings must be prepared for. Many, such as terrorist attacks, are man-made. The DHS/NIBS report considers blast resistance and ballistics as measures of building resiliency when it comes to security.

Determining a building’s level of protection against an explosive threat can be complicated. The U.S. Department of Defense’s (DoD’s) Unified Facilities Criteria (UFC) program[10] developed guidelines to minimize the threat of mass casualties in the event of a terrorist attack on federal buildings, but these guidelines have also been adopted by many private sector projects. Mitigation strategies include maximizing standoff distance, preventing building collapse, and minimizing hazardous flying debris. Recent research demonstrates[11] the overall stiffness and strength of cold-formed steel can be utilized to resist blast threats using conventional construction methods that add little cost to traditional designs.

Cold-formed steel’s performance against progressive collapse was recently demonstrated in the AIT Barracks project in Monterey, California. The 10,219-m² (110,000-sf) facility was commissioned by the U.S. Army Corps of Engineers to serve as a modernized, private-sector residence. In addition to seismic considerations, the structure was required to adhere to DoD Unified Facilities Criteria (DoD UFC) 4-010-01, Minimum Antiterrorism Standards for Buildings, requirements for blast loading applied to the exterior framing, as well as DoD UFC 4-023-03, Design of Buildings to Resist Progressive Collapse, With Change 3.

Progressive collapse is defined by the American Society of Civil Engineers (ASCE) as the spread of an initial local failure from element to element, eventually resulting in the collapse of an entire structure, or a disproportionately large part of it. To meet DoD UFC progressive collapse requirement for the exterior walls of the AIT Barracks, the project team ran the CFS joist framing from demising wall to demising wall. The end joists at the exterior wall were then designed to support the wall and roof loading above. This allowed large sections of exterior wall framing to be removed, while still providing vertical support to the structure above. Additionally, structural steel tubes were designed in line with the joist framing at three end locations where the floor and roof were supported by the stud framing, allowing for progressive collapse requirements to be met if the bearing wall below were removed.

Environment and energy conservation

In addition to offering safety, security, and durability, resilient buildings are also judged on their energy efficiency and impact on the environment. Air tightness, thermal transfer, and the use of renewable energy all play a part. Environmental impact, which is often defined through life-cycle analysis (LCA) and also encompasses issues such as acoustical performance, is another area where cold-formed steel can really make a difference.

Steel framing contains on average a minimum 25 percent recycled content and is 100 percent recyclable at the end of its life. It is highly unlikely to end up in a landfill as other framing materials might. According to the American Iron and Steel Institute (AISI)[12], nearly 70 million tons of domestic steel scrap is used each year in the production of new steel. When structures must be renovated or rebuilt after a devastating event, using a material that can be reused or recycled is beneficial from a cost, convenience, and sustainability standpoint.

Additionally, the steel industry as a whole has made great strides to reduce the environmental impact of manufacturing CFS products. Producing a ton of steel today in the United States requires less than half the energy that was needed to produce a ton of steel 40 years ago, resulting in a 50 percent reduction in greenhouse gas (GHG) emissions.[13]

Designing for a resilient future

A greater understanding of the many forces a building must be able to withstand to be truly resilient reveals why cold-formed steel is a wise investment from a material specification standpoint. Its consistent strength and durability, along with its high ductility and strength-to-weight ratio is uniquely suited to resist seismic and high wind events. The fact it is an inorganic material that will not burn, corrode, harbor mold growth, or serve as a food supply for pests further reinforce cold-formed steel’s resiliency.

As the industry continues to embrace resiliency, it is certainly worth considering how one can incorporate this versatile material into their upcoming projects.

Greg Ralph is vice-president of business development for ClarkDietrich. He is actively involved with the Association of Walls and Ceilings International (AWCI) Construction Technology Council and also serves on the Standards Council for the American Iron and Steel Institute (AISI), the AISI Committee on Framing Standards, and the AISI Committee on Specifications. Addition, he holds active memberships with the International Code Council (ICC) and the American Society for Testing and Materials (ASTM) on committees C11, A05, and E06. He can be reached at greg.ralph@clarkdietrich.com.[14]

Source URL: https://www.constructionspecifier.com/specifying-cold-formed-steel-for-resilient-buildings/

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