How tall can you build with cold-formed steel (CFS)?

by | Jun 20, 2024 | Structural, Thought Leadership

The answer is not so simple; there are two main components limiting the height of CFS structures. The first is related to the vertical (gravity) loads on the structure. This affects the axial capacity of the CFS bearing wall studs. The second factor is the lateral (wind or seismic) load resisting system for the structure. Both of these considerations are affected by several different factors including the weight of the floor system used for the structure. Heavier floor structures will max out the load bearing capacity of the walls studs faster than a lighter floor system, while lighter floor systems may max out due to the lateral capabilities of the structure.

Concrete Structures and Gypcrete Structures

Typically, there are two main types of structures: concrete structures and gypcrete structures. There are different types of concrete systems, including composite decks (decks that span without joists structure) and non-composite decks with long spanning joists. Joist systems can usually span further than composite decks, but the total weight on the load-bearing walls is relatively similar. Gypcrete structures also use joist systems. Concrete structures are heavier than gypcrete structures and can limit the height of the structure based on vertical loads, while gypcrete systems tend to max out height based on lateral loads.

Lateral Considerations

For lateral considerations, generally when using concrete floor systems lateral forces should be resisted by concrete masonry units (cmu) or concrete shearwalls. Shearwalls take forces from wind or seismic loads acting on the building down to the foundations. The cmu or concrete shearwalls for the structure are often the stair and elevator shafts in the building. For gypcrete or light-weight floor systems the floor is not as stiff as concrete floors, therefore the shearwalls are required to be more closely spaced within the building. This means that using the stair and elevator shafts for the lateral system is often not practical for cost of the structure. Generally, for light-weight floor systems the lateral forces are resisted by X-braced strap shearwalls. Strap shearwalls are similar in function to common steel braced framed structures, but are required frequently throughout the structure.

Clarendale of Clayton – St. Louis, MO (13 Stories of CFS)

Clarendale of Clayton – St. Louis, MO (13 Stories of CFS)

While both types of systems have different limiting factors, both types of CFS structures generally start to reach practical limitations around about 12-14 stories. It is most common to see CFS structures in the 7-10 story range in order to maximize the cost and efficiency of the structure to give developers the best return on their investments. It is a common misconception that CFS is limited to less height than this. CFS is often used for shorter buildings for reasons unrelated to the capacity of the structure such as fire ratings, owner’s insurance, occupancy, jurisdiction restrictions, etc. All of these items are unrelated to the ability of CFS buildings to reach their ideal height. It should also be noted that buildings utilizing CFS can be taller than 12-14 stories in combination with concrete podiums or other structures also. This renders the actual building height only limited by high-rise provisions and other code related limitations not related to CFS strength. Some manufacturers are pushing the limits even further with custom wall stud sizes and creative engineering methods to support higher loads.

Wayne State – Detroit, MI (10 Stories of CFS)

Wayne State – Detroit, MI (10 Stories of CFS)

Selecting your CFS system

There are many other considerations when selecting the correct CFS system for your structure, but it is rare for CFS to not be the best selection for multi-family family structures in the mid-rise construction height. It should be noted that structures in this height range require engineers with CFS experience as well as contractors with experience using CFS building materials. For buildings over 4 to 5 stories the CFS structure should be panelized (walls built in a shop) rather than stick-built (walls framed on site). This is necessary to prevent gaps in the structure that would otherwise compress when the building is fully loaded. Some panelizers also have systems that allow the strapwalls or concrete shearwalls to be erected in the shop and shipped to the job-site.

The Collegiate 929 P Street – Lincoln, NE (13 Stories of CFS)

The Collegiate 929 P Street – Lincoln, NE (13 Stories of CFS)

International Building Code (IBC): Section 504.4

The number one limiting factor for CFS structures is section 504.4 of the International Building Code (IBC), which limits buildings to 12 stories for type 1B construction. Going taller than 12 stories would require an increase in the fire rating for the structure from 1hour rated assemblies to 2 hour rated assemblies. This is generally cost prohibitive to the financing of the building and kill a project before it begins. Going to a 2 hour rated building increases the cost of materials for doors, windows, walls and other components. This would also add cost to the mechanical systems, which would require more expensive systems for ductwork, piping, enclosures, insulations, etc. Lastly, this would require the addition of fire suppression systems such as sprinklers.

Conclusion

So how tall can you build with cold-formed steel? The real answer is that there is no limit, but about 12-14 stories becomes a threshold for CFS as an efficient material, while the limit is usually due to the high-rise provisions in the International Building Code, which limit the height to 12 stories for 1 hour rated construction. These are the common threshold for wall panelizers, specialty engineers, and contractors that work with CFS every day. The key is to find the right team with the right experience to develop the project alongside the developers and architects working on mid-rise residential style buildings. CFS is typically the best non-combustible and most cost-efficient building material for almost any “mid-rise” structure in the height range of about 5-12 stories. Cold-form steel can also be the most economical building material for shorter buildings for insurance reasons or for taller buildings in combination with other building materials. Any other rhetoric regarding a stricter limitation of CFS building heights is selling CFS short.

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Marcus Himmelberg, PE

Marcus Himmelberg, PE

Marcus brings 15 years of experience as a structural engineer. His structural based education and design versatility has made him a valuable part of the structural team. Marcus has a knack for solving complex problems and out of the box solutions for new design challenges. Working with multiple materials including steel, masonry, concrete wood, and cold-formed framing on numerous types of projects has made him a well-rounded project manager for the structural team. Marcus has developed many client relationships with repeat and new clients over the years and continues to provide services to keep projects on track and on time. You can contact Marcus at mhimmelberg@mcclurevision.com.