What is Cold-Formed Steel Framing?
Cold-formed steel framing (CFS) is made from sheet steel that is formed into C-sections and other shapes by roll forming the steel through a series of dies at room temperature. Heat is not required to form the shapes, hence the name cold-formed steel. A variety of steel thicknesses are available to meet a wide range of structural and non-structural applications.
Cold-formed steel framing is lightweight, noncombustible, strong, and relatively easy to install. CFS has dominated the market for interior, non-loadbearing partition walls in commercial construction for years. With recent developments in panelized systems, the building community is using CFS for structural applications in mid-rise and multifamily buildings. Should you consider Cold-formed steel framing for your next construction project?
Should You Consider Cold-Formed Steel Framing in a Structural Application?
Like any product, there are pros and cons to using CFS in a structural application. 3F Construction recently developed a 6-story multifamily building at 851 W. Grand in Chicago with cold-formed steel framing and learned first hand the pros and cons. We are sharing them below and hope you can learn from our experiences.
Advantages of cold-formed steel framing in a structural application
- CFS is light, making it easy to ship, handle and assemble.
- Walls can be panelized off-site which reduces on-site labor costs and construction waste. Panelized walls can improve quality control and increase the speed of erection.
- Steel framing’s strength-to-weight ratio significantly exceeds that of wood or concrete. Being strong and relatively lightweight, CFS systems can reduce a building’s total weight. This allows the owner to potentially save money with the foundation system.
- The quality of CFS is very consistent. That consistency translates into less scrap to haul off because there is less waste. Especially when panels are fabricated off-site.
- Steel is dimensionally stable and does not expand or contract with changes in moisture content. Steel will not warp, split, crack or creep when exposed to the elements.
- CFS members have stamped openings making it easy to pass through conduit and plumbing.
Many of these advantages were summarized from the Building Steel website. They do a great job of highlighting all the advantages of steel buildings. However, there are disadvantages to CFS.
Disadvantages of cold-formed steel framing in a structural application
- The number of footings increases because demising walls are load-bearing walls. In addition, we increased the number of Geopier Rammed Aggregate Piers supporting the footings.
- The joist span limitations of a CFS building can impact unit layouts.
- The stamped openings in CFS joists are too small to pass flexible HVAC duct. Therefore, soffits are required for HVAC distribution ductwork which increases costs and slows construction.
- The structural engineer might require additional CMU shear walls for lateral strength.
- CFS wall installation requires trades to mobilize and demobilize for the installation of each floor. This slows down construction progress.
- The structural engineer might require a concrete roof structure. This will delay the installation of the roof by at least 21 days.
- In Chicago, there are very few non-union carpenters that can erect CFS buildings. The union carpenters with CFS experience have higher labor costs.
- Window rough openings MUST be correct. Rough openings cannot easily be changed in the field. They are engineered to dimension.
- Placing wall switches in a logical location can be challenging. Short walls generally have 2 or 3 studs together. Switches cannot be placed in these stud packs because they are structural.
- Carpenters cannot make field modifications to structural walls in the building. The structural engineer must review and approve all modifications to engineered studs.
- HVAC wall penetration must be engineered. This requires up-front coordination with the HVAC contractor. The engineered penetrations cannot be modified easily in the field.
- Actual wall thicknesses can vary based on protruding screw heads. If you are using Timely door frames, FIELD VERIFY THE WALL DIMENSIONS BEFORE ORDERING FRAMES. We learned this the hard way.
As a contractor, we generally recommend against cold-formed steel framing in a structural application. In our experience, the disadvantages increase costs and dramatically slow production. For a more detailed explanation of the disadvantages, read below.
Increased Number of Footings
Footings are constructed under most CFS walls because they are load bearing. Therefore, the number of footings increases. The increased number of footings offsets the savings from the building weight reduction. You can see the increased quantity of footings in the photos below. Unsuitable soil on site exacerbated the issue. We installed Geopier Rammed Aggregate Piers to improve the ground, further increasing costs.
Joist Span Limitations
The allowable joist span varies based on joist depth and spacing. In our experience, plan on a 12′ maximum span.
Based on our experience, spans longer that 12′ are cost prohibitive.
Stamped Openings in Joists
The stamped openings in the joists are not large enough for flexible HVAC ductwork to pass through them. Plan to run ductwork in soffits. This increases costs and slows the construction schedule.
CMU Sheer Walls
Before hiring a structural engineer for a CFS building, I recommend you discuss the design of the sheer walls. Our building had (2) 6 story stair towers in CMU PLUS a CMU sheer wall between them on all floors. We discussed the CMU sheer wall with the CFS designer and other structural engineers who indicated it was not necessary and excessively conservative. We toured several other CFS buildings in Chicago like ours that did not have a CMU sheer wall. Regardless, our structural engineer would not remove it further driving up the cost of the project.
Cross-bracing straps screwed to the studs can replace sheer walls in CMU. Our engineer would not allow straps because in his opinion they tend to buckle under load and lose their effectiveness. In addition, installing cross bracing after the building was erected would not satisfy his concerns. For these reasons, and more outlined below, I highly recommend using a structural engineer with CFS experience.
Cold Form Steel Construction Inefficiencies
Cold-formed steel framing in a structural application is simply slower than traditional steel and concrete construction. A lot slower. Here are the steps to erect a single floor:
- Mason lays one and a half floors of CMU and pulls off the site.
- Carpenter erects prefabricated cold-formed steel panels.
- Carpenter install joists and steel deck and then pulls off the site.
- Concrete sub installs wire mesh, pours the concrete floor, finishes the concrete and then pulls off the site.
- The mason remobilizes to lay CMU after the concrete floor cures for 2 or 3 days. Then pulls off again.
- Carpenter returns to install prefabricated panels, joists, and deck. Then pulls off again.
- Concrete sub returns to place the concrete. Then pulls off again.
This cycle repeats for every floor. And getting subs to remobilize exactly when you need them is impossible. Especially given weather and demands of other jobs. I estimate that we lost 6 weeks on the schedule compared to traditional steel and concrete construction.
Concrete Roof Structure and Potential Schedule Delays
Our structural engineer required a poured concrete roof deck with wire mesh. This design requirement alone delayed the project for approximately 3 weeks. Before a roofer can install a fully adhered roof membrane over a concrete deck they measure the moisture in the concrete. Typical concrete takes 21 days to cure before it can achieve 75% RH. Fast drying concrete is very expensive and does not always dry faster. And installing a temp roof does not allow the concrete to dry properly. We never solved this problem and had to wait 21 days before we could install the roof which delayed the start of insulation and drywall.
A poured concrete roof will delay the installation of a roof by approximately 21 days. Plan for this in your schedule.
Other developers were able to use typical cement roof board screwed into the deck. USG offers a product called SECUROCK Cement Roof Board. This would have solved the problem, but our structural engineer would not allow it. He argued the building diaphragm needed the rigidity of the poured concrete roof with wire mesh. However, other developers in the Chicago market are using cement board on their roofs. Once again, selecting the right structural engineer can save developers time and money with CFS buildings.
Engineered Openings and Penetrations
CFS structural walls openings must be engineered in advance and submitted with the shop drawings. That means the rough openings for windows must be selected, coordinated, and engineered. Make sure to plan for this advanced coordination. Also, HVAC penetrations for distribution must be engineered on the shop drawings. Plan to coordinate this with the CFS fabricator in the shop drawing chase and do it with the HVAC installer. These penetrations cannot be easily changed once the panels arrive, so take extra time to make sure they are correct from the start.
Placing Switches can be a Challange
As seen in this demising wall photo, the studs between the door openings are 3 or 4 studs bundled together. Wall switches cannot be placed in these stud packs, so placing a switch in a wall like the one in this photo can be a real challenge. We moved switches to locations that were not typical. I highly recommend you coordinate switch locations with the CFS submittal. The electrical engineer will place them in typical locations on their drawings. But CFS shops drawings are generally not available when the electrical engineer is finalizing their Issue For Construction drawings. This leads to switch location conflicts in the field.
Wall Thickness Can Vary
We learned that strapping screwed to the face of the steel stud for lateral support adds about 3/8″ to the wall thickness. Do NOT order door frames from the wall thickness identified on the drawings. Absolutely every opening needs to be field measured for wall thickness. Otherwise, door frames simply will not fit. We learned this the hard way.
Lessons Learned from Installing Cold-Formed Steel Framing in a Structural Application
- Hire a structural engineer with extensive experience using cold-formed steel in a structural application. Ask for references from their last project and call them. The wrong structural engineer can cost the project a ton of money and delay the project. I estimate that our engineer cost the project $250,000 and delayed the schedule at least 45 days in total.
- Push to use steel strapping for lateral bracing rather than CMU walls.
- Plan to coordinate rough window openings, HVAC penetrations, and wall switch locations during shop drawings preparation.
- Use cement roof board rather than poured lightweight concrete. A poured concrete roof deck will delay the project 21 days.
- Consider using USG Structural Panel Concrete Subfloor rather than lightweight concrete. Our structural engineer would not allow it, but it is used in CFS buildings. However, panel concrete subfloors do transfer more noise than lightweight concrete floors. The panels are not tongue and grove so noise passes through them more easily. In addition, the subfloor shifts which creates an issue for LVT flooring.
- Coordinate the optimal joist span when layout out the units. Coordinate this with the carpenter during design development.
- Building with CFS in a structural application is inefficient. Trades need to mobilize and demobilize for each floor. Consider ways to frame the entire building and pour concrete floors on the steel deck with walls erected. Our structural engineer would not approve this.
- Wall thickness does vary. Field verify wall thickness before ordering door frames.
- Plan to soffit HVAC distribution. Distribution trunks cannot fit through the openings.