Cold-formed steel framing offers a unique combination of stability, strength and longevity for a variety of real estate developments. Ideal for use in residential buildings and overbuild structures, cold-formed steel framing can be a good option for structural engineers to consider when selecting a structural system.
Cold-formed steel framed bearing walls are constructed with cold-formed steel structural stud members. Sheets of steel are formed into various shapes at ambient temperature (as opposed to the hot rolled process used in structural steel), to improve the strength to weight ratio in structures. The most common shapes are C, Z, U and box sections. These members are considerably thicker than the “light gauge studs” commonly used in non-structural partition walls.
When considering what structural system to choose, architects and designers might pick cold-formed steel framed bearing walls over other structural systems because it is lightweight. Specifically, cold-formed steel framed bearing walls are lighter than concrete or masonry, making it an excellent option to use when designing overbuilds of existing buildings and buildings with lower capacity foundation systems. The system is ideal for small to midsize buildings with up to about 10 stories of bearing wall-supported floors. Above 10 stories, the design of the bearing wall structural studs and the cold-formed shear wall lateral strength becomes inefficient.
Residential buildings are good applications for cold-formed steel framed bearing walls because the system can take advantage of apartment demising walls and corridor walls to structure the building. Traditional column-supported structural systems, such as cast-in-place concrete and structural steel framing, require columns, braced frames and shear walls that can be obtrusive to an apartment floor plan. Cold-formed steel framed bearing walls are hidden inside the demising and partition walls, resulting in more useable square feet per floor. The bearing walls can support both gravity and lateral loads from wind and seismic, all while remaining hidden and non-invasive for architects. This application is ideal for buildings that are not anticipated to change floor plan layouts in the future. Bearing walls limit future fitout modifications to the floor plans since the bearing walls cannot be removed as easily as traditional non-load-bearing demising and corridor partitions.
Many types of floor structural systems are used with cold-formed steel framed bearing wall systems. In the Philadelphia market, the most common floor system is the precast hollow core plank. Precast planks are typically eight inches thick and can span up to about 30 feet while supporting residential loads. The eight-inch thickness provides a thin floor structure with shallow floor-to-floor heights using a shop fabricated system that reduces field labor. Other common floor systems include open web bar joists with slab on metal deck, long-span deep metal deck with concrete fill and cold-formed steel floor joists with sheathing.
Cold-formed steel framed bearing walls are not without their challenges. Residential buildings often change their floor plans on the lower levels, often to more open layouts such as retail and parking that demand longer spans than that of the residential floor levels. Transfer podiums are often the solution for this change in the floor plate. These are typically structural steel framing with slab on metal deck or cast-in-place concrete flat plate supported on concrete columns. Overbuild projects can demand more creative transfer podium solutions.
The Harman Group designed a very unique overbuild project using cold-formed framed bearing walls as the structural system at 2040 Market Street in Philadelphia. 2040 Market Street was an existing five-story, cast-in-place concrete mixed-use office building that was renovated and expanded for conversion to a 13-story residential building with ground-level retail and underground parking. The Harman Group worked with our project partners to select cold-formed steel framed bearing walls as the structural system to maximize the number of floors on the vertical overbuild. While a typical cold-formed steel framed bearing wall system has cross sections in the shape of a C, this project incorporated a proprietary wall stud, using high-capacity cold-formed steel stud box sections, enabling the structure to rise to 13 stories.
The cold-formed steel framed bearing walls at 2040 Market Street provided a non-invasive lightweight structural system, allowing for three more additional overbuild levels than a traditional steel-framed system would have allowed. The bearing wall system on 2040 Market Street presented a challenge where the overbuild begins above the existing roof, which was only a 12-inch-thick concrete slab – inadequate to support the load of an eight-story overbuild. To solve this problem, The Harman Group applied a unique bearing wall system using hot rolled structural steel trusses located within the demising and corridor wall stud cavities just above the existing roof where the existing and new structure met to support the weight of the overbuild. These in-wall trusses transferred the weight of the overbuild to existing building columns at each end of the walls.
Another excellent example of a cold-formed steel framed bearing wall project is the award-winning Vantage, a 471,000 square foot mixed-use student housing building at the edge of the Temple University campus. Vantage uses cold-formed steel framed bearing walls in the 12-story West Tower and the four-story north connector bridge. Additional THG projects that use cold-formed steel framed bearing walls are Rodin Square, The Hamilton Phase I, Community College of Philadelphia, The National, 3939 Chestnut Street and the University of the Sciences Living and Learning Commons.
When it comes to choosing a structural system, cold-formed steel framed bearing walls are a good choice, particularly with adaptive re-use and residential building projects.