Comprehensive Guide to Cold-Formed Floor Framing Systems: Key Differences Explained
Scottsdale provides both cold-formed steel floor trusses and floor joists within our product lineup through the Scottsdale and Knudson by Scottsdale roll formers. The Scottsdale roll formers are capable of producing in-line trusses using Top Hat and C-section profiles, which are designed to offer high strength and stability for various construction applications. These profiles are particularly useful in creating lightweight yet robust structural components that can support significant loads. On the other hand, the Knudson series of roll formers can manufacture floor trusses both in-line (minor axis) and strongback (major axis). The in-line trusses are typically used for applications where the load is distributed along the minor axis, providing efficient load-bearing capabilities. Strongback trusses, produced along the major axis, offer enhanced rigidity and support for larger spans and heavier loads. Below are typical sectional views of the various floor trusses and joists types that can be produced through our roll formers, showcasing the versatility and adaptability of our cold-formed steel floor framing system to meet diverse residential and commercial construction needs.
Further information on the section dimensions and the capabilities of different roll formers is available in the attached article from our archives.
Cold-Formed Steel Floor Framing System: Practical Example with Strength and Cost Comparison
Different floor framing system offers different strength based on the configuration, choice of section, steel thickness and grade. Cold-formed steel lattice floor trusses and joists are considered for comparison. Images of floor trusses and joists are shown below
Picture showing cold-formed steel lattice floor trusses
Picture showing cold-formed steel floor joist - Image courtesy US Frame Factory
A typical residential floor spans of 6 m [~20 ft] between load bearing walls is considered for this study. The loads considered for the example include the following
- Dead load = 0.9 kPa
- Live load = 1.5 kPa
- Point load = 1.8 kN
Scottruss Cold-Formed Steel Hat Section Lattice Floor Truss: Key Features and Benefits
A 6050 [2 inch] Scottruss Hat section made from 0.95 mm [20 Ga] thick G550 [80 ksi] grade steel is required to pass structural adequacy for the given loads. This translates to 19.8 linear meters of steel per truss, with each joist weighing 25.5 kg. Additionally, assembling a single floor truss necessitates 44 bolts and nuts. Typical floor trusses to wall connection are shown in image below.
Knudson by Scottsdale KFS Framemaker: Cold-Formed Steel Floor Joists Explained
Conversely, a C-section cold-formed steel linear floor joist 1200S350-54 (SFIA / SSMA section) by using the KFS 1218 Framemaker, using G350 grade steel, can also meet the structural adequacy for the specified loads. This will weigh 35.36 kg for the 6m span. However, no additional fasteners are needs to assemble the webs as in case of a trussed floor joist making the manufacturing process a lot faster. Typical floor joist sitting on a cold-formed steel wall stud is shown below.
Typical floor joist to wall stud connection - Ref SFIA Technical Manual
Cold-Formed Steel Floor Trusses vs. Floor Joists: A Cost Analysis Summary
Using the internal cost comparison tool, developed from decades of experience in the metal framing industry, a truss joist produced with the Scottruss Hat section machine costs approximately $4.5 USD per linear meter, while C-section joists cost $5.5 USD. At first glance, it may appear that C-section joists are more expensive to manufacture than trussed joists. However, several factors should be considered when selecting the appropriate flooring system for your project.
- The cost comparison does not account for the labor efficiency in assembling the joists. Slower assembly speeds can significantly increase the cost of trussed joists. In contrast, this factor does not impact the cost of C-section joists, as they require little to no labor for assembly and can be directly rolled out of the roll formers.
- C-section joist profiles are thicker than trussed joists, with 1.5 mm steel used for C-section joists compared to 1.15 mm steel for trussed joists. The 26% difference in thickness results in a higher weight and slightly increased cost for the joists. However, the greater thickness also enhances the strength of the floor joists, allowing for longer spans compared to trussed joists.
- Another crucial aspect to consider is the speed of rolling the profile. The KFS Framemaker excels in this area, as it involves fewer operations to produce the section compared to the Scottruss roll former. The production speed of trussed floor joists also heavily depends on the efficiency of the assembly crews.
Therefore, all these points must be carefully evaluated when choosing the right flooring system for your project.
Contact us to learn more about the various cold-formed steel floor systems we offer to help you achieve significant savings on your next project.
