Modern Steel Construction • March 2025 • Go for the Joists

Consider these important steps when designing high-performance floors with open web steel joists.

OPEN WEB steel joists are an ideal solution for supporting high-performance floors with optimal vibration characteristics. They usually become even more appealing after considering their low cost of construction, low carbon footprint, conduciveness to low floor-to-floor height, ease of routing HVAC, electrical, and plumbing conduits, large floor areas free of columns, and a reduced construction schedule.

Designing a high-performance floor entails much more than merely using open web steel joists, though. Steel joists can perform well in a floor and achieve their benefits when used properly. When used poorly and if good design procedures are ignored, steel joists may not perform optimally. There are several important steps to consider when using joists in a floor system to make sure they function properly.

Spacing. The floor joist spacing should never be less than 4 ft, with an ideal joist spacing between 5 ft and 10 ft. A larger steel joist spacing means fewer joists to manufacture, ship, sort, and install, which saves time and money. Larger steel joist spacings also result in larger joists with larger chords, requiring fewer rows of bridging—another time and money saver.

Just as important, a larger steel joist spacing results in joists with more stiffness, a higher natural frequency, and more applied dead load from a larger tributary area, which substantially improves the floor vibration characteristics.

Depth. Select steel joists with sufficient depth to run the mechanical ducts through the open webs above the bottom chord of the joist. Even though the joist is deeper, the larger depth will potentially reduce the building floor-to-floor height by containing the ducts within the joist depth. Often, round ducts can be run through the standard triangular web openings. But if necessary, specify a rectangular Vierendeel web opening near the joist mid-span for larger duct passage.

Deeper steel joists are usually more efficient and lighter weight. They are also stiffer, which results in lower live load deflections and higher natural frequency for improved floor vibration characteristics.

Camber. Because steel joist camber is built into the manufacturing process, Steel Joist Institute (SJI) standard camber comes at no additional cost, and special camber comes with little added cost. Furthermore, open web steel joist camber is generally more precise and repeatable than wide-flange beam cambering processes.

Take full advantage of the joist as a cambered product by specifying that the joist be cambered for the dead load of the concrete and steel deck. Placing the concrete at a constant thickness over the cambered steel joist will result in adequate concrete coverage for shear studs, required minimum slab thickness per UL Fire Code, and flat finished floor over the entire joist span.

End Connections. While standard underslung joist bearing seats are handy for placing steel joists during erection, flush-frame shear tab connections (see Figure 1) also offer distinct advantages and should be considered. The bolted shear tabs are similar to simple beam shear connections (see Figure 2). When used with extended shear tabs, (see Figure 3) they also allow for unitized erection with steel joists and steel deck pre-assembled on the ground to be lifted and bolted into place. Steel joist and steel deck erection times can be reduced by more than 50% with panelized erection.

The flush frame connection also brings the bottom of the steel deck supporting the concrete into direct contact with the top flange of the girder, which substantially improves the floor vibration characteristics. The girder acts compositely with the slab for vibration excitation. Plus, it brings an additional 50% of the adjacent bay into play if the adjacent span is greater than 0.7 times the span under consideration. With the stiffer flush framed connection to the girder, vibrational energy is transferred into the adjacent span. The 50% increase in the effective panel weight, Wj, translates to a minimum reduction in floor acceleration of 33% given that the floor acceleration is inversely proportional to Wj.

If a designer uses standard clip-angle joist bearing seats, and the predicted floor acceleration from walking excitation is greater than 0.5% of g, the first thing to consider would be changing the joist bearing seats to flush-framed seats. If the building is in a high-seismic area, increasing the floor concrete slab thicknesses to reduce the floor acceleration will increase the seismic lateral loads on the structure. It is far more economical to use flush-framed seats.

Flush-frame connections allow the girder to be easily designed compositely, which translates into girder weight savings of up to 20%.

SJI CJ-Series Composite Steel Joists. With the concrete locked to the steel joist top chord via welded shear studs, compression loads normally carried by the joist top chord are transferred into the concrete slab. The transfer allows the top chord of a composite joist to be substantially smaller. Likewise, a composite joist’s greater effective depth permits a reduction in the bottom chord size. The result is a 20% to 30% reduction in steel joist weight to carry the same equivalent load.

Composite steel joists have a much greater moment of inertia, which translates into reduced composite live load deflections.

SJI Floor Bay Comparison Tool—With Vibration. SJI’s Floor Bay Comparison Tool assists the structural engineer of record with comparing weights, costs, and floor depths for selected bay sizes and framing configuration. The tool offers comparisons using non-composite and composite options for steel joists, wide-flange beams, and wide-flange girders, plus non-composite steel joist girders. Floor vibration criteria is checked for each framing option. The Floor Bay Comparison Tool is available for free download and can be found at www.steeljoist.org.

Following these guidelines will result in a high-performance steel joist floor system with excellent floor vibration characteristics, flatter finished floors with more consistent concrete thickness, lower floor-to floor heights, easy passage of mechanical systems, large floor areas free of columns, faster erection, a lower weight and cost, and a smaller carbon footprint than other floor framing options.

This article is a preview of the 2025 NASCC: The Steel Conference session “Efficient Steel Joist Design—Floors.” To learn more about this session and others, and to register for the conference, visit aisc.org/nascc. The conference takes place April 2 to 4, 2025 in Louisville, Ky.

Joe Pote (joe.pote@newmill.com) is the director of research and development at New Millenium.

Dave Samuelson (dave.samuelson@nucor.com) is a retired structural research engineer at Nucor and prior chair of the Steel Joist Institute’s composite joist subcommittee.