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Composite Steel Joists
Composite steel construction has been recognized for a number of years as one of the most economical systems for constructing building floors. Three options have evolved over the years to meet the requirements for building height limitations and the need to run complex heating, ventilating, electrical, and communication systems:
(1) Composite wide flange beams with web openings;
(2) Stub girders; and
(3) Open web steel joists and joist girders.
The term composite joist refers to an open web, parallel chord, load carrying member suitable for the direct support of floors in buildings, utilizing hot-rolled or cold-formed steel, including cold-formed steel whose yield strength has been attained by cold working.
Shear connection between the joist top chord and overlying concrete slab allows the steel joist and concrete to act together as an integral unit after the concrete has adequately cured. Currently the most commonly used forms of shear connection between the joist top chord and concrete slab include specially rolled cold formed steel "s" shaped top chords (Hambro), specially embossed back-to-back double angle top chords (Vescom), perforated structural tee top chords (Taftrus), and shear studs welded through the metal deck (Canam, SMI, and Vulcraft).
Fig.1:HVAC ducts and pipes passing through joist open webs.
Advantages of Steel Composite Joists
Benefits to be gained by using composite steel joists include the following:
- Ability to route the new mechanical heating, ventilating, plumbing, and electrical lines through the joist open webs as shown in Figure 1. Customized web openings and configurations can be provided for large ducts.
- Ease of relocating and/or moving future HVAC during the life of the building.
- Better plenum space utilization.
- Floor-to-floor height can potentially be reduced by not having to run the mechanical lines under the joists. Also the more efficient and stiffer composite design makes it possible to support a given load with a shallower joist.
- Weight savings resulting from the efficient joist design reduces overall building costs.
- Simplified erection and connections provide for fastest construction.
- Large column free areas give the building tenant maximum flexibility when selecting a floor layout plan.
- Ability to provide customized composite joist designs for any given loading and serviceability requirements.
