A steel joist, also called an open web steel joist, is a custom-built structural element which utilizes a truss configuration to act as a beam when supporting a floor or roof. Most common steel joists have a double angle top and bottom chord with angle or round web members and are designed according to the specifications of the Steel Joist Institute (SJI). SJI member companies design steel joists to meet SJI specifications after being provided loading diagrams by the design engineer. Steel joists are chosen to provide an ecomonic alternative to steel wide flange members when supporting both metal deck roofs and composite floor slabs.
Common joists are identified as open web bar joists (SJI S, J, H and K Series), longspan joists (SJI LJ, DLJ, LH and DLH Series), or joist girders. Identifying characteristics include the joist depth, joist span, chord sizes, end seat depth and web configuration. It is common for structural engineers to have to identify existing steel joists based on these characteristics when evaluating existing structures.
Steel joists often require modification or reinforcing when additional loading is applied beyond the original design loads such as found in fit-out, retrofit or renovation projects. There are three common methods to obtain additional capacity from a steel joist framing system: load redistribution to adjacent members, the addition of new infill joists, or reinforcing the existing joist.
Composite Joist-Slab Floor Systems
Just as beams are often times designed to act composite with the slab, joists can also be designed to act composite with the floor slab. One example is the Hambrosystem which uses composite action to give added strength to the structure. Hambro achieves composite action by extending the web members into the slab. Vulcraft also makes a composite joist-floor framing system which uses shear studs just as a composite beam-slab system.
When a joist has a concentrated point load it must be designed for this force. Concentrated loads should be located at a panel point. If a load diagram is not included on the design documents for this force then the force must be resolved to a panel point by reinforcing the web as shown in Figure 2.
Notes On Joists
Strength and Design
- Until the 1970's, joist web members were designed for a minimum shear capacity equal to 50% of the joist end reaction. Current joist web members are designed for a minimum shear capacity equal to 25% of the joist end reaction.
- When unique loading conditions are present, a joist loading diagram with loads and dimensions should be included on design documents to aid the joist manufacturer in design of the steel joists. Joist with a special loading should be indicated on plan with an 'SP' suffix.
- Joist end seat depths are typically 2 1/2 inches while longspan joist end seat depths are typically 5 inches deep.
- Joists can be identified by round stock web members while longspan joists typically have angle stock as web members.
- Joist end seats may be welded or bolted to their supports.
Construction - Steel Joists
Erection and Stability
Stability of joists during erection is an important element in steel joist framing construction. Erection stability is achieved by proper installed end connections and Bridging between members.
Prior to installation of joists, all supporting construction should be inplace and secure.
OSHA Standard 1926.757 as pertaining to steel joist erection requires:
"where steel joists are used and columns are not framed in at least two directions with solid web structural steel members, a steel joist shall be field-bolted at the column to provide lateral stability to the column during erection."
"A vertical stabilizer plate shall be provided on each column for steel joists. The plate shall be a minimum of 6 inch by 6 inch (152 mm by 152 mm) and shall extend at least 3 inches (76 mm) below the bottom chord of the joist with a 13/16 inch (21 mm) hole to provide an attachment point for guying or plumbing cables."
"The bottom chords of steel joists at columns shall be stabilized to prevent rotation during erection."
"Hoisting cables shall not be released until the seat at each end of the steel joist is field-bolted, and each end of the bottom chord is restrained by the column stabilizer plate."
More information on steel joists are available at the following links:
- Macomber VBeam Joist Technical Data
- Macomber VBeam Joist Catalog - 1968
- Macomber Bowstring Catalog - 1958
- Armco Joists - 1974
- Haven Busch DLH Joists