Composite Steel Joist

Want to reduce overall building costs?
Want to speed construction?
Want to create more rentable space for the building owners at no additional cost?
The evolving technology of composite steel joist construction can help you.

What is a composite steel joist?

The term composite steel joist refers to an open web, parallel chord, load-carrying member suitable for direct support of one-way floor or roof systems. Members may consist of 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 slab to act together as an integral unit after the concrete has adequately cured.

Why did the Steel Joist Institute (SJI) introduce the CJ-Series for composite steel joists?

The SJI, a nonprofit organization of active joist manufacturers, felt that it was in the Institute's and design communities best interests to develop a standard composite steel joist specification for all SJI member companies.

What are the Standard Specifications?

The SJI recently published the First Edition Composite Steel Joist Catalog that contains the Standard Specifications for Composite Steel Joists, CJ-Series. The specification covers the Load and Resistance Factor Design (LRFD), manufacture and use of simply-supported Open Web Composite Steel Joists. Other sections in the catalog include Accessories and Details, Responsibilities of the Design Professional, Code of Standard Practice for Composite Steel Joists, Weight Tables, Bridging Tables, Composite Joist Design Examples and a Composite Joist Floor Design Parameters checklist. Go to the "Publications" section of the website to get your copy.

Have composite steel joists been used prior to now?

Yes. Hundreds of projects utilizing composite steel joists have been constructed across the United States and Canada during the past 40 years. Now with SJI's recent standardization of a design method for composite steel joists (i.e. the CJ-Series), the structural engineer of record now has the assurance that multiple SJI member companies can provide composite steel joists for their specific projects. Plus there will be greater consistency between composite steel joist designs because all SJI member companies approved to manufacture CJ-Series joists are required to follow the same design specifications.

What are some advantages of using CJ-Series composite steel joists?

• Reductions in overall floor-to-floor height of the structure.
• Mechanical ducts and piping can be run through the open webs.
• Maximum span-to-depth ratio of L/30 permit the use of shallower joists for any given span.
• Efficient composite design makes it possible to span greater distances resulting in larger column spacing which increases the rental value of the floor space.
• Composite steel joists are more efficient allowing a reduction in the joist weight for any given joist depth. Lighter weight joists translate into potentially lighter weight columns and reduced foundation costs.
• Live load deflections are significantly reduced. With the overlying concrete slab locked to the steel joist, the resulting composite action provides a stiffer floor system.
• Efficient erection of the CJ-Series joist system reduces construction time and permits early occupancy of the building. Wider joist spacing reduces the number of joists to be erected and, when required, the number to be fireproofed.

What is the strength of a composite steel joist?

The design of CJ-Series joist chord sections are based on a yield strength of 50 ksi (345 MPa). Web sections are based on a yield strength of at least 36 ksi (250 MPa), but not greater than 50 ksi (345 MPa). Concrete strengths can vary from 4 to 5 ksi (27.6 to 34.5 MPa).

What about web design requirements?

Testing has verified that the web members of a composite steel joist behave in the same manner as web members found within a non-composite joist. Webs must be designed so that they have sufficient strength to transfer the vertical shear from the applied loads to the ends of the composite joist. The CJ-Series webs are designed for a minimum vertical shear equal to 25% of the factored end reaction.

Does the application of paint affect the installation process of shear studs?

Paint may potentially hinder the installation of welded shear studs to the joist top chord. For this reason, standard shop practice is to supply composite steel joists unpainted.

For a more comprehensive description of SJI's newest joist series go to the Technical Information section of the website and click on the December 2007 article reprint from Structural Engineer "The Steel Joist Institute's newCJ-Series Composite Steel Joist catalog." For a copy of the 2007 First Edition Steel Joist Institute Composite Steel Joist Catalog go the Publications section of the website.