Open web steel joists analysis and design

What open web steel joists are

Open web steel joists are standardized trusses manufactured to Steel Joist Institute (SJI) specifications for use in floor and roof framing. The name describes the structure: the web between the top and bottom chords is open, made up of diagonal members rather than a solid plate. This geometry makes joists extremely efficient in bending, and the open web provides a clear path for mechanical, electrical, and plumbing distribution.

The SJI organizes joists into series by span range. K-series covers spans up to 60 feet and is the most common choice for light commercial construction. LH-series covers longer spans. DLH-series (Deep Long-span Horizontal chord) covers spans up to 150 feet for long-span roof applications. KCS-series joists provide a constant shear capacity along the length. These series cover the full range of typical floor and roof framing applications, enabling long spans and open floor plans at lower weight than equivalent solid-web beams.

Design challenges: non-uniform loads and stress reversal

The SJI designation system is built around uniform distributed load, and standard joist selection from the SJI load tables assumes that loading pattern. Non-uniform loading, including concentrated point loads from mechanical equipment or partitions, falls outside the standard designation and requires a special design by the joist manufacturer. One important exception: concentrated loads under 100 pounds do not require chord bending checks under the SJI specification.

Stress reversal under wind uplift is a second design consideration that applies to roof joists. Under gravity loading the top chord is in compression and the bottom chord in tension. Under net wind uplift those forces reverse, and the bottom chord goes into compression. Joists in roof applications with meaningful uplift exposure must be checked for the reversed condition, which can govern chord and web member selection.

Reading SJI load tables

The SJI load tables provide allowable line loads for each joist designation, set by both strength and live-load deflection at L/360. Selecting a joist means identifying the designation whose tabulated allowable load meets or exceeds the design demand at the required span.

In the Calcs.com steel joists calculator, the blue capacity envelopes displayed alongside the applied load diagram represent these SJI allowable loads. The calculator flags when the applied load exceeds the envelope so the engineer can see immediately whether the selected designation is adequate at the required span and spacing.

Worked examples: uniform load and rooftop unit

The first design example in the session covered a uniform roof load on a 21-foot-4-inch span with 4-foot joist spacing. The appropriate designation was a 12K1 joist, selected directly from the SJI load tables for the governing combination of strength and deflection.

The second example introduced a 1,000-pound rooftop unit located 9 feet from the end of the same span. The concentrated load exceeded what the 12K1 could carry, requiring an upgrade to a 12K5. Through load linking in Calcs.com, the updated joist reaction was automatically passed to the connected girder, so the girder design reflected the revised loading without manual re-entry. The calculator is in beta with external validation against SJI data planned before full release.