Steel roof and floor joists are often used due to their flexibility in supporting ductwork, mechanical equipment, or architectural finishes.  However, loads added in the field can require costly modifications to joists, if they are unplanned or placed without proper consideration.  How can these costly additions be avoided when using steel joists?

In the August 2026 SEU session, Bruce Brotherson, PE, SE, an Innovation Architect from Vulcraft, and Walter Worthley, PE, SE, from SEQ Engineering, LLC, presented Evaluation, Modification, Retrofit: Steel Joists, Roof, and Floors.  Bruce demonstrated examples of joists that can be retrofitted and should not be retrofitted.  He also identified key characteristics of in-place joists and explained how to determine who the original manufacturer was and whether they can provide additional documentation.  Bruce described how to verify the original design loads and evaluate the joists for new loads.  Walt then covered methods and practices to modify existing joists to increase the load carrying capacity and shared some commonly used details to increase the load capacity of a joist component.

Bruce explained guidance offered by SJI based on testing of concentrated loads on steel joists.  As shown on the slide below and noted in the 45th Edition of the SJI Specification, the 100 pound rule says concentrated loads up to 100 pounds may be applied within any top or bottom chord panel, so long as the load was included in the uniform design load, and is applied concentrically to the chord.  This eliminates the need for an additional angle strut to be added and reduces labor costs when these minimal loads are added along the length of the joist.

Bruce also shared new research completed by SJI which is known as the 500 pound rule.  This rule allows for the addition of a single 500 pound load applied within 6 inches of any panel point without the need for a field installed strut as long as the load was accounted for in the uniform design load and it is applied concentrically to the joist chord  However, the 500 pound rule and 100 pound rule cannot be combined within the same panel zone.  To watch a short video from SJI explaining these two useful rules, click here.

Adding struts in the field for various hanging loads from joists is a common practice, however, this new guidance from SJI can help to minimize the need for these costly, labor-intensive additions.