• Mark Denavit, associate professor at the University of Tennessee, Knoxville and consulting engineer to the Steel Joist Institute.
• Travis Hanson, development team lead of engineering design tools at Vulcraft.
• Tim Holtermann, executive technical director of engineering for CSC – Canam Steel Corporation and past-president of the Steel Joist Institute.

What Is Bridging?

Bridging is a type of bracing that strengthens primary structural members (such as the top chord of a joist) but does not carry primary loads itself. “Bridging helps keep things aligned as the load is applied and prevents buckling,” Mark explains. 

This added support can make a major impact on your structure. Mark spoke of tests he conducted at the University of Tennessee, Knoxville. The study compared erection stability of joists with flush frame to those with traditional bearing seats. The joists were loaded without any lateral bracing and demonstrated lateral torsional buckling at hundreds of pounds, not tens of thousands of pounds as would have been expected if all the bridging was installed. Tim also added that in-plane joists have a spectacular load carrying capacity and are incredibly efficient, but they are far weaker out of plane. This illustrates why proper bridging is essential in any construction operation.

Understanding Different Bridging Roles

Bridging in a structure serves a role in three distinct stages: erection, construction and permanent. Tim explains, “Erection is the immediate stability at the point of unhooking a crane. Construction is providing enough loading capacity to complete the installation of the bridging system and provide for other construction loads like the metal deck. And permanent bridging plays a role in supporting loads or providing bracing even after the entire joist and deck system is completely installed.” Importantly, the same physical pieces of steel that form the bridging serve these different functions at different stages of construction.

Safety and Regulatory Considerations

OSHA addresses bridging requirements specifically due to critical safety concerns during construction. Without proper bridging, workers would be at risk of the joists becoming unstable. The regulations ensure sufficient capacity for workers to safely move along the joist length during installation. When required, erection bridging must be installed before releasing the joist from the crane.

SJI Specifications and Drawing Requirements

When it comes to documenting bridging requirements, engineers and architects have varying approaches. According to our experts, drawings may include bridging system details, but they caution against overly prescriptive specifications. Tim explains that contract drawings don’t always capture the complexity of joist design. “There are occasions where we can meet the SJI specification but might not have the same number of rows at the top and bottom chords, or specifically locate bridging rows so that top and bottom cords aren’t aligned,” he notes. This complexity is why the Steel Joist Institute recommends leaving detailed bridging specifications to joist professionals who understand the nuanced design requirements.

Coordination With Other Systems

One common challenge is coordinating bridging with other building systems. Travis points out that diagonal bridging often presents the biggest coordination challenge, frequently requiring adjustments to accommodate Early Suppression Fast Response (ESFR) sprinklers, HVAC ductwork, or equipment like basketball backboard assemblies. In some cases, temporary X-bridging may be used during installation and later removed to accommodate other systems.

It’s also important to consider standing seam roofs and other specialized installations that may require different approaches. Our experts all agreed that a standing seam roof may not provide lateral support the same way a conventional deck profile would. Similar considerations should be applied and thought of for areas with skylights or other roofing variations.

Tim says, “It’s not uncommon to have to deal with or work with some constraints with the bridging placement relative to other trades. So duct work is one example that comes up sometimes.” A particular coordination challenge is the ESFR sprinkler systems. Tim continues, “Constraints due to ESFR are very common because there are some pretty rigid rules about the potential interference of a bridging line and the water flow or spray from that sprinkler head with that type of system.”

Essential Takeaways

All engineers should remember that joists, deck and bridging form an interconnected system where each component is vital. “Sometimes, people want to dismiss the importance of bridging and potentially neglect it or remove it without asking,” Travis cautions. “It’s vitally important that they know it is a system and all pieces are needed.”

Bridging should never be used as an attachment point for hanging other systems as its sole purpose is to brace the joist. Additionally, proper termination of horizontal bridging, either through anchoring to external structures or using additional diagonal bridging members, is essential for system integrity.

Understanding and properly implementing bridging requirements ensures not just structural integrity but also the safety of workers and other building personnel alike.