Structural Engineering · Engineering Glossary
A joist is a repetitive, closely spaced horizontal framing member that supports the floor deck or roof deck and transfers loads to beams, girders, or bearing walls. Joists can be solid wood, engineered lumber (LVL, TJI), open-web steel bar joists, or light-gauge metal.
Wood joists are common in residential and light commercial construction; steel bar joists (manufactured to SJI standards) span efficiently over long distances in industrial and commercial buildings; engineered wood I-joists offer high strength-to-weight ratios for floor framing. The joist must be designed for bending strength, shear strength, and deflection limits — total deflection under live load typically limited to L/360 for floors and L/240 for roofs. Lateral bracing at the top chord and bridging at the bottom chord prevent joist rotation. In renovation projects, joists must be evaluated for their capacity to carry new loads, including HVAC equipment or rooftop solar panels.
Apex Grid Engineering maintains this glossary as a resource for architects, contractors, developers, and property owners who encounter unfamiliar engineering terminology on their projects.
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