A steel joist is an open-web, load-carrying structural member used to support floors and roofs in commercial and industrial buildings. Unlike a solid steel beam, a steel joist has a triangular lattice of steel pieces connecting its top and bottom bars, creating a lightweight truss that can span long distances while using significantly less material. You’ll find them in warehouses, office buildings, big-box retail stores, schools, and virtually any structure with a wide, open interior.
How a Steel Joist Is Built
A steel joist looks like a long, narrow truss. It has a top chord and a bottom chord (the horizontal bars running along the top and bottom) connected by diagonal and vertical web members arranged in a zigzag or “W” pattern. These web members are typically steel angles or round bars welded to the chords. The open triangular spaces between the web members are what give the joist its name: “open web.” This design carries loads efficiently through tension and compression in the individual web pieces rather than relying on the sheer mass of a solid beam.
The ends of a steel joist sit on bearing surfaces, usually the top of a steel beam, a wall, or a concrete masonry unit. Each end has a welded seat plate that rests on the support and gets bolted or welded in place. Steel joists are manufactured in a factory to precise specifications and delivered to the job site ready to install, which speeds up construction compared to field-fabricated framing.
Types and Series
Steel joists are classified into standard series based on their load capacity and span range. The most common are:
- K-Series: The workhorse of steel joists, used for the majority of roof and short-to-medium-span floor applications. K-Series joists handle spans up to about 60 feet and are the lightest and most economical option for typical loads.
- LH-Series (Longspan): Designed for longer spans and heavier loads than K-Series joists. LH joists can reach spans of 96 feet and are common in gymnasiums, auditoriums, and large retail spaces.
- DLH-Series (Deep Longspan): The heaviest-duty standard joist, capable of spanning up to 144 feet. These are used in large industrial facilities, airplane hangars, and convention centers.
Each series has published load tables that engineers use to select the right joist depth, weight, and designation for a given span and load combination. The Steel Joist Institute (SJI) publishes these tables. The current edition, the 45th Edition published in July 2020, expanded the LH-Series tables to cover shorter spans with higher loads, making it easier for designers to specify joists for floor systems.
Joist Girders: The Primary Support
A standard steel joist is a secondary framing member, meaning it spans between larger structural elements. A joist girder is the primary framing member it connects to. Joist girders typically span between two columns and support several standard joists within a given bay (the rectangular area between columns). Think of it this way: the joist girder is the main highway, and the individual joists are the on-ramps feeding into it.
While standard joists are optimized for uniform loads spread evenly across their length, joist girders are specifically designed to handle the concentrated point loads that joists deliver at regular intervals. Depending on joist spacing, the web members of joist girders can be configured differently to align with those load points.
Why Use Steel Joists Instead of Solid Beams
The open-web design gives steel joists several practical advantages over traditional wide-flange steel beams. The most significant is weight savings. Steel joists can achieve up to 35% less material weight compared to wide-flange beams for the same span and load, which translates directly into lower material costs, lighter foundations, and easier handling on site.
The open spaces between the web members also solve a major coordination headache in construction: routing mechanical, electrical, and plumbing (MEP) systems. Ductwork, pipes, and conduit can pass directly through the open web without anyone needing to cut holes in a solid beam or hang systems below it. This is a big deal for controlling floor-to-floor height. In a building with solid beams, MEP systems often run below the beams, adding inches or even feet to each story. With steel joists, those systems tuck inside the joist depth, keeping the overall building height down and reducing exterior cladding, elevator shaft length, and other costs that scale with building height.
Because joists can be made deeper without a proportional increase in weight (unlike solid beams, which get dramatically heavier as they get taller), designers can increase joist depth to gain stiffness and pass-through space simultaneously. Flush-frame connections that eliminate traditional joist seats can reduce story height even further.
Bridging: Keeping Joists Stable
Steel joists are tall and narrow, which makes them prone to buckling sideways, especially before the floor or roof deck is attached. Bridging solves this problem. Bridging consists of small steel members that run perpendicular to the joists, connecting them to each other and preventing lateral movement.
There are two main types. Horizontal bridging uses steel angles bolted between the top chords (and sometimes bottom chords) of adjacent joists. Diagonal bridging uses cross-braced members that form an “X” pattern between joists. Most installations use a combination of both.
Bridging is so critical to safety that OSHA regulates exactly when it must be installed during erection. For longer-span joists, a row of bolted diagonal erection bridging must be installed near the midspan before hoisting cables can be released. Until all bridging is installed and anchored, no more than one worker is allowed on those spans. This is one of the most tightly regulated aspects of steel joist construction because an unbridged joist can roll over with very little force.
How Steel Joists Get Installed
Erecting steel joists is a carefully sequenced process governed by both engineering requirements and OSHA safety regulations. When a joist is landed on a structure by crane, it must be immediately secured to prevent it from shifting before full attachment. The support structure itself must be stabilized before any joist is placed on it.
Each joist gets attached to its supports at both ends through welding or bolting. K-Series joists require a minimum of two 1-inch-long fillet welds or two half-inch bolts at each end. The heavier LH and DLH series require larger welds or three-quarter-inch bolts. For joists over 60 feet long, both ends must be fully attached and all required bridging installed before the crane releases its cables.
Where columns aren’t braced in two directions by solid steel members, the joist itself provides lateral stability to the column. In these cases, the joist must be field-bolted at the column, and a vertical stabilizer plate (at least 6 inches by 6 inches) extends below the bottom chord to provide an attachment point for guying cables. The bottom chord must also be restrained against rotation. These requirements exist because during erection, before the full structural system is connected, individual columns and joists are vulnerable to collapse.
Common Applications
Steel joists dominate single-story commercial construction in the United States. If you’ve been inside a warehouse, a strip mall, a grocery store, or a school gymnasium, you’ve almost certainly looked up at open-web steel joists. They’re the go-to choice for roof framing over large open areas because they span long distances at low cost and low weight.
In multi-story buildings, steel joists are increasingly used for floor systems as well. Composite floor joist systems, where a concrete deck bonds to the top chord of the joist, combine the spanning efficiency of the joist with the stiffness and mass of concrete. This approach handles floor vibration (a key comfort concern in office buildings) while still delivering the weight and cost savings that make joists attractive. The SJI offers a free Floor Bay Comparison Tool that lets designers compare joist-based floor systems against wide-flange alternatives across cost, weight, vibration, and depth.
Steel joists are not typically used in residential construction. Wood framing and engineered wood products fill that role at a lower cost for the shorter spans and lighter loads involved. You’ll also rarely see steel joists in heavy industrial settings where concentrated loads, dynamic forces, or extreme conditions call for solid steel or concrete framing instead.