A bar joist is a lightweight steel truss, made from a top chord, a bottom chord and a zigzag web of rods or angles, that holds up a floor or roof deck across long spans. The industry name is an open web steel joist, and it is the reason most warehouses, big-box stores and school gyms have that familiar ceiling of repeating steel triangles.
This guide covers how a bar joist carries load, how to read a joist designation, how joists differ from solid rolled beams, and the erection rules that make them safe to stand on.
What a Bar Joist Is Made Of
A bar joist is a small parallel-chord truss, built in a shop and delivered ready to set. It has four basic parts:
- Top chord. Usually a pair of steel angles set back to back with a gap between them. It is in compression under gravity load and it is what the deck sits on.
- Bottom chord. Another pair of angles. Under gravity load it is in tension.
- Web. Round bars bent into a continuous zigzag on lighter joists, or individual angles on heavier ones. The web carries the shear between the chords.
- Bearing seats. Short angle assemblies at each end that sit on a wall, a beam or a joist girder, and get welded or bolted in place.
The “bar” in the nickname comes from those bent round-bar webs on the lightest joists. Once the webs are angles, people still call them bar joists.
How a Bar Joist Carries Load
A solid beam and a bar joist resist bending in the same basic way: the top edge is squeezed, the bottom edge is stretched, and the further apart those two edges are, the more bending the member can take. The difference is what sits in the middle.
In a wide flange or I-beam, a solid web plate connects the flanges. In a bar joist that web is replaced by a triangulated pattern of diagonals, which does the same job of holding the chords apart and passing shear between them, but with far less steel. That is the whole trick. A joist can be made deep, which is what gives it stiffness and strength, without paying for a deep solid web.
The trade-off is that a joist is designed to take load in a particular way. It expects roughly uniform load spread along the top chord by a deck, and it expects to bear at its seats. Hanging a heavy concentrated load from the middle of a chord panel, between web points, bends that chord locally in a way it may not have been designed for. That is why rooftop units, hoists and sprinkler mains hung from joists are normally coordinated with the engineer and the joist manufacturer rather than just clamped on wherever they fit.
Joist Series and How to Read a Designation
Open web joists in the US are designed and specified to Steel Joist Institute standards. The SJI Standards and Specifications page (checked September 2026) lists the current documents: SJI 100-2025 for K-Series, LH-Series and DLH-Series open web joists and for joist girders, and SJI 200-2025 for CJ-Series composite joists. Each series has its own load and weight tables.
| Series | What it is | Typical use |
|---|---|---|
| K | Standard open web joist, the lightest series | Roofs and floors of ordinary commercial buildings |
| LH | Longspan joist | Longer spans and heavier loads than K joists |
| DLH | Deep longspan joist | The longest joist spans, often roofs of large open buildings |
| CJ | Composite joist | Floors where the joist works together with a concrete slab |
| Joist girder | A heavier truss that supports joists | Replaces a rolled girder beam between columns |
A joist designation reads in the same order every time: depth in inches, then the series letters, then a chord size number. So 18K4 is an 18 in deep K-series joist with chord size 4. A larger chord number within the same depth means heavier chords and more capacity. Joist girders use a different format that also lists the number of joist spaces and the panel point load, because a joist girder is designed for the specific joists landing on it.
The capacity of any particular joist comes from the SJI load tables or from the manufacturer’s design, not from its depth alone. Two joists of the same depth can have very different capacities.
Bar Joist vs Steel Beam
Joists and rolled beams often compete for the same job, and each wins in different buildings. If you want the bigger picture of where a joist sits in the framing hierarchy, see our guide to girders vs beams vs joists.
| Open web bar joist | Rolled W-shape beam | |
|---|---|---|
| How it is made | Shop-fabricated truss, designed per job by the joist supplier | Hot-rolled at a mill, cut and detailed by a fabricator |
| Weight for the span | Light, because most of the web is air | Heavier, because the web is solid |
| Depth | Deep for the load, which adds building height | Shallower for the same load |
| Services | Ducts, pipes and conduit pass through the web openings | Services run below, or through fabricated web holes |
| Concentrated loads | Must be coordinated; best at panel points | More tolerant of point loads along the span |
| Best fit | Long, repetitive, lightly loaded roof and floor bays | Shorter spans, heavy or irregular loads, tight depth limits |
For a sense of what a light rolled beam looks like, a W16x26 is 15.7 in deep with a 5.50 in flange and weighs 26 lb per foot (AISC Shapes Database v16.0). A joist of similar depth can be considerably lighter per foot for a roof load, which is why long roof bays tend to go to joists while short, heavily loaded floor bays stay with rolled beams. The wide flange beam sizes chart has the full list of W-shapes for comparison.
What Sits on a Bar Joist
Joists are almost always paired with steel decking. On roofs it is a fluted roof deck with insulation and membrane on top. On floors it is form deck or composite deck with a concrete slab. The deck does two jobs: it spreads load evenly along the top chords and, once fastened, it braces the top chords against sideways buckling.
That second job matters. A top chord in compression wants to buckle sideways, and until the deck is attached and the bridging is in, the joist has very little resistance to that. This is the root of most joist erection rules.
Bridging and Erection Safety
Bridging is the line of small angles or rods that runs perpendicular to the joists, tying them together so they cannot roll over or buckle one at a time. There are two kinds: horizontal bridging, which runs continuously along the top and bottom chords, and diagonal bridging, which forms an X between adjacent joists.
Federal construction rules take joist stability seriously because a lone joist is unstable during erection. OSHA 29 CFR 1926.757 on open web steel joists (checked September 2026) sets out, among other things:
- Joists at columns must be field-bolted at the column to provide lateral stability, and hoisting cables are not released until the seat at each end is field-bolted (1926.757(a)(1)).
- For spans over 60 ft up to 100 ft, two rows of bolted diagonal erection bridging are required near the third points (1926.757(d)(2)).
- For spans over 100 ft up to 144 ft, all rows of bridging must be bolted diagonal bridging, and hoisting cables stay attached until all bridging is installed (1926.757(d)(3)).
- No construction loads go on joists until all bridging is installed and anchored and all bearing ends are attached, with a narrow exception for deck bundles that meets conditions in the rule (1926.757(e)).
The rule also includes tables of the span at which erection bridging becomes mandatory for specific joist sizes, which is why the erection plan and the joist supplier’s drawings are part of the paperwork on any joist job.
Where Bar Joists Are Used, and Where They Are Not
Good fits
- Warehouse, retail and industrial roofs with long, repetitive bays.
- School gyms, churches and community halls that need open floor area.
- Office floors using composite joists with a concrete slab.
Poor fits
- Houses. A residential garage or basement beam is usually a rolled beam, LVL or flitch beam, sized for a short span and a point load from a wall or post.
- Bays with heavy concentrated loads, such as mechanical equipment or crane runways, unless the joists are specially designed for them.
- Floors with a tight overall depth limit, where a shallower rolled beam wins.
Modifying Existing Bar Joists
Renovations often run into existing joists: someone wants to add a rooftop unit, hang a heavier sprinkler main, cut a web member for a duct or weld on a support. Each of these changes how the truss carries load. Cutting a single web diagonal can remove the load path for that panel. Welding heat can affect thin chord angles. Adding a point load between panel points bends the chord locally.
Joists can usually be reinforced, but the reinforcement has to be designed. A licensed structural engineer should check the existing joist, usually with the original joist designation from the drawings or a tag on the joist, before anything is added, cut or welded.
Frequently Asked Questions
Is a bar joist the same as an open web steel joist?
Yes. “Bar joist” is the trade nickname, “open web steel joist” is the formal term used in SJI standards and OSHA rules. Both describe a shop-built steel truss used as a joist.
What does 18K4 mean on a joist?
It is an 18 in deep K-series joist with chord size 4. The load it can carry at a given span comes from the SJI K-Series load table, not from the name alone.
How far can a bar joist span?
It depends on the series, depth, chord size and load. K joists cover ordinary spans, LH and DLH joists cover progressively longer ones, and the SJI load tables give the actual limits. A licensed structural engineer or the joist manufacturer has to select the joist for a real project.
Can I hang things from a bar joist?
Light items are routinely hung at panel points. Heavy or concentrated loads, and anything hung between panel points, should be checked by an engineer because the joist was designed for a specific load pattern.
Why do bar joists need bridging?
The top chord is a slender member in compression. Without bridging and an attached deck it can buckle sideways or the joist can roll over, especially during erection. Bridging ties the joists together so they act as a stable system.
Reference information only, not engineering advice. Disclaimer.