Special Structures
Open-Web Steel Joists in Pre-Engineered Buildings
Definition: An open-web steel joist is a lightweight, shop-fabricated steel truss with top and bottom chords connected by a web of diagonal members, used to span between beams, walls or main frames to support roof or floor decks over longer distances than solid secondary members.
Key facts
- Used where secondary spans are longer than typical cold-formed purlins can carry economically.
- The open web allows ducts, pipes and cables to pass through the structural depth.
- Top chords are usually made from double angles or cold-formed sections, webs from rods, angles or tubes.
- Bridging rows are required for stability during erection and in service.
- Joist girders can be used as primary members that support joists at panel points.
Role in the structure
Joists carry roof or floor loads from the deck and pass them as end reactions to the supporting beams, girders or main frame members. The top chord, braced by the deck, resists compression, while the bottom chord resists tension and the web members carry shear. Bridging between joists holds the chords in position until the deck is fixed and helps resist load reversal from wind uplift.
When are open-web joists used in metal buildings?
In most pre-engineered buildings, Z or C purlins span between main frames at common bay spacings. When bays become wider, when the roof must carry heavy collateral loads such as sprinklers and services, or when a mezzanine floor needs long clear spans, open-web joists can be more efficient than solid members.
They are also used where the structure combines main frames with wide bays, such as warehouses designed for racking layouts, and in hybrid systems where joists span onto rafters or girders rather than conventional frame lines.
Parts of an open-web joist
A joist has a top chord, a bottom chord and web members arranged in a triangulated pattern. Bearing seats at each end sit on the supporting member and are bolted or welded in place.
Depth, chord sizes and web arrangement are chosen by the designer to suit span and load. Deeper joists are lighter for a given span but take more height, so the depth is balanced against eave height and clearance below.
Bridging, bearing and uplift
Joists are slender until the deck is attached, so horizontal or diagonal bridging is installed in rows across the joists as they are erected. Bridging is anchored at the ends to walls, braced frames or other rigid points.
Roof joists in windy regions may see net uplift, which puts the bottom chord into compression. The bridging layout and bottom chord bracing must be designed for this reversal, and the bearing connection must resist the uplift reaction.
Materials & coatings
- Hot-rolled angles or cold-formed sections for chords in structural steel grades to the specified code
- Round bars, angles or tubes for web members
- Bolted or welded bearing seats and bridging angles or rods
- Shop primer or galvanizing selected for the environment
Design considerations
- Define spans, spacing and all gravity, collateral, wind uplift and concentrated loads.
- Locate concentrated loads from equipment or hung services near panel points or design for them.
- Check deflection, especially for floors and roofs that support brittle finishes.
- Design bridging and bottom chord bracing for load reversal from uplift.
- Coordinate openings in the web zone for services before fabrication.
Fabrication
- Chords and webs are cut to length and assembled in jigs to control geometry and camber.
- Welds at web-to-chord joints are made and inspected to the specified procedure.
- Bearing seats are fitted and drilled to match the supporting member.
- Joists are marked with their mark number and orientation before dispatch.
Erection
- Lift joists at the points recommended by the manufacturer to avoid buckling.
- Fix bearing seats immediately after placing each joist.
- Install bridging rows before releasing the hoist or loading the joists.
- Do not place bundles of deck or materials on unbridged joists.
Inspection & quality
- Check bearing lengths and seat connections at each end.
- Confirm bridging is complete and anchored as shown on the drawings.
- Look for bent web members or damaged chords caused by handling.
- Verify that any field-added loads are at panel points or approved by the engineer.
What to include in your RFQ
- Joist spans, spacing and support conditions.
- Roof or floor use, design loads and collateral loads.
- Concentrated loads from equipment, cranes or hung services and their locations.
- Maximum depth available and required clearance below.
- Coating or galvanizing requirements.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcBeam (JOIST). Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Are open-web joists better than Z-purlins?
Neither is always better. Purlins are efficient for common bay spacings, while joists become economical for longer spans or heavier loads. The project engineer chooses based on span, load and cost.
Can services be hung from open-web joists?
Yes, if the loads are included in the design. Hangers are usually placed at panel points so the chords are not bent locally.
Why is bridging so important?
Joists are unstable sideways until bridged and decked. Bridging prevents the chords from buckling during erection and under uplift, and it must be installed as the joists are placed.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
