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Structuralaka: bar joistaka: open-web steel joist

Joist

In Plain English

A series of closely spaced beams that directly support a floor or roof deck.

Definition

A closely spaced, secondary horizontal framing member that directly supports floor or roof decking. Joists span between beams, girders, or bearing walls and are typically lighter than the primary framing they rest upon. Steel bar joists, wood dimensional lumber, and engineered wood joists are common types.

Why It Matters in Bidding

Joists drive significant quantities in structural takeoffs because they repeat across every floor and roof bay, so spacing, span, and joist type directly affect material counts, connection hardware, and labor hours. Estimators must read joist schedules carefully since substituting steel bar joists for engineered wood, or changing on-center spacing, swings both material pricing and the deck and fireproofing that ride on top of them.

Example

Taking off a warehouse roof, the estimator counts the open-web steel bar joists at the specified on-center spacing across each bay, then adds the joist girders, bridging, and bearing connections before pricing the roof deck that lands on them.

Related Terms

BeamGirderPurlinSpan

Frequently Asked Questions

Joists are closely spaced secondary members that directly carry floor or roof decking and span shorter distances. Beams and girders are heavier primary members that the joists bear on, collecting those loads and carrying them to columns or walls. In takeoff, joists are counted in repetitive series while beams are tallied individually by mark.
Tighter on-center spacing means more joists, more connections and bridging, and more installation labor, but it can allow thinner, cheaper decking. Wider spacing reduces joist count but demands heavier deck. Estimators price the system as a whole, since changing spacing shifts cost between the joists, the deck, and any fireproofing above.
Open-web steel bar joists are standard for commercial roofs and floors over long spans, often paired with joist girders. Engineered wood I-joists and dimensional lumber joists dominate residential and light commercial floors. The chosen type changes lead time, connection details, fireproofing needs, and pricing, so estimators confirm it against the structural drawings.

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