Open Web Steel Joists (Bar Joists)
Light steel trusses that span between beams or walls to carry roofs and floors.
Quick Answer
Open web steel joists, often called bar joists, are lightweight steel trusses with top and bottom chords joined by web members. The Steel Joist Institute designates them by depth, series letters (K, LH, DLH) and section number, such as 18K4. Estimators count them by piece and length, with bridging and accessories added.
The Full Picture
A bar joist is a small truss: top and bottom chords connected by a zigzag of web members, usually round bars or angles. The open web makes it light for its span and leaves room for ducts and pipes. Joists typically sit at regular spacing on beams, joist girders or bearing walls and carry metal roof deck or floor deck.
The Steel Joist Institute standard SJI 100 covers K-series, LH-series and DLH-series open web joists and Joist Girders; the current edition is SJI 100-2025. K-series are the standard shallow joists, while LH (longspan) and DLH (deep longspan) cover longer spans and heavier loads. A designation reads nominal depth, series and section number, for example 18K4 or 32LH10, or a load-based format that lists total and live load. The structural drawings usually carry a joist schedule or notes giving designations, spacing and special loads.
Bridging ties the joists together and braces the chords. Horizontal and diagonal bridging, plus bridging anchors, are counted separately from joists and sometimes shown only in a general note or detail. Erection also has safety rules: under 29 CFR 1926.757, connections of joists in bays of 40 feet or more must be fabricated for field bolting, with exceptions, and OSHA sets bridging requirements by span, so long spans carry extra bridging and erection steps.
Takeoff is by piece, grouped by designation and length, with total linear feet useful as a check. Bridging is taken off in linear feet by row, and accessories such as bearing seats, extended ends, outriggers, headers and ceiling extensions are counted by each. Joist suppliers often quote a package including joists, bridging and accessories, so scope comparison between bids should list those items, plus whether deck, painting and erection are included.
Real Examples
Common Misconceptions
People assume: Bar joists are priced like any other structural steel, by the ton.
Actually: Joists are often quoted by the piece or by the linear foot of each designation, with bridging and accessories separate, though some suppliers quote by weight. Ask which basis the quote uses.
People assume: Bridging is minor and can be ignored in a takeoff.
Actually: Bridging is required by the standard and by OSHA erection rules for many spans, and it adds both material and labor. Count it on its own line.
People assume: A joist designation is a weight.
Actually: It is a nominal depth, a series and a section number. Section numbers correspond to load capacities in published tables, not to a weight per foot you can use directly.
Does MeltPlan Solve This?
Partially — adjacentMeltPlan's takeoff pulls quantities from PDF plans, and US-based estimators review and adjust them, which covers counting joists and measuring bridging from framing plans. Joist design and supplier pricing are outside its scope.
Count joists and bridging rows from roof framing plans →Frequently Asked Questions
What does 18K4 mean on a joist?
It is a Steel Joist Institute designation: an 18 inch nominal depth, K series, section number 4. Load tables for that section give its capacity by span.
What is the difference between K, LH and DLH joists?
K-series are standard joists for shorter spans. LH and DLH are longspan and deep longspan series for longer spans and heavier loads, with different depth ranges.
How are bar joists quantified?
By piece and length for each designation, bridging in linear feet, and accessories by each. Many suppliers quote them as a package.
What is a joist girder?
A Joist Girder is a truss-type member that supports joists. It is covered in the same SJI standard but is a separate line in takeoff and quotes.
Where are joists shown on the drawings?
On roof and floor framing plans with a joist schedule or general notes, plus details for bearing, bridging and extended ends.