12.7 Steel Joists, Girders & Metal Deck
Key Takeaways
- Open-web steel joist designations such as 24K9 encode depth in inches, series, and chord section number
- Bridging holds joists laterally during erection and in service; the number of rows comes from the joist tables and bridging must be installed and anchored before workers stand on the joist
- No construction loads may be placed on a joist until all bridging is completely installed and anchored
- Metal deck is classified as form deck, roof deck, or composite floor deck, and composite deck acts structurally with the concrete topping through embossments
- Deck attachment — puddle welds, screws, or power-actuated fasteners plus sidelap fasteners — is what makes the deck a structural diaphragm
The Systems Above Your Head
Quick Answer: Open-web steel joists and joist girders span between beams and columns; metal deck spans between joists and forms the roof or floor surface. Together they account for a large share of the Metals (13 items) questions on the AZ ROC commercial exam, and they are the source of some of the most serious construction-phase collapses in the industry.
The economics are simple: an open-web joist gets the same span as a rolled beam at a fraction of the weight, because the web is mostly air. That efficiency is also the hazard — until a joist is bridged and decked, it has almost no lateral stability.
Reading Joist Designations
| Designation | Meaning |
|---|---|
| 24K9 | 24 in. deep, K series (open-web steel joist), chord section 9 |
| 28LH10 | 28 in. deep, LH series (longspan), chord section 10 |
| 52DLH15 | 52 in. deep, DLH series (deep longspan) |
| 44G8N9.6K | Joist girder: 44 in. deep, 8 joist spaces, 9.6 kip panel point load |
| Series | Typical span range |
|---|---|
| K — open web steel joists | Short to medium spans |
| LH — longspan | Longer spans |
| DLH — deep longspan | Long-span roof structures |
| Joist girders | Support joists at panel points, framing into columns |
The chord number is a relative section size, not a dimension: a 24K9 has heavier chords and greater capacity than a 24K4 at the same depth. Load tables published by the joist institute give allowable total and live loads by span for each designation, and the structural drawings identify the required designation plus any special loads such as suspended mechanical units.
Bridging — the Rule That Prevents Collapses
Bridging is the horizontal and diagonal bracing between joists. It does two jobs: it stabilizes the joists laterally during erection, and it braces the compression chord in service.
| Type | What it is |
|---|---|
| Horizontal bridging | Continuous angles or bars attached to the top and bottom chords |
| Diagonal (cross) bridging | X-braces between adjacent joists |
The number of rows required comes from the joist tables and increases with span. Two rules control the field:
- Bridging must be completely installed and anchored before any construction loads are placed on the joists. That includes workers, bundles of deck, welding equipment, and stacked material. An unbridged joist under a bundle of deck will roll over.
- Bridging terminus points must be anchored — the end of each bridging line must be fastened to a wall, beam, or other point that can take the force, or the line simply moves as an assembly.
Deck bundles are the specific hazard the industry keeps relearning. Landing a full bundle of deck on a few unbridged joists concentrates a load the system was never designed to take in that configuration. Bundles are placed only over or adjacent to bearing points, distributed as the erection plan directs, and only after bridging is complete.
Warning: joists are designed for uniformly distributed loads applied at the panel points. Hanging a mechanical unit, a pipe, or a scoreboard from a mid-panel web member is not a field decision — concentrated loads require the joist manufacturer's or engineer's direction, and often a reinforcing detail.
Metal Deck
| Deck type | Function |
|---|---|
| Form deck | Serves only as a stay-in-place form for a concrete slab; the concrete carries the load once cured |
| Roof deck | Spans between joists and supports roof insulation and membrane; typically type B (1½ in. rib) |
| Composite floor deck | Acts structurally together with the concrete topping through embossments rolled into the flutes that key into the slab |
| Cellular deck | Adds a flat bottom plate to create raceways for electrical distribution |
The composite distinction is the exam item. Composite deck is not a form; the embossed ribs transfer horizontal shear between deck and slab so the two act as one member. Substituting plain form deck where composite deck was specified produces a floor with a fraction of its design capacity and no visible difference from below once it is painted.
Attachment is what converts loose sheets into a structural diaphragm capable of carrying lateral wind and seismic load to the frame:
| Attachment | Where |
|---|---|
| Puddle (arc-spot) welds, often with weld washers on thin deck | Deck to supporting steel, at the pattern shown (e.g., 36/4 or 36/7) |
| Screws or power-actuated fasteners | Alternative support attachment on many projects |
| Sidelap fasteners — button punch, screw, or weld | Deck sheet to adjacent deck sheet, at the specified spacing |
Sidelap fasteners are the most commonly shorted item in the field and one of the most consequential: without them, adjacent sheets slide relative to one another and the diaphragm does not develop its rated shear.
Shear studs on composite beams are welded through the deck to the beam flange in the number and pattern the drawings specify. Miscounting studs is a capacity reduction that is invisible after the slab is poured.
What does the joist designation 24K9 describe?
When may construction loads such as deck bundles be placed on newly set open-web steel joists?
A submittal substitutes plain form deck for the specified composite floor deck. What is the consequence?
Why do metal deck sidelap fasteners matter?