8.3 Steel Joists, Deck & Cold-Formed Framing

8.3 Steel Joists, Deck & Cold-Formed Framing

Commercial buildings across Georgia rely heavily on lightweight, high-strength structural metal assemblies to create expansive column-free interior spaces and efficient floor/roof diaphragms. The design, erection, and inspection of these systems are governed by the Steel Joist Institute (SJI), the Steel Deck Institute (SDI), the American Iron and Steel Institute (AISI), and OSHA 29 CFR 1926.757. General contractors must master open-web steel joist series, bearing criteria, mandatory erection bridging tables, metal decking profiles, and cold-formed steel (CFS) framing nomenclature.

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Steel Joist Institute (SJI) Open-Web Steel Joists

Open-web steel joists are lightweight, shop-fabricated structural trusses consisting of parallel top and bottom chord angles separated by triangular zig-zag web lacing (round steel bars or crimped angles). They offer exceptional strength-to-weight ratios for supporting roof and floor decks.

                         OPEN-WEB STEEL JOIST SERIES (SJI)
                                         │
         ┌───────────────────────────────┼───────────────────────────────┐
         ▼                               ▼                               ▼
     K-SERIES                       LH-SERIES                       DLH-SERIES
  • Standard Short-Span          • Longspan Steel Joists         • Deep Longspan Joists
  • Depth: 8" to 30"             • Depth: 18" to 48"             • Depth: 52" to 72"
  • Spans: Up to 60 ft           • Spans: Up to 96 ft            • Spans: Up to 144 ft
  • Bearing: 2-1/2" on Steel     • Bearing: 4" on Steel          • Bearing: 4" on Steel
    4" on Masonry/Concrete         6" on Masonry/Concrete          6" on Masonry/Concrete

1. SJI Joist Classifications & Span Capabilities

• K-Series (Standard Short-Span Joists): Manufactured in depths ranging from 8 inches to 30 inches in 2-inch increments, capable of spanning up to 60 feet. Designed primarily for uniform floor and roof gravity loads. (Specialized KCS joists are engineered to resist uniform loads plus concentrated and non-uniform loads with constant moment capacity along their length).
• LH-Series (Longspan Joists): Manufactured in depths from 18 inches to 48 inches, accommodating clear spans up to 96 feet. Specified for heavily loaded commercial floor systems and long-span roof framing.
• DLH-Series (Deep Longspan Joists): Manufactured in depths from 52 inches to 72 inches, accommodating expansive clear spans up to 144 feet. Used in high-bay distribution warehouses, airport hangars, gymnasiums, and sports arenas.
• Joist Girders (G-Series): Primary structural framing members that span between building columns to directly support the bearing ends of open-web joists at distinct panel points. A joist girder designation such as 48G8N10K indicates a girder with a nominal depth of 48 inches, consisting of 8 joist spaces (panel points), 'N' standard joist loading, and a design concentrated load of 10 kips (10,000 lbs) at each joist panel point.

2. SJI & OSHA Minimum Bearing Length & Attachment Standards

Proper seating and physical attachment of joist bearing seats onto structural supports are critical to prevent slip and shear failure:

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Joist Bridging & Erection Safety (OSHA 1926.757)

Because open-web steel joists are extremely slender before decking is applied, their top compression chords are highly susceptible to catastrophic lateral-torsional buckling. Joist bridging provides lateral restraint during erection and permanent service loading.

Types of Bridging

1. Horizontal Bridging: Continuous structural angle or round bar lines attached to the top and bottom chords across adjacent joists. Must be anchored at wall terminals or cross-braced bays. Used primarily on short to moderate K-series spans.
2. Diagonal Bridging: Cross-bracing angles forming an 'X' pattern between the top chord of one joist and the bottom chord of the adjacent joist. Provides significantly higher torsional stiffness and roll resistance.
3. Bolted Diagonal Bridging: Heavy diagonal bridging connected with high-strength structural bolts at intersection points and chord connections.

  ┌─────────────────────────────────────────────────────────────────────────┐
  │               CRITICAL HOISTING RELEASE SAFETY RULES                    │
  └─────────────────────────────────────────────────────────────────────────┘
     │
     ├─► 1. OSHA 1926.757 / SJI BRIDGING TABLES
     │      Where joist spans fall within the SJI mandatory bridging tables,
     │      HOISTING CABLES CANNOT BE RELEASED until the specified bolted diagonal
     │      bridging is 100% installed and tensioned.
     │
     ├─► 2. BUNDLED MATERIAL PLACEMENT LIMITATIONS
     │      No construction loads or bundled metal decking may be placed on steel
     │      joists until ALL bridging is installed, anchored, and bearing ends attached.
     │
     └─► 3. FIELD MODIFICATION BAN
            Never cut, notch, heat, or splice joist chords or web lacing in the field
            without express, stamped engineering repair details from the Joist Manufacturer
            and the Structural Engineer of Record (SER).

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Steel Deck Institute (SDI) Metal Decking Systems

Corrugated metal decking is roll-formed from structural sheet steel (ASTM A653 or A1008) to create structural floor and roof diaphragms that transfer gravity, wind, and seismic shear loads to the steel framing.

                                STEEL DECK PROFILES (SDI)
                                            │
            ┌───────────────────────────────┼───────────────────────────────┐
            ▼                               ▼                               ▼
      ROOF DECKING                 COMPOSITE FLOOR DECK              FORM DECKING
  • Type A (Narrow Rib)          • Embossed Deformations         • Non-Composite Formwork
  • Type B (Wide Rib - Std)      • Interlocks with Concrete      • Concrete requires full
  • Type F (Intermediate Rib)    • Acts as Tensile Rebar           positive moment rebar
  • Type N (Deep Rib - 3")       • Depths: 1.5", 2", 3"          • Fast, economical pour

1. Metal Roof Deck Profiles

Roof decking acts as a structural diaphragm and substrate for rigid insulation and membrane roofing:

• Type B (Wide Rib): The industry standard commercial roof deck. Nominal depth is 1-1/2 inches with a wide 2-1/2 inch top rib opening, providing high section modulus and an excellent adhesive/fastener substrate for rigid roof insulation.
• Type A (Narrow Rib): 1-1/2" deep with a narrow 1" top rib opening. Used in reroofing or historical matches.
• Type F (Intermediate Rib): 1-1/2" deep with a 1-3/4" rib opening.
• Type N (Deep Rib): 3 inches deep with 6" to 8" rib spacing. Engineered for long spans between joists (up to 12–16 feet), reducing the required number of structural joists.

2. Composite Floor Deck Systems

Composite floor decking features rolled mechanical embossments, ribs, or indentations along the vertical web flutes. When concrete is placed over the deck, these deformations physically interlock with the cured concrete matrix:

• Composite Action: The steel deck serves a dual purpose—it acts as formwork during the wet concrete pour, and after curing, it acts as the primary positive bending tensile reinforcement for the slab.
• Gauge & Depth: Commonly specified in 1-1/2", 2", and 3" depths, in thicknesses ranging from 22 gauge (0.028") to 16 gauge (0.059").
• Welded Wire Mesh (WWM): Secondary welded wire reinforcement (e.g., $6\times6\text{ W1.4/W1.4}$) or synthetic fibers must be placed in the upper slab section to resist concrete shrinkage and temperature cracking.

3. Deck Fastening & Diaphragm Attachment Methods

• Puddle Welds (Arc Spot Welds): High-heat welding through the metal deck into the underlying joist or beam flange, creating a circular fused weld nugget 5/8" to 3/4" in diameter. On thin deck gauges (22 gauge and lighter), weld washers (steel washers with a pre-punched hole) must be placed in the rib bottom prior to welding to prevent arc blow-through and ensure structural fusion.
• Mechanical Fasteners: Powder-actuated fasteners (PAF) and pneumatically driven structural steel pins shot directly through the deck into structural steel flanges; high-speed self-drilling structural screws (#10, #12, #14 hex-head Tek screws).
• Side-Lap Fastening: Adjacent metal deck panels must be stitched together along their longitudinal interlocking side laps at 12" to 36" on center using mechanical button punching (crimping tools), stitch screws, or seam welds to create a continuous lateral shear diaphragm.

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Cold-Formed Steel (CFS) Light-Gauge Metal Framing

Cold-Formed Steel (CFS) framing utilizes thin-gauge galvanized sheet steel (ASTM A653 / A1003) shaped at room temperature through continuous roll-forming presses. CFS is noncombustible (IBC Type I, II, and non-bearing Type III/V walls) and will not rot, shrink, or warp.

AISI Standard Member Designation Nomenclature

The American Iron and Steel Institute (AISI) utilizes a standardized four-part alphanumeric coding system for all cold-formed steel members:

                                600 S 162 - 43
                                 │  │  │     │
     Web Depth (in 1/100"): ────┘  │  │     └──── Minimum Mil Thickness: 43 mils
     600 = 6.00 inches             │  │           (43 mils = 0.043" = 18 Gauge)
                                   │  │
     Member Profile Type: ─────────┘  └────────── Flange Width (in 1/100"):
     S = C-Stud (with stiffening lip)             162 = 1.625 inches (1-5/8")
     T = Track (unlipped channel)
     U = Cold-Rolled U-Channel
     F = Furring / Hat Channel
     L = Angle / L-Header

Standard Mil Thicknesses & Corresponding Gauge Equivalents

Critical CFS Structural Detailing: Deflection & Lateral Bracing

1. Slotted Deflection Tracks & Slip Clips: Concrete floor slabs and structural steel beams naturally deflect downward under live loads. If exterior curtain wall studs or interior partition studs are rigidly fastened directly to the overhead structure, that deflection transfers tremendous axial compressive loads onto the slender studs, causing wall buckling and drywall blowout. Contractors must install slotted deep-leg deflection tracks or slotted slip clips at the top plate, where screws are driven through vertical slots into the stud web, allowing $\pm 1/2"$ to $\pm 1"$ of vertical slab movement while resisting lateral wind forces.
2. Lateral Torsional Bridging: Slender CFS studs under axial compressive loads will buckle torsionally unless braced. Contractors must install continuous Cold-Rolled Channel (CRC) mechanically clipped through stud punchouts at 4-foot to 5-foot vertical intervals, or screw flat tension strapping with solid stud blocking across stud faces.