Composite Steel Deck — Shear Stud Design Guide

Q: How is shear stud capacity calculated for composite decks?

Per AISC 360 I3-2d, the nominal shear strength of a stud in composite deck is: Qn = 0.5Asc√(fc'Ec) ≤ AscFu. For deck ribs perpendicular to the beam, an additional reduction factor of 0.85/√Nr applies where Nr ≤ 3 studs per rib. Stud height must extend at least 1.5 inches above the top of the deck after welding.

Q: What deck profile limitations affect composite action?

Per SDI and AISC 360 I3-2a: the deck rib depth cannot exceed 3 inches (75 mm), the average rib width must be at least 2 inches (50 mm), and the slab thickness above the deck flute must be at least 2 inches (50 mm) for fire rating and 2.5 inches (64 mm) for composite action. Studs can be welded through the deck if the paint/coating is less than 1.5 mils total.

Q: How are construction loads handled in composite deck design?

During construction, the steel deck alone must support wet concrete weight, construction live load (20 psf per SDI), and self-weight. Deflection limits of L/180 or L/240 apply to the unshored deck. Per AISC 360 I3-1, the steel beam must also be checked for non-composite construction loads before the concrete cures. Shoring may be required for longer spans.

Q: What is the minimum concrete cover above the steel deck flute?

Per AISC 360 I3-2a and SDI standards, the minimum concrete cover above the top of the steel deck flute is 2 inches (50 mm) for fire rating and 2.5 inches (64 mm) for composite action. For a 2-inch deck with 4-inch total slab thickness, the cover above the flute is 2 inches (4 - 2 = 2 inches). The cover requirement ensures: (1) adequate fire resistance per IBC 2021 Table 601, (2) sufficient concrete above the deck for shear stud embedment (minimum 1.5 inches above the top of the deck after welding), and (3) crack control under service loads.

Q: How are shear studs welded through the steel deck?

Shear studs can be welded directly through the galvanized steel deck using the drawn-arc stud welding process per AWS D1.1 Clause 7. Key requirements: (1) total paint/coating thickness on the deck must not exceed 1.5 mils (0.0015 inches) at the weld location, (2) the deck must be in tight contact with the beam flange (gap ≤ 1/16 inch), (3) through-deck welding requires a ceramic ferrule to contain the weld puddle, (4) stud diameter limited to 3/4 inch or 7/8 inch for through-deck welding, and (5) minimum stud height of 4 inches to ensure adequate embedment. Studs are tested per AWS D1.1 by bending 30 degrees from vertical.

Composite steel deck systems combine cold-formed steel decking with a reinforced concrete slab, connected by shear studs for composite action. This guide covers design provisions per AISC 360 Chapter I3 and SDI (Steel Deck Institute) standards.

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Understanding Composite Steel Deck Systems

Composite steel deck systems are widely used in steel-framed buildings for floor and roof construction. The system consists of cold-formed steel deck panels spanning between steel beams, a reinforced concrete topping slab cast over the deck, and shear studs welded through the deck to the steel beam flange. The deck serves as: (1) a working platform during construction, (2) formwork for the concrete slab, (3) positive moment reinforcement for the slab, and (4) part of the composite system through mechanical interlock with the concrete ribs.

The composite action between the steel beam and the concrete slab significantly increases the flexural strength and stiffness of the floor system compared to a non-composite design. A composite beam can carry 30-60% more load with the same steel section, or alternatively, a lighter steel section can be used for the same load.

AISC 360 Chapter I3 Provisions

AISC 360 Section I3 provides the design requirements for composite beams with formed steel deck:

Shear stud strength (I3-2d): The nominal shear strength of one headed stud in composite deck is the lesser of:

Qn = 0.5 × Asc × √(fc' × Ec) and Qn = Asc × Fu

Where:

Asc = cross-sectional area of the stud shank (πd²/4)
fc' = concrete compressive strength (ksi)
Ec = modulus of elasticity of concrete = ωc^1.5 × 33√(fc') (psi) or wc^1.5 × 0.043√(fcm) (MPa)
Fu = minimum specified tensile strength of stud steel (typically 65 ksi for standard studs)

Reduction factors for deck ribs:

For deck ribs perpendicular to the beam: Rg = 1.0 (one stud per rib), 0.85 (two studs), 0.70 (three studs). Rp = 0.75 (welded through deck), 1.0 (prepunched or predrilled)

For deck ribs parallel to the beam: Rg = 1.0, Rp = 0.75 (studs welded through deck)

The reduced stud strength: Qn,reduced = Rg × Rp × Qn

Limitations per I3-2a:

Maximum deck rib depth: 3 inches (75 mm)
Average rib width: minimum 2 inches (50 mm)
Stud diameter: maximum 3/4 inch (19 mm) for studs welded through deck (except 7/8 inch permitted with special welding procedures)
Stud height after welding: minimum 1.5 inches (38 mm) above top of deck
Concrete slab thickness above the deck flute: minimum 2 inches (50 mm)

Composite Beam Neutral Axis Analysis

The composite beam section is analyzed by transforming the concrete area into an equivalent steel area using the modular ratio n = Es/Ec. The effective width of the concrete slab is determined per AISC 360 I3-1a:

Interior beams: beff = min(L/4, s, 12×ts + bf/2) where L is the span, s is the beam spacing, ts is the slab thickness, and bf is the beam flange width.

Edge beams: beff = min(L/8 + distance to edge, s/2 + distance to edge, 6×ts + bf/2)

Partial composite action: The stud strength may limit the composite action. The degree of partial composite action ΣQn/(C or T) determines whether full or partial composite action is achieved. Per AISC I3-1d, minimum 25% composite action is required (ΣQn ≥ 0.25C).

At the strength limit state, the nominal moment capacity Mn is determined from the stress distribution, with concrete at 0.85fc' and steel at Fy. The plastic neutral axis (PNA) location depends on the degree of composite action:

Full composite action: ΣQn ≥ min(As×Fy, 0.85fc'×Ac). The PNA may be in the slab or the steel section.

Partial composite action: ΣQn < min(As×Fy, 0.85fc'×Ac). The PNA is in the steel section, and the horizontal force at the steel-concrete interface is ΣQn.

Construction Load Design

During construction, the steel deck alone must resist:

Wet concrete weight — Typically 100-120 psf (4.8-5.7 kN/m²) for 5-inch slab on 3-inch deck, or 75-90 psf (3.6-4.3 kN/m²) for lightweight concrete

Construction live load — 20 psf (0.96 kN/m²) minimum per SDI, plus 20 psf for work crews and equipment for a total of 40 psf during concrete placement

Construction dead load — Deck self-weight (2-4 psf), reinforcing steel (1-2 psf), and miscellaneous

Deflection limits during construction: Per SDI, the combined deck + concrete deflection must not exceed L/180 for spans up to 10 ft, and L/240 for longer spans. Deflection of the steel beam during concrete placement must be checked for non-composite section strength. Per AISC I3-1, the steel beam alone must support the wet concrete weight and construction live load.

Shoring may be required when: (1) the steel beam deflection during construction exceeds L/240, (2) the beam flexural stress during construction exceeds 0.9Fy, or (3) the slab thickness exceeds 6 inches. Shoring is typically removed after the concrete reaches 75% of its design strength.

Deck Profile Requirements

Cold-formed steel deck comes in various profiles categorized by rib depth:

1.5-inch deck (WR/G deck): Used for short spans (up to 6 ft) and light loads. Typical slab thickness: 4 inches total. Rib width: approximately 6 inches. Common in residential and light commercial.

2-inch deck (B deck): Most common profile for composite floors. Span range: 6-10 ft. Typical slab: 4.5-5 inches total. Rib spacing: 6 inches.

3-inch deck (N deck): Used for longer spans (8-12 ft) and heavier loads. Typical slab: 5-6 inches total. Rib width: approximately 6 inches at top.

The deck gauge (thickness) ranges from 22 ga (0.030 inch) to 16 ga (0.060 inch), selected based on span and construction loads. The SDI Design Manual provides span-load tables for each deck profile and gauge.

Shear Stud Welding Through Deck

Studs are welded through the deck to the beam flange using a drawn arc stud welding process. The welding gun holds the stud against a ceramic ferrule, an arc is struck between the stud and the beam flange, and the stud is plunged into the molten pool. Key requirements:

Paint/coating limitations: Total coating thickness (deck paint + beam primer) must not exceed 1.5 mils (0.0015 inches) for through-deck welding. Thicker coatings require studs to be welded to bare flanges in prepunched holes.

Stud orientation: Studs must be positioned within the rib — not on the rib crest. For deck ribs perpendicular to the beam, studs can be staggered or inline.

Quality control: Per AWS D1.1 Clause 7, each stud must be tested by: (1) visual inspection — full 360-degree fillet, no underfill exceeding 1/16 inch, (2) bend testing — 2 studs per 100 bent 30 degrees from vertical, and (3) torque testing — minimum 500 in-lbs for 3/4-inch studs.

Deflection and Vibration Control

Composite beam deflection — The transformed moment of inertia Itr is used for deflection calculations. For partial composite action, an effective moment of inertia Ieff is computed per AISC I3-1c: Ieff = Is + √(ΣQn/C) × (Itr - Is).

Long-term deflection — Under sustained loads, concrete creep reduces the effective stiffness. The modular ratio is increased by a factor of 2-3 for long-term loading, depending on the aggregate type and concrete age at loading. The tool computes both short-term and long-term (creep-adjusted) deflections.

Floor vibration — The natural frequency of the composite floor system is computed as fn = 0.18 × √(g/ΔDL), where g is gravity (386 in/s²) and ΔDL is the dead load deflection (inches). A minimum natural frequency of 4-6 Hz is recommended for walking vibration comfort per AISC Design Guide 11.

Frequently Asked Questions

How is shear stud capacity calculated for composite decks? Per AISC 360 I3-2d, the nominal shear strength of a stud in composite deck is: Qn = 0.5Asc√(fc'Ec) ≤ AscFu. For deck ribs perpendicular to the beam, an additional reduction factor of 0.85/√Nr applies where Nr ≤ 3 studs per rib. Stud height must extend at least 1.5 inches above the top of the deck after welding.

What deck profile limitations affect composite action? Per SDI and AISC 360 I3-2a: the deck rib depth cannot exceed 3 inches (75 mm), the average rib width must be at least 2 inches (50 mm), and the slab thickness above the deck flute must be at least 2 inches (50 mm) for fire rating and 2.5 inches (64 mm) for composite action. Studs can be welded through the deck if the paint/coating is less than 1.5 mils total.

How are construction loads handled in composite deck design? During construction, the steel deck alone must support wet concrete weight, construction live load (20 psf per SDI), and self-weight. Deflection limits of L/180 or L/240 apply to the unshored deck. Per AISC 360 I3-1, the steel beam must also be checked for non-composite construction loads before the concrete cures. Shoring may be required for longer spans.

What is the minimum concrete cover above the steel deck flute? Per AISC 360 I3-2a, the minimum concrete slab thickness above the top of the steel deck flute is 2.0 inches (50 mm) for fire resistance rating and 2.5 inches (64 mm) for composite action to develop the shear stud capacity. For fire-rated assemblies, the cover may need to increase to 2.5-3.0 inches depending on the required fire rating (per UL design listings). Total slab depth (deck flute + cover) typically ranges from 4.5 to 6.5 inches for composite floors.

How are shear studs welded through the steel deck? Shear studs are welded through the deck using the drawn arc stud welding process. A ceramic ferrule concentrates the arc heat and shapes the weld fillet. The welding gun lifts the stud, an arc is struck melting the stud tip and beam flange, and the stud is plunged into the molten pool to form the weld. Per AWS D1.1 Clause 7, the minimum weld fillet must be continuous around the stud base, and the stud must be free of underfill exceeding 1/16 inch. Studs welded through deck require the deck to be tightly clamped to the beam flange (gap ≤ 1/8 inch) and clean of moisture, oil, and heavy coatings.

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