What is the difference between unshored and shored composite slab construction?

Unshored construction is the standard method for steel composite floors — the steel deck spans between beams and alone supports the wet concrete weight plus a 20 psf construction live load (workers, equipment, concrete buggies). The deck deflection during concrete placement must be limited to L/180 or 3/4" maximum per SDI. After the concrete reaches 75% of f'c (typically 3-7 days), the composite slab acts compositely with the steel beams for superimposed dead and live loads. Shored construction uses temporary supports (shores) under the deck during concrete placement, allowing the deck to span longer distances between beams. When shores are removed after curing, the full dead load is resisted by the composite section rather than the bare deck. Shored construction allows thinner decks (or longer spans for the same deck) because the construction dead load doesn't size the deck — but the shores add 10-20% to the concrete placement cost and slow the construction cycle. Selection guideline: use unshored when the deck span is 4-12 ft for 1.5" deck, 6-14 ft for 2" deck, or 8-18 ft for 3" deck. Use shored when the deck span exceeds these ranges, when minimizing slab depth is critical (shored 1.5" deck can span 15 ft vs 10 ft unshored), or when the deck is only a form (not composite) and is removed or absorbed into the composite section without carrying construction loads. For a typical 10 ft beam spacing with normal weight concrete and 3" total slab depth: 20-gauge 1.5" composite deck works unshored (span rating 10-12 ft). Same spacing with 14 ft: requires either 18-gauge 1.5" deck or 20-gauge 2" deck, or unshored 1.5" deck with a mid-span shore.

How are shear studs designed for composite steel beams?

Shear studs provide the horizontal shear transfer between the concrete slab and steel beam that enables composite action. Per AISC 360-22 Section I8.2, the nominal shear strength of one steel headed stud anchor embedded in solid concrete or composite deck is Qn = 0.5 × Asc × √(f'c × Ec) ≤ Asc × Fu, where Asc = cross-sectional area of stud (0.442 in² for 3/4" dia, 0.601 in² for 7/8" dia), f'c is concrete compressive strength (psi), Ec = wc^1.5 × √(f'c) is the concrete modulus (psi), and Fu = 65 ksi minimum (ASTM A108/A29). For a 3/4" dia stud with f'c = 4,000 psi normal weight concrete (wc=145 pcf, Ec = 3,490 ksi): Qn = 0.5 × 0.442 × √(4,000 × 3,490,000) = 0.5 × 0.442 × 118,200 = 26.1 kips. Upper limit: Asc × Fu = 0.442 × 65 = 28.7 kips. So Qn = 26.1 kips per stud. φ = 0.65 (AISC I8.2a for headed stud anchors), φQn = 17.0 kips. The number of studs required between the point of zero moment and the point of maximum positive moment = V'h / (φ × Qn), where V'h is the total horizontal shear = the smaller of 0.85 × f'c × Ac (concrete crushing) or As × Fy (steel yielding). For a W18×35 beam spanning 30 ft with 4.5" slab (effective width 90"): Ac = 90 × 4.5 = 405 in², 0.85 × 4 × 405 = 1,377 kips; As × Fy = 10.3 × 50 = 515 kips. V'h = 515 kips (steel yielding governs). Number required = 515 / 17.0 = 30.3 → 31 studs from end to midspan. Studs are placed in the deck ribs (one per rib for perpendicular deck, two per rib for parallel deck) and must extend at least 1.5" above the deck for composite action per AISC I8.2.

What concrete thickness is needed above the steel deck?

Minimum concrete cover above the steel deck is governed by three requirements. (1) Structural: AISC 360 I8.2 requires at least 2" of concrete above the top of the steel deck for headed stud anchorage — the stud must extend 1.5" above the deck and have 0.5" minimum clear cover above the stud plus any topping requirements. For a 3/4" dia × 4.5" long stud through 1.5" deck: concrete = 1.5" (deck) + 1.5" (min above deck) = 3" minimum total, but the stud is 4.5" long, so embedment = 4.5 − 1.5" deck = 3" in concrete above deck. With 0.5" clear cover: total = 1.5 + 3 + 0.5 = 5" minimum. (2) Fire rating: IBC Table 720.1 requires minimum slab thickness for fire ratings — 1-hr rating requires 2.5" slab (cellular deck) or 3.2" (fluted deck); 2-hr requires 3.2" (cellular) or 4.1" (fluted). The concrete above the deck flutes counts toward the total thickness. For a 1.5" deck with 2-hr rating: total slab = 1.5 + 2.6" cover = 4.1" minimum. (3) diaphragm shear: greater concrete thickness increases the diaphragm unit shear capacity (per SDI DDM04), important for seismic design. Typical total slab depths: 4.5" (1.5" deck + 3" concrete) for non-fire-rated offices with moderate spans; 5.5" (2" deck + 3.5" concrete) for 1-hr offices; 6.25" (1.5" deck + 4.75" concrete) for 2-hr. Heavier slabs (6.5-7.5" total) provide better vibration performance for sensitive occupancies (hospitals, labs) and longer spans.

How does composite deck contribute to diaphragm strength?

The composite steel deck-concrete slab functions as the building's horizontal diaphragm, transferring wind and seismic forces from the building envelope to the vertical lateral-force-resisting system. Per SDI DDM04 (Steel Deck Institute Diaphragm Design Manual), the diaphragm shear capacity comes from two mechanisms: (1) The concrete slab in compression — the concrete acts as a strut (analogous to a deep beam web) carrying compression diagonally across the diaphragm to the chord members. The concrete contribution is limited by its shear strength: Vc = 2 × √(f'c) × b × d (ACI shear equation adapted for in-plane shear), where b is the diaphragm depth and d is the effective depth. (2) The steel deck as tension reinforcement — the deck profile provides continuous tensile resistance in the diaphragm's in-plane direction. Capacity depends on the deck profile depth, gauge, fastener type/spacing, and side-lap connections. Typical composite diaphragm shear capacities: 20-gauge 1.5" deck with 3.25" LW concrete provides 800-1,400 plf; 18-gauge 1.5" deck with 3.25" LW concrete provides 1,200-1,800 plf; 16-gauge 2" deck with 4.5" NW concrete provides 1,800-2,500 plf; 3" deep deck with 4.5-6.5" NW concrete provides 3,000-4,500+ plf. Critical design detail: the deck-to-structure connection (puddle welds to the supporting beam flange, or powder-actuated fasteners/pins) must be designed for the required diaphragm shear — a deck with 2,000 plf capacity but insufficient welds to the collectors will fail at the connections. SDI requires minimum 5/8" diameter puddle welds at 12-18" spacing for composite deck diaphragms.

What are the key composite beam design checks per AISC 360?

Composite beam design per AISC 360 Chapter I checks the combined steel beam + concrete slab acting as a composite section. Construction phase (unshored): the steel beam alone resists the wet concrete weight + 20 psf construction live load. Check steel beam flexural yielding (φMn ≥ Mdead+construction), shear, and deflection (L/360 for total construction dead load). The bare steel beam typically has 40-60% of the composite section capacity, so construction is often the controlling phase for beam size. Composite phase: the cured concrete slab acts as the compression flange, the steel beam as the tension element. Check: (1) Positive moment capacity — the plastic stress distribution assumes the concrete slab in compression (0.85f'c × beff × a) and the steel beam in tension (As × Fy). The depth of the compression block a = As × Fy / (0.85 × f'c × beff). For a W18×35 with 90" effective width and f'c=4 ksi: a = 10.3×50/(0.85×4×90) = 515/(306) = 1.68". The nominal moment Mn = As × Fy × (d/2 + tslab − a/2) = 515 × (17.7/2 + 4.5 − 1.68/2) = 515 × (8.85+4.5-0.84) = 515 × 12.51 = 6,443 kip-in = 537 kip-ft. φMn = 0.90 × 537 = 483 kip-ft. (2) Shear — identical to non-composite steel beam shear check (concrete slab doesn't significantly contribute to shear). (3) Deflection — the effective moment of inertia is the lower bound value from AISC Commentary Eq. C-I3-1: Ieff = Is + (Itr − Is) × (ΣQn/Cf)^(1/2), accounting for partial composite action if fewer studs than full-composite are provided. (4) Stud strength — the number of studs provided must be ≥ V'h / (φQn). (5) Concentrated loads — studs may need closer spacing near point loads to transfer the local force into the slab. For a 25% partial composite design (fewer studs), the moment capacity is approximately 15-20% lower than full composite — the economic trade-off is between fewer studs and a slightly heavier beam.

Composite Slab Design — Steel Deck & Concrete

Composite slabs consist of corrugated steel deck with a concrete topping, connected by mechanical interlock and optionally by shear studs welded through the deck. This is the most common floor system in steel-framed buildings. This page covers deck types, design methods, span tables, and shear stud requirements.

Composite Slab System Components

Component	Function	Typical Size
Steel deck	Permanent form + tensile reinforcement	9/16 to 3 in depth
Concrete topping	Compressive element + fire rating	2.5 to 6 in above deck
Wire mesh	Temperature/shrinkage reinforcement	WWR 6×6-W1.4×W1.4
Shear studs	Composite action (beam + slab)	3/4 in dia, 3 to 5 in
Stud weld	Attaches studs through deck to beam	Arc stud weld

Steel Deck Types

Roof Deck (Non-Composite)

Profile	Depth (in)	Span Range (ft)	Gauges	Use
B-Deck	1.5	4-10	22 to 16	Most common roof
F-Deck	1.5	4-8	26 to 20	Light roof
N-Deck	3.0	8-16	22 to 18	Long-span roof
A-Deck	1.5	3-7	28 to 22	Architectural

Composite Floor Deck

Profile	Depth (in)	Span Range (ft)	Gauges	Embossment
1.0 in wide rib	1.0	4-8	22 to 18	Yes
1.5 in wide rib	1.5	5-12	22 to 16	Yes
2.0 in wide rib	2.0	6-14	22 to 16	Yes
3.0 in deep	3.0	8-18	20 to 16	Yes

Deeper profiles allow longer unshored spans. Embossments (dimples rolled into the deck) provide mechanical interlock between the deck and concrete.

Design Methods

Unshored Construction (Most Common)

The steel deck alone supports the wet concrete and construction loads. After the concrete cures, the composite deck-concrete system carries the service loads.

Deck stress check (construction):

Dead: deck weight + wet concrete + mesh + construction live (20 psf)
Deck span = spacing between beams (unshored span)
Check deck bending, shear, and deflection

Composite slab check (service):

Dead: full slab + finishes + MEP + partitions
Live: per ASCE 7 (typically 50-100 psf for office)
Check positive moment, negative moment (continuous), shear, and deflection

Shored Construction

Temporary shores support the slab during concrete placement. Shoring allows thinner decks and longer spans but adds cost and schedule time.

Concrete Requirements

Parameter	Value
Minimum above deck	1.5 in (fire-rated: 2 in)
Typical total slab	4.5 to 7.5 in
Concrete strength	3,000 to 5,000 psi (norm)
Lightweight concrete	110-115 pcf (reduces weight)
Normal weight concrete	145-150 pcf
Minimum mesh	WWR 6×6-W1.4×W1.4

Fire Rating Requirements

Rating	Minimum Slab (in)	Concrete Above Deck	Notes
Unrated	4.0 total	1.5	Minimum practical
1 hour	4.5 total	2.0	Most offices
2 hour	5.5 total	3.0	Many building codes

Span Tables (Unshored, Normal Weight Concrete)

1.5 in Composite Deck, 20-Gauge

Slab Total (in)	Single Span (ft)	Double Span (ft)	Load (psf)
4.5	7.5	8.5	150
5.0	7.0	8.0	150
5.5	6.5	7.5	150
6.0	6.0	7.0	150

2.0 in Composite Deck, 20-Gauge

Slab Total (in)	Single Span (ft)	Double Span (ft)	Load (psf)
5.0	9.0	10.0	150
5.5	8.5	9.5	150
6.0	8.0	9.0	150
6.5	7.5	8.5	150

3.0 in Composite Deck, 20-Gauge

Slab Total (in)	Single Span (ft)	Double Span (ft)	Load (psf)
5.5	12.0	14.0	150
6.0	11.5	13.0	150
6.5	11.0	12.5	150
7.0	10.5	12.0	150

Values are approximate. Consult the deck manufacturer's load tables for specific products.

Shear Stud Requirements

Shear studs provide composite action between the slab and steel beam, increasing the beam's moment capacity by 30-50%.

Stud Specifications

Parameter	Value
Diameter	3/4 in (most common)
Length	3 to 5 in (must embed 1.5 in min into concrete above deck)
Welding	Through-deck arc stud welding
Steel	ASTM A108 (low carbon)
Tensile strength	61 ksi minimum
Yield strength	49 ksi minimum

Number of Studs Required

Per AISC 360 Chapter I, the number of studs depends on the required composite action (partial or full):

Full composite action: N_studs ≥ V_h / Q_n

where V_h = total horizontal shear force = min(0.85 × f'c × Ac, Fy × As), Q_n = nominal shear strength of one stud.

Q_n per stud (3/4 in dia, normal weight concrete):

f'c (psi)	Q_n (kips)
3,000	17.7
4,000	20.4
5,000	22.8

For lightweight concrete, reduce Q_n by the concrete density factor.

Stud Layout Patterns

Pattern	Description	Application
Uniform	Equal spacing along beam	Simple spans
Concentrated	More studs near supports	Heavy loads
Grouped in ribs	1 or 2 studs per deck rib	Standard practice

Maximum stud spacing: 8 × total slab thickness, ≤ 36 in Minimum stud spacing (longitudinal): 6 × stud diameter = 4.5 in (for 3/4 in studs) Minimum stud spacing (transverse): 4 × stud diameter = 3.0 in

Studs Through Deck

When studs are welded through the deck:

Parameter	Requirement
Deck thickness	≤ 16 gauge (0.060 in) for through-deck welding
Deck orientation	Flutes perpendicular to beam (strong axis)
Studs per flute	1 preferred (2 max with reduction factor)
Weld quality	Visual inspection per AWS D1.1

Slab Weight per Square Foot

Total Slab (in)	Normal Weight (psf)	Lightweight (psf)
4.5	47	38
5.0	52	42
5.5	57	46
6.0	62	50
6.5	67	54
7.0	72	58

Values include steel deck weight (approximately 2-3 psf for 20-gauge).

Frequently Asked Questions

What is a composite slab? A composite slab is a floor system made of corrugated steel deck filled with concrete. The steel deck acts as permanent formwork and bottom reinforcement, while the concrete provides compressive strength and fire rating. Mechanical interlock (embossments) or shear studs connect the deck to the concrete.

Does a composite slab need rebar? Composite slabs require wire mesh (WWR) for temperature and shrinkage crack control. Structural rebar is needed for: (1) continuous slabs over multiple supports (negative moment reinforcement), (2) concentrated loads, (3) slab openings, and (4) cantilevers. The steel deck itself provides the primary bottom reinforcement.

How thick should a composite slab be? Typical office construction uses 4.5 to 5.5 in total thickness (including the deck profile). The minimum concrete above the deck top is 1.5 in for structural purposes, but 2.0 in minimum for a 1-hour fire rating. Heavier loads or longer spans require thicker slabs.

What is through-deck welding? Through-deck welding is the process of welding shear studs through the steel deck directly to the beam flange without pre-drilling holes. A specialized arc stud welding gun melts through the deck and fuses the stud base to the beam. The weld quality is verified visually and by bend testing.

Steel Shear Stud Design — Stud capacity data
Beam Capacity — Composite beam design
Steel Weight Calculator — Weight by dimensions
Steel Beam Load Capacity — W-shape span tables
Connection Types Explained — Connection overview

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.