--------------------- | ------------ | -------------- | ------------ | ---------------------------- | | Composite beam + metal deck | 30–45 ft | 20–30 in total | 1 (lowest) | Office, residential | | Non-composite beam + deck | 20–35 ft | 18–24 in | 2 | Parking, industrial | | Composite cellular beam | 40–60 ft | 24–36 in | 3 | Long-span office, open plans | | Steel joist + deck | 30–60 ft | 18–30 in | 2 | Roof, warehouse | | Composite truss | 40–60 ft | 30–48 in | 4 | Heavy load, long span |

Typical floor construction: 3-1/4" lightweight concrete on 2" or 3" composite metal deck, with total slab thickness of 5-1/4" to 6-1/4".

Composite beam design per AISC 360-22 Chapter I

The composite beam strength is determined by the compressive force that can be developed in the concrete slab and transferred through the shear studs. Three force components compete:

C_concrete = 0.85 × f'c × beff × tc    (concrete crushing capacity)
T_steel = Fy × As                       (steel yielding capacity)
V'_studs = n × Qn                       (total shear stud capacity)

The composite beam strength is controlled by the minimum of these three. Full composite action occurs when the stud capacity equals or exceeds the smaller of C_concrete and T_steel. Partial composite action (25–100% of full) uses fewer studs and is permitted by AISC 360-22 Section I3.2d, with a minimum composite ratio of 25%.

Effective flange width (AISC 360-22 Section I3.1a):

beff = min(span/4, beam spacing, distance to edge)

Worked example — composite beam

Given: W16x26 at 10 ft spacing, span = 35 ft simply supported. 3" deck with 3.25" LW concrete topping (tc = 3.25 in), f'c = 3 ksi, 3/4" headed studs. A992 steel (Fy = 50 ksi). Superimposed dead = 20 psf, live = 50 psf.

Step 1 — Steel and concrete capacities: As = 7.68 in², T_steel = 50 × 7.68 = 384 kips. beff = min(35×12/4, 10×12) = min(105, 120) = 105 in. C_concrete = 0.85 × 3 × 105 × 3.25 = 870 kips. Controlling: T_steel = 384 kips (PNA in slab, steel fully yielded).

Step 2 — Stud capacity: Qn = 21.0 kips per stud (3/4" stud, LW concrete, from AISC Table 3-21). For full composite: n = 384/21.0 = 18.3, use 19 studs per half-span (38 total).

Step 3 — Composite moment capacity: The distance from the steel centroid to the concrete force resultant determines the moment arm. a = T_steel / (0.85 × f'c × beff) = 384 / (0.85 × 3 × 105) = 1.43 in (compression block within slab). Moment arm = d/2 + hr + tc - a/2 = 15.7/2 + 3.0 + 3.25 - 1.43/2 = 13.39 in. phi × Mn = 0.85 × 384 × 13.39 / 12 = 364 kip-ft.

Step 4 — Demand check: wD = 10 ft × (57 psf slab + 26 plf beam/10 + 20 psf SDL) = 797 plf wL = 10 × 50 = 500 plf wu = 1.2 × 797 + 1.6 × 500 = 1756 plf = 1.756 klf Mu = 1.756 × 35² / 8 = 269 kip-ft < 364 kip-ft — OK

Partial composite design

Using fewer studs saves stud installation cost. At 50% composite ratio, the beam weight typically increases one size (e.g., W16x26 to W16x31) but 19 studs are saved per half-span. The economic optimum for most office buildings is 50–75% composite action.

AISC 360-22 Section I3.2d requires a minimum 25% composite ratio. Below this, the plastic stress distribution model becomes unreliable. Deflection checks become critical for partial composite beams because the lower composite stiffness increases live load deflection.

Lower-bound moment of inertia for deflection (AISC 360-22 Commentary Section I3.2):

ILB = Is + sqrt(Sigma_Qn / C_f) × (Itr - Is)

Metal deck selection

Deck profile Depth Concrete above Total slab Typical span (unshored)
1.5" composite 1.5" 3.25" 4.75" 6–9 ft
2" composite 2" 3.25" 5.25" 8–11 ft
3" composite 3" 3.25" 6.25" 10–15 ft

Key rule: Deck ribs perpendicular to the beam allow shear studs in every rib (maximum stud spacing = rib spacing, typically 6" or 12"). Deck ribs parallel to the beam require studs in the flat pan, limited to one or two studs per rib width.

Code comparison

AISC 360-22 Chapter I (USA): Plastic stress distribution for strength. Full and partial composite with minimum 25% ratio. Shear stud capacity Qn from Chapter I equations or Table 3-21. phi = 0.85 for composite flexure. Headed studs per AWS D1.1.

AS 2327.1-2003 (Australia): Covers composite beam design with metal deck (now superseded by AS/NZS 2327:2017). Uses similar plastic stress distribution with phi = 0.80. Minimum composite ratio not explicitly stated but practical minimum is 0.4. Shear connector capacity based on Australian test data (slightly different from AISC values for the same stud size).

EN 1994-1-1 (Eurocode 4): Composite beam design with partial safety factor gamma_v = 1.25 for shear connectors. Minimum degree of shear connection depends on span: for Le ≤ 25m, minimum eta = max(1 - (355/fy) × (0.75 - 0.03Le), 0.4). Eurocode permits slip at the interface and accounts for it in the stiffness calculation, providing more accurate deflection predictions than AISC's lower-bound method.

Common mistakes engineers make

  1. Forgetting to check construction-stage loading. Before the concrete hardens, the steel beam alone carries the wet concrete, deck, and construction loads. Many beams that pass the final composite check fail the construction-stage check. AISC 360 Commentary Section I3.1 requires checking the bare steel beam for construction loads.

  2. Using strong-axis stud capacity when studs are in deck ribs. AISC 360-22 Section I8.2a applies a group reduction factor Rg and position factor Rp for studs in deck ribs. A single stud in a rib with the deck perpendicular gets Rg × Rp = 1.0 × 0.75 = 0.75, reducing capacity by 25%.

  3. Ignoring live load deflection for partial composite beams. Partial composite beams at 25–50% ratio have significantly less stiffness than full composite beams. The L/360 live load deflection limit (AISC Table L1.1) frequently governs over strength for partial composite design.

  4. Specifying studs through deck ribs parallel to the beam without checking rib geometry. When deck ribs run parallel to the beam, studs must be placed in the flat pan. If the pan width is too narrow for the stud diameter plus clearances, studs cannot be installed. Check deck manufacturer's details for stud placement limits.

Steel floor system types: detailed comparison

Steel buildings can use several floor system types, each with distinct advantages in span capability, weight, fire rating, cost, and construction speed. The choice depends on the building occupancy, span requirements, vibration sensitivity, and budget.

Composite slab on metal deck

The dominant floor system for steel-framed commercial and residential buildings. A corrugated steel deck serves as both permanent formwork and bottom reinforcement for the concrete slab. Headed shear studs welded through the deck to the steel beams create composite action. Lightweight concrete (110-120 pcf) is typically used to reduce dead load.

Non-composite slab on metal deck

Similar to composite construction but without shear studs. The steel beams and concrete slab act independently. This system is used when stud welding is impractical (e.g., painted or galvanized beams), when the steel beam is already sized for other constraints, or for mezzanine floors where deflection is not critical.

Precast concrete on steel framing

Precast concrete planks (hollow-core or solid) are set on steel beams, with or without a composite concrete topping. The planks are manufactured off-site and erected by crane, providing immediate working surface. Shear studs can be embedded in a poured concrete topping over the planks for composite action.

Steel deck only (no concrete)

Corrugated steel deck panels span between beams without a concrete topping. Used for roof systems, mezzanines with light loads, and industrial platforms where fire rating is not required. The deck provides diaphragm action for lateral load distribution.

Raised access floors

A raised floor system consists of removable floor panels supported by adjustable pedestals on top of the structural floor. The cavity between the structural floor and the raised floor provides space for underfloor air distribution, power and data cabling, and fire suppression. Common in data centers and high-end office buildings.

Floor system comparison table

System Span (ft) Weight (psf) Fire Rating Relative Cost Construction Speed Best Use
Composite slab on deck 30-45 45-55 2 hr (SFRM) Low Fast Office, residential, hospital
Non-composite on deck 20-35 55-70 2 hr (SFRM) Moderate Fast Industrial, retrofit
Precast on steel 30-50 60-80 2-3 hr Moderate-High Medium Residential, parking
Steel deck only 4-10 2-4 None Very Low Very Fast Roof, canopy, mezzanine
Raised access floor N/A 8-15 Varies High Slow Data center, Class A office

Composite construction overview: shear studs and design per AISC Chapter I

Composite beam design leverages the concrete slab as the compression flange of a steel T-beam, dramatically increasing the flexural capacity and stiffness compared to the bare steel beam alone. AISC 360-22 Chapter I provides the complete design framework for composite beams, composite slabs, and composite columns.

Shear stud mechanics

Headed shear studs (typically 3/4 in diameter, 3 to 5 in long) are the most common shear connector. The stud head prevents pullout from the concrete, and the weld at the base transfers horizontal shear from the steel beam flange to the concrete slab. The stud capacity Q_n per AISC 360-22 Section I8.2a is the minimum of stud shank capacity and concrete crushing capacity:

Q_n = 0.5 x A_sc x sqrt(f'c x Ec) <= A_sc x F_u

where A_sc is the cross-sectional area of the stud, f'c is the concrete compressive strength, Ec is the concrete modulus of elasticity, and F_u is the stud tensile strength (65 ksi for ASTM A108 studs).

For a 3/4 in diameter stud in 3 ksi normal weight concrete:

A_sc = 0.442 in^2
Ec = 57,000 x sqrt(3000) = 3,122 ksi
Q_n = 0.5 x 0.442 x sqrt(3 x 3122) = 0.221 x 96.8 = 21.4 kips per stud
Check: A_sc x F_u = 0.442 x 65 = 28.7 kips > 21.4 kips (concrete governs)

Partial vs full composite action

Full composite action occurs when enough studs are provided to develop the full compressive capacity of the concrete slab or the full tensile capacity of the steel beam (whichever is smaller). The number of studs required for full composite action is:

n_full = min(C_concrete, T_steel) / Q_n

Partial composite action uses fewer studs, resulting in the steel beam carrying more of the flexural demand and the concrete providing reduced compression resistance. AISC 360-22 Section I3.2d permits partial composite design with a minimum composite ratio of 25%. The composite ratio is defined as:

Composite ratio = Sigma_Qn / min(C_concrete, T_steel) >= 0.25
Composite Ratio Studs Saved vs Full Beam Weight Increase Typical Application
100% (full) 0% Baseline Heavy loads, deflection-critical
75% 25% ~5% increase General office buildings
50% 50% ~15% increase Cost-optimized design
25% (minimum) 75% ~30% increase Very light loads only

The economic optimum for most office buildings is 50-75% composite action. The cost savings from fewer studs (each stud requires welding time) typically outweigh the cost of the slightly heavier beam. However, for deflection-sensitive floors (laboratories, medical equipment rooms), full composite action may be necessary to achieve the required stiffness.

Plastic neutral axis location

The plastic neutral axis (PNA) location determines the stress distribution at the ultimate limit state:

The AISC 360-22 Manual Table 3-19 provides pre-calculated composite beam properties (phi x M_n and I_lb) for standard W-shapes with various composite ratios, avoiding the need to manually calculate the PNA location for each case.

Metal deck types: profiles, depths, and spans

Steel floor and roof deck is manufactured from cold-formed steel sheet in a variety of profiles. The profile geometry determines the structural capacity, concrete volume, and span capability. The Steel Deck Institute (SDI) and the deck manufacturer's catalog provide the design tables.

B-deck (1.5 in depth)

B-deck is the most common narrow-rib deck profile, with a 1.5 in flute depth and 6 in flute spacing. It is widely used for both roof and floor applications.

Profile:   /\/\/\/\/\
           |  |  |  |
         1.5" depth, 6" rib spacing

F-deck (1.5 in depth, wide rib)

F-deck has a 1.5 in depth with wider flutes (wider rib spacing) than B-deck. The wider flat between ribs provides more area for shear stud placement and attachment of finishes.

Profile:   /  \  /  \  /  \
           |    ||    ||    |
         1.5" depth, wider ribs

W-deck (3 in depth)

W-deck (also called 3W, 3-inch composite deck) has a 3 in flute depth, providing significantly greater span capability and higher composite action due to the deeper concrete ribs around the studs.

Profile:   /    \    /    \    /    \
           |      |  |      |  |      |
         3" depth, 12" rib spacing

Deck type comparison table

Deck Type Depth Rib Spacing Max Unshored Span (ft) Concrete Above Ribs Total Slab Stud Placement
B-deck 1.5" 6" 6-8 3.25" 4.75" 1 per rib (max)
F-deck 1.5" 8-10" 6-9 3.25" 4.75" 1-2 per rib
W-deck 3" 12" 10-15 3.25" 6.25" 1-2 per rib
Deep deck 4.5" 12" 14-20 3.25" 7.75" 2 per rib

Deck orientation and stud layout

The orientation of deck ribs relative to the supporting beam has a significant impact on stud capacity and layout:

Deck ribs perpendicular to beam: Studs are placed in the ribs, one or two per rib depending on rib width. This is the preferred orientation because studs are uniformly distributed and the deck provides lateral bracing to the beam top flange during construction.

Deck ribs parallel to beam: Studs must be placed in the flat pans between ribs, limited by pan width. When the pan is too narrow for the required stud diameter plus edge clearance, a "steel header" or "stud rail" detail may be needed. AISC 360-22 Section I8.2a applies reduction factors for studs in deck ribs that limit capacity to 75% of the flat-slab value for a single stud.

Floor Fire Protection for Steel Beams

Steel loses strength at elevated temperatures — approximately 50% of its yield strength at 550 degrees C and 90% at 800 degrees C. For fire-rated floors (typically 1 or 2 hours per IBC Table 601), steel beams require fire protection unless the building is unprotected (e.g., Type II-B construction with sprinklers). Common fire protection methods include:

Sprayed fire-resistive material (SFRM): Cementitious or mineral-fibre spray applied to the steel surface. Thickness depends on the required fire rating and the heated perimeter-to-mass ratio (Hp/A) of the section. For a typical W16 composite beam with 1-hour rating, SFRM thickness ranges from 3/8 to 3/4 inch. SFRM is the most economical system for concealed steel (above ceilings, in floor cavities) but is unsuitable for exposed architectural steel.

Intumescent paint: Thin-film coating that expands (intumesces) when heated, forming a thick insulating char layer. Applied at 1-5 mm dry film thickness depending on the required fire rating and section Hp/A. Intumescent coatings are specified for architecturally exposed structural steel (AESS) where the steel appearance must be preserved. More expensive than SFRM by a factor of 3-5 but essential for exposed steel in atria, lobbies, and feature staircases.

Concrete encasement: Traditional method — the steel beam is fully encased in concrete, providing both fire protection and composite action. Rare in modern construction due to weight, cost, and construction time, but still used for heavy industrial buildings and where impact resistance is required.

Board fire protection: Calcium silicate or vermiculite boards mechanically fixed around the steel section. Provides a clean, boxed appearance and can be installed after the steel is erected. Common for columns in exposed locations and for retrofitting fire protection to existing steel.

Fire engineering (performance-based design): For large open-plan buildings with sprinklers, fire engineering may demonstrate that the steel temperature remains below the critical temperature (typically 550 degrees C) without applied fire protection. This is assessed using computational fluid dynamics (CFD) fire modelling per BS 7974 or the Eurocode fire parts (EN 1991-1-2, EN 1993-1-2). Passive fire protection can often be omitted for secondary beams in sprinkled buildings with high ceilings, saving significant cost.

Key design considerations: (1) fire protection must be continuous over connections (beam-to-column, beam-to-beam) — gaps create heat paths; (2) the heated perimeter-to-mass ratio (Hp/A) is the critical parameter for fire protection thickness; UC sections (lower Hp/A) require less protection than UB sections of the same mass; (3) composite slabs with the deck acting as permanent formwork provide inherent fire protection to the beam top flange; (4) fire protection must be compatible with applied corrosion protection — SFRM over intumescent primer is not permitted.

Floor Vibration Design — AISC DG11 and SCI P354

Floor vibration is a serviceability limit state that frequently governs the design of long-span, lightly loaded composite floors in offices, hospitals, and laboratories. Annoying vibrations from footfall can render an otherwise adequate floor unusable, and retrofitting vibration damping is expensive.

AISC Design Guide 11 (DG11): The American approach evaluates floor vibration using fundamental frequency and peak acceleration criteria. For walking excitation, the floor fundamental frequency f_n should ideally exceed 9-10 Hz to avoid resonance with the first three harmonics of walking (1.6-2.2 Hz, 3.2-4.4 Hz, and 4.8-6.6 Hz). The peak acceleration ratio a_p/g should not exceed 0.5% for offices and 0.2% for sensitive equipment. The effective moment of inertia for composite beams under vibration loading is typically 1.5-2.0 times the bare steel value, reflecting partial composite action and the stiffening effect of the slab.

SCI P354 (UK): The UK approach uses the response factor R as the vibration criterion, with limiting values of R = 4 for general offices and R = 2 for hospital operating theatres. The SCI P354 method calculates the root-mean-square (RMS) velocity response of the floor to a standard walking excitation. Composite floor design for vibration requires a minimum slab depth of approximately 130 mm (5 inches) and a minimum floor mass of approximately 50-60 kg/m^2 (10-12 psf) to provide sufficient damping.

Practical vibration control strategies:

For a quick preliminary check: a composite floor with f_n above 4 Hz, mass above 50 kg/m^2, and damping ratio above 3% (typical for open-plan offices with partitions and services) will generally meet the SCI P354 R=4 criterion. Floors with f_n below 3 Hz require detailed analysis.

Frequently Asked Questions

What is the optimum beam spacing for a composite steel floor?

For multi-storey office buildings, the economic optimum beam spacing is typically 2.5-3.6 m (8-12 ft). At this spacing, the slab can span between beams without intermediate supports, the beams are close enough that vibration is not critical, and the number of beam lines is minimised. Wider beam spacing (up to 5 m or 16 ft) is achievable with deeper deck profiles (3" W-deck) but increases the slab thickness and dead load. Narrower spacing (<2.5 m) increases the number of beams and connections, increasing fabrication and erection cost.

When should I use cellular (castellated) beams instead of solid UB sections?

Cellular beams (beams with regular circular openings in the web, often called "castellated" or "cellular" beams) are specified when: (1) services (ducts, pipes, cable trays) must pass through the beam depth rather than below it, reducing floor-to-floor height; (2) long spans (15-25 m) require greater depth than standard rolled sections provide; or (3) architectural expression of the beam is desired. Cellular beams are fabricated by cutting and re-welding a UB section, producing a beam 40-60% deeper than the parent section. The openings must be checked for Vierendeel bending, web post buckling, and shear at the openings per SCI P355 or AISC DG31. Cellular beams typically cost 20-30% more than the parent UB section but save 150-200 mm per floor in building height.

What are the fire protection requirements for composite metal deck slabs?

The concrete slab itself provides inherent fire resistance, typically achieving 1-2 hours without additional protection for a 130 mm (5 inch) total slab depth. The metal deck acts as permanent reinforcement and provides tensile capacity during the fire (the deck temperature may reach 800-900 degrees C but only contributes 10-15% of the cold-state capacity). The slab reinforcement (typically A142 or A193 mesh for crack control in the UK, 6x6 W2.9/W2.9 WWF in the US) provides the primary tensile capacity at elevated temperatures. Intumescent or SFRM is required on the underside of the deck only for very high fire ratings (3+ hours) or where the slab is unusually thin (below 100 mm total).

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This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.