How does composite beam design work per AISC 360?

Composite beam design per AISC 360 Chapter I couples the steel beam and concrete slab through shear studs so they act as a single T-beam. The design process: (1) Effective slab width beff = min(L/8, sp/2) on each side of the beam centerline. For a 30 ft beam at 10 ft spacing: beff = min(30x12/8=45, 10x12/2=60) = 45 in total slab width contributing to composite action. (2) Calculate compressive force in concrete: C = 0.85 x f'c x beff x a, where a = depth of compression block. (3) Determine plastic neutral axis (PNA) location: If C > As x Fy, PNA is in slab (most economical — full composite action). If C < As x Fy, PNA is in steel beam (partial composite or deeper PNA). (4) Nominal moment Mn = C x (d/2 + t_slab - a/2) when PNA in slab. For a W18x40 with 5 in slab (f'c=4 ksi): C = 0.85 x 4 x 45 x a = 153a. As x Fy = 11.7 x 50 = 585 kips. Set equal: 153a = 585, a = 3.82 in. Mn = 585 x (17.9/2 + 5 - 3.82/2) = 585 x (8.95 + 5 - 1.91) = 585 x 12.04 = 7,043 kip-in = 587 kip-ft. phi Mn = 0.90 x 587 = 528 kip-ft. Compare to bare steel phi Mp = 0.90 x 50 x 68.4/12 = 257 kip-ft. Composite action DOUBLES the moment capacity with the same beam weight — that's why composite floors are near-universal in steel buildings.

How are shear studs designed for composite beams?

Shear studs transfer horizontal shear between slab and beam — they enforce composite action. Per AISC 360 I8.2: (1) Nominal strength of one stud: Qn = 0.5 x Asc x sqrt(f'c x Ec) <= Rg x Rp x Asc x Fu. For a 3/4 in diameter stud (Asc=0.442 in^2) in 4 ksi normal-weight concrete: Qn = 0.5 x 0.442 x sqrt(4 x 3,605) = 0.221 x sqrt(14,420) = 0.221 x 120.1 = 26.5 kips. Upper bound: Rg x Rp x Asc x Fu. Rg = 1.0 (1 stud per rib), Rp = 0.75 (studs in deck ribs perpendicular to beam — most common). Fu = 65 ksi. Upper Qn = 1.0 x 0.75 x 0.442 x 65 = 21.5 kips. Controls. (2) Number of studs required between point of zero moment and maximum moment: N_req = Vh / (phi x Qn), where Vh = min(0.85 x f'c x beff x t, As x Fy) = horizontal shear to be transferred. For W18x40: Vh = As x Fy = 585 kips (steel controls because PNA is in slab). N_req = 585 / (0.65 x 21.5) = 585 / 14.0 = 42 studs total, or 21 studs each side of midspan (since studs are placed from zero moment to max moment on each half). (3) Spacing: max 8 x slab thickness = 40 in, min 6 x stud diameter = 4.5 in. For transverse deck: 1 stud max per rib. For 6 in rib spacing: 21 studs x 6 in/rib = 126 in (10.5 ft) required each side.

What floor vibration criteria apply to steel floors?

Floor vibration governs steel floor design more often than strength. Per AISC Design Guide 11 (Floor Vibrations Due to Human Activity): (1) Walking excitation — the critical case for offices, residential, hospitals. Minimum natural frequency fn >= 3 Hz (offices) to avoid resonance with the first harmonic of walking (1.6-2.2 Hz). Second harmonic at 3.2-4.4 Hz also excites floors. fn = (pi/2) x sqrt(g x EI / (w x L^4)), simplified to fn = 0.18 x sqrt(g / delta), where delta = midspan deflection under the supported weight (DL + 10-25% LL). For a 30 ft composite beam: if delta = 0.5 in under DL+0.25LL, fn = 0.18 x sqrt(386/0.5) = 0.18 x sqrt(772) = 0.18 x 27.8 = 5.0 Hz — OK. (2) Peak acceleration criterion: ap/g <= 0.5% for offices, 1.5% for shopping malls, 5% for pedestrian bridges. The acceleration depends on the walking force (92 lbs for offices), floor damping (2-5% for bare steel, 6-12% with partitions/ceiling), and the effective modal mass. (3) Stiffness criterion: for offices, floor deflection under 225 lb concentrated load = 5 Hz for rhythmic. These criteria explain the rule of thumb: steel floor depth >= L/24 for simply supported composite beams (30 ft x 12 / 24 = 15 in → W16-W18 range). Vibration, not bending strength, sets the minimum depth for most steel floors.

What types of steel deck are available and how to choose?

Steel deck types per SDI (Steel Deck Institute): (1) Composite deck — has embossments/indentations that mechanically interlock with concrete. Types: 1.5WR (1.5 in deep, wide rib), 2WR (2 in deep), 3WR (3 in deep). 1.5WR is the most common for floors. Span: 6-15 ft between beams depending on concrete thickness and construction loads. The deck acts as permanent formwork AND positive moment reinforcement after concrete cures. (2) Non-composite (form) deck — smooth profile, no embossments. Acts only as stay-in-place formwork. Types: 9/16 in, 1.0 in, 1.5 in. Cheaper than composite deck but requires more beam reinforcement. (3) Cellular deck — closed bottom with integrated wire raceways. Used where underfloor power/data distribution is needed. Deeper and heavier than standard deck. (4) Acoustic deck — perforated with acoustic batting for sound absorption. Used in theaters, auditoriums, open offices. Selection: composite 1.5WR on 18 or 20 gage is the default for typical commercial floors. 3WR or deeper deck when beam spacing > 12 ft (reduces number of beams needed). Cellular where raised floor access is cost-prohibitive. All decks must be puddle-welded or screwed to beam flanges per SDI erection standards — the attachment provides lateral bracing to the beam top flange during construction.

When should steel beams be cambered?

Camber is an upward curvature fabricated into the steel beam to offset dead load deflection. Per AISC 303 (Code of Standard Practice), camber should be specified when: (1) Dead load deflection exceeds 0.5 in — this is the practical threshold below which camber isn't worth the fabrication cost. (2) Flatness-critical occupancies — hospital operating rooms, high-end retail, laboratories with sensitive equipment. (3) The beam depth is less than L/24 — shallower beams deflect more, camber helps maintain ceiling flatness. (4) PONDING risk — flat roofs where excessive deflection can create water-collecting depressions. Camber specification: typically 75-80% of the calculated dead load deflection (not 100% — some settlement occurs in connections). For a W18x40 at 30 ft: DL deflection = 5 x w_DL x L^4 / (384 x E x I) = 5 x (0.5/12) x 360^4 / (384 x 29000 x 612) = 0.62 in. Camber = 0.75 x 0.62 = 0.47 in, specified as 1/2 in camber. Camber is fabricated by cold-bending (three-point bending at the mill) or heat-cambering. Beams over ~W30 or plate girders are cambered by cutting the web to a curved profile. Camber tolerance per ASTM A6: -0 to + (1/8 in per 10 ft of length, max +1/2 in). The mill only guarantees camber won't be less than specified but can be more — the erector shims to match. Do NOT camber beams with significant end restraint (moment connections, braced frames) — the camber introduces unintended moments.

Steel Floor Systems — Composite Design, Deck Selection & Span Optimization

Q: How are shear studs designed for composite beams?

Shear studs transfer horizontal shear between slab and beam — they enforce composite action. Per AISC 360 I8.2: (1) Nominal strength of one stud: Qn = 0.5 x Asc x sqrt(f'c x Ec) <= Rg x Rp x Asc x Fu. For a 3/4 in diameter stud (Asc=0.442 in^2) in 4 ksi normal-weight concrete: Qn = 0.5 x 0.442 x sqrt(4 x 3,605) = 0.221 x sqrt(14,420) = 0.221 x 120.1 = 26.5 kips. Upper bound: Rg x Rp x Asc x Fu. Rg = 1.0 (1 stud per rib), Rp = 0.75 (studs in deck ribs perpendicular to beam — most common). Fu = 65 ksi. Upper Qn = 1.0 x 0.75 x 0.442 x 65 = 21.5 kips. Controls. (2) Number of studs required between point of zero moment and maximum moment: N_req = Vh / (phi x Qn), where Vh = min(0.85 x f'c x beff x t, As x Fy) = horizontal shear to be transferred. For W18x40: Vh = As x Fy = 585 kips (steel controls because PNA is in slab). N_req = 585 / (0.65 x 21.5) = 585 / 14.0 = 42 studs total, or 21 studs each side of midspan (since studs are placed from zero moment to max moment on each half). (3) Spacing: max 8 x slab thickness = 40 in, min 6 x stud diameter = 4.5 in. For transverse deck: 1 stud max per rib. For 6 in rib spacing: 21 studs x 6 in/rib = 126 in (10.5 ft) required each side.

Q: What floor vibration criteria apply to steel floors?

Floor vibration governs steel floor design more often than strength. Per AISC Design Guide 11 (Floor Vibrations Due to Human Activity): (1) Walking excitation — the critical case for offices, residential, hospitals. Minimum natural frequency fn >= 3 Hz (offices) to avoid resonance with the first harmonic of walking (1.6-2.2 Hz). Second harmonic at 3.2-4.4 Hz also excites floors. fn = (pi/2) x sqrt(g x EI / (w x L^4)), simplified to fn = 0.18 x sqrt(g / delta), where delta = midspan deflection under the supported weight (DL + 10-25% LL). For a 30 ft composite beam: if delta = 0.5 in under DL+0.25LL, fn = 0.18 x sqrt(386/0.5) = 0.18 x sqrt(772) = 0.18 x 27.8 = 5.0 Hz — OK. (2) Peak acceleration criterion: ap/g <= 0.5% for offices, 1.5% for shopping malls, 5% for pedestrian bridges. The acceleration depends on the walking force (92 lbs for offices), floor damping (2-5% for bare steel, 6-12% with partitions/ceiling), and the effective modal mass. (3) Stiffness criterion: for offices, floor deflection under 225 lb concentrated load = 5 Hz for rhythmic. These criteria explain the rule of thumb: steel floor depth >= L/24 for simply supported composite beams (30 ft x 12 / 24 = 15 in → W16-W18 range). Vibration, not bending strength, sets the minimum depth for most steel floors.

Steel floor systems in multi-story buildings consist of a concrete slab on metal deck supported by steel beams and girders. The most efficient configuration uses composite action between the slab and the steel beams, where headed shear studs transfer horizontal shear so the concrete acts as the compression flange of a T-beam. Composite construction typically reduces steel beam weight by 30–40% compared to non-composite design for the same span and load.

Floor system configurations

System	Typical span	Depth	Cost ranking	Best for
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

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.
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%.
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.
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.

Span capability: 6-15 ft (deck span between beams), 30-45 ft (composite beam span)
Weight: 45-55 psf (deck + concrete only, no beams)
Fire rating: 2-hour with sprayed fire-resistive material (SFRM) on beams and deck underside; concrete slab provides inherent fire resistance
Cost: Lowest overall (fewest beams, efficient material use)
Typical application: Office buildings, residential towers, hospitals, schools
Construction speed: Fast (deck acts as working platform, concrete placed in one pour)

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.

Span capability: 20-35 ft (non-composite beam span)
Weight: 45-55 psf (slab), but steel beams must be 30-40% heavier to compensate for lack of composite action
Fire rating: Same as composite (SFRM required)
Cost: Slightly higher than composite (more steel weight)
Typical application: Industrial mezzanines, parking structures, retrofits where studs cannot be welded

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.

Span capability: 20-40 ft (plank span), 30-50 ft (steel beam span)
Weight: 60-80 psf (plank + topping)
Fire rating: 2-3 hour inherent in the precast concrete (no SFRM needed)
Cost: Moderate to high (precast manufacturing and transport)
Typical application: Residential buildings, parking structures, schools, buildings requiring inherent fire resistance
Key advantage: Immediate floor surface for construction traffic

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.

Span capability: 4-10 ft (unshored deck span between supports)
Weight: 2-4 psf (deck only)
Fire rating: None without concrete topping (or intumescent coating)
Cost: Lowest material cost but limited to light loads
Typical application: Roofs, canopies, industrial mezzanines, equipment platforms

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.

Span capability: 2-3 ft (pedestal spacing, not a structural span)
Weight: 8-15 psf (panels + pedestals)
Fire rating: Varies by panel type (steel, aluminum, or calcium sulfate core)
Cost: High ($15-40/sf installed)
Typical application: Data centers, server rooms, trading floors, Class A office space

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:

PNA in the slab: When T_steel < C_concrete, the entire steel section yields in tension and part of the slab is in compression. This is the most common and most efficient case.
PNA in the steel flange: When T_steel approximately equals C_concrete (or for partial composite), the PNA falls within the steel beam top flange.
PNA in the steel web: When partial composite action is low (25-40%), the PNA moves down into the web, and only the steel below the PNA yields in tension.

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

Typical spans: 5-8 ft unshored, 6-10 ft shored (floor)
Concrete volume: 0.31 cy/sf for 3.25 in total slab (2" above ribs)
Common use: Short-span floor systems, roof deck, canopies
Gauge range: 22 ga (0.0295 in) to 16 ga (0.0598 in)

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

Typical spans: 5-9 ft unshored
Concrete volume: 0.28 cy/sf for 3.25 in total slab
Common use: Floor systems where wider flute accommodates studs or ceiling attachments
Advantage: Easier stud placement, better for finishes attached to deck underside

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

Typical spans: 10-15 ft unshored (floor), up to 18 ft shored
Concrete volume: 0.43 cy/sf for 6.25 in total slab (3.25 in above ribs)
Common use: Long-span floor systems, minimizing beam lines
Stud advantage: Deep ribs allow larger stud engagement; 3/4 in studs are 3-4 in long, well-anchored in 3 in deck
Weight penalty: Heavier concrete slab (approximately 75 psf for 6.25 in LW concrete vs 50 psf for 5.25 in on 2" deck)

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.

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Related references

Disclaimer

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.