Steel Deck Design Guide — SDI Profiles, Composite Deck, and Diaphragm Design

Steel deck serves three roles in modern building construction: (1) permanent formwork for concrete slabs during construction, (2) positive tensile reinforcement in composite slab systems after the concrete hardens, and (3) a horizontal diaphragm transferring wind and seismic forces to the lateral force-resisting system. The Steel Deck Institute (SDI) publishes the governing standards in North America — the SDI Floor Deck Design Manual and Roof Deck Design Manual — complemented by AISC 360 Chapter I for composite design and AISI S310 for diaphragm design.

This guide covers composite vs. non-composite deck profiles, form deck and roof deck span tables, shear stud requirements for composite beams, diaphragm shear capacity per SDI DD3, and fire-rated deck assemblies. A worked example determines the deck gage and span for a typical office floor.

PRELIMINARY — NOT FOR CONSTRUCTION. This guide is for educational reference. All designs must be independently verified by a licensed Professional Engineer.

Deck profiles and their roles

Composite deck (SDI profiles: 1.5C, 2C, 3C)

Composite steel deck has formed embossments (small ribs, indentations, or dovetails) in the webs and top flange. These embossments mechanically interlock with the hardened concrete, transferring horizontal shear between the steel deck and the concrete slab. The deck acts as permanent positive reinforcement, eliminating the need for bottom rebar mats in many applications (though temperature and shrinkage reinforcement is still required per ACI 318).

Standard composite deck depths and coverage widths:

WR = wide rib. The ribs create longitudinal void formers that reduce concrete volume (and weight) by 20–40% compared to a flat slab of equivalent flexural depth.

Form deck (SDI profiles: 1.0F, 1.5F, 2.0F)

Form deck (also called non-composite deck) supports the wet concrete during construction but does not participate in the composite action after concrete hardening. The deck provides formwork only — all reinforcement for the slab must come from rebar or welded wire fabric. Form deck profiles are simpler (no embossments) and less expensive per square foot than composite deck.

Roof deck (SDI profiles: NR, WR, B, LS)

Roof deck spans between open-web steel joists or purlins and supports insulation, membrane roofing, and live/snow loads. Roof deck is typically thinner (22–18 gage) and has a wider rib spacing to accommodate mechanical fasteners for insulation and roofing membrane attachment. Standard depths are 1.5 in, 3.0 in, and 4.5 in, with the deeper sections used for longer spans.

Acoustical / cellular deck

Cellular deck has a flat bottom plate welded to the underside of the flutes, creating closed cells that can house electrical wiring, communication cables, or air distribution. The flat underside also improves acoustical performance by eliminating the ribbed ceiling profile. Cellular deck costs approximately 30–50% more than standard composite deck and is typically specified for high-end office and laboratory buildings.

Deck span tables: construction loads vs. superimposed loads

Steel deck must be checked for two distinct load conditions:

Construction loads (unshored)

During concrete placement, the deck alone carries the wet concrete weight (typically 145–150 pcf for normal-weight concrete) plus a 20 psf uniform construction live load or a 150 lb concentrated load over a 12 in square area, whichever produces the larger effect (SDI C-2017 Section 2.4.B and ANSI/ASCE 37). This is the unshored span condition — no temporary shoring props support the deck.

For 1.5C composite deck, 20 gage (0.036 in / 0.91 mm base metal thickness), with 3.25 in total slab depth:

• Wet concrete weight = 3.25/12 × 150 = 40.6 psf
• Construction live load = 20 psf
• Total construction load = 60.6 psf
• Maximum unshored span from SDI tables: approximately 8 ft single-span, 10 ft triple-span

If the span exceeds the unshored limit, temporary shoring must be installed at mid-span or third-points until the concrete reaches 75% of specified compressive strength (typically 3–7 days).

Superimposed loads (composite stage)

After the concrete has cured, the composite slab (deck + concrete) carries the occupancy loads. The SDI composite slab load tables give the superimposed load capacity based on:

• Total slab depth (deck depth + concrete above deck)
• Concrete compressive strength f'_c (typically 3,000 or 4,000 psi)
• Deck gage and profile
• Span condition (single-span or multi-span)

For 1.5C × 20 gage with 3.25 in total depth and f'_c = 3,000 psi at 10 ft span:

• Superimposed load capacity from SDI tables: approximately 125 psf
• Office floor live load (IBC Table 1607.1): 50 psf + 20 psf partition allowance = 70 psf
• Dead load of finishes (ceiling, MEP, floor covering): 10–15 psf
• Total superimposed load: 70 + 15 = 85 psf < 125 psf — OK.

Shear stud design for composite beams (AISC 360 Chapter I)

Headed shear studs welded through the steel deck to the beam flange transfer longitudinal shear between the concrete slab and the steel beam. The number of studs required depends on whether full or partial composite action is desired.

Nominal strength of one stud (AISC 360 Eq. I8-1):

Q_n = 0.5 × A_sa × sqrt(f'_c × E_c) ≤ R_g × R_p × A_sa × F_u

Where:

• A_sa = cross-sectional area of stud shank (π/4 × d²)
• E_c = w_c^1.5 × sqrt(f'_c) (concrete elastic modulus)
• R_g = reduction factor for stud group geometry (1.0 for one stud per rib, 0.85 for two, 0.70 for three)
• R_p = reduction factor for stud position within deck rib (0.75 for studs welded in ribs perpendicular to beam, 0.60 for parallel)

The R_g × R_p reduction is critical — a stud welded through a composite deck rib that runs perpendicular to the beam axis has R_g × R_p = 0.75 × 1.0 = 0.75, meaning 25% reduction in capacity compared to a stud on bare steel. This is why composite beam tables in the AISC Manual separate "deck perpendicular" and "deck parallel" cases.

Number of studs for full composite action

The total horizontal shear to be transferred between the point of zero moment and the point of maximum positive moment:

V' = min(0.85 × f'_c × A_c, F_y × A_s)

Where A_c is the effective concrete area (b_eff × slab depth above deck) and A_s is the steel beam area. The number of studs N = V' / Q_n per half-span.

For a typical 30-ft W18×55 composite beam with 3.25 in slab on 1.5C deck at 10 ft spacing:

• Effective width b_eff = min(30×12/8, 10×12/2) = min(45, 60) = 45 in
• A_c = 45 × 3.25 = 146 in² (concrete above deck; concrete in ribs is ignored)
• 0.85 f'_c A_c = 0.85 × 3 × 146 = 372 kips (concrete crushing governs)
• F_y × A_s = 50 × 16.2 = 810 kips
• V' = 372 kips (concrete governs)
• Q_n per 3/4" stud (deck perpendicular): approximately 17.2 × 0.75 = 12.9 kips
• N = 372 / 12.9 = 29 studs per half-span → 58 studs total

In practice, this is reduced because partial composite action (providing ≥ 25% of full composite studs per AISC 360 Section I3.2d) often achieves adequate strength with fewer studs.

Diaphragm design (SDI DD3)

The steel deck, when properly attached to the supporting steel framing, acts as a horizontal diaphragm transferring wind and seismic forces to the vertical lateral force-resisting system (braced frames, moment frames, shear walls). The Steel Deck Institute Diaphragm Design Manual (DD3) provides the design methodology.

Diaphragm shear strength

The nominal diaphragm shear strength S_n depends on:

1. Deck profile and gage — deeper decks and thicker gages have higher shear capacity.
2. Deck attachment — puddle welds, screws, or powder-actuated fasteners at the deck-to-support connection.
3. Side-lap connection — screws or button punches connecting adjacent deck panels at the side lap.
4. Span and number of spans — continuous multi-span decks have higher diaphragm stiffness than single-span.

A typical 1.5C × 20 gage composite deck with 36/4 attachment pattern (puddle welds at 36 in at supports and 4 side-lap screws per span) provides:

• Nominal shear strength S_n ≈ 1,000–1,500 plf (pounds per linear foot of diaphragm depth)
• Design shear strength φ × S_n with φ = 0.55 for wind, 0.50 for seismic
• For a 100 ft deep diaphragm: total shear capacity = 100 × 1,000 × 0.55 = 55,000 lb = 55 kips

Diaphragm flexibility classification

Per ASCE 7 Section 12.3.1, a diaphragm is classified as flexible if the maximum in-plane diaphragm deflection exceeds 2× the average story drift of the adjoining vertical elements. Steel deck diaphragms on bar joists or light beams are typically classified as flexible, meaning lateral forces are distributed to vertical elements by tributary area rather than by relative stiffness.

Fire-rated deck assemblies

Fire resistance ratings for steel deck assemblies are provided by UL (Underwriters Laboratories) designs. The key variables that determine the fire rating are:

• Concrete type and thickness — normal-weight concrete provides better fire resistance than lightweight per unit thickness.
• Deck type and depth — deeper decks provide more concrete cover over the beam flange.
• Beam protection — spray-applied fireproofing (SFRM) on the beam itself or the beam encased in concrete.
• Restrained vs. unrestrained — restrained assemblies (where thermal expansion is resisted by the surrounding structure) achieve higher ratings per IBC Table 722.

Common fire-rated deck assemblies:

• D902 (1-hr restrained): 1.5C × 20 gage, 3.25 in normal-weight concrete, no SFRM on beam (beam protected by concrete cover only)
• D916 (2-hr restrained): 2C × 20 gage, 4.5 in normal-weight concrete, beam with 1/2 in SFRM
• D925 (3-hr restrained): 3C × 18 gage, 6.25 in total depth, beam fully encased

Worked example: deck gage selection for an office floor

An office floor uses composite steel deck on W18×55 beams spaced at 10 ft on center. The deck spans 8 ft between beams (perpendicular to beam axis). Determine the minimum deck gage and confirm the composite slab capacity.

Step 1 — Construction loads (unshored):

Select 1.5C composite deck. Total slab depth: 3.25 in (1.5 in deck + 1.75 in concrete topping). Wet concrete weight = 3.25/12 × 150 = 40.6 psf. Construction live load = 20 psf. Total = 60.6 psf.

From SDI C-2017 uniform load tables for 1.5C deck at 8 ft triple-span:

• 22 gage (0.030 in): allowable = 55 psf (insufficient)
• 20 gage (0.036 in): allowable = 72 psf > 60.6 psf — OK.

Step 2 — Superimposed loads (composite stage):

Occupancy: office = 50 psf live load + 20 psf partition + 15 psf finishes = 85 psf total superimposed load.

From SDI composite slab load tables for 1.5C × 20 gage, 3.25 in total depth, f'_c = 3,000 psi at 8 ft span:

• Allowable superimposed load = 165 psf > 85 psf — OK. (The table value includes a 1/240 deflection limit, so stiffness is verified.)

Step 3 — Shear stud count:

Per Step 2 above, full composite action requires approximately 29 studs per half-span. Partial composite action at 50%: N = 15 studs per half-span. Check AISC 360 Section I3.2d for minimum: N_min = 0.25 × 29 = 8. At 15 studs (52% partial composite), the moment capacity from AISC composite beam tables for W18×55 with 1.5C deck: φM_n ≈ 420 kip-ft.

Factored moment (assuming 85 psf total load × 10 ft tributary × 30 ft span): w_u = 1.2 × 15 + 1.6 × 70 = 18 + 112 = 130 psf → 130 × 10 = 1,300 plf = 1.3 klf. M_u = 1.3 × 30² / 8 = 146 kip-ft. Utilization: 146 / 420 = 0.348 — OK.

Step 4 — Diaphragm check:

Diaphragm depth = building width = 100 ft. Tributary wind force at roof level: w = 25 psf (ASCE 7 MWFRS, typical for 60 ft tall office in Exposure B). Total wind shear at diaphragm = 25 × 60 × 100 / (2 × 100) = 750 plf. From SDI diaphragm tables for 1.5C × 20 gage, 36/4 pattern: S_n = 1,200 plf. φ × S_n = 0.55 × 1,200 = 660 plf < 750 plf — FAIL.

Revise: Use 36/7 pattern (puddle welds at 36 in at every support + edge, screws at 7 side-lap connections per span). S_n increases to approximately 1,600 plf. φ × S_n = 0.55 × 1,600 = 880 plf > 750 plf — OK. The additional side-lap screws add approximately $0.15/sf to the installed deck cost.

Key takeaways

• Steel deck serves three roles simultaneously: permanent concrete formwork, tensile reinforcement in composite slabs, and a horizontal diaphragm. Each role imposes different design requirements, and the governing condition can be construction loads (unshored span), occupancy loads (superimposed capacity), or lateral forces (diaphragm shear).

• The critical deck design condition is typically the unshored construction span. The wet concrete weight (40–70 psf depending on slab thickness) plus 20 psf construction live load determines the maximum span before temporary shoring is required. If construction loads govern, increase the deck gage or add shoring — increasing gage from 20 to 18 adds approximately 15% cost per square foot but extends the unshored span by 25–30%.

• Shear stud capacity is reduced by R_g × R_p when studs are welded through deck ribs. For deck perpendicular to the beam axis (most common case), R_p = 0.75, meaning a 25% reduction per stud. This reduction is accounted for in AISC Manual composite beam tables, so always use the table that matches your deck orientation.

• Composite deck contributes to fire resistance by providing concrete cover over the beam flange. UL assembly listings specify the governing concrete thickness, deck profile, and beam protection for each fire rating. The standard 1.5C deck with 3.25 in total slab depth can achieve a 1-hour rating without additional beam fireproofing in restrained assemblies.

FAQ

What is the difference between composite deck and form deck?

Composite deck has embossments (ribs, indentations) in the webs and top flange that mechanically interlock with the concrete, allowing the deck to serve as permanent positive reinforcement. Form deck has a smooth profile and serves only as concrete formwork — all slab reinforcement must come from rebar or welded wire fabric. Composite deck is approximately 10–15% more expensive per square foot but eliminates the bottom mat of rebar, which typically costs $0.50–1.00/sf. For spans under 15 ft, composite deck is usually the more economical choice.

When do I need to shore steel deck during construction?

Shoring is required when the clear span exceeds the unshored capacity listed in the SDI span tables for the selected deck gage and profile. As a rule of thumb, 1.5C × 20 gage composite deck can span 6–8 ft unshored with 3.25 in normal-weight concrete. 3C × 18 gage deck can span 12–16 ft unshored. Temporary shoring adds $1–3/sf to construction cost and extends the construction schedule by 3–7 days (until concrete reaches 75% f'_c). For large floor plates, the shoring cost can exceed the cost savings of using a lighter gage deck.

How are shear studs installed through steel deck?

A drawn-arc stud welding gun positions the stud against the deck, initiates an arc that melts both the stud tip and the deck surface, then plunges the stud into the molten puddle. The weld burns through the deck and fuses to the beam flange beneath. The process takes approximately 0.5 seconds per stud. For deck thicker than 20 gage, a pre-heat pass or larger-diameter ferrule may be needed. QC testing: bend 1% of studs to 30° from vertical without fracture (AISC 360 Section I8.2b and AWS D1.1 Section 7.8).

What is the minimum side-lap connection for diaphragm action?

SDI DD3 requires a minimum of one side-lap connection per span for all deck panels acting as diaphragms. Standard practice for low-seismic applications is 36/4 (welds at 36 in at supports, 4 side-lap screws per span). For moderate and high seismic design categories (SDC C–F), SDI requires a minimum of 36/7 pattern, and the actual number of side-lap connections is determined by the diaphragm shear demand. Button punches (mechanical interlock without a fastener) are not permitted as diaphragm connections in SDC D and above because they can disengage under cyclic loading.