CSA S16:24 Composite Beam Framework

Composite beams combine a steel beam (acting in tension) with a concrete slab (acting in compression) via headed shear studs. The result is a section with 2-3x the strength and stiffness of the bare steel beam alone. CSA S16:24 Clause 17 governs all aspects of composite beam design in Canada.

Design follows three sequential checks:

Effective Slab Width — CSA S16:24 Clause 17.3

The effective concrete flange width b_e for a composite beam is the minimum of:

b_e = min(span/4, b_0 + 16h_s + b_f)

For interior beams: b_e = min(span/4, b_0 + 8h_s for each side)

Worked Example — Effective Width

Given: W460x52 beam, span 9000 mm, beam spacing 3000 mm, slab 75 mm above 76 mm deck ribs.

b_e = min(9000/4, 3000) = min(2250, 3000) = 2250 mm

Check: b_0 + 8h_s = 3000 + 8 x 75 = 3600 mm > 2250 mm. OK, b_e = 2250 mm governs.

Headed Shear Stud Design — Clause 17.7

Stud Capacity

Per CSA S16:24 Clause 17.7.2, the factored shear resistance of a single headed stud embedded in a solid concrete slab:

q_r = 0.5 x phi_sc x A_s x sqrt(f'_c x E_c) <= phi_sc x A_s x F_u

Where:

Studs in Deck Ribs — Reduction Factors

For studs in steel deck ribs (the common case), CSA S16 reduces capacity by:

Reduction factor R_g = 0.85 x (w_r/h_r) x (h_s/h_r - 1.0) <= 0.75

And a separate geometry reduction R_p applies when studs are welded through the deck:

R_p = 0.60 for single stud per rib R_p = 0.75 for two studs per rib

Worked Example — Stud Capacity

Given: 19 mm diameter x 125 mm headed stud (A_s = 284 mm^2), f'_c = 25 MPa, deck rib height 76 mm, slab above 75 mm.

Step 1 — Solid slab capacity: E_c = 4500 x sqrt(25) = 22,500 MPa q_r solid = 0.5 x 0.80 x 284 x sqrt(25 x 22,500) = 0.40 x 284 x 750 = 85,200 N

Upper limit: 0.80 x 284 x 450 = 102,240 N. 85.2 kN < 102.2 kN. OK.

Step 2 — Deck reduction: h_s = 75 mm, h_r = 76 mm, w_r = 152 mm (average deck rib width). R_g = 0.85 x (152/76) x (75/76 - 1.0) = 0.85 x 2.0 x (-0.013) => use 0.75 when positive

Since (h_s/h_r - 1.0) = -0.013 (negative for thin slab), the reduction is severe — the stud is partially above the rib zone. For h_s/h_r >= 1.5, the reduction is minimal.

For single stud per rib: q_r deck = 85.2 x 0.75 x 0.60 = 38.3 kN

Full vs Partial Composite Action

Full Interaction

Full composite action requires sufficient shear studs to develop the full plastic capacity of the composite section:

n_full = min(C_c, T_s) / q_r

Where C_c = 0.85 x f'_c x b_e x h_s (concrete compression) and T_s = phi x A_s x F_y (steel tension).

Partial Interaction

Partial composite action with fewer studs (cost savings, up to 40% fewer studs) is permitted per Clause 17.5. The reduced moment capacity is interpolated linearly:

M_r partial = M_r steel + (n/n_full) x (M_r composite - M_r steel)

Minimum degree of shear connection: n/n_full >= 0.40 for beams with symmetrical sections and spans <= 12 m, AND the stud capacity q_r is reduced by 10% for n/n_full < 0.50.

Worked Example — Number of Studs

Given: W460x52 beam, 9000 mm span, composite section with C_c = 3344 kN, T_s = 2330 kN (steel governs).

n_full = 2330 / 85.2 = 27.3 => 28 studs (14 pairs at 300 mm spacing)

For partial interaction at 60%: n = 0.60 x 28 = 17 studs (9 pairs). Check minimum: 0.40 x 28 = 11. 17 > 11. OK.

Canadian Steel Deck Profiles

Common Deck Types

Stud Placement in Deck

For composite beams with ribs perpendicular to the beam:

• One stud per rib: R_p = 0.60
• Two studs per rib: R_p = 0.75
• Stud diameter <= 20 mm for deck thickness >= 0.76 mm
• Stud projection above deck: minimum 40 mm

For ribs parallel to the beam: no deck reduction factor (R_p = 1.0), but minimum clear spacing between studs = 6d along the beam and 4d transversely.

Construction Stage — Unshored Construction

In unshored construction (most common in Canada), the bare steel beam must support:

Per CSA S16 Clause 17.6, the steel beam alone must satisfy:

M_f (construction) <= M_r (steel beam)

If the bare steel beam is inadequate, temporary shoring is required (shored construction). Shoring is removed after concrete reaches 75% of f'_c (typically 7 days for 25 MPa concrete at 20 degrees Celsius).

Worked Example — Construction Check

Given: W460x52 beam (M_r steel = 272 kN·m at phi = 0.90), 9000 mm span, beam spacing 3000 mm.

Construction loads:

• Dead: deck (0.15 kPa) + wet concrete 150 mm (3.60 kPa) = 3.75 kPa
• Live: 1.0 kPa
• Factored: 1.25 x 3.75 + 1.5 x 1.0 = 4.69 + 1.50 = 6.19 kPa

Line load on beam: 6.19 kPa x 3.0 m = 18.57 kN/m

M_f = 18.57 x 9.0^2 / 8 = 188.0 kN·m

188.0 < 272.0 kN·m. OK. Unshored construction is adequate.

Deflection — Composite Action

Composite beam deflections under live load are reduced compared to bare steel because of the larger transformed moment of inertia I_tr:

I_tr = I_s + A_s x (y_s - y_bar)^2 + (b_e/n) x h_s^3 / 12 + (b_e/n) x h_s x (h_s/2 + d/2 - y_bar)^2

Where n = E_s / E_c (modular ratio, typically 8-10 for normal-density concrete).

For partial interaction, the effective moment of inertia is:

I_eff = I_s + (n/n_full)^0.5 x (I_tr - I_s)

This accounts for the reduced stiffness when fewer studs are provided.

Frequently Asked Questions

What is the effective slab width for composite beams per CSA S16:24?

Per CSA S16:24 Clause 17.3, the effective slab width b_e is the minimum of span/4 and beam spacing b_0 for interior beams. For edge beams, b_e = span/8 + b_0/2. The effective width accounts for shear lag in the concrete flange: concrete further from the steel beam web contributes less to composite action. For beams at 3000 mm spacing spanning 9000 mm, b_e = min(2250, 3000) = 2250 mm typically governs.

How many shear studs are needed for full composite action in a typical Canadian office floor?

For a typical W460 beam at 3000 mm spacing spanning 9000 mm with 150 mm total slab (75 mm concrete above 76 mm deck), the steel tension capacity T_s typically governs over concrete compression C_c. With T_s approximately 2000-2500 kN and 19 mm stud capacity of 50-85 kN (after deck reduction), 28-45 studs are typically needed for full interaction. This equates to 14-23 pairs spaced at 300-400 mm. Partial interaction at 60% can reduce this to 17-27 studs.

Can composite beams be designed with partially encased sections per Canadian standards?

Yes. CSA S16:24 permits partially encased composite beams and composite columns. For partially encased beams, concrete is cast between the beam flanges with transverse reinforcement. The concrete provides fire protection, local buckling restraint for the web, and additional compressive capacity. The composite column provisions are in Clause 18, and the fire resistance rating for partially encased sections can reach 120 minutes without additional protection.

What Canadian deck profiles are commonly used for composite floor systems?

The Canam P-3606 (76 mm dovetail rib, 152 mm average rib width) and P-3615 (76 mm wide rib, 305 mm rib width) are the most common. Vicwest 75 mm trapezoidal deck is also widely used. The P-3615 wide rib allows two studs per rib with minimal reduction. For thin slabs, Vicwest 38 mm shallow deck is used in retrofit and residential applications where total floor depth must be minimized. Deck selection must consider the stud diameter limit (<= 20 mm for 0.76 mm deck), minimum 40 mm stud projection, and fire rating requirements.

Related Pages

• Canadian Shear Stud Guide — CSA S16 Headed Studs
• CSA S16 Beam Design — Flexure & Shear
• Canadian Composite Column Design — Clause 18
• Canadian Steel Grades — G40.21 Reference
• Beam Deflection Calculator — CSA S16
• Steel Beam Capacity Calculator — Free Tool
• All Canadian Steel Design References

Design Resources

Calculator tools

• Beam Capacity Calculator
• Beam Displacement and Sag Tool
• Steel Column Calculator
• Shear Stud Capacity Tool

Design guides

• CSA S16 Beam Design Guide
• CSA S16 Column Design — Buckling & Capacity
• Canadian Steel Fire Protection Guide
• AISC Composite Beam Design Guide
• EN 1994 Composite Beam Design

This page is for educational reference only. Composite beam design per CSA S16:24 Clause 17 and CSA A23.3 for concrete. All results are PRELIMINARY — NOT FOR CONSTRUCTION. All structural designs must be independently verified and sealed by a licensed Professional Engineer registered in the province or territory of the project.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.