Structural design to NBCC, CSA S16 and CSA A23.3

In Canada, the National Building Code of Canada (NBCC) sets the loads and how they combine, while the CSA Group publishes the material standards: CSA S16 for steel and CSA A23.3 for concrete. Choosing CSA in Struxt3D applies the NBCC 2020 combinations and loads, Canadian W-shapes and grades, and checks members with CSA S16-19 and CSA A23.3-19. This tutorial explains each part and checks some numbers by hand.

#Standards used

#Load combinations

NBCC uses a principal load (factored fully) and companion loads (factored less). Struxt3D generates:

Each wind combination is made for the four directions, and the seismic one for ±(E_X + 0.3E_Y) and ±(0.3E_X + E_Y). Snow (S) and 1.5L + 1.0S style companion terms are not generated. The seismic weight includes 25 % of the live load; NBCC itself counts 25 % of the snow load plus storage loads, so adjust if your building has heavy snow.

#Live loads

#Wind

NBCC tabulates a reference velocity pressure q per municipality. Struxt3D asks for the equivalent mean wind speed instead (25 m/s by default) and computes the pressure with an open-terrain exposure profile:

For example, q_1/50 = 0.40 kPa gives V = √(400/0.625) = 25.3 m/s. The importance factor I_w, gust factor C_g and topographic factor C_t are not applied separately.

#Seismic

Enter the 5 %-damped spectral accelerations S(0.2) and S(1.0) for your site class. Struxt3D builds a two-point spectrum of the same shape as ASCE 7 (flat at S(0.2), then falling as S(1.0)/T) and divides it by R_dR_o:

#Materials and sections

Beams come from W310×39 up to W920×201 and columns from W200×46 to W360×262. Steel uses φ = 0.90 with the CSA S16 column formula (n = 1.34). Concrete uses the material resistance factors φ_c = 0.65 and φ_s = 0.85 and the equivalent stress block α₁f′_c.

#Member and element checks

• Steel: CSA S16-19 section class, compression, lateral-torsional buckling, shear and the beam-column interaction of 13.8. See steel beam design.
• Concrete: CSA A23.3-19 flexure, shear, columns with slenderness, slabs with punching shear, walls and footings. See slab design.
• Timber: EN 1995-1-1 rules for every code (k_mod 0.8, γ_M 1.25), with Canadian glulam strengths.

#Worked example: a four-storey office frame

1. Open the Multi-storey frame template.
2. In Materials, choose CSA: steel 350W, concrete 35 MPa, W-shapes in metric designation.
3. In Supports & loads, pick Office, wind 25 m/s, and turn on Earthquake with S(0.2) = 0.5 g, S(1.0) = 0.25 g, ductile moment frame.
4. Press F5 then F6, and open the calculation report to read each CSA S16 clause.

#Check 1: factored floor load

#Check 2: factored wind on the windward wall at 10 m

#Check 3: spectrum plateau

#How CSA changes the design

NBCC's 1.25D + 1.5L is the principal-load form of the gravity combination. On a heavy concrete floor it gives a factored load close to the Eurocode's and slightly above the US one. The companion factor of 0.5 on live load with wind is the same idea as ψ₀ in EN 1990, but larger than the 0.4 of AS/NZS.

CSA A23.3 applies resistance factors to the materials (φ_c = 0.65 on concrete, φ_s = 0.85 on steel) rather than to the whole member as ACI does. The result is similar for lightly reinforced beams, but columns with high axial load are more sensitive to the concrete strength: try f′_c 40 MPa before enlarging a column. In steel, the CSA S16 column formula with n = 1.34 sits close to EN 1993 curve b for rolled W-shapes.

Because R_dR_o combines ductility and overstrength, a ductile moment frame (7.5) attracts about a third of the seismic force of a limited-ductility frame (2.6) at the same period. Pick the system you will actually detail for; the higher factor needs the capacity design and detailing of CSA S16 Clause 27 and A23.3 Chapter 21.

#What you still need to check

• Snow and rain loads, including drifting, and the companion-load combinations with S.
• Importance factors I_w, I_E, I_S for your importance category.
• Seismic: M_v, the full multi-point spectrum, scaling to the static base shear, torsional sensitivity, drift and capacity design (CSA S16 Clause 27, A23.3 Chapter 21).
• Wind exposure, topography and dynamic effects for tall or slender structures.

More on the solver's assumptions: accuracy and limitations.