CSA S16 Load Combinations — NBCC 2020 ULS & SLS Guide

Quick Reference: Canadian steel design uses NBCC 2020 Division B Part 4 for load combinations, not ASCE 7. ULS combinations use principal and companion load factors. SLS combinations use service-level loads. Importance factors (I_E, I_W, I_S) vary by building importance category.

This guide covers the full set of NBCC 2020 load combinations for CSA S16 steel design, including ULS (Ultimate Limit States) and SLS (Serviceability Limit States) combinations, load factors for dead (D), live (L), snow (S), wind (W), earthquake (E), and temperature (T) loads, practical worked examples, and a comparison with ASCE 7 load combinations for engineers working cross-border.

NBCC 2020 Load Combination Framework

NBCC 2020 Division B Clause 4.1.3 provides the load combination rules for Canadian structural design. Unlike ASCE 7, which organises load combinations by load type, NBCC 2020 uses a principal load / companion load framework:

• Principal load: The load being considered as the primary action (full factor applied)
• Companion loads: Other loads acting simultaneously (reduced factor applied for probability of coincidence)

The companion load concept reflects the low probability that multiple extreme loads occur at the same time. For example, a once-in-50-year wind event is unlikely to coincide with a once-in-50-year snow event, so the companion factor for snow in a wind-governed combination is 0.5 (or 0.25 for seismic).

Importance Categories

NBCC 2020 assigns every building an importance category based on its use and occupancy:

The importance factor multiplies the specified load effect. For a post-disaster building, the factored wind load is 1.25 times the reference wind load, while the SLS deflection check uses the unfactored load (I_E_SLS = 1.00).

ULS Load Combinations (Table 4.1.3.2)

NBCC 2020 Table 4.1.3.2 specifies the following Ultimate Limit States combinations for steel design per CSA S16. Each combination uses the format: principal load factor × principal load + companion factor × companion loads.

Notes:

• Comb. 7a/7b are identical to 6a/6b — both are included in the table for clarity. Designers typically use the governing case and label it accordingly.
• The 1.25 factor on dead load applies when dead load adds to the principal load effect. The 0.9 factor on dead load applies when dead load resists overturning or uplift (stability case).
• Earthquake combination uses I_E = 1.0 for Normal importance. For High importance, I_E = 1.15 and the earthquake load is 1.15 × E.
• Temperature loads (T) apply only when significant restrained thermal effects exist (long-span roofs, exposed structures in northern climates).

Load Factors for Different Importance Categories

The importance factor I_E multiplies the specified load for snow, wind, and earthquake. The factored combination becomes:

Comb. 4 (Snow principal): 1.25 × D + 1.5 × I_S × S

Comb. 6 (Wind principal): 1.25 × D + 0.5 × L + 0.5 × I_S × S + 1.4 × I_W × W

Comb. 8 (Earthquake): 1.0 × D + 0.5 × L + 0.25 × I_S × S + 1.0 × I_E × E

For a Normal importance building, I_E = I_W = I_S = 1.0, so the factors are as shown in the base table. For a High importance building (school), I_W = I_S = 1.15, so Comb. 6 uses 1.4 × 1.15 W = 1.61 W and 0.5 × 1.15 S = 0.575 S.

SLS Load Combinations

Serviceability Limit States (SLS) combinations use unfactored or reduced loads. These are used for deflection, drift, vibration, and foundation settlement checks.

Deflection Limits Per CSA S16 / NBCC

CSA S16 vs ASCE 7 — Load Combination Comparison

Engineers working on both US and Canadian projects need to understand the differences. While both are ultimate-strength design codes, the load combination formats differ significantly:

Practical Differences:

1. Dead load factor: NBCC uses 1.25 vs ASCE 1.2 — 4% higher for Canadian designs. This means self-weight has a slightly larger influence.
2. Live load factor: NBCC 1.5 vs ASCE 1.6 — NBCC is 6% lower. For office buildings where live load governs, ASCE gives slightly higher factored loads.
3. Wind load factor: NBCC 1.4 vs ASCE 1.0 — this is not directly comparable because NBCC and ASCE define reference wind pressures differently (NBCC uses q × C_e × C_g × C_p based on hourly wind, ASCE uses q_z × G × C_p based on 3-second gust). The total factored wind effect is similar despite different factors.
4. Companion load concept: NBCC's companion factors reduce simultaneously acting loads (e.g., 0.5L when snow is principal). ASCE uses additive full factors for all loads in the combination, making some ASCE combinations more conservative.

Worked Example: Steel Beam Under Factored Loads (NBCC 2020)

Problem: A W460x74 floor beam in a Normal importance office building has the following unfactored loads. Determine the governing factored load for ULS design per NBCC 2020.

Given:

• Dead load: D = 12 kN/m (including self-weight)
• Live load: L = 18 kN/m (office occupancy)
• Snow load: N/A (interior)

Step 1 — Identify applicable combinations:

Comb. 1: 1.4 × D = 1.4 × 12 = 16.8 kN/m Comb. 2: 1.25 × D + 1.5 × L = 1.25 × 12 + 1.5 × 18 = 15.0 + 27.0 = 42.0 kN/m

Comb. 3: 1.25 × D + 1.5 × L + 0.5 × S — S = 0 (interior), so this reduces to Comb. 2.

Step 2 — Governing load:

For this interior beam, the governing factored load is Comb. 2: w_f = 42.0 kN/m.

Step 3 — Design moment and shear:

M_f = w_f × L² / 8 = 42.0 × 8.0² / 8 = 42.0 × 64 / 8 = 336.0 kN·m

V_f = w_f × L / 2 = 42.0 × 8.0 / 2 = 168.0 kN

Step 4 — Compare with service loads (SLS check):

w_s = D + L = 12 + 18 = 30.0 kN/m

δ = 5 × w_s × L⁴ / (384 × E × I) = 5 × 30.0 × 8000⁴ / (384 × 200,000 × 333 × 10⁶) = 6.14 × 10¹⁶ / 2.56 × 10¹³ = 24.0 mm

L/δ = 8000 / 24.0 = 333 > 300 ✓ (live load deflection governs at L/300)

Worked Example: Roof Beam with Snow

Problem: A steel roof beam in Edmonton (Normal importance) has the following loads:

• Dead load: D = 5 kN/m (steel deck + insulation + roofing + self-weight)
• Live load: L = 2 kN/m (maintenance access)
• Snow load: S = 8 kN/m (NBCC ground snow S_s = 2.2 kPa, roof factor applied)
• Wind load: W = 3.5 kN/m (NBCC wind pressure calculation — upward and downward)

Step 1 — Determine governing combination:

Downward case: Comb. 4 (Snow principal): 1.25 × 5 + 1.5 × 8 = 6.25 + 12.0 = 18.25 kN/m Comb. 5 (Snow principal + live): 1.25 × 5 + 0.5 × 2 + 1.5 × 8 = 6.25 + 1.0 + 12.0 = 19.25 kN/m ← governs

Uplift / stability case: Comb. 6b (Wind principal, stability): 0.9 × 5 + 1.4 × 3.5 = 4.5 + 4.9 = 9.4 kN/m (net downward — wind not strong enough to cause net uplift for this example)

If W were 8 kN/m upward: 0.9 × 5 + 1.4 × (-8) = 4.5 - 11.2 = -6.7 kN/m net uplift → governs uplift connection design.

Step 2 — Governing factored load:

Downward: w_f = 19.25 kN/m (Comb. 5) Uplift: — (check connections if net uplift exists)

M_f = 19.25 × 6.0² / 8 = 86.6 kN·m

Snow Load Factors (NBCC 4.1.6)

NBCC 2020 Clause 4.1.6 provides the snow load calculation:

S = I_S × [S_s × (C_b × C_w × C_s × C_a) + S_r]

Where:

• I_S = Importance factor for snow (0.8 to 1.25)
• S_s = 1-in-50-year ground snow load (kPa) — from NBCC Appendix C
• S_r = 1-in-50-year associated rain load (kPa)
• C_b = Basic roof snow factor = 0.7 (most roofs)
• C_w = Wind exposure factor (1.0 or 0.75 depending on exposure)
• C_s = Slope factor (1.0 for flat roofs, reduces with pitch)
• C_a = Accumulation factor (account for drifting, sliding)

For the Edmonton example with I_S = 1.0, S_s = 1.8 kPa (Edmonton Zone 2), C_b = 0.7, C_w = 1.0, C_s = 1.0 (flat roof), C_a = 1.0 (no drift), S_r = 0.3 kPa:

S = 1.0 × [1.8 × (0.7 × 1.0 × 1.0 × 1.0) + 0.3] = 1.0 × [1.26 + 0.3] = 1.56 kPa

With tributary width = 5 m: w_snow = 1.56 × 5 = 7.8 kN/m → rounds to ~8 kN/m as used above.

Wind Load Factors (NBCC 4.1.7)

NBCC 2020 Clause 4.1.7 provides the wind load calculation:

p = I_W × q × C_e × C_g × C_p

Where:

• I_W = Importance factor for wind (0.8 to 1.25)
• q = Reference velocity pressure (kPa) — based on 1-in-50-year hourly wind speed
• C_e = Exposure factor (accounts for height and terrain)
• C_g = Gust effect factor (1.0 for rigid buildings, dynamic for flexible)
• C_p = External pressure coefficient (depends on building shape and zone)

The reference velocity pressure q = 0.5 × ρ × V², where ρ = 1.292 kg/m³ (air density at 10°C) and V is the 1-in-50-hourly wind speed in m/s. At V = 90 km/h (25 m/s), q = 0.5 × 1.292 × 25² = 404 Pa ≈ 0.40 kPa.

Related Pages

• Canada CSA S16 Steel Design Guide — Full CSA S16 design reference
• Canadian Steel Beam Sizes — W Shapes, HSS — Complete section tables
• Canadian Steel Grades — G40.21 300W to 480W — Material properties
• CSA S16 Beam Design — Flexure, LTB & Shear — Beam design per CSA S16
• Canadian Wind Load — NBCC & CSA S16 Reference — Wind load calculation guide
• Canadian Snow Load — NBCC Ground & Roof Loads — Snow load calculation guide
• Canadian Seismic Design — CSA S16 Clause 27 — Seismic design provisions
• Load Combinations Calculator — CSA S16 & NBCC — Free Canadian combination calculator
• Beam Capacity Calculator — Free multi-code beam calculator

Frequently Asked Questions

How do NBCC 2020 load combinations differ from ASCE 7 for steel design?

NBCC 2020 uses a principal load / companion load framework where only one load at a time is considered the "principal" load with its full factor applied, while other loads receive reduced companion factors (typically 0.5). ASCE 7 applies full factors to all loads in the combination simultaneously. This means NBCC combinations are not directly comparable to ASCE 7 combinations — they are structured differently. For example, the dead load factor is 1.25 in NBCC vs 1.2 in ASCE 7, but live load is 1.5 vs 1.6. The companion load concept (0.5 × L when S is principal) is absent from ASCE 7, which uses 0.5 × L as a direct additive term. For cross-border projects, recalculate using the local code rather than applying conversion factors.

What is the importance factor I_E and when does it apply?

I_E is the importance factor for earthquake loads, ranging from 0.80 (Low importance — barns, storage) to 1.25 (Post-Disaster — hospitals, fire stations). It multiplies the earthquake load E in ULS combination 8. Separate importance factors exist for wind (I_W) and snow (I_S). The values for Normal importance buildings — the most common category — are I_E = I_W = I_S = 1.0, meaning the base factors in the ULS table apply without modification. High and Post-Disaster buildings require increased factored loads. For example, a school (High importance) has I_W = 1.15, so the wind load in Comb. 6a becomes 1.4 × 1.15 W = 1.61 W.

What serviceability limits does CSA S16 reference for deflection?

CSA S16-19 Clause 25.3 references NBCC 2020 and project specifications for deflection limits. Common limits: L/300 for live load deflection of floor beams, L/360 for floors supporting brittle finishes (terrazzo, tile), L/240 for total load deflection of roof beams under snow, L/180 for cantilevers, and H/300 for inter-storey drift under wind. Vibration-sensitive floors (offices with long-span open areas) may require L/480 or a specific vibration analysis per CSA S16 Appendix G. These limits are typically specified by the structural engineer of record as part of the design criteria and may be more stringent than code minimums.

When does the 0.9 dead load factor govern for Canadian steel design?

The 0.9 dead load factor appears in stability combinations (Comb. 6b and 7b) where dead load is beneficial — it resists overturning, uplift, or sliding. The reduced 0.9 factor accounts for the probability that dead load is less than its nominal value. This governs for: roof beams with net uplift from wind, cantilever retaining walls (overturning check), portal frames (gravity load reduces uplift at leeward column), and foundations (sliding resistance). The 0.9 factor represents the 5th percentile dead load — there is a 95% probability that actual dead load exceeds 90% of the nominal value.

This page is for educational reference. All load factors per NBCC 2020 Division B Part 4 and CSA S16-19. Verify all load combinations against the current building code applicable in your jurisdiction. Canadian provinces and territories may have amendments to NBCC 2020. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent P.Eng. verification.