Wind Loads - Main Structural System (NBCC 2025)

How it calculates

This calculator determines wind pressures on the main wind-force-resisting system (MWFRS) of a low-rise building using the static procedure of the National Building Code of Canada 2025 (Division B, Part 4, Subsection 4.1.7). It builds the velocity pressure, applies exposure, gust and topographic effects, then resolves the Table 4.1.7.6 zone coefficients for both orthogonal load cases.

Reference velocity pressure and importance factors

The reference velocity pressure q500 is the value with a 1-in-500 annual probability of being exceeded at the site, taken from the project defaults or entered as an override. This is the change that most affects results carried over from an NBCC 2020 workflow, which was built on the 1-in-50 value.

The building importance category (Low, Normal, High or Post-disaster, per Table 4.1.2.1) selects the wind importance factor. The ultimate limit state factor IW,ULS varies with the category from Table 4.1.7.3; the serviceability factor IW,SLS is 0.6 for every category under NBCC 2025. Both are carried through the calculation in parallel so ULS and SLS pressures are produced in one pass.

Building geometry and reference height

Roof type (flat or gable) and roof pitch, entered as rise per 12 units of run, give the roof angle θ. From the roof top height and the building width, the eave height he and the actual mean roof height Hmean follow. The reference height h used for the exposure factor is the mid-height of the roof, or the eave height where the roof angle is shallow.

Hmean is also what the low-rise screening test uses. A separate height H, the eave height, is carried for the Figure 4.1.7.6.-A zone geometry, which is deliberately distinct from the reference height h.

End-zone widths

The gable-wall end-zone width z is taken as

z = max( min(0.10 × Bmin, 0.40 × H), max(0.04 × Bmin, 1 m) )

where Bmin is the least horizontal dimension in plan and H the eave height used in the figure. The wider end-zone dimension for Load Case A follows as

y = max( 6 m, 2z )

These two dimensions set where the end-zone coefficients 1E through 6E apply in place of the interior zone values.

Exposure, gust and topographic factors

The exposure factor Ce is computed from the reference height and the terrain type (open, rough or intermediate). The intermediate case interpolates on the extent of rough terrain upwind, xr, and is subject to the code floor on the factor. The exposure factor applied to internal pressure, Cei, is taken equal to Ce, since the dominant-opening exception is outside the scope of this calculator.

The gust effect factor Cg for the building as a whole is 2.0, and the internal gust effect factor Cg,i is likewise 2.0. The alternative internal gust calculation based on opening area is not covered.

Where a hill or escarpment is present, the effective upwind distance Lh,eff is taken as the greater of the measured distance to the half-height point and twice the feature height, and the shape selection sets the decay constants k and α. The maximum speed-up ΔS,max at the crest is derived from the feature height and Lh,eff, then attenuated by horizontal distance from the crest and height above ground to give ΔS at the building. The topographic factor is

Ct = (1 + ΔS / Cg) × (1 + ΔS)

Where no feature is present, or the hill is too shallow to trigger the provisions, Ct is 1.0.

External pressure coefficients

The combined external gust-pressure coefficient CgCp is read from Table 4.1.7.6 against the Figure 4.1.7.6.-A zone system. The gust effect factor is already embedded in these tabulated values and is not applied a second time.

For Load Case A, wind normal to the ridge, the coefficients vary with roof angle and are interpolated between the tabulated slopes, so a pitch falling between listed angles is handled without rounding. The ratio of building width to eave height B/H is evaluated to determine whether Note (6) to Figure 4.1.7.6.-A governs the extent of Zone 2 and 2E. For Load Case B, wind parallel to the ridge, the coefficients are constant across all roof slopes.

Internal, external and net pressures

The external pressure on each zone is

p,ext = IW × q500 × Ce × Ct × CgCp

The building opening condition sets the positive and negative internal pressure coefficients Cpi,+ and Cpi,−, which are factored by the internal exposure factor Cei and the internal gust factor Cg,i to give the internal pressure. The net pressure on a zone is the external pressure less the internal pressure:

p,net = p,ext − p,i

Because the internal pressure can act outward or inward, every zone is reported against both the positive and the negative internal condition, at both ULS and SLS. That gives four net pressure values per zone per load case, from which the governing value for a given frame element is selected.

Scope and limits

The calculation is valid only for low-rise buildings as NBCC 2025 defines them: height H not greater than 20 m, and H less than the smaller horizontal plan dimension (Cl. 4.1.7.3.(6)(a) and 4.1.7.6.(1)). Results outside that range are not valid.

Only the static procedure of Cl. 4.1.7.3 is implemented; the dynamic procedure for wind-sensitive or flexible structures is not covered. Roof types are limited to flat and gable, and roof slopes to a maximum of 45 degrees, so the 90 degree row of Table 4.1.7.6 is never used. Components and cladding pressures, from Figures 4.1.7.6.-B to -H, are out of scope and belong to the separate components and cladding calculator.