Wind Loads - Main Structural System (NBCC 2020)

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 2020 (Division B, Part 4, Subsection 4.1.7). It builds the velocity pressure, applies exposure, gust and topographic effects, then resolves the Figure 4.1.7.6 zone coefficients for both orthogonal load cases.

Reference velocity pressure and importance factors

The reference velocity pressure q50 is the 1-in-50 annual probability value for the site, taken from the project defaults or entered as an override. The building importance category sets the wind importance factor, which differs between ultimate and serviceability limit states, so both are carried through the calculation in parallel.

Building geometry and reference height

Roof type and pitch give the roof angle, from which the eave height, mean roof height and the reference height used for the exposure factor are derived. The building width perpendicular to the ridge and length parallel to it set the least horizontal dimension, which in turn drives the end-zone geometry.

End-zone widths

The 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 and H the building height used for the figure. The wider end-zone dimension y follows as max(6 m, 2z). These two dimensions define where the end-zone coefficients 1E through 6E apply rather than the interior zone values.

Exposure, gust and topographic factors

The exposure factor Ce is computed from the reference height and the terrain type, with the intermediate terrain case interpolating on the extent of rough terrain upwind and subject to the code floor on the factor. The gust effect factor Cg for the main structural system is taken as the code value for this procedure.

Where a hill or escarpment is present, the maximum speed-up ratio is derived from the hill height and the effective upwind distance for the selected shape, then attenuated by the horizontal distance from the peak and the height above ground. The resulting speed-up is combined with the gust factor to give the topographic factor:

Ct = (1 + ΔS / Cg) × (1 + ΔS)

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

External pressure coefficients

The gust-pressure coefficient product CgCp is read from the Figure 4.1.7.6 zone system. Load Case A covers wind normal to the ridge using zones 1, 2, 3 and 4 plus end zones 1E to 4E. Load Case B covers wind parallel to the ridge and extends the set with zones 5 and 6 plus end zones 5E and 6E. Coefficients vary with roof angle and are interpolated across the code breakpoints, so a roof pitch between tabulated angles is handled without rounding to the nearest listed value.

External and net pressures

The external pressure on each zone is

p,ext = IW × q50 × Ce × Ct × CgCp

The internal pressure is derived from the building opening condition, which sets the positive and negative internal pressure coefficients, factored by the internal exposure and internal gust factors. The net pressure on each zone is then 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 twice, once against the positive internal condition and once against the negative, at both ULS and SLS. This gives four net pressure values per zone per load case, from which the governing value for any given frame element can be selected.