Wind Loads - Components and Cladding (NBCC 2020)

How it calculates

This calculator determines peak local wind pressures on components and cladding of low-rise buildings using the static procedure of the National Building Code of Canada 2020 (Division B, Part 4, Subsection 4.1.7). The workflow moves from the site reference velocity pressure, through exposure, topographic and pressure coefficients, to net design pressures reported per zone at both ultimate and serviceability limit states.

Reference velocity pressure and importance factors

The reference velocity pressure for NBCC 2020 is the 1-in-50 annual probability value, q50. It is taken from the project defaults for the site or entered directly as an override. The building importance category sets the wind importance factor, applied separately at ULS and SLS, so both limit states are produced from the same geometry and coefficient set rather than requiring a second run.

Exposure and topographic factors

The exposure factor Ce is computed from the reference height and the terrain condition, subject to the code floor on the factor for low buildings. Where a hill or escarpment is present, the speed-up ratio is evaluated from the hill height, the upwind horizontal distance and the shape, and combined with the gust effect factor to give the topographic factor Ct. When no topographic feature applies, Ct is unity and drops out of the pressure equation.

External pressure coefficients

Wall and roof surfaces are divided into the code zones: wall interior and wall end zone, and roof interior, roof edge and roof corner. Each zone carries a gust-pressure coefficient product CgCp that depends on the roof angle, which the calculator resolves into the NBCC bands of θ ≤ 7°, 7° < θ ≤ 27°, and steeper roofs. Positive and negative coefficients are tracked separately so that both pressure and suction cases are reported.

The coefficient for a given member depends on its tributary area. Each zone defines a coefficient at a lower area limit and at an upper area limit, and the calculator interpolates logarithmically between them:

CgCp = CgCp,low − (CgCp,low − CgCp,up) × log(A / A,low) / log(A,up / A,low)

Tributary areas at or below the lower limit take the lower-limit value, and areas at or above the upper limit take the upper-limit value.

Internal pressure

The building opening condition selects the internal pressure coefficients Cpi, positive and negative, from the NBCC categories ranging from a sealed building with small uniformly distributed openings through to a building with large or dominant openings. The internal pressure is factored by its own exposure factor Cei and the internal gust effect factor Cgi.

Net design pressures

The net pressure on a component combines the external and internal contributions:

p,net = IW × q50 × Ct × (Ce × CgCp − Cei × Cgi × Cpi)

The positive net pressure pairs the positive external coefficient with the negative internal coefficient, and the negative net pressure pairs the negative external coefficient with the positive internal coefficient, so each zone reports the governing inward and outward case. Substituting the ULS and SLS importance factors produces both limit states.

Overhangs

Where roof overhangs are included, they are evaluated in their own interior, edge and corner zones using the overhang coefficients and the overhang tributary area, with the same logarithmic area interpolation. Overhang pressures are reported as net uplift at ULS and SLS, since the underside and topside contributions are already combined in the overhang coefficient.

Parapets

Parapets are handled as two separate cases. Parapet members take windward and leeward coefficients by region, General for the wall interior and roof edge combination and Corner for the wall end zone and roof corner combination. Parapet cladding is reported as an outer face positive pressure, an outer face negative pressure, and an inner and top face pressure, each combined with the parapet internal pressure coefficients. Both cases use the parapet height and the parapet member tributary area as inputs.