Topographic Factor (Ct) in NBCC 2015/2020

In the National Building Code of Canada (NBCC) 2015 and 2020, the topographic factor (denoted as Ct) adjusts wind pressure to account for speed-up effects caused by terrain features such as hills, ridges, and escarpments. This factor is important for sites located near or on elevated terrain where wind acceleration occurs due to topographic amplification.

Code Reference and Purpose

Ct is referenced in Clause 4.1.7.2.(5) of NBCC 2015 and in Commentary I of the NBCC User’s Guide. It accounts for localized increases in wind velocity due to flow over elevated terrain. If not considered when applicable, wind loads may be significantly underestimated for buildings located on or near hills or escarpments.

When is Ct Required?

Ct must be considered when the following conditions are met:- The building is located within 2H (twice the height of the hill or escarpment) from the crest.- The slope of the terrain exceeds a threshold angle (commonly > 3° to 5%).If these conditions are not met, or if the site is flat, then Ct = 1.0 may be used.

Typical Values of Ct

Terrain Feature	Slope Angle	Position on Feature	Approximate Ct
Flat terrain	—	Anywhere	1.0
Hill, gentle slope	~5–10°	Crest	1.1 – 1.2
Hill, steep slope	>10°	Crest	1.2 – 1.4
Escarpment or ridge	15–20°	Within 2H of crest	1.4 – 1.6
Windward mid-slope	—	Not at crest	1.0 – 1.2
Leeward slope	—	Anywhere	1.0

Design Notes

- If site-specific topographic data is not available, assuming Ct = 1.0 is conservative.- For critical structures or structures on prominent terrain, consider a wind tunnel study or consultation with a wind engineer to evaluate Ct more precisely.- Ct is multiplicative with other wind load factors and can increase total wind pressure significantly.

Example

A school is built near the crest of a hill with:- Slope: 20°- Height of hill: 20 m- Building located 30 m from crestSince 2H = 40 m, the site is within the influence zone.Estimated Ct = 1.3 – 1.5 → Wind pressure increases by 30–50% due to terrain speed-up.