Essential structural considerations in roof design

Figure 1 Gust front.

However, each jurisdiction having its own building code led to situations where different municipalities used different editions of the IBC. Currently:

  • Four states have adopted IBC 2021, ASCE-7-16.
  • 26 states have adopted IBC 2018, ASCE-7-16.
  • 16 states have adopted IBC 2015, ASCE-7-10.
  • Four states have adopted IBC 2012 or older,
    ASCE-7 -10 or older.

Figure 1A (page 16) shows the present use of different editions of the IBC.

The use of various editions of IBC presents a challenge. ASCE-7-22 has made significant changes to how rain and snow loads are handled. ASCE-7-10 changed the specified wind speed from a 1/50-year wind speed to an ultimate wind speed for each category of building importance.

Figure 1 Tornado.

Canada’s population is lesser than California’s, but it is slightly larger in area size. The smaller population size has led to better involvement of the government in the development of building codes. In Canada, building codes are also a municipal responsibility. Most municipalities do not want to have their own code, so they rely on the provincial government product and maintain a provincial building code. Most of the provinces and territories use the National Building Code of Canada (NBC) as a basis for their building codes. The NBC is produced by Codes Canada, a government agency.

Similar to the U.S., having different provincial building codes has led to different editions of the NBC being used. Fortunately, the structural provisions of the NBC 2020 are very similar to NBC 2010. While these editions are not accepted in all locations, this article will discuss the provisions from the NBC 2020 and ASCE-7, Minimum Design Loads for Buildings and Other Structures. The provisions of ASCE-7-22 have been adopted for the next edition of the IBC 2024.

Figure 1 Microburst.


Several types of winds are behind the wind loads used in Canada. Synoptic winds (winds associated with large-scale events, such as warm and cold fronts) make up everyday weather. In the center of the continent, short-term storm events cause the major wind events.

Short-term storms start as thunderstorms, but they can cause microbursts which can turn into tornadoes or hailstorms. In Alberta, on the east side of the Rocky Mountain Range, in Canada and the U.S., there are areas where downslope winds (known as Chinooks) contribute to extreme wind events. A comparable situation occurs on the maritime coasts. The U.S. (and rarely the Atlantic maritime provinces) have tropical storms, hurricanes, and/or typhoons.

Code provisions

The wind load provisions in U.S. and Canada are different, but they also have some similar provisions.

Figure 1 Hailstorms.

In the U.S., ASCE-7 has several chapters dealing with wind provisions. Chapter 26 has the general calculation for wind loads. The wind design load, qz, is the design wind pressure on a structure occurring over a three-second period. The wind load calculated using equation is:

  • qz = 0.00256KzKztKeV2 [lb/ft2]
  • p = [lb/ft2]
  • Kz = velocity pressure exposure coefficient
  • Kzt = topographic factor
  • Ke = ground elevation factor
  • V = basic wind speed, [mi/hr]
  • p = qKd GCp – qiKdGCpi
  • Cpi = pressure coefficient for interior element
  • Cp = pressure coefficient for exterior element
  • G = gust factor
  • Kd = wind directionality factor

This article focuses on four of the variables:

The design velocity in the U.S. is taken over a sample for a time of three seconds, with a probability of exceedance of 0.33 percent, 0.14 percent, 0.06 percent and 0.03 percent (return period of 300, 700, 1,700, 3,000 years), at a height of 33 ft (10 m) above ground, depending on the risk category of the structure.

Over the years, the criteria for basic velocity have changed. This has been for several reasons, the improvement of the measurement of wind speeds (anemometers) and unifying the design approach for both winds in hurricanes and synoptic winds. The change from just using one return period wind to using return periods of 300, 700, 1,700, or 3,000 years, was introduced in the 2010 edition of ASCE-7.

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