Essential structural considerations in roof design

Exposure factor (Ce): the exposure factor accounts for the effect of the surrounding terrain. Rough and open terrain are the only two exposure categories considered.

Over the years, the wind provisions have evolved with these significant changes:

  • NBC 2005: The three return periods of 10, 30, and 100 years were replaced by one of 50 years. IW (importance factor for wind) was used to replace the three return periods.
  • NBC 2010: The minimum reference velocity pressure was changed to 0.30 kPa (0.04 psi); and the exposure factors were modified to consider only open and rough terrain.

In the U.S., ASCE-7 has several chapters dealing with wind provisions. Chapter 26 has the general calculation for wind loads.

The wind load calculated using the equation is:

  • qz = 0.00256KzKztKdKeV2 [lb/ft2]
  • Kz = velocity pressure exposure coefficient
  • Kzt = topographic factor
  • Kd = wind directionality factor
  • Ke = ground elevation factor
  • V = basic wind speed, [mi/hr] in the U.S. is the average wind pressure taken over a sample for a time of three seconds with a probability of exceedance of 0.33 per cent, 0.14 per cent, 0.06 per cent (return period of 300, 700, 1700 years), at a height of 10 m (33 ft) above ground.

Over the years, the criteria of the basic velocity have changed. Table 1 shows the ASCE-7 edition with the basic wind speed and a conversion equation between ASCE-7 to NBC.

In the U.S., it is accepted (by the construction and design communities) that areas where the basic wind speed is greater than 201 km/hr (125 mph) 1/700-year wind, ASCE-7-22, (160 km/hr [100 mph] ASCE-7-05), the area was considered a high-wind zone requiring special detailing. Converting design wind speed for use in Canada, the same criteria is used for buildings in areas where the 1/50-year pressure is greater than 0.55 kPa (11.5 psf). Special framing needs to be considered.

Figure 2 shows a roof section of a project in southern Alberta. The detail shows attention being paid to building science consideration. The project was constructed in an area where the design wind pressure is 0.9 kPa (18.8 psf). This pressure is the equivalent to winds from a category 3 hurricane or an EF-2 tornado.

Tornadoes

Tornadoes are rotating columns of air from the ground to the base of a thunderstorm. They cut a narrow path of destruction about 100 m (328 ft) wide, extending out as wide as 1500 m (1640 ft). The greatest wind intensity is in the central path. Wind speeds decrease rapidly away from the vortex of the tornado, but they can still be damaging.

With large tornadoes, more damage can occur in the periphery than from the central path. Field surveys performed after tornadoes have discovered common structural failures. Some of these include:

  • Bottom chord buckling of roof trusses, column base plates
  • Loss of roof decks and wall siding due to inadequate connection to the structural supports
  • Inadequate connection of walls to the foundation

Tornadoes are classified based on the level of damage they cause. In the 1970s, the Fujita scale was developed and enhanced in the 2000s (Enhanced Fujita scale). Figure 4 provides a description of each scale, estimated wind speeds, and equivalent gust pressures.

Table 2  indicates tornado categories, wind speeds, and wind pressure.

Design for tornadoes is an evolving situation with extensive ongoing research. Tornadoes are different than other wind events as their highest wind speeds are near the ground, are very localized, and cause up to three times larger uplift forces compared to other types of wind events. With the high winds at the ground, windborne debris becomes a major concern.

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