In Canada, tornadoes are addressed in the “Structural Commentaries of NBC 2015.” The following is the key guidance for the commentary:
- Tornadoes account for the greatest incidence of death and serious injury of building occupants due to structural failure and cause considerable economic loss. However, while the probability of tornado occurrence per km2 can well exceed 1 x 10-5 per year, the probability of any one particular building being hit by a tornado is very small (less than 10-5 per year). With some exceptions, such as nuclear power plants, it is generally not economical to design buildings for tornadoes beyond what is currently required by NBC Subsection 4.1.7.
- Key details such as those indicated above should be designed on the basis of a factored uplift wind suction of 2 kPa (41.8 psf) on the roof, a factored lateral wind pressure of 1 kPa (20.9 psf) on the windward wall, and suction 2 kPa on the leeward wall.
Structural commentaries of the NBC 2015 list three levels of risk for tornado-prone regions in Canada as follows:
(1) “regions prone to significant tornadoes” are defined as regions where the estimated probability of occurrence of a significant tornado (F2–F5 with three-second wind gust speeds in excess of 180 km/h [111 mph]) per km2 per year exceeds 10-5.
(2) “regions prone to tornadoes” are defined as regions where the estimated probability of occurrence of a tornado (F0–F2 with three-second wind gust speeds in excess of 60 km/h [37 mph]) per km2 per year exceeds 10-5.
(3) “regions where tornadoes are possible” are defined as regions where tornadoes have been observed, but where the estimated probability of tornado occurrence per km2 per year is not more than 10-5.
In the U.S., ASCE-7 covers the design for tornadoes in Chapter 32. The first part of the chapter describes the types of buildings which do not need to be specially designed for tornado winds:
- Structures outside tornado-prone areas can rely on the wind provisions for the area (Figure 6).
- Low occupancy and regular buildings can rely on the wind provisions for the area.
- If the tornado wind speed is below 96 km/hr (60 mph), ASCE-7 states can rely on the wind provisions for the area.
- If the tornado wind speed is below a specified speed for each exposure, ASCE-7 states can rely on the wind provisions for the area.
If the building is required to be specially designed against tornadoes, Chapter 32 provides the basic provisions to calculate the design wind speeds. The other wind parameters in Chapters 27, 29, and 30 can be used in the same manner as with other wind loads to calculate wind loads on the structure.
Hail is not considered a structural load. However, the occurrence of hailstorms raises one of the key specifiers’ criteria: Is the product durable?
In Canada, hail occurs in the same areas as thunderstorms. Figure 7 shows the location of hail activity, with Alberta being the most active area.
In the U.S., Factory Mutual has identified the high-risk areas in Property Loss Prevention Data Sheets 1-34 (Figure 8).
Designing for rain is vastly different compared to other roof loads, and for several reasons. The structural engineer is dependent on decisions made by the mechanical engineer and architect on water drainage from the roof. Water moves and follows the geometry of the roof structure. The code provisions give the designer a volume of water, for example, a one-day rain of 100 mm (4 in.) over a roof of a 1000 m2 (10,764 sf) gives 100 m3 (26,417 gal) of water. The designer is then responsible to work out how the water would sit on the roof accounting for roof slopes and how much the roof structure deflects. The rain loading is dependent on the stiffness of the roof system and its components, if the decking and supporting members are not stiff enough, the water will flow to that area and overload the structure. This is called a ponding failure.
Ponding calculations are quite complex and iterative. In the past, guidance was provided on which minimum stiffness to use and what geometry should be avoided. With the additional computing capabilities now available, it is feasible to calculate ponding loads.