Statistically, there are few tornados above F2 on the Fujita scale (maximum wind speeds of 113-157 mph). This is a tremendous wind load to hit a structure and most buildings are not typically designed to survive those kinds of loads. The one good thing about tornados is that they are typically short-lived, on the ground only a few minutes, and averaging about 1,000 feet across. Regardless, the concentrated force of these rotating winds at that speed hitting the building for a short period can cause significant damage. Here, we will look at what happens to a structure when it is hit by a tornado.
Wind hitting the building façade creates a positive pressure on the building materials on the windward side; the leeward side of the building experiences negative pressure (suction). This interaction changes the pressure inside the building, contributing to the destruction. This dynamic is seen on all buildings, though heavier buildings, like old mill buildings with thick masonry walls, tend to resist the wind forces better than wood-framed or light steel-framed buildings, such as typical high rises.
Tornadic winds increase internal pressure by entering the building through open windows and doors, and non-airtight components on the windward side, while pulling air out of the building on the leeward side. When more air comes into the building, even from something like a small broken window, it escapes on the leeward side and the building becomes positively pressurized. This can blow down interior walls or the side or back walls of the structure, break windows and doors, and also take the roof off.
When openings in the side and back walls of the building allow more air out than is coming into the building on the windward side, it creates negative pressure. The force of this suction can pull the roof down into the building, or pull the windward wall toward the building’s interior, which makes those walls less able to withstand the force of the wind hitting them.
Debris interacting with the building
Flying debris during a tornado (referred to as missiles) can cause significant damage to structures. Most building materials are not designed to resist windborne debris, such as trees, signs, outdoor equipment, or pieces of other structures, like a 2x4 piece of lumber, which can be blown through walls and windows.
In areas where tornadoes are more common, there are often code requirements for stronger building materials. These codes typically refer to windows on lower stories of buildings, since most debris blows around near ground level, up to about 30 feet.
Since there is little advanced warning for tornados, the most important building design element is to ensure there are interior or basement rooms, away from windows and glass doors, where occupants can shelter in place during a storm.
After the winds die down
Wind is the major destructive force in tornados. However, after the storm has passed, buildings and people may still be at risk. Gas feeds can be damaged causing gas leaks and potentially fires. Outside electrical lines create the danger of personal injury or fire, as well.
With our expertise in architecture, structures, and engineering, the team at CCA can help you ensure your buildings will withstand the forces of nature. Call us today for an evaluation of your property.