A tornado is a violently rotating column of air extending from a thunderstorm to the ground. The question of whether such a destructive force can destroy an apartment building is complicated, as the answer is not a simple yes or no. Many factors influence a large building’s ability to remain intact during a severe weather event, including the tornado’s intensity and the structure’s specific engineering. The survival of multi-unit dwellings depends heavily on the maximum wind speeds encountered and the building materials used in their construction.
Structural Survival Based on Tornado Intensity
A tornado’s destructive power is categorized using the Enhanced Fujita (EF) scale, which rates intensity based on the resulting damage to structures and vegetation. This scale ranges from EF0, representing the lowest wind speeds, up to EF5, which signifies the greatest destruction.
Most apartment buildings are constructed to resist straight-line winds, but they face a significant threat from tornadoes rated EF3 or higher. An EF3 tornado features wind gusts between 136 and 165 miles per hour and can cause severe damage, including tearing off the roofs and some walls of well-built structures. At this level, a low-rise apartment building could experience significant structural compromise.
The potential for catastrophic failure increases dramatically with EF4 and EF5 tornadoes, which are statistically rare events. An EF4 tornado generates wind speeds from 166 to 200 miles per hour, capable of leveling well-constructed frame houses. Apartment buildings constructed with lighter materials, such as wood-frame low-rises, may be completely destroyed under these conditions.
An EF5 tornado, with wind speeds exceeding 200 miles per hour, is associated with the most extreme damage, including sweeping away entire strong frame houses from their foundations. Even engineered multi-story buildings, while offering better resistance than standard homes, are not designed to withstand these forces without suffering extreme or total destruction.
Mechanisms of Apartment Building Failure
A tornado compromises an apartment building through a combination of three primary physical forces acting upon the structure.
Wind Pressure and Suction
The first force is the extreme wind pressure exerted laterally against the exterior walls. Positive pressure attempts to push the windward side of the building off its foundation. Simultaneously, the opposite side experiences strong negative pressure, or suction, which works to pull components away from the frame. The roof is particularly vulnerable to uplift forces because wind flowing over the top creates an aerodynamic effect similar to a wing, producing significant negative pressure. This is often where structural failure begins, and once the roof is removed, the remaining walls lose their upper support and become highly susceptible to collapse.
Internal Pressure Differential
A second mechanism of failure involves the pressure differential created between the air inside and outside the building. Once a tornado breaks windows or tears off exterior doors, air rushes into the structure, rapidly increasing the internal pressure. This sudden internal pressurization can cause walls and the roof to explode outward, as the structure is unable to contain the force from within.
Debris Impact
The third destructive mechanism is the impact of windborne debris, often referred to as missiles. These high-velocity impacts can breach the building envelope by shattering windows and penetrating exterior walls, especially on the lower floors where most debris travels. High-rises with large expanses of glass are susceptible to failure from debris before the maximum wind pressure even hits the core structure.
Safety Planning for Multi-Unit Dwellings
Residents of multi-unit dwellings must have a clear, pre-determined plan for seeking shelter, as the average warning time for a tornado is short. The safest location in any apartment building is typically an interior space on the lowest possible floor, ideally a basement or a designated storm shelter. If an on-site shelter is not accessible, moving to the ground floor is a priority, as upper floors are more exposed to wind and debris.
Shelter Guidelines
Once on the lowest level, occupants should seek an interior room with no windows, such as a closet, bathroom, or central hallway. These spaces are surrounded by more walls, which offer greater protection from collapsing exterior walls and flying glass.
For those in high-rise apartments, descending to a lower floor may not be possible due to time constraints, and elevators should be avoided due to the risk of power loss. In this situation, the best option is to take shelter in an internal stairwell or the core of the building.
When seeking shelter, residents should:
- Avoid large open areas, which are more prone to structural failure.
- Avoid areas directly beneath heavy objects on the floor above.
- Stay away from all exterior windows and curtain walls.
- Cover the head and body with a mattress, blankets, or pillows to provide a protective layer against falling debris.