Why? Because the main structure will fail and we don’t want it to take the tornado safe room with it. And the tornado safe room should be separated from the rest of the structure. As we mentioned above, it’s important to create a structural ceiling to your tornado safe room in addition to the walls. Also called FEMA P-320, this document gives details and construction drawings for creating several different types of storm shelters. If you’ve decided to build your new home with a basement, or if you would like to make your existing basement safer, you may want to check out the FEMA publication, Taking Shelter From the Storm: Building a Safe Room for Your Home or Small Business. Now think about the size of the average home…it’s not uncommon for a side wall of a small, modest home to be 30 feet long.much bigger than the 8 feet we used in our example! This helps us better understand why the average home disintegrates at these wind speeds.įorming Up a Poured Concrete Basement Wall Basement Safe Room This force is equivalent to the weight of three mid sized sedans… an absolutely huge force for such a small surface area. The total force created on the positive pressure side of this small building is… 9030.86 pounds! 200 MPH wind speed similar to the estimated maximum wind speeds last week in Oklahoma.Negative Pressure – Much like an airplane wing, there is also a negative pressure created by the wind on the leeward side of the building…but these aren’t included in this example.Direct Wind – We will also assume the wind hits the side of the structure directly at a 90 degree angle for simplicity.Size – The side of the building will be 8 feet wide and 8 feet tall.Flat Roof – This allows us to focus entirely on the force placed on the side wall of the structure.For our example, we will assume the following… Let’s take a look at the effects that a wind of this magnitude can have on a small structure. Just how big can the actual pressures be on the side of a building? Initial estimates say the Oklahoma twisters created winds of up to 200 MPH. This sounds small, but remember the typical tornado only lasts a few minutes. Size – Minimum size for adults…five square feet per adult.Why ceilings you say? Remember, objects that are picked up by the wind will eventually come down so a structural ceiling is an important part to any tornado safe room. This requires the tornado shelter walls to withstand perforation by the missile at 100 MPH…and for ceilings, it’s 67 MPH. So the performance criteria for walls assumes that the 250 MPH wind propels a 15 lb.
Entire structure shall resist failure from overturning, sliding, or uplift (see photo above).Walls, ceilings and floors shall withstand design pressures so that no element will separate from the other (examples = floor to wall or wall to ceiling).According to FEMA’s (Federal Emergency Management Agency) National Performance Criteria for Tornado Shelters, wind velocity of 250 MPH shall be used when designing a tornado safe room. Tornado safe rooms need to withstand tremendous forces created from pressures that build up from high wind speeds. Here we take a look at some of the options along with the cost to add a tornado safe room. With wind speeds in the 200 miles per hour range, shelter is critical to survival.
The question of the week is… what options do I have for creating a tornado safe room in my home? The Oklahoma tornadoes that touched down last week reminded us just how devastating mother nature can be.
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