Tornado season is upon us, and once again, we in the Midwest and Southeast will see thunderstorms forming and rolling across the plains, and we will look up and wonder if that is just a low hanging cloud, or something more ominous.
Safety is of primary concern during a tornado. It is an immensely powerful, destructive force, which can devastate anything in its path. Historically we in the plains states have been told to move to the middle of your home and either open or close windows depending on who you have listened to. If you had the luxury of a cellar or basement, get into that. It is important to have a safe place to go during a storm, such as safe rooms or community shelter.
There are many types of safe rooms, ranging from below grade concrete structures, and steel or fiberglass “water tanks” with reinforced doors, above grade filled CMU, ICF, and wood and steel composite structures. There are advantages to each of these styles of protection.
A reinforced concrete structure, placed above grade, and built to FEMA specifications is fully rated for Near-Absolute Protection. The following information is taken from the NOAA Severe Weather Workshop – March 2009:
“Near-absolute protection means that, based on our current knowledge of tornadoes and hurricanes, the occupants of a safe room built according to this guidance will have a very high probability of being protected from injury or death… to date, a wind event exceeding the maximum design criteria in this publication has not been observed.”
The wind speeds at the ground have not been decisively measured, and the estimates given by Doppler radar are for 500 feet above the ground. At ground level, there are obstructions, which can affect the wind velocity, limiting it in areas and funneling into higher velocities in others. Typically, tornado strengths are measured, not by Doppler wind speeds above ground, but by the damage assessment on the ground, making the measurement of the strength of the tornado a subjective rather than objective matter.
Most underground shelters have only one entrance/exit. Their door opens upward to prevent debris falling in and injuring or further trapping the occupants while they are trying to exit, but this can pose a serious problem if, for instance in a garage, collapsed building materials are sitting on top of the door. The occupants would be trapped until someone could remove this material. This may pose an additional risk, especially for those with health problems.
Safe rooms built above grade also avoid other issues that can plague underground shelters. An above ground safe room has a door that opens inward, which is supported by 3 or more locking points in the door frame. There is a much lower chance of debris falling in and trapping someone in an above ground shelter, and with the door opening inward, it is not necessary to wait for rescuers to exit after the storm.
Localized flooding can be a problem in any thunderstorm, and those that spawn tornadoes, super-cells, are also prone to produce heavy rainfall. Below ground shelters are more prone to take on water, and without a clear drain system, can fill up.
Above ground shelters are much less likely to become submerged, and are much easier to drain. The following list is taken from the NOAA Severe Weather Workshop
Safe rooms should be located outside of the following high risk flood hazard areas:
- Coastal High Hazard Area (VE Zones)
- Coastal A Zones
- Areas behind levees that have not been certified
- Outside Category 5 hurricane surge zones (Residential only)
- For Residential safe rooms, within floodplain areas subject to flooding of 3 feet or less
Safe Room Design Testing
FEMA guidelines relating to missile speeds for tornadoes are 15lb, 13’6” long at 100mph horizontal (2200lb*ft/s), and 67mph vertical (1572lb*ft/s). The Hurricane missile speeds are 9lb, 8’0” long at 127mph horizontal (1676lb*ft/s), and 26mph vertical (381lb*ft/s). FEMA 361 Appendix E lists a number of wall sections and their performance during the missile tests. The missile tests performed at Texas Tech University all have shown conclusively that above ground safe rooms will withstand the design criteria that is listed in the FEMA documentation.
If you have an existing home, there are a number of options still left to you for an above ground safe room. Build the safe room as an out building, as a retrofit into an existing room (will require significant modifications) or as an addition to the house. There are a number of options for walls which are approved by FEMA for use as a safe room, and can make it easier to incorporate into a retrofit.
If you are building a new home the same options will apply, but you have a distinct advantage over retrofitting. You have more room to work, and can build the full structure of the home to incorporate the safe room. An ICF safe room has multiple uses within a home. It doesn’t have to look like a safe room, except for the additional locks on the door. Homes with a safe room in used as a closet, bathroom, bedroom or pantry have all been built. The safe room can be finished to match the rest of the house, and many people would likely not even notice it in passing.
An ICF safe room can be used in more situations than just severe weather events. In a home invasion this is a solid, protected, and secure location. Door designs can even prevent bullets from penetrating, keeping you safe until help can arrive.
Another overlooked option is to use it as a safe, for storing valuables while on vacation, firewarms, and other items. Multiple locks take longer to open and can protect your belongings from theft and offer up to 4 hours protection from fire.
For more information on BuildBlock ICF Safe Rooms: https://buildblock.com/icfs/safety/safe-rooms/