With the National Hurricane Center forecasting an active and potentially deadly 2010 hurricane season, it is important to know that protection is available during these storms.
The Federal Emergency Management Agency (FEMA) has long recognized that safe rooms constructed from cast-in-place and precast concrete provide effective protection against the most dangerous storms.
Safe rooms built according to FEMA publication 320, Taking Shelter from the Storm: Building a Safe Room for Your Home or Small Business, will provide near-absolute protection against wind and wind-borne debris associated with tornadoes and hurricanes. FEMA 361, Design and Constructive Guidance for Community Safe Rooms, contains technical guidance for the design and construction of community safe rooms intended for larger groups of people. Both were updated in 2008.
The changes to the prescriptive designs contained in FEMA 320 are the result of post-disaster investigations into the performance of safe rooms and shelters after tornadoes and hurricanes. In addition to FEMA 320 and 361, there is now a consensus standard from the International Code Council (ICC) and the National Storm Shelter Association (NSSA) for storm shelters, called the ICC/NSSA Standard for the Design and Construction of Storm Shelters (ICC-500).
The ICC-500, also published in 2008, is based on design criteria contained in earlier editions of FEMA 320 and 361 (published in 2000), and was coordinated with the 2008 update of the FEMA publications.
Building safe rooms with concrete
The inherent physical characteristics of properly constructed reinforced concrete make it ideal to withstand wind-induced pressures and wind-borne debris impacts. With the addition of an exterior finish capable of preventing water infiltration from wind-driven rain, these systems can provide exceptional protection.
Accordingly, concrete is one of the most preferred construction materials for safe rooms. Most safe rooms are constructed with conventional cast-in-place concrete and/or precast concrete units.
FEMA 320 provides examples of how concrete can be used to build residential and small community safe rooms. When this document's precriptive designs are not used for the construction of a safe room, the design criteria in FEMA 361 should be followed.
The FEMA 361 criteria contain all of the necessary information to design a safe room that provides near-absolute protection using reinforced concrete. The prescriptive designs in FEMA 320 have been designed to withstand pressures from 250 mph, three-second gust winds and to resist wind-borne debris represented by a 15-pound, 2x4 wood board traveling horizontally at 100 mph.
Cast-in-place and precast concrete
FEMA 320 provides prescriptive solutions for safe rooms using cast-in-place and precast concrete. For a standard 8x8-foot safe room, the wall thickness is 6 inches minimum, with #4 vertical bars at 12 inches on center (each way). However, FEMA 320 now provides prescriptive solutions for a 14x14-foot residential or small, community safe room.