March 30, 2018

Residential Safe Room Design

A residential safe room protects the occupants of a home or small business from the high wind forces of a tornado or hurricane. Safe rooms are essential in the United States where an average of 1,253 tornadoes occur yearly, creating wind speeds of 200 mph or more. Between the years of 1991 and 2010, Texas alone experienced an annual average of 155 tornadoes, more than any other state. The impact of tornadoes and hurricanes is devastating. The National Oceanic and Atmospheric Administration (NOAA) reports that tornadoes cause an average of 60-65 fatalities and 1,500 injuries each year. In 2017, Hurricanes Harvey and Irma took 143 lives. CNN reports that 500 people have died as a result of Hurricane Maria, with 113 people still missing. An added benefit of a safe room is it increases a home’s value. In fact, a study by Professor Kevin Simmons, an economist with Austin College and the Federal Alliance for Safe Homes concluded safe rooms increase a home’s value by approximately $4,200, or on average 3.5 percent.  Residential safe rooms are a good investment and essential for protecting the occupants of a home or small business from disaster during high wind events, like tornados and hurricanes.

Design Standards and Guidelines for Residential Safe Room Design

The ICC-500 is the International Code Council’s and the National Storm Shelter Association (NSSA) standard (ICC/NSSA) for the Design and Construction of Storm Shelters (safe rooms). The 2014 ICC-500 is the most current ICC/NSSA standards for construction and design and construction of residential and community storm shelters. Since 2009, the  International Building Code (IBC, Section 423) and the International Residential Code (IRC, Section R323) have utilized ICC-500 as their reference standard for building storm shelters.

The Federal Emergency Management Association guidelines are found in FEMA P-320,Taking Shelter from the Storm: Building a Safe Room for Your Home or Small Business and FEMA P-361, Safe Rooms for Tornadoes and Hurricanes: Guidance for Community and Residential Safe Rooms. Both FEMA documents use ICC-500 as a referenced standard, however, the FEMA standards for safe rooms are considered slightly more conservative than the standard for ICC-500 storm shelters. The ICC-500 is the reference standard for the design and construction of storm shelters that will protect the occupants of a business or home during wind storm events. Residential safe rooms designed to FEMA guidelines offer occupants of homes and small businesses “near-absolute protection” against severe wind events.

Designing an ICC-500 Residential Storm Shelter

An ICC-500 residential storm shelter is a solid structure designed to meet IRC/ICC specifications and provide the occupants of a home a degree of life safety in extreme weather events, including tornadoes and hurricanes.The design and construction of an ICC-500 residential storm shelter must follow the guidelines specified in the ICC-500 standard (here is a free summary provided by FEMA). Key elements of an ICC-500 residential storm shelter are a strong continuous load path, and resistance to overturning, uplift, and windborne debris.

Residential Storm Shelter Design must have a Continuous Load Path

A strong continuous load path is critical to holding the roof, walls, and foundation of a residential storm shelter together during an extreme wind event. The shear walls1 of the storm shelter is the primary component of the continuous load path that combats the lateral-loads (horizontal wind forces). High winds travelling over the roof of the storm shelter creates uplift forces that pull on the walls. Winds hitting the windward side of the safe room impart lateral pushing forces on the wall and the movement of air around the leeward side of the structure causes suction pulling on the walls in the direction of wind travel. The walls of the storm shelter must maintain their integrity and transfer all of these loads to the foundation. The foundation must also be designed to resist any overturning of the structure due to the combined forces of the wind.

The Bautex™ Block Wall System is an excellent product for a residential storm shelter. The steel reinforced concrete poured inside the Bautex Blocks have the continuous load path required to resist extreme wind events during tornadoes and hurricanes.  Bautex Blocks meet the ICC-500 guidelines in areas with maximum wind speeds up to 225 mph. Essential to the design of a residential storm shelter is a continuous load path. Crucial to the continuous load path of a residential storm shelter is a strong wall system that can also support the roof, like the  Bautex™ Block Wall System.

Residential Storm Shelter Design Resists Overturning and Uplift

The foundation of a residential storm shelter must be anchored so to resist overturning and uplift as it receives the wind loads from of the walls. The design and anchoring of the foundation must follow the guidelines in the ICC-500 standard (Section 308.1.1.2) which “mandates that the design of slab-on-grade foundations be designed for the applicable loads and must at a minimum be 3.5 inches thick, contain steel reinforcement, and take into account the presence of slab joints” . During severe wind events, the foundation of a residential safe room must resist overturning, uplift, and sliding forces.

Residential Storm Shelters Must Protect Against Flying Debris

The roofs, walls, and doors of a residential storm shelter must resist the impact and penetration of flying debris during a severe wind event. The roofs of a safe room must be built to resist the impact of a 15-pound wood 2x4 shot at 67 mph. The doors of a safe room should have documented proof that they are compliant with the most current version of FEMA P-361 and FEMA P-320 or the ICC 500 for tornado wind speed of 250 mph.  Walls of an ICC-500 storm shelter must be built to resist the impact of a 15-pound wood 2x4 shot at 100 mph. The entire envelope of a residential safe room must protect the room’s occupants from flying debris during hurricanes and tornadoes.

The Bautex Block Wall Assembly resists the impact and penetration of flying debris during a severe wind event, which makes the Bautex Block an ideal material choice for a residential safe room. Specifically, Bautex insulated concrete block has the mass and strength to resist the impact to wind-driven debris at speeds more than 100 mph. Also,  The Bautex Blocks meets or exceeds the following ICC-500 FEMA standards for debris impact.

  • Series 1 FEMA 320/361 Bautex Block Panel with Brick Veneer.
  • Series 2 FEMA 320/361 Bautex Block Panel with CMU Block Veneer.
  • Test projectile 15 lb. wooden 2-inch X 4-inch propelled at 100 mph.

A residential safe room protects the occupants of a home or small business during severe wind events. A residential safe room constructed with Bautex Blocks meets and exceeds the criteria for a FEMA residential safe room design for continuous load path and impact resistance.  Visit Bautex Block Wall Systems for more information on residential safe room design.

1Shear walls are a structural system that provides lateral resistance to a building home, safe room, etc.


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