By Evan Gurley
Every year the potential looms for a natural disaster involving extreme wind events. Tornadoes, hurricanes, tropical storms, typhoons and other extreme wind events strike with lethal force. For the United States, some years are far worse than others.
Hurricane Katrina of the 2005 Atlantic hurricane season was the costliest natural disaster, as well as one of the five deadliest hurricanes in U.S. history. At least 1,836 people died, and damages were estimated at $81 billion.
When we look back at the stories that made the headlines in 2011, we will certainly see mention of Tuscaloosa, Ala., and Joplin, Mo., where tornadoes devastated both cities and caused more than 400 deaths.
In the summer of 2011, the East Coast of the United States braced for the impact of Hurricane Irene with evacuations and the first mass-transit shutdown in the history of New York City. Irene was downgraded to a tropical storm by the time it made landfall in New Jersey and the Coney-Island area of Brooklyn, N.Y., but by then the hurricane had caused almost $15 billion in damage and had taken 56 lives.
When extreme wind events approach, we are warned to seek cover. Some communities, homes and buildings are equipped with shelters, basements or rooms designated as safe areas during such events; however, many communities and individuals do not have storm shelters and remain at risk as a result. This article explains why certified and approved precast concrete safe rooms and storm shelters can help provide protection and peace of mind for families and communities when dangerous storms approach.
FEMA and ICC/NSSA safe room criteria
When extreme weather threatens, individuals and communities need a structurally sound safe room to provide protection. To ensure that safe rooms are structurally sound and will provide near-absolute protection from devastating storms, the Federal Emergency Management Agency (FEMA) has developed design, construction and operation criteria for architects, engineers, building officials, local officials, emergency managers and prospective safe room owner/operators. The two design guidelines developed are FEMA 320 and FEMA 361. FEMA 320 outlines the design criteria for the development of residential safe rooms (16 persons or less) located in residential basements, garages, interior rooms and stand-alone structures (buried or exposed). FEMA 361 designates the design criteria for construction and operation for both community and residential safe rooms (greater than 16-person capacity).
Using the FEMA guidelines as a pre-standard, design and construction professionals led by the International Code Council (ICC) and the National Storm Shelter Association (NSSA) have joined forces to produce the first ICC/NSSA Standard for the Design and Construction of Storm Shelters (ICC-500). Precasters who manufacture safe rooms meeting this standard assure prospective owners and occupants that precast concrete safe rooms will provide life-safety protection. While fully supporting this effort, FEMA has continued to promote its 320 and 361 guidelines to communities and individuals seeking further guidance on best practices.
Due to the implementation of the ICC-500 standard and other national, state and local protection initiatives, FEMA identified a need to distinguish between a “safe room” and a “shelter,” as the terms have been used almost interchangeably in the past. While FEMA and ICC criteria are both designed to ensure life-safety protection, only precast concrete units meeting FEMA criteria provide “near-absolute” protection from extreme wind events. Therefore, FEMA coins the term “safe room” as all shelters, buildings or spaces that are designed to the FEMA criteria. Buildings, shelters or spaces designed to the ICC-500 standard are termed “shelters,” so all precast concrete safe rooms designed to the FEMA criteria meet or exceed the ICC-500 requirements.
Specifying precast concrete safe rooms
Precast concrete safe rooms and shelters are classified according to their location: above-ground (stand-alone) or in-ground (internal safe room). There are inherent advantages to each type of structure.
When specifying a safe room, the structural integrity of the unit itself is the primary consideration. While additional occupant life-safety and health requirements are also a fundamental part of the selection process, the first consideration when selecting a safe room is to verify that the unit can withstand the direct and secondary forces of wind and wind-borne debris.
FEMA 320, FEMA 361 and ICC-500 outline the requirements for the main wind-resisting structural system, components and cladding of these units. FEMA provides requirements for occupant life-safety and health requirements including: lighting; ventilation; sanitation; fire safety; means of egress; and minimum floor space for occupants. Precast concrete safe rooms provide many advantages – not only with the heightened structural integrity of the unit itself – but also meet the requirements for occupant life safety and health requirements.
It is extremely important to conduct a thorough flood hazard analysis on the proposed site for a safe room. The possibility of flooding from a hurricane event is very high, and as occupants might be in the safe room for an extended period of time (perhaps longer than 24 hours), the scenario of saving people from death or injury caused by extreme winds only to cause them harm by flooding must be avoided. Residential safe rooms should not be located in an area subject to storm-surge inundation.
Six advantages of precast concrete safe rooms
1. Structural integrity.
Structural integrity is the philosophy behind the engineered design of sound structural components that comply with, or exceed, applicable standards and codes. Structural integrity implies that the strength of the structure will be greater than the maximum anticipated service stresses or storm loading. The structural integrity of a precast concrete safe room makes it capable of supporting service loads independently, without relying on adjacent support materials or soils. If the designed strength of a precast concrete safe room is greater than the expected maximum applied stresses by an appropriate factor of safety and meets the additional FEMA 320 and FEMA 361 requirements, the structure is considered to be adequate to fulfill its function. Structural integrity of a safe room is essential to provide dependable shelter from extreme wind events, and precast concrete safe rooms do just that. Precast concrete safe rooms can provide life-safety protection for occupants for durations of two hours or more for tornado events and more than 24 hours for hurricane events.
2. Extreme wind protection.
Extreme winds created by tornadoes, hurricanes and other such events are anything but constant. Wind speeds are continually fluctuating and changing direction, increasing the pressure and stresses on the parts of a structure. A safe room structure will also be affected by wind forces acting on both sides (inside and outside) of the structure.
While swirling winds from the vortex of a tornado or hurricane are thought to produce the most damage, much of the damage actually done to the building envelope is due to straight-line winds rushing toward, and being pulled into, the tornado itself.
Other stresses that winds project on safe rooms include:
• Airflows that can cause structural separation at surfaces near sharp edges and at points where the structural geometry changes
• Localized suction or negative pressures at ridges, edges, eaves, and the corner of roofs and walls that are caused by turbulence and flow separation
• Wind pressures and the impact of windborne debris on windows, doors and other openings.
If wind speeds reach a certain threshold level, there is even the potential for the swirling winds to launch objects through the air like missiles with enough force to penetrate a structure’s building components and harm the occupants within. FEMA 320 and FEMA 361 address windborne missiles with numerous design minimums.
Precast concrete safe rooms designed to meet the FEMA 320 and FEMA 361 requirements will be able to withstand all extreme wind forces including projectiles, and will provide complete security for safe room occupants. Seeking shelter from an extreme wind event in a structure that is not made with a FEMA-compliant precast concrete design can prove to be a life-altering choice.
3. Access and entry
Access points in safe rooms are an important factor to consider when specifying a safe room. An access point (a door or window) must be easily accessible, able to withstand missile-type forces and be properly attached to a structural backing material; all requirements that are easily met in a precast concrete design. Access and entry points designed for approved precast concrete safe rooms are effectively designed to withstand tornado events as follows: doors compliant with ICC-500, Section 306.3.1; windows compliant with ICC-500, Section 306.3.2; and all other openings compliant with ICC-500, Section 306.4.
4. Safe room transportation & installation
Precast concrete safe rooms can be manufactured well in advance of site installation. They are fully cured prior to delivery to the site, so there is no need to wait for cast-in-place concrete structures to set or cure. Precast concrete safe rooms are ready for service upon delivery, are competitively priced and in ready supply. These precast benefits can save valuable days, weeks or even months on a project compared with other materials.
Manufacturers of precast concrete safe rooms that are NSSA-approved (meet FEMA 320 and/or FEMA 361) are located throughout North America, and precast concrete safe rooms can be ordered and shipped to any city or region at a moment’s notice. In addition to ample supply, convenient delivery and readiness for service, inclement weather at the installation site does not pose a problem for precast (See “Extreme Precast: Freezing out the Competition” on page 10). Precast concrete safe rooms can be installed in extreme weather conditions and in remote locations.
5. Thermal mass
Thermal mass is used in the building industry to define the inherent property of a material to absorb heat energy. A wall material with a high thermal mass can moderate daily temperature variations in the building’s interior spaces. Precast concrete has a high thermal mass, which means a lot of heat energy is required to change its temperature (See “Thermal Mass: Precast Concrete can take the Heat” on page 30). Wood or metal walls, by comparison, have a lower thermal mass (relative to precast concrete) and transfer heat more readily. High thermal mass precast concrete walls act as thermal sponges, absorbing heat during the day and then slowly releasing the heat during the cooler nighttime hours.
So, why is the thermal mass of a safe room important? Precast’s ability to moderate temperature fluctuations is important, because safe rooms are designed to hold a number of people for a duration of time, and the ventilation and the temperature inside play a critical role in assuring occupant health and safety. Extreme outside temperatures can adversely affect the living conditions in safe rooms constructed with alternative materials. Materials with a low thermal mass can act like ovens when sitting out in the hot sun for a long duration of time. Precast concrete safe rooms help keep the temperature inside safe rooms moderated and manageable.
Ventilation in approved precast concrete safe rooms complies with the building codes or ordinances adopted by the local jurisdiction; the 2006 IBCand IRC codes1 are applicable if the local jurisdiction has not adopted a building code. Ventilation is provided either through the floor or the ceiling of precast concrete safe rooms. A protected shroud or cowling meeting the missile-impact criteria testing is mandatory to protect any ventilation openings. If the safe room is designed for occupancy of more than 50 people, the room must be ventilated by mechanical means, a requirement that is conveniently met when specifying a precast concrete safe room. Ventilation openings are easily fabricated during the precast manufacturing process and can be installed to custom specifications.
While there are many options in the marketplace from which to select when looking for a safe room for a residential home or community setting, only one safe room option stands out for quality, structural soundness, versatility, strength and protection – and that is an approved, precast-concrete quality, life-saving unit designed to resist even extreme wind conditions.
1 IBC is The International Building Code and IRC stands for The International Residential Code.
Residential safe rooms that are designed to meet FEMA 320 criteria comply with the following guidelines:
• Located in an area that is accessible as quickly as possible
• Built in an area where flooding will not occur
• Readily accessible from all parts of a home, business or critical facilities (building or facility occupied by large numbers of people)
• Free of clutter and obstacles
• Adequately anchored to resist overturning and uplift (if specified by design) wind forces
• Connections that can resist failure
• Walls and roof that can withstand windborne missiles (designed @ 250 mph for tornado hazards)
• Designed to resist a 15-lb, 2-in. x 4-in. wood board missile traveling horizontally at 100 mph and vertically at 67 mph (ASCE 7-05)2
FEMA 361 (additional criteria to FEMA 320):
• Designed for all cases as partially enclosed buildings
• Special life-safety protection elements when there is occupancy of 50 and greater
2 SEI/ASCE 7-05, Minimum Design Loads for Buildings and Other Structures
Evan Gurley is a technical services engineer with NPCA.