For precasters who are thinking of diversifying their product lines, storm shelters may be just the thing. Most areas of the country experience nasty weather from time to time, but where there is high wind there is greater need for protection.
When we look back at the stories that made headlines in 2011, we will certainly see mention of Tuscaloosa, Ala., and Joplin, Mo. On April 27 and May 22, respectively, tornadoes touched down and devastated both cities. The death toll from both events exceeded 400.
As of Aug. 1, there have been 1,175 confirmed tornadoes reported in the United States. At least 560 people have perished, and damage reaching billions of dollars has been inflicted by the wrath of the tornadoes. This has been the deadliest year in the United States for tornadoes since 1936.
There has to be a better way to protect people from these storms. In fact, certified and approved precast concrete safe rooms and storm shelters can provide a safe area and peace of mind for families and communities..
To ensure that safe rooms are structurally sound units that provide near-absolute protection from adverse elements, FEMA has developed design, construction and operation criteria for architects, engineers, building officials, local officials, emergency managers and prospective safe room owners/operators. The two design guidelines are FEMA 320 and FEMA 361.
FEMA 320 outlines the design criteria for the development of residential safe rooms (16 persons or less), while FEMA 361 covers the development of public and community safe rooms (more than 16 people).
Using the FEMA guidelines as a 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). Manufacturers of products meeting this standard assure prospective owners that their safe rooms will be able to provide life-safety protection. While fully supporting this effort, FEMA has continued to promote FEMA 320 and FEMA 361 guidelines to communities and individuals seeking further guidance.
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 units meeting FEMA criterion provides“near-absolute” protection from extreme wind events. Therefore, FEMA refers to 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 as “shelters.” So all safe rooms designed to the FEMA criteria meet or exceed the ICC-500 requirements.
Damage from extreme wind events
When addressing the damage that extreme winds can cause to a building or structure, there must be a fundamental understanding of the forces and stresses inflicted during extreme-wind events.
Extreme winds created by tornadoes and hurricanes are anything but constant. Wind speeds are continually fluctuating and changing direction, increasing the pressure and stresses on parts of the structure and potentially leading to the failure of connections between the building components. The safe room structure will also be affected by wind forces acting on both the inside and outside of the structure, which must be accounted for in the design. When wind is allowed to enter the structure through an open door, wall section or window, it will apply a pushing (downward) force and pulling (upward) force on the windwalls, leeward walls and roof of the safe room.
A common misconception is that buildings in extreme wind events explode due to the dissimilarity in atmospheric or wind pressures between the inside and outside the structure. As a result, many people open the windows or doors of a building in the event of a tornado to ease the pressure variance. Doing this can actually double the wind pressure exerted on the building compared with a sealed structure, in turn leading to potential failure from the additional pushing and pulling forces.
While swirling winds from the vortex of a tornado are thought to produce the most damage, much of the damage to the building envelope is due to straight-line winds being pulled into the tornado itself. Therefore, the designs of safe rooms must take into account these additional variables.
Other effects of wind on buildings can be summarized as follows:
• Airflow separates from building surfaces at sharp building edges and at points where the building geometry changes.
• Localized suction or negative pressures at ridges, edges, eaves and corners of roofs and walls are caused by turbulence and flow separation.
• Windows, doors and other openings are subjected to wind pressures and the impact of windblown debris.
If wind speeds reach a certain threshold, they can lift objects from their original locations and launch them like missiles with enough force to penetrate a structure’s building components and harm its occupants. To ensure that the windblown objects do not penetrate the walls or roof of the structure, FEMA 320 and FEMA 361 have outlined design minimums.
Precast concrete safe rooms and shelters are classified according to location: above-ground (stand-alone) or in-ground (internal safe room). Each has several inherent advantages.
When designing a precast concrete safe room, structural integrity is the primary design consideration. Although human safety and health are fundamental to design, the first consideration should be a structurally sound unit that can withstand the direct and secondary forces of wind and windblown debris.
FEMA 320, FEMA 361 and ICC-500 outline the design requirements for the main wind-resisting structural system and components as well as cladding of these units. They also provide safety and health considerations such as lighting, ventilation, sanitation, fire safety, means of egress and minimum floor space.
Residential and community safe rooms designed to meet FEMA 320 and FEMA 361 criteria follow these basic principles:
• FEMA 320
– Located in an area that is quickly accessible
– Built in an area where flooding will not occur
– Readily accessible from all parts of the 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)
– Built with connections that can resist failure
– Built with walls and roof that can withstand windblown objects (designed for 250 mph winds)
– Designed to resist a 15-lb wooden 2×4 in. board traveling horizontally at 100 mph and vertically at 67 mph (ASCE 7-05).
• FEMA 361 (additional criteria to FEMA 320)
– Designed for all cases as partially enclosed buildings
– Special life-safety protection elements when occupancy is 50 or more
Since tornadoes are typically short-term events, comfort is not a big factor in design, although precast concrete safe rooms provide additional comfort over other safe room designs in terms of ventilation, insulation and ease of installation. FEMA 320, FEMA 361 and ICC-500 state that safe rooms designed to withstand tornadoes must provide a minimum of 5 sq ft of floor area per person. For long-term events, a minimum of 7 to 20 sq ft of floor area per person is required.
Design: foundation types
Slabs-on-grade, basements and crawlspaces are foundation types suitable for the installation of an internal safe room as identified in FEMA 320. A significant benefit of using a precast concrete safe room is that no matter where the safe room is located, the structure will be able to provide the protection outlined by FEMA guidelines and ICC-500 standards.
Therefore, a precast concrete safe room will not necessarily have to be tied into an existing structure or foundation to meet the design criteria, making a precast concrete safe room a
• Slab-on-grade foundation. For slab-on-grade foundations, FEMA 320 recommends increasing the concrete thickness and including additional reinforcement in the design of the foundation below the safe room. If the slab-on-grade foundation is part of an existing building, a precast concrete safe room can be anchored to an adequately reinforced existing concrete foundation.
• Basement foundation. If a home has a basement, FEMA 320 suggests placing the safe room in this location (if the safe room is precast concrete, it can be placed in any location). It also states that the safe room can be constructed as a stand-alone structure with its own walls (precast concrete) or can use the existing basement walls. The downside to using existing walls is that they may not provide sufficient protection from windblown objects and extreme wind loads. The safe room must have its own reinforced ceiling.
• Crawlspace or pile application. Building a safe room on an existing crawlspace foundation is possible, but as with the other applications it poses additional design considerations such as flooring and ventilation.
• Stand-alone, above-ground safe rooms. Safe rooms can also be designed as stand-alone additions to the outside of a building. To be structurally sound, the room must have a watertight roof, properly designed footings, and adhere to the wall, roof and footing thickness guidelines set forth in FEMA 320 and FEMA 361.
The design professional should first determine the loads acting on the safe room using the load combinations and conditions for normal building use as outlined in Section 2 of ASCE 7-05. The designer will then need to determine all additional loadings such as the forces transferred from the safe room through the connections, overturning moments and impact forces from windblown objects.
The main resisting forces that precast concrete safe rooms possess are the design of the concrete slab, the weight of the structure and suction force from the soil beneath the concrete slab. With all the overturning forces of an extreme wind event, an uprooting or upward movement is common in lightweight structures that can’t resist these forces. In addition to uprooting, flexural failures and pullout failures at connection points are common as extreme wind events produce concentrated loading at connection points, creating high tensile stresses in the concrete components.
Design: openings in tornado safe rooms
Openings in a precast concrete safe room designed to withstand tornado events should be protected by doors complying with ICC-500, Section 306.3.1; windows complying with ICC-500, Section 306.3.2; other openings complying with ICC-500, Section 306.4.
Design: flexural failure
Flexural failure/cracking develops in precast concrete slabs when tensile strength is exceeded. In order to resist these tensile stresses, the precast concrete slab must be designed with adequate reinforcement size, spacing and depth as outlined in FEMA 320 and FEMA 361. Flexural cracking reduces the structure’s load-carrying capacity, in turn leading to potential failure.
Design: connection failure
Factors that affect connection failures are the magnitude of tensile force, spacing between anchor bolts, edge distance of the anchor bolt, strength of the concrete and embedment depth of the anchor bolts. Cone failures due to pulling forces acting on the connections are the most common type of connection failure. A combination of shallow embedment depths and high-strength anchor bolts are generally the cause of pullout failures.
If you’ve thought of diversifying your product line, this may be a direction to consider depending on where your plant is in relation to storm-prone areas of the country. You could be providing a potentially lifesaving product while helping your bottom line.
Following the design criteria set forth in FEMA guidelines and ICC standards will ensure that precast concrete safe rooms will provide protection from the deadly forces exerted by tornadoes.
If you have any questions regarding precast concrete storm shelters or safe rooms, contact Evan Gurley at (800) 366-7731 or [email protected]