FEMA prescribes building for “near-absolute protection.”
FEMA’s New Manual
Deaths, injuries and property damage caused by tornadoes and hurricanes can be prevented. That’s the primary and most important conclusion FEMA (Federal Emergency Management Agency) reaches in its manual, Design and Construction Guidance for Community Shelters.
But this manual doesn’t stop there. It not only says that structures strong enough to survive tornadoes and hurricanes can be built, it actually tells people how to do that.
Understandably, FEMA claims the book is a first. Prior to its publication there simply were no known buildings, fire or life safety codes or engineering standards that gave detailed information for the construction of protective structures.
Monolithic’s President David B. South agrees. He says, “For years now, FEMA has been studying the after-effects of natural disasters. They have seen so much loss of life and destruction caused by tornadoes and hurricanes that they are now really encouraging people to build homes, public structures and especially community shelters with what they call ‘near-absolute protection,’ and they are setting a precedent by actually telling people how that can be done.”
But exactly what is near-absolute protection?
FEMA defines its term: Shelters designed and constructed in accordance with the guidance presented in this manual provide “near-absolute protection” from extreme-wind events. Near-absolute protection means that, based on our knowledge of tornadoes and hurricanes, the occupants of a shelter built according to this guidance will be protected from injury or death (P.1-2).
FEMA has its reasons for calling this protection near-absolute rather than absolute. The agency says that its knowledge of hurricanes and tornadoes is based on meteorological records and damage investigations of “extreme-wind events,” such as tornadoes with wind speeds of up to 250 mph. Yet, hypothetically, even more severe winds can occur that make absolute protection an impossibility (P.1-2).
“But the fact that people can have near-absolute protection is really important information for schools, mobile home parks – any community in a high-risk area,” says South. “Just think how many lives could be saved, how much property damage could be prevented if every community had at least one school gymnasium, community center or church that could provide near-absolute protection. Tornadoes and hurricanes hit often enough, so it is a goal worth working towards.”
Tornadoes and Hurricanes
Since 1995 FEMA has been receiving reports of more than 1200 tornadoes nationwide each year. Those twisters are categorized on The Fujita Scale according to the damage they cause. The Fujita Scale has a 0-to-5 range, with F0 as a light tornado that might damage roofs and topple young trees. At the other end of the scale, the F5 produces incredible devastation, lifting homes off foundations and turning cars into flying projectiles.
As for hurricanes, FEMA says that about five hurricanes strike our mainland every three years. Two of those five will be major hurricanes creating extensive damage, or at least a Category 3 on the Saffir-Simpson Hurricane Scale. This scale uses a 1-to-5 range, with a C1 as a minimal hurricane and a C5 as catastrophic.
FEMA encourages each community to do its own risk assessment to determine its need for a community shelter and suggests the steps that should be followed in making that assessment.
David South agrees and says, “Put in really simple terms, the number one question that a community must answer is, ‘Do we live in a high-risk area?’ And the question that naturally follows that is, ‘Do we want to protect our children and ourselves from potential natural disasters?’ The FEMA manual has tools that can help answer such questions.”
One of those assessment tools is a “Design Wind Speed Map For Community Shelters.” This map presents four wind zones showing which U.S. areas are most prone to tornadoes and hurricanes. With this information, an engineer or architect can select the right design for a particular community.
For example, Wichita, Kansas in Zone IV is in the likeliest area for winds of 250 mph, while Rocky Mount, North Carolina in Zone III could have winds of 200 mph. So, each needs shelters that can withstand those velocities (Fig. 2-2, P. 2-5).
An Expert’s Input
Dr. Arnold Wilson, MDI’s Consulting Engineer, elaborated on the importance of the Design Wind Speed concept. He said, "The forces on buildings caused by wind are related to the velocity of the wind. Local conditions, such as mountains and valleys or trees or other buildings or open spaces, all affect the wind forces.
“Measurements of wind velocity have been made for many years at many locations, such as airports. By knowing the wind velocity and how it varies with height and location, it is possible to estimate the force that results on a particular type and shape of structure or building.
“When the forces on the building are known, the engineer can then design the building to withstand those forces.”
According to FEMA, for a structure to protect its occupants during a high-wind event, it must be designed to withstand the wind’s velocity, and it must be constructed with a continuous load path.
Dr. Wilson explained this concept as well. He said, "When forces are placed upon a structure by wind or seismic events, the forces are transferred through all parts of the structure through continuous contact between its members.
“In other words, the exterior forces have a continuous load path through joints, connections, beams, columns, walls and slabs until the forces are transferred into the ground.”
Shape and Resistance
FEMA points out that the very shape of a structure significantly affects how it weathers a storm. David South agrees and says, “Angels, sharp corners and flat surfaces give the wind something to lift or push against; so, buildings that are smoother and rounded survive better.”
What FEMA calls Resistance to Missile Impact is also important. To provide near-absolute protection, a building must be able to resist a missile blown into it. That missile could be a brick, a tree limb, patio furniture – just about anything. But FEMA has determined that if a structure can resist a two-by-four, weighing 15 pounds and traveling at 100 mph horizontally or 67 mph vertically it has the needed strength.
“A Monolithic Dome can do that – easily,” South says. “We had a semitrailer truck, fully loaded with rocks, careen down a hill and smash full force into a Monolithic Dome. It knocked a hole in the shell, but there was no serious penetration – which means that people inside the dome were protected. But other structures probably would have been leveled and their occupants either hurt or killed.”
FEMA defines a hurricane as “An intense tropical weather system with a well-defined circulation and sustained winds of 74 mph or higher” (P. 3-5). Hurricane winds create storm surge — gigantic walls of sea water, often carrying debris, that crash against buildings.
Although water produces more pressure against a structure than wind, South says, “A hurricane does not exert enough pressure on a dome to be even noticed. A Monolithic Dome at Port Arthur, Texas has now successfully survived three hurricanes. However, storm-surge debris could cut the dome’s surface membrane. Given the right conditions, a large timber or metal object could possibly puncture the dome. But the puncture would be very local and would not collapse the dome.”
Dr. Wilson says, “When it comes to disaster survivability, the Monolithic Dome’s rounded shape and its steel-reinforced concrete put it head and shoulders above most other structures.”
Wilson points out that a wind of 300 mph pushing against a structure exerts 404 pounds of pressure per square foot (404 psf). But Monolithic Domes have been built underground, buried up to 30 feet deep. They withstand pressure up to 1 ton per square foot (2000 psf).
A Comparison of Two Structures
In its manual, FEMA includes much detailed information relating to the design and construction of a building that can provide near-absolute protection. After reviewing FEMA’s criteria, both South and Wilson agree that Monolithic Domes meet or exceed each of FEMA’s standards. In other words, Monolithic Domes do provide the near-absolute protection that an agency of our federal government now, more actively than ever, is encouraging communities to seek.
In its Design and Construction Guidance for Community Shelters manual, FEMA details the design of a community shelter planned for North Carolina, but not yet built (Appendix C). This particular community has been hit by hurricanes and is in Wind Zone III, so it can experience winds of up to 200 mph.
We compared the proposed North Carolina structure to the Monolithic Dome gymnasium built in Italy, Texas, a Wind Zone IV area prone to winds of up to 250 mph.
|Feature||Proposed FEMA Shelter—North Carolina||Monolithic Dome—Italy, Texas|
|Size||3,600 SF Structure is approx. 12’ high, 72’ long and 50’ wide.||17,000 SF Diameter of 142 feet. Includes classrooms, auditorium and gymnasium|
|Construction Materials||Reinforced Concrete||Steel-reinforced concrete, 6" thick at the bottom and 3.5" thick at the top|
|Design Strength||Designed to survive winds of up to 200 mph.||Designed to survive winds of up to 250 mph.|
|Unit Cost Per Sq. Ft.||$98.00 (estimated)||$85|
|Total Construction Costs||$354,000 (estimated)||$1.8 Million|