Abstract:
This invention is related to the development of and installation of a shelter structure suitable for severe storm safety. The shelter is a composite structure constructed of precast and post tensioned structural elements or shells, the tendon post-tensioning is designed to maximize strength and minimize material costs by implementing a specific geometric shape, the inverted catenary, in three dimensions. The shelter is easily constructed and then assembled from readily available structural materials that are both economic and mechanically efficient.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from provisional application 60/804,840 dated Jun. 15, 2006 by Dr. Henry Crichlow, titled “Severe Storm Shelter”. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of The Invention 
         [0003]    The present invention relates in general to the field of shelters, more specifically, to above ground shelters for hurricanes, tornadoes, and other high wind events that occur in severe storms. 
         [0004]    2. Prior Art 
         [0005]    U.S. Pat. No. 4,126,972 approaches the problem by showing an in-house structure built on a reinforced concrete base. It is a massive building like structure which looks like a miniature house complete with toilet facilities. The majority of the sidewalls are covered with sheet metal of sufficient strength and rigidity to withstand the tornado force impacts. 
         [0006]    U.S. Pat. No. 4,787,1 81 is a complex of two shell units which are connected together to provide the support for the structure. 
         [0007]    Patent D466,220 describes a design patent of a storm shelter in which a hemispherical “flying-saucer-like” structure with an opening is utilized. 
         [0008]    U.S. Pat. No. 4,615,158 is specially designed for mobile homes. It is a buried structure connected to the mobile home with a slanted slide to allow the occupants to enter via a stairway directly from the mobile home into the shelter. 
         [0009]    U.S. Pat. No. 5,794,389 is an elaborate system which completely encapsulates the home and allows it to be raised and lowered by a specialized lifting mechanism. This mechanism is a scissors like system connected to a moveable platform on which the house rests. 
         [0010]    U.S. Pat. No. 5,408,793 describes a process for constructing a prestressed composite structure which utilizes a prestressing of circumferentially wrapped material around a frame. The patent teaches a dome structure with a membrane sandwiched between layers of rigidifying material such as “shotcrete” or reinforced composite which also serve to embed radial wires and circumferential tensioned prestressing. Various types of circumferential tensioned prestressing can be applied to minimize bursting stresses and can consist of steel wire as well as fiber or steel-reinforced tape. Further layers of rigidifying material can then be applied over the circumferential prestressing as a final protection and cover. The radial wires can contain spacers or hooks to preclude the circumferential prestressing from riding up on the structure. 
         [0011]    U.S. Pat. Nos. 5,953,866 and 6,393,776 show reinforced rectangular modular systems with composite walls. 
         [0012]    U.S. Pat. No. 6,1 61,345 describes a rectangular parallelepiped with a door set up so that it can be slid horizontally to enter and exit the structure. 
         [0013]    Commercial shelters that are available are generally provided in two categories. Underground i.e buried or aboveground. Underground structures are simple geometric shapes either rectangular or cylindrical usually made of steel, reinforced concrete or plastic. These structures are placed in a large excavation made in the ground and then covered by the excavated earth. The structures are then fitted with a doorway and a ladder or stairway to enter and egress the structure. Though efficient as physical shelters public acceptance has been limited because of aesthetic concerns and primitive fear in some individuals when one has to physically leave your home and run through the wind, rain, hail and lightning of the oncoming tornado to an outside shelter and enter a hole in the ground. 
         [0014]    Above ground shelters like the “Oz”, Ref. 1, is massive monolithic cement structure poured around forms placed on location. A 5×5 foot structure weighs 21 tons or 42,000 lbs, and costs about $8,000, an 8×5 structure weighs 60,000 pounds and cost close to $10,000. 
         [0015]    After careful consideration of the above noted problems and prior art solutions, the inventor has herein a novel and improved method and system that allows the design, manufacture and installation of a more effective and severe storm shelter with minimal costs. 
       SUMMARY OF THE INVENTION 
       [0016]    An objective of this invention is to provide a safe severe storm protective system that is easily constructed and that meets the requirements for protection from impact loads, wind loads and weather phenomena found in a large tornado. The US Federal Emergency Management Agency, FEMA, Ref. 2 details the minimum requirements for such a structure. 
         [0017]    Another important objective is to provide the type of structure that is user friendly and inviting to the household occupants who can be in a state of near hysteria when the tornado approaches. In such a structure, being close to the house, the occupants of the house will feel comfortable and confident and welcome in using the structure in and under all conditions as opposed to buried outdoor structures which can be uninviting to the individual in times of stress after the tornado alarm has sounded. 
         [0018]    In one embodiment the structure has a structurally secure primary anchor which is augmented by multiple secondary anchors and supported by multiple interlocking post tensioned structural shells. In another embodiment, there is no primary anchor and the shells are held together at the top by a massive circular steel member in addition to the secondary anchors. The structure is completed with an easily opened lockable door which is preferably balanced such that minimum effort is required to open it. This door is solidly constructed of steel or built from laminated impact resistant material like Kevlar. 
       DESCRIPTION OF THE INVENTION 
       [0019]    These embodiments of the invention provide a safe room-like structure which is easily reached by all occupants of the house. A preferred location is in or near an outside patio which has a concrete base. In other embodiments it can be installed inside a garage or a large room. 
         [0020]    The invention consists of a specialized dome shaped structure comprising of connected structural shells made of post-tensioned structural shells which are viably and suitably anchored to the substratum. The surface shape of the structure is that of an inverted catenary which is rotated in three dimensions. 
         [0021]    In one embodiment the shells are made of pre-stressed concrete panels connected to the each other and suitably reinforced by steel rebar and further strengthened with a plurality of circumferential tendons made of steel cables which are post tensioned or stressed to enhance the structural integrity of the system. The shell elements form both the sides and the roof in a form that resembles a dome or Quonset with no clear demarcation of side and roof. The floor of the structure can be a cement pad or any suitable substratum since it has no load bearing function. In one embodiment, the bottom edge of the structure in contact with the surface or ground, is buried below grade a few inches and a small skirt is constructed to prevent winds from getting under and creating an overturning load on the structure. In another embodiment, a concrete skirt can be poured around the periphery to cover the bottom of the structure and form a skirt a few inched high. This also prevents winds from entering the structure from the ground/shell contact. 
         [0022]    Another innovation of this invention is use of existing technology for structural concrete as a basic element in the design and construction of the system. This embodiment provides for an economical and ease of design and rapid deployment in the field. A further embodiment which uses pre-stressed and post tensioned concrete elements allows for a variety of design while still maintaining simplicity and the same impact and structural integrity of the shelter. The manufacturing process enables the structure to be fabricated offsite and then to be assembled economically at the requisite location. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  shows the overall view of one embodiment of the invention. The structural shells attached by fasteners and are interconnected and form a symmetric dome-like structure. In one embodiment there is implemented a major primary anchor at the center of the dome and several secondary anchors at the periphery. These anchors are suitably buried in the substratum and connected by steel connectors to the reinforced structural shell elements. In addition circumferential steel tendons connect shell elements and apply a post tensioning stress to the structure as discussed later in the application. 
           [0024]      FIG. 2   a  shows a cross-sectional view of a structural shell which is connected both to the central primary anchor structure and a peripheral secondary anchor. Both anchors are suitably embedded in a cement substructure for added strength. 
           [0025]      FIG. 2   b  shows a cross-sectional view of one embodiment with no central support. The system is designed with a robust circular structural support element designed to withstand the lateral loads. The shells are secured to this central structural element. 
           [0026]      FIG. 3  shows a top view of the structure showing 8 shell elements. 
           [0027]      FIG. 4  shows a side view of one embodiment wherein the shells are expanded to show the post-tensioning tendons and shell connectors to the secondary anchors. 
           [0028]      FIG. 5   a ,  5   b ,  5   c  and  FIG. 5   d  show embodiments of the interlocking of the shells where they are joined to provide strength and a solid locking mechanism. 
           [0029]      FIG. 6  shows one embodiment of the shell construction where the rebar system is shown and the brackets, male pins and female sockets necessary for alignment of the shells. 
           [0030]      FIG. 7   a  shows a vertical cross-section of one embodiment with a circular contoured bench seating arrangement for several persons inside the shelter. 
           [0031]      FIG. 7   b  shows a vertical cross-section of one embodiment of the post-tensioning tendons attached and held in place on the structural shell. 
           [0032]      FIG. 7   c  shows a vertical cross-section of one embodiment of the shell with a circular tendon implemented inside the shell concrete structure. In this embodiment the tendon is pulled through each shell during assembly on location before being tensioned. 
           [0033]      FIGS. 8   a ,  8   b  show a top view schematic of the flow of tornadic winds and debris around the invention as compared to flow around a typical block shaped shelter showing the beneficial effect of reduced drag flow around the new invention. 
           [0034]      FIGS. 9   a ,  9   b  show a side view schematic of the flow of tornadic winds and debris around the invention as compared to flow around a typical block shaped shelter showing the beneficial effect of reduced drag flow around the new invention. 
           [0035]      FIG. 10  shows an isometric rendition of one embodiment of a completed severe storm structure with 2 circumferential tendons implemented for post tensioning. 
       
    
    
     BACKGROUND OF THE INVENTION 
       [0036]    The invention consists of a specialized dome shaped structure comprising of connected structural shells made of post-tensioned structural elements which are viably and suitably anchored to the substratum. The surface shape of the structure is that of an inverted catenary which is rotated in three dimensions. The catenary is mathematically described in two dimensions by the following equation: 
         [0000]        Y= a cosh ( k x )  (1)       where:   Y is the vertical distance   x is the horizontal distance,   k, a are constants.         
         [0041]    The catenary has several attributes that are effectively incorporated into the design and construction of the severe storm shelter. These attributes are discussed in the specifications. 
         [0042]    The wind research laboratory at Texas Tech University, Ref. 3, states that a safety shelter must be able to resist the forces that extreme winds or interacting structural components place upon it. The safety shelter should be designed to withstand wind speeds of 250 mph, which accounts for virtually all tornadoes, which have occurred in the US 
         [0043]    Wind load calculations are based on some variation of the following computation model: 
         [0000]        F=Q   z   ×G   h   ×C   f   ×A   f   (2)       Where:   F is Force generated by the wind   Q z  is wind velocity pressure   G h  is Gust response factor   C f  is Force Coefficient   A f  is area exposed to wind force
 
and the wind velocity pressure is calculated as follows:
         
         [0000]        Q   z =0.00256 ×K   z   ×[I×V]   2   (3)       where:   Kz is the exposure coefficient   I is the importance factor   V is the wind velocity.         
         [0054]    By calculating the wind forces “F” above, acting on the walls of the shelter the unit is scaled to keep the stress loads within the limits required by statute and justified by good engineering practice. In addition the embodiments indicated herein have utilized the available high strength materials available to the industry to optimize design and efficiency of the system. The invention discussed herein in addition to strength of materials uses specific engineering design to minimize wind load and drag forces on the structure to lower the forces acting on the structure. The selected shape, which is an inverted catenary in cross-section, provides for minimal loading of the structure by allowing the high velocity flow to careen off the structure because of the streamlined form of the new invention and the lack of sharp edges open to the wind. In addition, a catenary structure is self-supporting, for example in a structure built of blocks, because every element of the catenary is held in place by the neighboring blocks, the blocks don&#39;t slide off each other, even at the top, because the forces between the blocks are along the curve of the catenary itself. The blocks at the bottom are more vertical because they have more weight to support from the blocks above. The structure in this invention has its own intrinsic stability. 
         [0055]    The combination of forces provided by inches of 5,000-psi reinforced concrete shells and the post tensioning of the structure prevents any penetration by flying missiles in excess of a hundred miles per hour. In addition the catenary structure provides the smallest cross section surface area for a given volume of cement material so that the subject structure provides a maximum strength for a minimal amount of construction material. Further more compared to other existing block shaped severe storm shelter structures which provide a rectangular cross-section to wind flow and missile contact, the subject invention provides a streamlined cross-section and a continuously curved cross-section to wind flow and only a fractional component of the wind force acts on the structure proposed in this invention. Missile and wind impact is minimized in the case of the new invention and in the existing block structures of the prior art the missile and wind impact is maximized. 
         [0056]    Windborne debris and falling objects are two major risks to people in severe wind driven storms. It is generally accepted that tornado-generated missiles as flying debris, create the greatest threat to occupants of homes that are struck by severe winds. These missiles, very often, perforate conventional walls and roofs. A 2×4 beam flying at 200 mph can produce deadly consequences on contact with an unprotected human being. In order to provide a high degree of occupant protection, the shelter must be designed to prevent perforation by missiles on all surfaces, walls, roof, and door. Windborne debris can be adequately described by their mass, shape, impact velocity, angle of impact and motion at impact, i.e linear or tumbling. Impact momentum calculated from Equation 4 and impact energy compute from Equation 5 provide reasonable estimated of the momentum and energy effects of windborne debris that is striking perpendicular to the surface of the structure. 
         [0000]        I   m =( W/g )  (4)       where   I m  is impact momentum   W is the weight of debris   g is the acceleration of gravity   V is the impact velocity         
       And 
       [0062]        I   e =½( W/g )(V 2 )  (5)       where   I e  is impact energy   W is the weight of debris   g is the acceleration of gravity   V is the impact velocity         
         [0068]    They also provide reasonable information estimates when there is no rotation of the flying body. For off-angle impacts of windborne debris, the normal component of the impact momentum and energy component must be replaced by an effective velocity which includes the cosine of the angle of impact as a product. Since the cosine is maximum at zero degrees with a value of 1.00 and minimum at 90 degrees with a value of 0.00, the larger the angle the smaller the effective velocity and consequently the smaller the forces on impact. Additionally, for slender rigid-body missiles such as wood structural elements, pipes or rods with length to diameter ratio greater than 4.0, research by Pietras in Ref. 4, has shown that the missile begins to rotate on impact and that the impact force drops off much more rapidly than the cosine formula would predict. 
         [0069]    The impact of windborne debris can produce extremely high forces on a structure in a very short time. Dynamics teaches us that the magnitude of the force depends on the type of impact, whether it is elastic or inelastic and the duration of the impact. 
         [0070]    The subject invention is designed such that a perpendicular hit is minimized because of the shape of the structure and that there is maximum rotation of the object that is hitting the side of the structure since flow around the structure tends to provide a rotation of the trailing edge of the projectile at the moment of impact of the leading edge of the debris. 
         [0071]    Adequate strength, impact resistance and penetration resistance are the key to satisfactory structural performance of the shelter. The roof must be securely anchored to the walls, the walls to each other, and the walls to the foundation. These connections are necessary to insure structural integrity and to prevent the shelter from overturning. 
         [0000]    The current invention indicated herein meets all the requirements of a safe severe storm shelter especially the primary areas are the structural integrity and impact protection are maximized because of the combination of intrinsic protective properties of reinforced concrete, by the enhancements afforded by the pre-casting, pre-stressing and post tensioning in this invention and innovative design using a catenary geometry which guarantees minimum surface area for wind driven missile impact.
 
To fully appreciate the benefits of pre and post-tensioning in enhancing a structure, it is helpful to know a little bit about concrete. As discussed in Ref 5, concrete is very strong in compression but weak in tension, (i.e. it will crack when forces act to pull it apart). In conventional concrete construction, if a load is applied to a slab or beam, the slab or beam will tend to deflect or sag. This deflection will cause the bottom of the beam to elongate slightly; even a slight elongation is enough to cause tensile forces and cracking.
 
         [0072]    Steel reinforcing bars “rebar” are typically embedded in the concrete as tensile reinforcement. Rebar is what is called “passive” reinforcement however; it does not carry any force until the concrete has already deflected enough to crack. Post-tensioning on the other hand is considered “active” reinforcing. Because it is pre-stressed, the steel is effective as reinforcement even though the concrete may not be cracked. Post-tensioned structures can be designed to have minimal deflection and cracking, even under full load. In the present invention, a post-tensioning “tendon” is defined as a complete assembly consisting of the anchorages, the prestressing strand or bar, the sheathing or duct and any grout or corrosion-inhibiting coating (grease) surrounding the prestressing steel. There are two main types of post-tensioning: bonded (grouted) and unbonded. An unbonded tendon is one in which the prestressing steel is not actually bonded to the concrete, which surrounds it and its compressive force is transferred to the concrete by its anchorages only. 
         [0000]    The most common unbonded system (single strand) tendons which are used in slabs and beams for buildings, parking systems and slabs-on-ground. A monostrand tendon consists of a seven-wire strand that is coated with corrosion-inhibiting grease and encased in an extruded plastic protective sheathing. The anchorage consists of an iron casting in which the strand is gripped by a conical, two-piece wedge. Bonded systems are more commonly used in bridges, both in the superstructure (the roadway) and in cable-stayed bridges, the cable-stays. In buildings they are typically only used in heavily loaded beams such as transfer girders and landscaped plaza decks where the large number of strands required makes them more economical. In bonded strand systems, two or more strands are inserted into a metal or plastic duct that is embedded in the concrete. The strands are stressed with a large, multi-strand jack and anchored in a common anchorage device. In one method of stressing a turnbuckle type device can be used to stress the cables or tendons. The duct is then filled with a cementitious grout, which provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct. In the anticipated use of this invention, bonded or unbonded systems can be used in the construction and installation. 
         [0073]    The inherent stiffness of the concrete shell means that pre and post tensioning could be used efficiently. This reduced the long-term effect of deflection due to creep and other effects on the structural integrity of the current embodiment. 
         [0074]    Another embodiment of the invention comprises the use of high impact laminate material instead of reinforced concrete as the impact resistant material of the structural shells. In this embodiment a carbon fiber type material with suitable epoxy matrix material can be used instead for the strengthening of the shell. Modern material like Kevlar™ can be used for penetration prevention and impact absorption but since these are extremely expensive their routine use is not expected until prices have been reduced considerably. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0075]    As shown in  FIG. 1 , in the preferred embodiment, the severe storm shelter is a geometric shaped structure with a primary support element  1  to which are connected structural concrete shells  2  and these shells  2  are connected by horizontal tendons  7  with upper and lower tendon anchor elements  3  and  4  located circumferentially around the structure on the shells  2 . The shells  2  are also connected by fasteners  5   a  to each other and the shell bases are connected by tie down brackets  5  to the shell secondary support structures  6  which are anchors that are set in a concrete base  19  implemented in the substratum  18 . The top of the structure has a cover  8  which allows for ventilation of the structure.  FIG. 2   a  shows how the top of the shell  2  is connected by tie down brackets  5  which are securely bolted to the primary support  1  by tie down bolts  22 . This figure shows the detail of the bottom anchoring of the shell  2  via the shell lip  10  through the tie-down brackets  5  which are connected to the secondary support  6 . A protective skirt  9  covers the base of the shells  2  and prevents wind from blowing under the structure. The floor  11  of the structure is shown above the substratum  18 . In this figure the access system is implemented by a sliding door  15  attached to a doorframe  29  by a hinge system  16 . The door  15  is opened by door opener  17 . 
         [0076]      FIG. 2   b  shows another embodiment in which the central primary support  1  is replaced by a high strength circular or polygonal shaped central support collar or element  30 . This embodiment allows more internal free space to the structure and thus greater human occupancy of the structure. The individual shells  2  are securely fastened to the central support element  30 . This central support  30  provides rigidity to all the concrete shells  2 . 
         [0077]    Further,  FIGS. 2   a ,  FIG. 2   b  show a side view and  FIG. 3  is a top view which shows the secondary anchors  6  which are constructed at the base of each concrete structural shell  2  in a symmetric manner around the severe storm shelter. The shell  2  is connected to the anchor  6  by a steel tie-down fastener  5 . As shown in  FIGS. 2   a ,  2   b,  the lower end of the shell  2  is buried about  6  inches below grade of the substratum  18  and a circumferential skirt  9  of cement is poured forming a berm and this prevents wind from blowing under the structure and creating an overturning force on the structure and its occupants. One or more shells  2  are modified to allow for an easily opened structurally competent door  15  and a door opening mechanism  17  which can be opened from the inside and outside. 
         [0078]      FIG. 4  which shows the structural shells  2  in a “stand-up” view. Each shell  2  is connected to its adjacent partner by means of shell fasteners  5   a.  In the middle of the base of each shell  2 , the shells are connected to the secondary support anchors  6  which are connected to the concrete base  19  which is further buried in the substratum  18 . The horizontal tendons  7  and the respective anchors  3  and  4  are also shown traversing the shell elements. 
         [0079]      FIG. 5   a ,  FIG. 5   b ,  FIG. 5   c  and  FIG. 5   d  show embodiments of detail connections between adjacent shell elements  2 . In one embodiment, the shell  2  has tongue  14  fits into the groove  13  of the adjacent shell. In another embodiment, the connection can be made by overlapping the shells  2  as shown in  FIG. 5   c . In  FIG. 5   d , since the outer edge of the shell  2  is longer than the inner edge of the shell, this embodiment behaves as a keystone and this keystone arrangement further interlocks the contiguous shells  2 . Each shell  2  behaves like a keystone and thus transfers the load forces laterally to the other shell elements  2  that make the structure more rigid. 
         [0080]    As shown in  FIG. 6 , the concrete elements  2  are constructed or “formed” from a combination of steel rebar  12  and high strength cement with approximately 5000 psi strength rating. The forming process is well known in the industry and the pre-casting can be efficiently done by many skilled in the field. These shells are almost an article of industry and given adequate “shop drawings” can be manufactured to the desired specifications by any competent pre-casting cement company at a centralized location and then shipped to the construction location. The shell element  2  uses a rebar frame  25  which is fabricated and curved to meet the design requirements of the severe storm shelter catenary surface structure. In one embodiment, at selected parts of the rebar frame  25  steel pins  23  are added as male inserts, either by extending the rebar  12  or by welding a steel pin  23 . At the same time female sockets  24  are made in the shell form such that the male pins  23  will later fit when the shells  2  are locked together. 
         [0081]      FIGS. 7   a ,  7   b ,  7   c  show cross-sections of one embodiment with the central support element  30 . In these figures the steel tendons  7  are installed circumferentially around the structure either externally on the shell  2  as shown in FIG.,  7   b  and held in place by tendon anchors  3  and  4 . The external positions of the tendon are also maintained by tendon guides  32   a  and  32   b . In the case of internal tendons  7  these are implemented inside the shell element  2  as shown in  FIG. 7   c.    
         [0082]    The steel tendons  7  are used to post-tension the structural elements. These tendons  7  can be bonded or un-bonded. The tendons are tensioned by well-known industry practices such as a screw mechanism or a hydraulic jack device, which allows the tendon  7  to be pulled together and stretched thus imparting a load on the shells  2  and thus increasing the unit strength of the structure. Post tensioning is a well-known process in the construction industry and by itself is not part of the invention but its implementation in this type of shell element  2  is novel to the severe storm shelter industry. In this application tendon, cable and tensioning element are used interchangeably to describe the element that is post tensioned.  FIG. 7   a  also shows a circular bench  31  for the occupants during their stay in the structure. 
         [0083]    In assembling the structure by referring to  FIG. 1 , the primary support  1  is planted in place and anchored with cement. In a similar manner the secondary supports  6  are emplaced and anchored in place in one embodiment in a circumferential trough about 6 inches below grade as shown in  FIG. 2   a  and  FIG. 2   b . The first structural shell  2  is attached to its secondary anchor  6  using bracket  5  and tie-down bolt  22 . The top of the shell is bolted to the top of the primary supports  1  or  30  using a similar bracket  5  and bolt  22 . A second shell  2  is put in place and attached to the first shell  2  by fitting the male insert  23  into the female socket  24  and allowing the tongue  14  to fit into the groove  13 . A further embodiment shown in  FIG. 5   c , of the invention allows an interlocking groove and recess system in which adjacent shells are assembled to form the shelter body. A waterproof chemical bond compound  20  is inserted between the shell elements  2  to provide a sealant and to maintain an impermeable seal against wind blown water entering the structure. Each shell is attached at both the top of the primary supports  1  or  30  and bottom secondary support by the brackets  5  using the tie-down bolts  22 . The shells  2  are also connected by the fasteners  5   a  which laterally connect the shell elements. All tie-down bolts  22  are tightened to the required torque and the skirt  9  is poured from cement around the structure. The door  15  is installed and bolted to its frame  29 . 
         [0084]    The post tensioning is implemented with methods current in the industry usually by hydraulic or mechanical means to stress the tendons. The amount of post tensioning is calculated and designed to maximize the strength of the structure. This is usually done by a computer program well known in the industry and available to many skilled in the art. In one embodiment, the horizontal tendons  7  are threaded through the tendon anchors  3  and  4 . The upper and lower tendons are emplaced and the tendons are stretched to the requisite limit. The tendons are anchored off using standard industry hardware and components as shown in Ref. 6 by SureStress™. A plurality of structural shells are modified as need to allow the post tensioning hardware and components to be installed and implemented. The fact that the tendons are kept in a permanently elongated state causes a compressive force to act on the cement shell elements. This pre-compression which results from the post tensioning counter balances the tensile forces created by the subsequent loading during a severe storm event and increases the load carrying capacity of the subject structure. 
         [0000]    The combination of strength provided by several inches of 5,000-psi reinforced concrete shells and the post tensioning of the structure prevents any penetration by flying missiles at more than a hundred miles per hour. In addition the catenary structure provides the smallest cross section surface area for a given volume of cement material so that the subject structure provides a maximum strength for a minimal amount of construction material. Further more compared to other existing block shaped severe storm structures which provide a rectangular cross-section to wind flow and missile contact, the subject invention provides a streamlined cross-section and a continuously curved cross-section to wind flow and only a fractional component of the wind force acts on the structure proposed in this invention. Missile and wind impact is minimized in the case of the new invention and in the block structure the missile and wind impact is maximized. 
       Procedure: 
       [0085]    The procedure used in installation of this invention is as follows. 
         [0086]    1. The selected location for installation is made and the design template for the anchor piles is used to fix the holes for the anchor supports. 
         [0087]    2. Support holes are excavated or drilled for the primary anchor if needed, and the secondary anchors. The support anchors are inserted into the holes. 
         [0088]    3. High strength cement is poured into the holes to anchor the steel supports. 
         [0089]    4. The cement is allowed to cure to maximum strength or for at least 2 days. 
         [0090]    5. The shelter is then assembled by emplacing each shell in turn ensuring alignment by fitting the steel pins into the female sockets. 
         [0091]    6. The steel tendons are connected using the tendon hardware attached to each shell or by threading the tendon circumferentially around the structure. 
         [0092]    7. The adjacent concrete shells are aligned, connected and bolted together into the primary and secondary supports anchors. In those embodiments where a flange is used instead of a primary vertical anchor the shells are bolted to the top steel flange. All connections are made to the required torque to ensure rigidity and strength. 
         [0093]    8. The structure is post tensioned by stressing the tendons and anchoring the tendons to the required level of stress. The stress level is at least 30% of the maximum allowable strength of the tendon. 
         [0094]    9. The structure joints are then sealed using a commercially available sealer or caulk especially at the floor slab and shelter connection planes. 
         [0095]    10. The door is installed on its frame. 
         [0000]    Further modifications and alternative embodiments of various aspects of the invention may be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the claims. 
       List of Elements of Invention. 
       [0096]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 No 
                 Item Description 
               
               
                   
                   
               
             
             
               
                   
                  1 
                 Primary Support Structure 
               
               
                   
                  2 
                 Concrete Precast Post tensioned Shell element 
               
               
                   
                  3 
                 Tendon Anchor upper 
               
               
                   
                  4 
                 Tendon anchor lower 
               
               
                   
                  5 
                 Shell Tie down Bracket 
               
               
                   
                  5a 
                 Shell Fasteners 
               
               
                   
                  6 
                 Secondary Support Structure 
               
               
                   
                  7 
                 Horizontal Tendon 
               
               
                   
                  8 
                 Ventilator dome cover 
               
               
                   
                  9 
                 Protective skirt 
               
               
                   
                 10 
                 Shell Lip 
               
               
                   
                 11 
                 Floor 
               
               
                   
                 12 
                 Cement reinforcing rebar 
               
               
                   
                 13 
                 Upper and lower portion of shell edge 
               
               
                   
                 14 
                 Male insert portion of shell edge 
               
               
                   
                 15 
                 Sliding Door System 
               
               
                   
                 16 
                 Hinge system 
               
               
                   
                 17 
                 Door opener system 
               
               
                   
                 18 
                 Substratum 
               
               
                   
                 19 
                 Concrete base for primary and secondary anchors 
               
               
                   
                 20 
                 Chemical bonding compound 
               
               
                   
                 21 
                 Welded metal bracket 
               
               
                   
                 22 
                 Tie down bolt 
               
               
                   
                 23 
                 Male insert pin 
               
               
                   
                 24 
                 Female socket 
               
               
                   
                 25 
                 Rebar grid frame 
               
               
                   
                 26 
                 Wind blown missile 
               
               
                   
                 27 
                 Block shaped tornado structure 
               
               
                   
                 28 
                 Tornadic wind flow 
               
               
                   
                 29 
                 Sliding Door Frame 
               
               
                   
                 30 
                 Central Support Collar 
               
               
                   
                 31 
                 Contoured Seats 
               
               
                   
                 32a 
                 Tendon guide - upper 
               
               
                   
                 32b 
                 Tendon guide - lower 
               
               
                   
                 33a 
                 Vortex at front of structure 
               
               
                   
                 33b 
                 Vortex at back of structure 
               
               
                   
                   
               
             
          
         
       
     
       REFERENCES 
       [0097]    (1): Oz Storm Shelter, www.ozsaferooms.com 
         [0098]    (2) FEMA Booklet: Design and Construction Guidance for Community Shelters.http://www.fema.gov/fima/tsfs02.shtm 
         [0099]    (3): Texas Tech Wind Research Lab. http: / /www.wind.ttu.edu/Research/research.asp 
         [0100]    (4): Pietras, B. K. 1 997. “Analysis of Angular Wind Borne Debris Impact Loads.” Senior Independent Study Report. Department of Civil Engineering, Clemson University, Clemson, S.C. 
         [0101]    (5): Post Tensioning Institute Publications: http://www.post-tensioning.org/ 
         [0102]    (6): SureStress Company: http://www.dur-o-wal.com/surestress.html