You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION  
         [0001]    U.S. Pat. No. 6,253,655 discloses a method of making an armor to resist projectiles, with a foam plastic inner layer that absorbs energy during impacts. The same principle is applied in the present invention, with notable contrasts in the simplicity of the design, methods of construction and configuration of the final product.  
           [0002]    U.S. Pat. No. 6,240,686 teaches that foam plastic blocks can be effectively arranged to support building loads. The invention requires the use of tensioning skins and includes multiple joints that are avoided in the present invention.  
           [0003]    U.S. Pat. No. 6,218,002 discloses a concrete mixture containing a barrel shaped polystyrene bead as the aggregate. The concrete in this formulation contrasts with the present invention. It uses only one size of virgin polystyrene beads while the present invention uses recycled aggregates of varied sizes, allowing small particles to fill voids between larger particles and contribute to the resiliency of the structure.  
           [0004]    U.S. Pat. No. 6,151,841 discloses a pyramid shaped shelter that is erected above ground with Plexiglas(TM) windows. The shelter demonstrates the merits of triangular sides in resisting wind, but it fail to address the problem of joints, which are eliminated in the present invention.  
           [0005]    U.S. Pat. No. 6,131,343 discloses a parabolic-like shaped dome shelter that may be constructed of or covered with corrugated metal. This patent teaches the value of dome shapes in resisting wind, but it does not provide the simplicity, insulation and other attributes of the present invention.  
           [0006]    U.S. Pat. No. 5,953,866 discloses a storm shelter that is partly buried underground with provisions for an emergency jack to forcibly open the sliding door. It is an attribute of the present invention that it is located above ground and is less likely to entrap occupants. In addition, ordinary carpenter tools can be used to cut an emergency escape if the door becomes inoperable. This feature does not diminish the fact that the lightweight concrete in the present invention resists severe impacts, including bullets.  
           [0007]    U.S. Pat. No. 4,879,855 discloses a building method that incorporates polystyrene in structure walls. Walls built with this method perform satisfactorily in extreme wind conditions, but failures have occurred at wall-roof intersections. In addition, the foam plastic produces toxic gasses when exposed in building fires. The concrete in the present invention is remarkably fire proof and the design eliminates joining problems and most of the concrete required in U.S. Pat. No. 4,879,855. In the present invention polystyrene beads are coated with a formulation to create a lightweight material that performs all of the functions of the heavy concrete and steel in the above-cited patent.  
           [0008]    People living in undeveloped and developed countries occupy structures, including emergency shelters, that are vulnerable to collapse and wind-blown missile penetration during extreme wind conditions. Few residential structures are designed to resist the highest velocity winds, and every year injuries and death occur due to intrusion of flying objects and building collapse. Shelters made of steel and concrete are often placed at floor level or below the floors inside residences. These shelters save lives, but they have recognized disadvantages. Their cost exceeds the budget limitations of many families, and occupants may be trapped under collapsed building materials. Underground shelters require expensive excavation and forming, and occupants are sometimes at risk from flooding. Stairways take up shelter space and they are inconvenient and dangerous. Typical engineering techniques used to design shelters ignore such risks and concentrate on overcoming maximum hurricane or tornado wind pressures. Structures are built with a safety factor that proves to be inadequate when the siding and roof assemblies separate and crash into adjacent property and when they collapse under loads of debris carried onto them by high winds.  
           [0009]    In this invention less dense materials create a structure that exceeds the performance of typical shelters. This is accomplished by a smooth exterior design that prevents the wind, regardless of velocity or direction, from achieving a direct hit. Most flying objects glance off the structure in this invention without imposing the shocks felt by conventional shelters. Direct hits by lengths of 2×4 lumber moving at 150 miles per hour do not penetrate this structure. Unlike steel, the lightweight concrete in this invention has a low coefficient of thermal expansion. It does not rust, decay when exposed to below-ground moisture, or react adversely to freeze-thaw cycling. It is a good insulator and withstands exposure to heat and flames in excess of 1700 degrees F.  
         SUMMARY OF THE INVENTION  
         [0010]    It is thus an object of this invention to provide a low cost structure that is safe during extreme weather, durable and easily transported. This invention, because of its superior shape, material formulation, manufacturing method and door design meets the objectives. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is an elevation view of the shelter.  
         [0012]    [0012]FIG. 2 is a view of the shelter dome showing the radio antenna, ventilation ports, interior diverters and debris collection bags.  
         [0013]    [0013]FIG. 3 is a view of the shelter dome from directly above showing ventilator port openings.  
         [0014]    [0014]FIG. 4 is a view of the preferred tie-down method.  
         [0015]    [0015]FIG. 5 is a view of an alternate tie-down method.  
         [0016]    [0016]FIG. 6 is a view of the molds used to construct the structure.  
         [0017]    [0017]FIG. 7 is a view of the shelter floor with a ramp for handicapped access.  
         [0018]    [0018]FIG. 8 is a view of the sliding door mechanism with the door opened to the inside.  
         [0019]    [0019]FIG. 9 is a view of the sliding door mechanism with the door opening to the outside.  
         [0020]    [0020]FIG. 10 is a view of the door latching mechanism in the open or unlatched position.  
         [0021]    [0021]FIG. 11 is a view of the door latching mechanism in the closed or latched position.  
         [0022]    [0022]FIG. 12 is a view of the door hardware and overhead track mechanism.  
         [0023]    [0023]FIG. 13 is a view of the latching mechanism isolated on the door. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    This description provides illustrative information in support of the claims and should not be regarded as defining the language in the claims. Those familiar with the art will recognize that other structure dimensions and variations in shape of the dome and in the concrete mix design are within the scope of this invention.  
         [0025]    [0025]FIG. 1 illustrates a monolithic dome-shaped structure  1  with the general location of the door latches and catches  4 , some of the air vents  2  and viewing ports  3 . The lightweight concrete comprising this structure provides extraordinary resistance to weather and the impacts of wind driven objects. The shape of the structure and the smooth exterior finish  1  minimize wind friction. These features and the large area of contact at the base work together to resist overturning. The viewing ports  3  are made of heavy transparent plastic or glass to allow safe viewing.  
         [0026]    [0026]FIG. 2 displays a cutaway section of the domed roof with ventilation ports  7 . These ports are nominally 2 inch diameter pipes made of plastic or steel. They replace larger window-like vents normally used. The number of vents is determined by the size of the structure and occupant load. Sufficient vents  7  to sustain healthy breathing conditions and equalize interior air pressure with outside atmospheric pressure can be easily installed by boring holes in the shelter. Because of their small diameter multiple vent pipes do not impair the structural integrity or smoothness of the structure&#39;s exterior. The pipes  7  are inclined upward from the outside to reduce infiltration by water and to provide drainage. Removable diverters  8  are connected to the vent pipes  7  on the inside. The sharp angles of the diverters  8  and the small diameter of the pipes reduce air velocity and prevent all but the smallest flying objects from entering the structure. Fine debris is caught in removable bags  6  that are permeable to admit required airflow. The bags serve as visual indicators of air movement and pressure conditions. The bags  6 , diverters  8  and pipes  7  can be easily cleaned. A radio antenna  9  allows one-way or two-way communications depending on the type of receiver-transmitter  10  selected. The antenna  9  penetration through the roof is sealed to prevent water intrusion.  
         [0027]    [0027]FIG. 3 is an overhead view illustrating the preferred placement of ventilation ports  12  below the top of the domed roof  11  and the radio antenna  13 . The door  14  is equipped with hinging and latching devices of sufficient strength to develop wind and impact resistance equal to other parts of the structure. In this view the door  14  slides into the structure to open and outward to close. In this configuration the door temporarily occupies interior space. It provides protection against weather during closing.  
         [0028]    [0028]FIG. 4 illustrates the preferred tie-down method with the bent ends  20  of ½ inch diameter anchor bolts  18  embedded in the concrete floor  19  and the threaded ends penetrating the structure near the base. Tamper-proof nuts  17  protect against vandalism and provide the structure with resistance against uplift and overturning during the most severe winds. The anchor bolts  18  are inserted through the holes in the base of the wall prior to pouring the concrete floor  19 . Large washers  16  and tamper-proof nuts  17  are installed after the concrete floor is cured.  
         [0029]    [0029]FIG. 5 illustrates an alternate tie-down method. Holes large enough to permit insertion of ½ inch diameter bolts are provided at a minimum of 8 locations around the perimeter at the base of the structure. After the structure is permanently placed on the ground the inside wall serves as a form for pouring the concrete floor  28  that has nominal compressive strength of 2500 PSI. An L-shaped anchor bolt  27  is placed vertically in the wet concrete adjacent to each hole in the structure base. 3-inch by 3-inch slotted angle plates  26  are placed over the anchor bolts  27  and the horizontal wall bolts  25 . On the exterior large washers  22  are placed over the bolts to distribute the load on the lightweight concrete. Tamper-proof nuts  23  permanently attach the wall to the concrete floor.  
         [0030]    [0030]FIG. 6 illustrates the method of forming and pouring the lightweight concrete to construct the monolithic structure. In the preferred embodiment the dome-shaped lower mold  29  is inverted and coated on the interior surface with form-release material. Cured spacer blocks  31  six inches thick made of the lightweight concrete are secured along the sides of the mold. One block  32  is secured to the bottom of the mold at the center. These blocks are identical in composition to the lightweight concrete comprising the rest of the shelter. They bond to it and become part of the structure. The lower mold  29  is partially filled with the fluid lightweight concrete creating a reservoir. A second upper mold  34  six inches smaller in diameter but identical in shape to the lower mold is forced down into the fluid concrete, descending until it rests on the center spacer block  32  and against the other spacer blocks  3 l. The pressure of the upper mold  34  pressing into the fluid concrete  33  is sufficient to cause it to rise and fill the six inch space between the two molds. Vibrators are applied to the outside of the lower mold and the inside of the upper mold to cause fine particles to accumulate against the molds, creating smooth surfaces. After sufficient curing the upper mold  34  is pulled up and out of the inverted structure. The lower mold  29  and the rigid structure are inverted together so the structure is resting on its base. The lower mold  29  is then removed from the outer surface of the structure, completing the process.  
         [0031]    [0031]FIG. 7 is a side view with a shelter wall segment showing the handicapped access ramp  36 . A ramp is required because the door entrance is nominally 4 inches above ground level  39 . Flexible gaskets  37  attached to grooves in the floor and in the bottom of the door compress against mating gaskets to prevent water intrusion. In a typical environment the concrete floor and handicapped access ramp are poured on solid natural ground  39 . If installed on flood plains the structure must be placed on elevated ground.  
         [0032]    [0032]FIG. 8 is a view of the shelter door  40  and the operating mechanism used to open the door to the inside. The door  40  is a cutaway segment of the structure wall  41 . The top of the door is connected to 2 Unistrut(TM) tracks  44  that are attached by 2 brackets  47  to the roller assemblies  42 . The rollers run inside the track and cannot be derailed. Two flat plate brackets  47 , one on each face of the door are attached with bolts on each upper corner of the door. The roller assemblies  42  are equipped with ball bearings. The rollers  42  align the door  40  for precise fit against the gaskets  43  surrounding the door. The bottom door control assembly consists of notched ball bearing-equipped rollers  45  fitted onto 90 degree steel angle tracks  46  secured to the concrete floor with the v facing upward and the open angle down. Notched rollers  45  fit precisely over the steel angle tracks to control the position of the door and share the weight with the upper track and roller assembly. All door edges and mating structure surfaces are equipped with flexible gaskets  43  that firmly seal the door. The upper track assembly  44  is located above the door opening to provide head clearance to standing shelter occupants. The lower track assembly  46  is configured to allow passage of wheel chairs.  
         [0033]    [0033]FIG. 9 is a view of the door and door operating mechanism with the door configured to be opened by moving it to the outside. This configuration may be selected in low wind areas. This configuration avoids temporary loss of interior space caused by the door opening in, but it causes the door and the upper track assembly  53  to be exposed to wind-driven debris. The lower  58  and upper  53  track mechanisms are identical to the track mechanisms in FIG. 8 except for being extended outside. The tracks holding the door are supported on the inside by two columns  55  made on fiberglass, pvc pipe or other material that is non conductive to avoid risk from lightening strikes. In this configuration the ramp  60  on one side of the shelter provides handicapped access to the structure. When closed, as shown in lighter color, the door  56  is held by 2 heavy latches  52 , one on each side of the door. The latches can be operated from inside or outside of the structure. These latches  52  consist of steel plates mounted on ½ inch diameter bolts fitted through pipes penetrating the door to form bearings. The lower roller assembly  54  consists of notched rollers fitted over angle tracks  58 . The door closes against replaceable gaskets  57  firmly sealing all edges.  
         [0034]    [0034]FIG. 10 is a view of the door  63  and its 2 latch assemblies in the open or unlatched position. These latches  61  consist of ¼ inch thick steel plates nominally 4 inches wide and 8 inches long. 4 identical latches are used, 2 on the inside and 2 on the outside of the door  63 . 2 pipes the same thickness as the door are inserted through a close fitting hole in the door and a ½ inch diameter steel bolt threaded on both ends is fitted through the pipe creating a bearing. The bolt is firmly attached to the latch plates  61 . Catches  62  are installed on the shelter walls  64  adjacent to the door on both sides. This configuration allows easy rotation of the latches which are operable from inside or outside of the structure.  
         [0035]    [0035]FIG. 11 is a view of the door and its latch assembly  65  in the closed position. The latch is easily operated from either the inside or outside by grasping the latch handle  67  and turning the latch to a horizontal position inside the catch plate. Replaceable gaskets where the door closes  68  into the structure wall provide weatherproof seals.  
         [0036]    [0036]FIG. 12 is a view of the door and overhead track assembly  70  with material above the door cut away to show the tracks  70 . Also shown are overhead rollers  71 , door hanging brackets  72 , outside catches,  73  and outside latches  74 . The door  75  is carried on the rollers  71  to a distance of required opening, such as three feet into the shelter. The door is carried out to close against flexible gaskets on all edges  76 .  
         [0037]    [0037]FIG. 13 is a view of the two latch plates  78  showing a door segment  77 , the bearing pipe  79  and latch bolt  80 .

Summary:
Disclosed is a monolithic prefabricated structure that is wind and impact resistant. This special pre-cast lightweight concrete structure is a blend of special aggregates and additives combined to resist the impacts of flying debris and extremes of weather. The curved exterior surface minimizes wind friction and deflects debris. The circular structure has a low center of gravity and firm attachment to a floor made of concrete. It has protected ventilation openings, viewing ports and a strong smooth-fitting door.