Patent Publication Number: US-2007095006-A1

Title: Lightweight portable concrete enclosure and associated method of construction

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to portable enclosure construction. More particularly, the present invention relates to a method for constructing an enclosure from a frame covered by a lightweight concrete mix.  
      2. Description of the Background Art  
      Presently, the use of pre-fabricated building elements is known. It is further known to spray such building elements with a layer of concrete. An example of such a building element is the EVG-3D® construction system produced by EVG Entwicklungs-U. Verwertungs-Gesellschaft mbH of Raab, Austria.  
      An EVG building element is described in U.S. Pat. No. 6,705,055 to Ritter et al. The &#39;055 Patent describes a building element having two parallel welded wire grid mats and associated web wires that hold the wire grid mats at a distance from one another. An insulating body is arranged between the wire grid mats. The web wires extend through the insulating body and support it inbetween the wire grid mats. An outer and inner shell of concrete can be sprayed on the wire grid mats. To improve the adhesion of the concrete to the insulating body, the insulating body may include roughened cover surfaces. The resulting structure can be used as a wall or ceiling element.  
      Another EVG building element is disclosed in U.S. Pat. No. 6,185,890 to Ritter. The building element in the &#39;890 Patent is aimed at improving sound damping in prefabricated building elements. Again, the building element of the &#39;890 patent includes two wire mesh mats interconnected by web wires that enclose an insulting body therebetween. Concrete is then applied to cover and inner and outer wire mesh mats and form concrete shells. The concrete shells are then interconnected by forming the insulating body with through-holes, which, upon application of the concrete, will form concrete webs or plugs that interconnect the two shells. By interconnecting the concrete shells resonant sound vibrations are prevented within the concrete shells.  
      Although each of the above referenced inventions achieves its individual objective, none of the described systems is intended for use with a lightweight concrete mix or for use in constructing a portable box.  
     SUMMARY OF THE INVENTION  
      It is therefore one of the objectives of this invention to provide a construction method which uses a number of building elements that allow lightweight enclosures to be quickly and efficiently constructed.  
      It is also an object of this invention to utilize a lightweight concrete mix in the construction of a portable box or enclosure.  
      Still another object of this invention is to utilize a number of building elements in forming an enclosure wherein the building elements are adapted to be sprayed with concrete to yield a lightweight monolithic design.  
      These and other objectives are carried out in a method for constructing a concrete enclosure. The method includes providing a number of structural building elements, with the individual building elements having exposed wire reinforcement. The various building elements are thereafter assembled together to form the enclosure. Thereafter, a lightweight concrete mix is prepared and sprayed to cover the exposed wire reinforcement of the building elements. As the concrete hardens it is reinforced by the exposed wire reinforcement.  
      The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:  
       FIG. 1  is a view of one anticipated application of the concrete enclosure of the present invention.  
       FIG. 2  is a detailed view taken from  FIG. 1 .  
       FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 2 .  
       FIG. 4  is a side elevational view of a concrete shelter constructed in accordance with the present invention.  
       FIG. 5  is a side elevational view of the concrete shelter of  FIG. 4   
       FIG. 6  is a view of the concrete shelter taken along line  6 - 6  of  FIG. 4 .  
       FIG. 7  is a view of the concrete shelter taken along line  7 - 7  of  FIG. 5 .  
       FIG. 8  is a flow chart illustrating the various steps associated with the method of the present invention.  
       FIG. 9  is a partial sectional view of the interior of the enclosure of the present invention illustrating the underlying wire mesh grid.  
       FIG. 10  is a sectional view taken along line  10 - 10  of  FIG. 9 .  
       FIG. 11  is a view of an upper corner of the assembled building elements.  
       FIG. 12  is a view of a lower corner of the assembled building elements.  
       FIG. 13  is a top plan view of the floor frame used in constructing the floor of the shelter.  
       FIG. 14  is a side elevational view taken along line  14 - 14  of  FIG. 13 .  
       FIG. 15  is a detailed top plan view taken from  FIG. 13 .  
       FIG. 16  is a side view taken along line  16 - 16  of  FIG. 14 .  
       FIG. 17  illustrates the floor frame being filled with concrete.  
       FIG. 18  is a view of an assembled shelter being lowered into the concrete of the floor frame.  
       FIG. 18 ( a ) is a view of concrete being applied to the inside floor of the shelter.  
       FIG. 19  is a view illustrating ceiling, upper edge frames and column frames being filled with concrete.  
       FIG. 20  is a view taken along line  20 - 20  of  FIG. 19 .  
       FIG. 21  is a view of the assembled shelter being sprayed with concrete.  
       FIG. 22  is a view taken along line  22 - 22  of  FIG. 21 .  
       FIG. 23  is a top plan view taken along line  23 - 23  of  FIG. 21 .  
       FIG. 24  is a view taken alone line  24 - 24  of  FIG. 21 .  
       FIG. 24 ( a ) is a view of a joint.  
       FIG. 25  is an illustration of a completed shelter being transported via a flat bed truck.  
       FIG. 26  is a view of a bottomless embodiment of the shelter of the present invention.  
       FIG. 27  is a view of a smaller embodiment of the shelter of the present invention.  
       FIG. 28  is a view taken along line  28 - 28  of  FIG. 27 . 
    
    
      Similar reference characters refer to similar parts throughout the several views of the drawings.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The present invention relates to a lightweight, portable concrete enclosure and an associated manufacturing method. The manufacturing method employs a structural member made up of two wire mesh grids that are spaced via web wires. The structural member further includes an intermediate layer of insulation positioned along the web wires. The structural members are interconnected to form a shelter. A lightweight concrete mix is then sprayed over the external surface of the structural members to yield a lightweight enclosure, or box, that can be easily transported. The various aspects of the enclosure of the present invention are described in greater detail hereinafter in conjunction with  FIGS. 1-28 .  
      An enclosure constructed in accordance with the principles of the present invention can be used in a number of environments. By way of non-limiting example, the shelter can be used as a portable classroom, as a temporary enclosure for a construction site, as a low cost home, or as a temporary house for natural disaster victims. Enclosures of the present invention can also be used as non-habitable enclosures, such containers or equipment boxes. The present invention finds use in any environment where inexpensive, portable, quickly constructed enclosures are needed.  
      With reference now to  FIG. 1 , another possible intended use is depicted. Here the enclosure is a shelter  20  that is used to house the telecommunications equipment associated with a cellular telephone tower  22 . This equipment is found at the base of all cellular telephone towers and includes radio transmitters and receivers and the associated electronics for routing incoming and outgoing telephone traffic. All of this equipment must be protected both from the elements and from vandals.  
      As illustrated in  FIGS. 4-7  shelter  20  is constructed with a door frame  24  in place to allow the subsequent installation of a door  26  for secure access to the shelter&#39;s interior by authorized personnel. Shelter  20  is also constructed with openings for use in installing one or more air conditioning units  28 . Such air conditioners are needed to keep the temperature of the internal equipment at a regulated level. Two air conditioners  28  are included to provide redundancy. Additional openings  32  (note  FIGS. 6 and 7 ) are included to provide power and ground wires to the equipment. One or more waveguides  34  (note  FIGS. 4 and 5 ) are also formed in the sides of shelter  20  to route the cabling associated with cell tower  22 .  
      In a manner described in more detail hereinafter, shelter  20  is constructed at an off site location and thereafter transported to the location of cell tower  22 . Shelter  20  is secured to a pre-formed concrete foundation  36  by way of anchor bolts  38  and plates  42  (note  FIGS. 2-3 ). A substantial savings in construction costs is realized by manufacturing the shelters off- site. Furthermore, the cellular telephone equipment can be installed within the interior of the shelter prior to its delivery to cell tower  22 . This makes the entire shelter “plug-and-play.” Namely, after shelter  20  is anchored at its intended destination, the cabling of cell phone tower  22  is merely coupled to the equipment within shelter  20 . Again, this results in a substantial savings in man power as all of the internal equipment associated with shelter  20  can be put together in an assembly line manner at an offsite location. Thereafter, once the equipment is installed, the entire shelter  20  is moved into place.  
      Method of Construction  
      The construction method of the present invention is next described in association with the flowchart of  FIG. 8 . In the first step of the method, the floor, walls and ceiling ( 44 ,  46 , and  48 , respectively) of portable structure  20  are assembled from a number of structural building elements  52 . The size and shape of the structural building element  52  employed will depend upon the size and shape of the intended shelter. In the embodiment depicted in  FIGS. 1 through 7 , a rectangular shelter  20  is constructed from six building elements, representing the four walls  46 , ceiling  48  and floor  44 . A greater or lesser number of building elements  52  of varying size and shape can be used depending upon the configuration of the intended shelter.  
      In the second step, building elements  52  are secured to one another along their respective edges by way of corner splices  54  and joints  54   a.  Corner splices  54  and joints  54   a  are depicted interconnecting adjacent building elements  52  in the cross sectional views of FIGS.  22 - 24 ( a ). Corner splices  54  are used to join building elements  52  at an angle, whereas joints  54   a  can be used to form a continuous flat surface from adjacent building elements  52 . The assembly of building elements  52  results in a shelter  20  with exposed wire grids and without concrete reinforcement. This structure is further defined by a number of upper edges  56  ( FIG. 11 ) about the periphery of the ceiling  48 , a number of lower edges  58  ( FIG. 12 ) about the periphery of the floor  44 , and by a number of side edges  62  ( FIGS. 11 and 12 ) that are formed by the abutting walls  48  of shelter  20 . As will be explained in conjunction with future steps, each of these edges receives additional concrete reinforcement during construction.  
      During assembly, ferring strips  102  can be installed within the interior of shelter  20  to facilitate the subsequent placement of wallboard within the interior. As noted in the cross section of  FIG. 10 , the ferring strips  102  are installed within the interior of shelter  20  along both walls  46  and ceiling  48 .  
      The preferred building element for the walls  46 , ceiling  48  and floor  44  is manufactured by EVG Entwicklungs-U. Verwertungs-Gesellschaft mbH of Raab, Austria. This building element is described in U.S. Pat. Nos. 6,706,055 to Ritter et al. and 6,185,890 to Ritter., both of which are assigned to EVG. The contents of both these patents are fully incorporated herein.  
       FIG. 10  is a cross section of a suitable building element  52 , which includes interior and exterior wire grids ( 64  and  66 , respectively) that are spaced from one another by web wires  68 . Wire grids ( 64  and  66 ) are formed from a series of individual wires that are welded to each other at 90 degree angles and which together form a mesh or wire screen, as noted in the partial sectional view of  FIG. 9 . In the preferred embodiment, web wires  68  are welded to adjacent wire grids ( 64  and  66 ) at approximately a 30 degree angle, with opposing web wires  68  having opposite orientations (note  FIG. 10 ). However, it is within the scope of the present invention to use web wires  68  with other configurations, such as web wires  68  that are formed at a 90 degree angle to the wire grids ( 64  and  66 ), or web wires  68  that are all angled in a uniform direction.  
      With continuing reference to  FIG. 10 , the insulation layer  72  of building element  52  is next described. More specifically, building elements  52  include an insulation layer  72  positioned between the interior and exterior wire grids ( 64  and  66 ). This insulation can consist of, for example, foam plastic such as polystyrene or polyurethane foam or other foam materials which have desirable insulating characteristics. Preformed holes are included through the width of insulation  72  to allow it to be positioned along the length of web wires  68 . As noted in the cross section of  FIG. 10 , insulation layer  72  is equidistant from both the interior and exterior wire grids ( 64  and  66 ). However, the insulation layer  72  can be positioned closer to either of the two wire grids ( 64  and  66 ) if desired. As noted in  FIG. 10 , ferring strips  102  are inserted in-between the interior wire grids  64  and the insulation  72  and between adjacent web wires  68 . These ferring strips are subsequently used in the installation of wallboard upon the interior of the shelter.  
      The third step of the method is next described. Here, as illustrated in  FIGS. 11 and 12 , additional reinforcement  74  is placed along the edges of the assembled shelter  20 . More specifically, after the walls  46 , ceiling  48  and floor  44  have been assembled via corner splices and joints, concrete reinforcing bars  74 , or “rebar,” are inserted along the edges of the shelter. Two reinforcing bars are preferably inserted along the side edges  62  of shelter  20  ( FIG. 11 ) and a single reinforcing bar  74  is inserted along the upper and lower edges ( 56  and  58 ) ( FIGS. 11 and 12 ). Reinforcing bars  74  can be conventional rebar with the gauge being selected based upon the .intended application for shelter  20 . Reinforcing bars  74  are inserted between the exterior wire grids  66  and insulation material  72 . If necessary, rebar  74  can be tied to the exterior wire grids  66  to prevent movement while the concrete is being poured.  
      Additional rebar  74  can be likewise inserted at spaced intervals along the floor  44  of the shelter ( FIG. 12 ). Again, reinforcing bars  74  are positioned intermediate the exterior wire grid  66  and the insulation  72  and can be tied, if necessary, to prevent movement. Floor reinforcing bars  74  are desirable in the event shelter  20  will house exceptionally heavy equipment. When used for telecommunications equipment, for example, shelter  20  often houses large power supplies. In such instances, additional reinforcing bars  74  are included along the length of floor  44  and especially in areas over which the power supplies will be placed. After all the building elements  52  have been assembled (step  1 ), fastened together (step  2 ) and the additional reinforcing bars  74  added (step  3 ), shelter  20  is ready to be coated with concrete.  
      The first step in applying the concrete is illustrated in  FIG. 17 . Here, ground form  76  is assembled in a size and shape that corresponds to floor  44  of shelter  20 . Care should be taken to ensure this step is carried out on a flat surface  78  to ensure resulting floor  44  of shelter  20  is uniform. A coating can be applied to area bounded by ground form  76  to prevent concrete from hardening to the ground  78  where shelter  20  is being assembled. Thereafter, a layer of wet cement  82  is added to the form as noted in  FIG. 17 .  
      Any of a variety of concrete mixes can be used for this purpose. However, in the preferred embodiment, an especially made lightweight concrete mix is utilized. The details of this concrete mix are described in greater detail hereinafter. After concrete  82  is poured, but before it hardens, assembled shelter  20  is lowered into form  76  and the wet concrete  82 . The volume of the concrete within the form is sufficient to fully immerse the lower edges  58  of shelter  20 . In order to make sure all the side surfaces of floor  44  are adequately covered in concrete, the floor building element is inwardly stepped  84  in relation to surrounding walls  46 . The stepped nature of the floor is noted in  FIG. 18 . By stepping floor  44  inward, all of the outer surfaces of floor  44  are covered by concrete so as to ensure that exterior wire grid  66  is fully encased in concrete  82 .  
      Thereafter, as noted in  FIG. 18a , concrete  82  is delivered into the interior of shelter  20  to form the interior floor. A sufficient amount of concrete is pumped into shelter  20  to completely cover the exposed interior wire grids  64  of building element  52 . The interior and exterior floor surfaces are thus formed by allowing concrete  82  to harden on both the exterior and interior wire grids ( 66  and  64 ) of the building element  52 .  
      In the embodiment illustrated in  FIGS. 13 through 16 , lifting plates  86  are positioned in the four corners of floor form  76  prior to concrete  82  being poured. Lifting plates  86  are secured to an adjoining frame member by way of a bolt  38  that is positioned within a coil insert  92 . Lifting plates  86  further include a number of nelson studs  94  to promote a firm bond between the plate  86  and surrounding concrete  82 . Once the concrete of floor  44  hardens, lifting plates  86  are used to transport shelter  20 . Namely, a lifting lug can be fitted to bolt  38  within the lifting plate  86 . These lugs, in turn, can be fastened to a crane to enable shelter  20  to be easily maneuvered. Coil inserts  92  diffuse any stresses encountered by the lifting plates and enable shelter  20  to be picked up without any shearing of the associated bolts  38 . After construction is completed, anchor plates  42  can be fitted over the bolts  38  to secure shelter  20  to a concrete foundation  36  as noted in  FIGS. 2 and 3 .  
      In the fifth step, after the concrete hardens to form floor  44  of shelter  20 , the floor forms  76  are removed. Thereafter, additional forms are added about the upper edges and the corner edges. Namely, an upper edge form  96  is attached to the periphery of ceiling  48  and corner edge forms  98  (or column forms) are added to the edges between adjacent walls  46 . These additional forms are depicted in  FIG. 19 . These concrete forms ( 96  and  98 ) correspond to the areas where the additional reinforcing bars  74  were previously inserted in step three. With continuing reference to  FIG. 19 , after additional forms ( 96  and  98 ) are assembled, concrete is sprayed upon ceiling  48 . The same concrete mix used for floor  44  is likewise used with ceiling  48 .  
      This concrete is sprayed from a concrete pump using any of a number of known concrete spraying techniques, such as those employed in the application of Spray-crete™ or Shot-crete™. Although sprayed, due to the horizontal orientation of ceiling  48 , it can just as easily be poured. A sufficient amount of concrete is provided to cover exterior wire grid  66  of ceiling  48 . A sufficient amount of concrete is also provided so as to completely fill the forms of the upper edges  96  and the forms of the corner edges  98 . Once the concrete covers ceiling  48  and forms ( 96  and  98 ), the flow of concrete is stopped and the concrete is allowed to harden.  
      As will be obvious to those skilled in the art, prior to concrete being applied to ceiling  48 , it may be necessary to temporarily reinforce the ceiling from within the interior of shelter  20 . This can be accomplished by installing one or more ceiling joists and associated jacks to fully support the weight of the poured concrete until it hardens.  
      After the concrete hardens the forms of the upper edges and the corner edges ( 96  and  98 ) are removed. These areas of shelter  20  will be solid reinforced concrete. As noted in  FIG. 8 , the step of removing these forms ( 96  and  98 ) is carried out in step nine. Thereafter, additional concrete is sprayed, again using a concrete pump, over exterior walls  48  of shelter  20 . Again, the same lightweight concrete mix that is used for ceiling and floor ( 48  and  44 ) can be used for walls  46 . The concrete mix is fluid enough to allow it to be sprayed but not so fluid as to prevent it from adhering to the vertical walls  46  of shelter  20 . The spraying of the walls with concrete is shown in  FIG. 21 . A sufficient volume of concrete is provided to fully cover exposed wire grids  66 . Thereafter, walls  46  can be screeded to smoothen the concrete and eliminate any internal voids. If desired, additional steps can be taken to texturize the outer surface of shelter  20  for aesthetic purposes. The addition of other architectural finishes is within the scope of the present invention. Thereafter, the concrete is allowed to harden and cure. After the concrete hardens, the exterior surface of the shelter can be sealed or painted to any desired color.  
      Next, the interior of shelter  20  is completed. This is accomplished by nailing conventional wallboard  104 , such as Sheetroc®, to the previously installed ferring strips  102 . As noted above in conjunction with previous steps, the ferring strips  102  are inserted within the interior of the shelter (note  FIG. 10 ) and allow wallboard  104  to be secured to the walls and ceiling.  
      The resulting construction is a lightweight concrete shelter  20  that can be transported via flatbed truck  106  to its intended destination ( FIG. 25 ). Also, because the concrete is sprayed, the structure is monolithic and does not include any joints. Although some additional reinforcement is provided along the edges, the majority of the concrete is reinforced by the exposed wire grids  66  of building elements  52 . The concrete reinforced by the exterior wire grids  66  constitutes the primary load bearing element of the resulting construction.  
      Also, the inventive construction method allows window and door frames, such as  24 , to be formed within the building elements  52  prior to concrete being applied. One of the advantages of the present method is that, because it is not a molding process, it can be used to construct a shelter of any desired size or configuration. For instance, although larger shelters can be constructed as noted in  FIGS. 4-7 , the method can just as easily be adapted to construct smaller shelters  112  as noted in  FIGS. 27 -28 . Shelter  112  is approximately eight feet by eight feet in size. Furthermore, in the present invention, because the walls, ceiling, and floor ( 46 ,  48 , and  44 ) are not poured in place, as with convention designs, the shelters can be assembled off-site in relatively tight surroundings, such as in a warehouse. Finally, the entire method can be carried out in a time frame of about three to five days with a minimum of labor.  
      Other alternative constructions can also be achieved using the construction method of the present invention. For example, as noted in  FIG. 26 , the method can be employed in constructing a floorless embodiment  114 . This embodiment finds application in situations where a pre-formed foundation is made at the intended destination and the foundation will also serve as the floor of the shelter. In this embodiment, no building element is needed to form the floor and no floor form is utilized. Instead, building elements  52  are used to form just the walls  46  and ceiling  48 . Forms are used for the upper edges and corners ( 96  and  98 ) and concrete is applied as noted in the above described method. In this embodiment, the upper peripheral edge  116  of shelter  114  acts as the primary load bearing element. Also, the upper edges  116  will be formed with lifting anchors, similar to the lifting anchors described above. This floorless embodiment can be use in “capping” the existing electrical components associated with a cell phone tower. Here, a floor can be poured around the existing equipment prior to being “capped” with shelter  114 , or the equipment can be capped first with the floor being subsequently poured.  
      Lightweight Concrete Mix  
      The preferred concrete mix for constructing the shelter is as follows:  
                                               Quantity   Percent By Weight   Description                                                188   lbs.   34.62%   Cement       75   lbs.   13.81%   Lime       14   lbs.    2.57%   Perlite       160   lbs.   29.47%   Aggregate       105   lbs.   19.34%   Water       12   oz.    .14%   Plasticizer       4   oz.    .05%   Retarder                  
 
      The first ingredient is cement. In the preferred embodiment, a Type I Portland cement is employed. However, other types of cements can be readily employed in the mix. Lime is also added to the mix to improve the adhesion of the final mix. Namely, the lime enables the sprayed concrete to stick and adhere to vertical surfaces. Perlite is the next ingredient and it is added to improve the smoothness and pumpability of the final mix and reduce weight.  
      Aggregate is the next component. The preferred aggregate is a coal based beneficiated aggregate. However, the use of other types of beneficiated aggregates is within the scope of the present invention. For example, the invention can be used in conjunction with aluminum, shale or slate based aggregates. Beneficiated aggregates are aggregates that have undergone any number of known treatment steps to concentrate valuable constituents.  
      Again, in the preferred embodiment a coal based beneficiated aggregate is used in the mix. It is also preferred that the beneficial aggregate is a “fine” with particle sizes of between ¼ inch and 200 mesh screen. As will be appreciated by those skilled in the art, “mesh screen” is a reference to a filter having 200 openings per inch. Thus, using a 200 mesh screen is one way to select fines of a suitable size. By using fines of this size, the resulting concrete has less porosity and less capacity to absorb water. This, in turn, means the resulting concrete is lighter without sacrificing any strength. As is typical in concrete mixes, a volume of water is also added to the mix.  
      The plasticizer is preferably “ADVA® 100 Superplasticizer” manufactured by Grace Construction Products. ADVA® 100 is a water-reducing admixture that produces a low water/cement ratio and a high strength concrete. At the same time, ADVA® 100 also promotes an extremely flowable concrete that provides superior workability and pumpability. As those skilled in the art will undoubtedly appreciate, the use of other plasticizers is within the scope of the present invention.  
      The final ingredient is a retarder. The preferred retarder is Daratard®  17 , which is also manufactured by Grace Construction Products. The retarder delays the setting time of the concrete to allow it to be pumped and sprayed to its intended location. Again, those skilled in the art will undoubtedly appreciate that the use of other retarders is within the scope of the present invention.  
      This concrete mix produces concrete that, when hardened, weights between 100-110 pounds per cubic foot (lbs/ft 3 ). Despite this light weight, the concrete is also strong and can withstand pressures of between 4,000-6,000 pounds per square inch (psi). The above referenced mix most commonly produces concrete that can withstand 5,000 psi. Although the chart above illustrates preferred weights and percentages, beneficial characteristics can still be achieved using other weights and percentages as well.  
      When applied to building elements  52  described above, the wire grids on the exterior surface  66  act to reinforce the sprayed on concrete. The resulting structure is lightweight. Many of the structures built in accordance with the method can be picked up by relatively small 70 ton cranes. At the same time, the structures are very strong. Shelters constructed in accordance with the present invention adhere to ballistics standard UL 752 from United Laboratories and meet seismic zone 4/D requirements.  
      The present disclosure includes that contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention.  
      Now that the invention has been described,