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FIELD OF THE INVENTION 
   The present invention refers to the construction of pools and, more specifically, to pools made up of metallic modules of standardized dimensions. 
   BACKGROUND OF THE STATE OF THE ART 
   The growing popularity of pools for recreational, therapeutic and domestic use has resulted in the creation of a plurality of types and models, intended to meet the market&#39;s large variety of expectations. Among others, the most widely known are the following:
         concrete pools, lined with tiles, miniature tiles or vinyl linings;   fiberglass pools, manufactured according to standard dimensions and shapes;   mixed-type pools, with a concrete base (bottom) and walls made of blocks, clay bricks or metallic sheets, usually waterproofed with vinyl lining or fiberglass skin.       

   However, constructing pools of the above mentioned types is a relatively complex, slow and expensive process, since, in addition to requiring specialized labor, they have disadvantages inherent to their nature. 
   In fact, it is known that concrete structures require the manufacture of molds that, once used, are disposed of, resulting in a substantial waste of material. 
   Fiberglass pools, although not having this shortcoming, require digging a hole in the ground with the proper dimensions, as well as the provision of a concrete support bottom. 
   Additionally, neither concrete nor fiberglass pools can be moved to another location, nor can they have their dimensions altered, leaving no choice for their owners but to live with the original dimensions forever. For instance, in a pool built for small children, it becomes impossible to increase its depth when these children grow up. 
   Conventional pools have other shortcomings, such as the need for special techniques to install underwater lighting (which must be planned before the construction begins), as well as the impossibility of altering the number or positions of these lighting fixtures after the construction is finished. 
   The above-mentioned inconveniences have led to the search for solutions based on modular techniques, in order to result in more accessible costs, as well as to reduce assembly time and to facilitate said assembly work. This trend is exemplified by patent documents U.S. Pat. No. 3,798,857 to Barrera (hereinafter “Barrera”), U.S. Pat. No. 3,820,174 to Rozanski (hereinafter “Rozanski”), U.S. Pat. No. 4,047,340 to Witte et al (hereinafter Witte) and DE 1264031 to Dr. Theodor Kootz (hereinafter “Kootz”). 
   The inventions described in the above-mentioned documents, however, have shortcomings that limit their usefulness, as discussed below. Barrera discloses a pool whose walls consist of modules made of steel sheets, equipped with coupling means between the vertical edges of adjacent modules, whose assembly results in the pool&#39;s side walls, as shown in  FIG. 1 . Said coupling means comprise rectangular slots into which tabs are inserted, the retention between modules being provided by locking pins. The shapes of these tabs require expensive manufacturing processes. Moreover, as shown in  FIG. 1 , the pool has to be placed inside a hole dug into the ground. 
   In addition, the modules of Barrera do not apply to the bottom of the pool, which is made of concrete and requires specialized as well as costly labor, which is also needed to manufacture the concrete blocks that provide support to the walls&#39; anchor beams. 
   Rozanksi discloses a pool whose walls are made of steel sheet modules, complemented by a three-dimensional lattice structure as shown in  FIGS. 2 and 3 . The object of this is to provide a supporting structure for the ladder&#39;s handrails, as well as supporting a concrete deck or pavement surrounding the edge of the pool. As in the previous example, the bottom of the pool requires specialized labor, which is also necessary to lay the concrete pavement. These operations are time consuming, due to the time needed for the concrete to harden. 
   Witte discloses a pool with walls made up of modular plate-shaped elements that have, in their vertical edges, grove and tongue joints. The horizontal forces are supported by X-shaped prefabricated elements, as shown in  FIGS. 4 and 5 . As shown in  FIG. 5 , a concrete bottom  90  must be provided to support the wall modules  32  as well as the internal edge of the deck  20  that surrounds the pool. The external edges of said deck rests upon one of the arms  50  of the X-shaped elements, whose bottom arm  50   d  rests on a metallic footing  80  that is secured to the ground or according Witte—a concrete base, not shown in the drawing. Said base is necessary due to the fact that the thrust resulting from the water&#39;s pressure upon the walls is also unloaded on this footing. In the subject matter of this patent, the same considerations regarding the delay in the construction time are also applicable. 
   Kootz teaches a swimming pool having its bottom, as well as its sides, formed of metallic tray-like modules that are bolted together. The pool has two parts with different depths: in the shallower part, the walls are composed of a single row of panels, whereas in the deeper portion, the walls are higher, being formed of two superposed rows of panels. In the bottom of the pool, the tray-like panels are placed with their flanges facing up (i.e., the inside of the pool). This configuration is necessary due to the fact that said panels must be bolted together to form the bottom. However, to attain a uniform bottom surface, the trays must be filled with concrete covered with a fiberglass layer. Therefore, the pool cannot be disassembled, as the bolts which join said bottom panels are be encased in concrete. Moreover, said bottom panels have to be laid on a leveled surface, preferably, one that has been compacted or overlaid with a layer of concrete. Additionally, the horizontal thrust upon the side panels, due to the water pressure, may result in the outward bending of the walls of said deeper portion, mainly along the joints between the upper and lower rows of panels. This sets a limit to the number of panels that can be superposed to increase the height of said side walls and, therefore, the pool&#39;s depth. 
   The above-mentioned examples of the state of the art suffer from serious shortcomings due to the possibility of structural damages due non-uniform resistance from the ground on which the pool lies. Such is particularly the case with the objects of Barrera and Witte. 
   OBJECTS OF THE INVENTION 
   In view of the above, a first object of the invention is to provide a modular pool that is not affected by irregularities of the soil&#39;s compression resistance. 
   Another object is to provide a modular pool adapted to be easily and quickly assembled, without recourse to specialized labor. 
   Another object is to provide a modular pool adapted to be easily assembled and disassembled. 
   Yet another object is to provide a modular pool whose construction does not require the use of concrete walls or bottom, blocks or bricks. 
   Yet another object is to provide a modular pool that allows the inclusion of a deck. 
   Another additional object is to provide a modular pool that can be easily changed in dimensions and shape. 
   Another object is to provide a modular pool adapted to be assembled either below or above ground level. 
   SUMMARY OF THE INVENTION 
   The above-mentioned objects, as well as others, are attained by the present invention through a modular pool in which the bottom edges of the metallic modules that form the walls are attached by semi-permanent attaching means to a latticed base structure composed of a plurality of metallic sleepers placed crosswise at right angles to a plurality of parallel metallic beams running lengthwise, said metallic sleepers and said beams being “U” section shaped with the central portion being vertically oriented. 
   In accordance with an additional feature of the invention, the metallic modules that form the walls are made from sheet metal comprising a rectangular shaped center portion provided with flanges along the vertical and horizontal edges of said central portion. 
   According to another feature of the invention, said flanges are bent at a right angle relation to said central portion. 
   In accordance with an additional feature of the invention, said semi-permanent attaching means comprise angle irons having their vertical flange attached to the vertical central portion of said sleepers and beams, and their horizontal flange attached to the bottom flanges of the metallic modules that form the bottom tier of the pool&#39;s side walls. 
   According to another feature of the invention, the bottom of the pool comprises a plurality of modular bottom panels placed crosswise to the above-mentioned sleepers with their end portions being supported by the horizontal upper flange of said sleepers. 
   According to another feature of the invention, said modular bottom panels comprise modules provided with bottom draining openings. 
   According to yet another feature of the invention, said sleepers, beams and modular wall panels as well as said angle irons are joined by nuts and bolts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the invention will be better understood through the description of a preferred embodiment and the accompanying drawings, in which: 
       FIGS. 1 ,  2 ,  3 ,  4  and  5  show pools built according to the previous art. 
       FIG. 6  is a general perspective view of the pool of the invention. 
       FIGS. 7 ,  8 ,  9  and  10  show side views of various combinations of modular side wall panels, providing different pool depths. 
       FIG. 11  shows a typical wall panel before folding of the flanges. 
       FIG. 12  shows the wall panel of the previous drawing, with the flanges folded and strengthening it in order to resist the water pressure. 
       FIG. 13  shows, by means of a top view, a corner where the side walls meet at an angle different from 90°. 
       FIG. 14  shows an exploded view of the assembly of the side walls of a pool. 
       FIG. 15  shows, by means of a partially exploded view, the assembly of sleepers and beams that comprise the latticed base structure of the pool. 
       FIG. 16  shows, by means of a perspective view, part of the assembled base structure of the pool and the positions of the angle irons, as well as the panels that form the bottom of the pool. 
       FIG. 17  shows by means of a cross-section view, the joint formed by the side panels of the pool, an angle iron and a sleeper. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 6  which shows a pool  10  built according to the invention, it can be seen that said pool comprises two side walls  11  and  12 , two head walls  13  and as well as a bottom  15 , all these parts consisting of constructive modular panels, as detailed below. 
   Considering that the exemplary embodiment shown in  FIG. 6  has a standard depth of 1 meter, said side walls an said end walls are formed by three tiers of panels, all having the same standard length, such as, for example, one meter. Upper panels  17  have a useful height of 500 millimeters, intermediate panels  18  are 300 millimeters high and bottom panels  19  are 200 millimeters high. In order to leave a clearance of 130 millimeters between the water surface and the pool&#39;s upper edge, panels  17  are 630 millimeters high overall. 
   Still according to  FIG. 6 , the pool&#39;s bottom surface is made up by panels  16 , hereinafter called “tiles”, which completely line the bottom&#39;s surface and are supported by the latticed base (not shown in this drawing). 
   Additionally, all said pools component parts have dimensions that allow them to be loaded in pick-ups or small trucks for low cost transportation. So, in the exemplary embodiment herein described, the largest parts are the beams that comprise the lengthwise beams, which are only 2 meters long. This allows them to be transported in building&#39;s elevators, substantially reducing vertical transportation costs for pools assembled in penthouses. 
     FIGS. 7 to 10  depict some exemplary arrangements of side panels of different heights, showing how the invention allows the construction of pools with various depths.  FIG. 7  shows how a depth of 1 meter is attained by the superposition of panel  17 , with a useful height of 500 mm, panel  18  with a height of 300 mm and panel  19  which is 200 mm high. In  FIG. 8 , a depth of 1.2 meters is attained by superposing a 500 mm panel  17 , a 400 mm panel  21  and a 300 mm panel  18 . A depth of 1.3 meters results from the superposition of one panel  17 , two 300 mm panels  18  and at the bottom one 200 mm panel  19 , as shown in  FIG. 9 . Finally,  FIG. 10  exemplifies a 1.5 meter depth attained by overlaying one panel  17 , one panel  21  and three panels  19 , having heights of 500 mm, 400 mm and 200 mm, respectively. 
   As a general rule, the larger panels should be placed closer to the surface, progressively narrower panels being used at greater depths, so that the panels having smaller height (such as panels  19 ) are placed next to the floor. It is also noted that in the present exemplary embodiment panel  17  has a height greater than 500 mm, the excess  24  corresponding to the clearance between the water surface  23  and the top  22  of the pool&#39;s side walls. 
     FIG. 11  shows how a panel  30  is formed from a rectangular metallic sheet. As shown, this sheet comprises a rectangular central portion  31  whose sides are contiguous with stripes  32 ,  33 ,  34 ,  35 , the boundaries between said central portion and said stripes being the folding lines  31   a ,  31   b ,  31   c ,  31   d . Said stripes have the same width and through-holes  36  placed at standardized positions, and are folded in the directions shown by arrows  37 . 
     FIG. 12  depicts the same panel after folding along said lines  31   a ,  31   b ,  31   c ,  31   d , where it can be seen that the horizontal and vertical stripes act as flanges which absorb the bending stresses due to the water pressure  38  acting upon the central portion  31 . Additionally, the through-holes in said stripes are used to connect adjacent panels to form the pool&#39;s side walls. 
   The general layout of the elements that form part of the wall as well as a rectangular corner are shown in  FIG. 14 . The first side wall comprises sets  40 ,  41  and  42 , each one being formed by the superposition of modules  17 ,  18  and  19 . As shown in this drawing, said vertically adjacent modules are joined by means of bolts  44  and nuts  45 . The same nut-bolt elements are used to join the vertical flanges of said sets. 
   The drawing in  FIG. 14  is exploded horizontally to show a vertical member  65  which is interposed and bolted between adjacent assemblies  40  and  41  (formed by panels  17 ,  18  and  19  vertically joined). This member  65  consists of a plate that may have the same width as the vertical flanges of said modular panels, and its height encompasses the total height of said assemblies. Said plate acts as a reinforcing member that resists the outward stresses acting upon the side walls, which tend to push outwardly central panels  18 . Lengthwise reinforcement at the top of the pool&#39;s wall is provided by a metal plate  64  whose through-holes  36 ′ are coincident with the through-holes of the upper flanges of the upper panels. Said metal plate  64  may be used to support a deck floor along the walls. 
     FIG. 14  also depicts a right-angle joint between two side walls of the pool. As shown, the vertical flanges of the identical modules at the free ends of assemblies  42  and  43  are bolted to the flanges of an angle iron  46 , which is provided with through-holes in positions coincident with the holes of said flanges. 
   It should be stressed that the pool&#39;s layout is not limited to right angles α=90° as shown in  FIG. 14 . For instance, an angle of α=120° for hexagonal shaped pools is shown in  FIG. 13 . In this case, side flanges  35 ′ adjacent to said angle iron  46  are folded at angles of 75° in relation to the central part of panels  17 ′. The corresponding upper and lower flanges of the panels  17 ′ have also been cut in accordance with this angle. 
   The floor of the pool comprises a supporting structure upon which the closing panels or “tiles” are placed. As depicted in  FIG. 15 , said base structure consists of a rectangular lattice comprising a plurality of parallel metallic U-shaped beams  51  having metallic U-shaped sleepers  56  placed crosswise between them. In a preferred embodiment, said beams and said sleepers may have the same height. As shown, beams  51  are composed of several modular elements  52  (in the central part of the beams) and  53  (at the ends of the beams) butt-joined with side-plates  54 . Sleepers  56  are attached to said beams by means of L-shaped plates  55 . Standardized bolts and nuts (not shown) of the same type of the ones used for side wall assembly are used throughout. 
     FIG. 16  depicts part of the assembled pool&#39;s base structure, forming a rectangular lattice or grid with module m. This grid supports the pool&#39;s floor tiles  57 ,  58 , whose ends rest upon the upper flanges of sleepers  56 . As shown in the drawing, said tiles are not bolted to their supporting members, and comprise flanges  57   a  along the greater part of their long sides, said flanges providing the necessary rigidity to resist the vertical thrust due to the water pressure upon the bottom. In addition to the regular tiles  57 , special tiles are provided for various specific functions, such as tile  58  that has a central opening  59  to receive the bottom&#39;s draining valve assembly. 
     FIG. 16  also shows the angle irons  61  that comprise the semi-permanent attaching means between the side walls and the base structure. Said angle irons have through-holes  62   a  on their vertical flanges, placed in coincident positions with holes  62   b  on the sleepers as well as on the beams, to which they are attached by means of bolts  44  and nuts  45 . The horizontal flanges of said angle irons are provided with through holes  63 , which are coincident with holes  36  on the lower flange of the wall panels. According to the cross-section view of  FIG. 17 , the bottom flanges of the panels of the lower tier  19  of the side walls are bolted to the horizontal flanges of said angle irons  61 , which have their vertical flanges bolted to the perimeter beams such as sleepers  56  of the base structure, allowing the horizontal stresses upon said side walls to be unloaded on the base structure. 
   Although the preceding description refers to swimming pools, the invention has a wider range of applications such as iced water reservoirs for air conditioning systems retrofitted into existing buildings. In this case, the reservoir can be placed over existing floors, such as in garages or courtyards, thermal insulation being provided by polyurethane or polystyrene sheets inserted between the walls and bottom and the inner vinyl lining of the reservoir. 
   Therefore, the object above described may be modified within the conceptual limits of the invention, being only limited by the following set of claims.

Summary:
Modular pool constructive design whose walls are constituted by metallic panels ( 17, 18, 19 ), made up by folding metallic sheets, comprising a bottom including a structure that supports a plurality of metallic panels-tiles ( 16 ) —said walls ( 11, 12, 13, 14 ) are connected to said bottom&#39;s structure, making up a unique and non-deformable structure, all the pool&#39;s elements are interlinked by semi-permanent connecting means, such as screws and nuts. The dimensions of the pool&#39;s elements allow its easy transport in small vehicles or buildings&#39; elevators.