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FIELD OF INVENTION 
     The invention relates to movable structures generally, and particularly to doors for such structures. 
     BACKGROUND 
     Many types of temporary, movable structures are known in the art. One kind of movable structure comprises a plurality of spaced apart frame members, such as arches, and a membrane stretched over the frame members to create an interior space underneath. 
     Such structures can have many different types of doors to allow access to the interior space. One type of door known in the art is a so-called “clamshell” door, in which a rounded end of the structure is rotated upward, providing access to the interior space of the structure. Clamshell doors can be implemented by providing a frame member that forms a rounded surface at one end (or both ends) of the structure. The frame member is movable between a closed position, in which the rounded surface closes off an opening to the structure, and an open position, in which the frame member is moved upwardly and either collapses into a compressed position adjacent to one of the plurality of arches, or is positioned above the structure. In either case, in the open position the rounded surface created by the frame member does not close off the opening to the structure. To complete the clamshell door, the membrane of the structure is extended over the rounded surface formed by the frame member when the frame member is in the closed position. 
     One problem with known clamshell doors for movable structures is that the membrane covering the clamshell door is untensioned, and allowed to drape or baffle when the door is opened. Slack in the membrane is a problem because it makes the membrane much more susceptible to damage. This problem is magnified in structures deployed in hot climates, where the clamshell door is often kept in the open position, with a slack membrane, for extended periods of time. Under such conditions, the useful life expectancy of the membrane is greatly reduced, often by as much as 90%. This problem also is prevalent in cold climates, where the slack membrane is susceptible to cold cracking. 
     SUMMARY OF THE INVENTION 
     The present invention provides a clamshell-type door for a temporary, movable structure having multiple modules movable between open and closed positions. Each module has its own membrane, separate from the membranes of the structure, which is stretched taught over the surface of the module. In the open position, the modules are nested one under another in an essentially vertical position, uncovering the entire opening in the end of the structure. In this way, the membranes of both the structure and the modules are always under tension; the membranes are never slack and more susceptible to damage. 
     In one embodiment of the invention, there is provided a door system for a movable structure, the movable structure comprising a plurality of spaced apart frame members and at least one tensioned structure membrane extending between at least two of the spaced apart frame members and forming an interior space under the spaced apart frame members, the interior space having at least one open end. The door system for the movable structure comprises first and second axles arranged coaxially and attached to the structure at first and second sides of the open end of the interior space, a first door module, and a second door module. The first door module comprises a first frame extending from a first end of the first frame hingedly attached to the first axle, to a second end of the first frame hingedly attached to the second axle, and a first tensioned door membrane supported by the first frame. The second door module comprises a second frame extending from a first end of the second frame hingedly attached to the first axle adjacent to the first end of the first frame, to a second end of the second frame hingedly attached to the second axle adjacent to the second end of the first frame, and a second tensioned door membrane supported by the second frame. The first and second door modules are rotatable about the first and second axles between a closed position in which the first door module extends across an upper portion of the open end and the second door module extends across a lower portion of the open end, and an open position in which the open end of the interior space is substantially unobstructed by the first and second door modules. 
     In one embodiment, the door system further comprises a system for rotating the first and second door modules about the first and second axes. 
     In one embodiment, the system for rotating the first and second door modules comprises at least one cable attached at one end to the second door module, and at the other end to a winch for selectively extending or retracting the cable and raising or lowering the second door module. 
     In one embodiment, the door system further comprises a cable extending from a first cable end attached to the first door module to a second cable end attached to one of the plurality of spaced apart frame members. The cable is adapted to hold the first door module in its closed position extending across the upper portion of the open end of the interior space. 
     In one embodiment, the first and second door modules are adapted to make contact with each other when the second door module is being rotated from the closed position to the open position, to move the first door module from the closed position to the open position. 
     In one embodiment, a plurality of arms extend from the first door module, the plurality of arms being adapted to make contact with the second door module when the second door module is being rotated from the closed position to the open position, to move the first door module from the closed position to the open position. 
     In one embodiment, the first and second door modules are rotatable about the first and second axles to a partially open position wherein the second door module extends across the upper portion of the open end, and the lower portion of the open end is substantially unobstructed by the second door module. 
     In one embodiment, the first and second frames of the first and second door modules each comprise a plurality of arch members extending from the first end of the frame to the second end of the frame. The plurality of arch members are spaced apart between the first and second ends, and the first and second frames each further comprise a plurality of spreader members extending between the plurality of arch members. 
     In one embodiment, the first and second frames further comprise a plurality of cross spreader members extending substantially diagonally between connections points between the arch members and the spreader members. The plurality of cross spreader members provides bracing between the spreader members. 
     In one embodiment, the system further comprises a locking mechanism for holding the first and second door modules in the closed position. In one embodiment, the locking mechanism comprises an articulated arm pivotally mounted to the second door module, the articulated arm being adapted to make contact with an attachment point on a surface on which the movable structure is erected, for holding the second door module in the closed position. 
     In one embodiment, the system further comprises a pair of catches, one of said pair of catches provided on each of the first door module and second door module, the catches adapted to interlock with one another and hold the first door module in the closed position when the second door module is in the closed position. 
     In another embodiment of the invention, there is provided a kit of parts for a door system for a movable structure. The kit comprises a first frame for supporting a membrane, the first frame extending from a first end of the first frame to a second end of the first frame. The kit also comprises a second frame for supporting a membrane, the second frame extending from a first end of the second frame to a second end of the second frame, wherein a distance from the first end of the second frame to the second end of the second frame is less than a distance from the first end of the first frame to the second end of the first frame. The kit further comprises first and second membranes to be tensioned and supported by the first and second frames, respectively. The kit further comprises first and second axles for hingedly supporting the first and second ends, respectively of the first and second frames. 
     In one embodiment, the kit further includes at least one lifting cable and winch, the lifting cable for attachment at one end to the second frame and an opposite end to the winch, the winch for selectively extending or retracting the lifting cable to raise or lower the second frame. 
     In one embodiment, the kit further includes at least one cable for attachment at one end to the first frame and at an opposite end of the cable to the movable structure. 
     In one embodiment, the first frame of the kit comprises an arm adapted to contact the second frame while the second frame is in motion, to move the first frame. 
     In one embodiment, the first and second frames of the kit each comprise a plurality of arch members extending from the first end of the frame to the second end of the frame, the plurality of arch members being spaced apart between the first and second ends, the first and second frames each further comprising a plurality of crossbar members extending between the plurality of arch members. 
     In one embodiment, the first and second frames of the kit each further comprise a plurality of cross spreader members extending substantially diagonally between connection points between the arch members and the spreader members. The plurality of cross spreader members provides bracing between the spreader members. 
     In one embodiment the second frame comprises an articulated arm pivotally mounted thereon, the articulated arm being adapted to make contact with an attachment point, for holding the second frame adjacent to the attachment point. 
     In one embodiment, the first and second frames each comprise one of a pair of catches adapted to interlock with one another and hold the first and second frames adjacent to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a movable structure having a door system according to the present invention. 
         FIG. 2  is a side view of the movable structure of  FIG. 1 . 
         FIG. 3  is a side view of an arch used in the construction of the movable structure of  FIG. 1 . 
         FIG. 4  is a cross-sectional view taken along the line IV-IV in  FIG. 3 . 
         FIG. 5  is a cross-sectional view taken along the line V-V in  FIG. 3 . 
         FIG. 6  is a perspective view of the door system shown in  FIG. 1 . 
         FIG. 7  is a side view of the door system shown in  FIG. 1 . 
         FIG. 8  is an end view of the door system shown in  FIG. 1 . 
         FIG. 9  is a perspective view of a door footing of the door system shown in  FIG. 6 . 
         FIG. 9A  is a perspective view of a door footing base plate of the door system shown in  FIG. 6 . 
         FIG. 9B  is a perspective view of a door mounting bracket of the door system shown in  FIG. 6 . 
         FIG. 9C  is a side view of the door mounting bracket shown in  FIG. 9B . 
         FIG. 10  is a perspective view of a canopy footing of the door system shown in  FIG. 6 . 
         FIG. 11  is a perspective view of the area of  FIG. 8  marked XI. 
         FIG. 12  is a cross-section view taken along the line XII-XII in  FIG. 8 . 
         FIG. 13  is a perspective view of an alternative embodiment of the door system of the present invention. 
         FIG. 14  is a perspective view of a winch and cable system for use with the door system of  FIG. 8 . 
         FIG. 15  is a perspective view of a stopping flange for use with the door system of  FIG. 8 . 
         FIG. 16  is a side sectional view of a pair of interlocking safety catches for use with the door system of the present invention. 
         FIG. 17A  is a perspective view of a locking mechanism for use with the door system of the present invention. 
         FIG. 17B  is a side view of the locking mechanism of  FIG. 17A , in an unlocked configuration. 
         FIG. 17C  is a side view of the locking mechanism of  FIG. 17A , in a locked configuration. 
         FIG. 18A  is a plan view of an alternative locking mechanism for use with the door system of the present invention. 
         FIG. 18B  is a side view of the alternative locking mechanism shown in  FIG. 18A . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Example embodiments of the invention will now be described with reference to the figures.  FIG. 1  depicts a movable structure  100  having a foldable door system  200  in accordance with the present invention.  FIG. 2  depicts a side view of movable structure  100 . The structure  100  comprises a series of arches  110  spaced apart along the length of the structure  100 , acting as a frame for the structure  100 . A plurality of membranes  150  (shown in  FIG. 1  but not in  FIG. 2  extend between the arches  110 , forming an interior space within the structure  100 . The membranes  150  extending between arches  110  also form open ends  160 ,  170  of the interior space under the membrane. In the illustrated embodiment, open end  160  is selectively closed off with the foldable door system  200  of the present invention. Open end  170  is closed off with an additional membrane (not shown). 
     Each of arches  110  extend from a first foot portion  112  to a peak  114  and back to a second foot portion  116 , as shown in  FIG. 3 . In the example shown, the arches  110  have a span W of 90 feet, and a height H of approximately 35 feet. 
     Each of the arches  110  includes a plurality of beams, some curved and others substantially straight. The beams (both curved and straight) used to construct the arches  110  are shown in cross-section in  FIG. 4 . The beams have an I-shaped cross section, and additional features that will be described in greater detail below. The curved beams are approximately 13.4 feet in length as measured along their outer edge, and rise approximately 4 feet (again, as measured along the outer edge) from the foot portions  112 ,  116  before curving inwardly toward the peak  114 . The straight beams extend from the end of the curved beams to the peak  114 . The distance from a curve transition point  118 , at which the curvature of the curved beam ends and the straight portion of the arches  110  begins, to the peak  114  is approximately 42.6 feet. 
     Other embodiments of the arches serving as a frame for the structure can have different dimensions and shapes than those described here. For example, the arches can be composed of a series of straight beams joined end to end at various angles to one another. For example, the arch can be formed by straight beams rising from the foot portions at an angle of 83 degrees (all angles relative to a floor of the structure), followed by beams extending therefrom at an angle of 57 degrees, followed by further beams extending at an angle of 34 degrees, followed by two final beams extending at an angle of 11 degrees and meeting at a peak. 
     As mentioned previously, the arches  110  have an I-shaped cross section, shown in  FIG. 4 . The arches  110  have a central web  120  with an integral first flange  122  at one end, and an integral second flange  124  at the other end. The flanges  122  and  124  have bifurcated ends that define rope chases  126  having rope chase openings  128 . For reasons that will be explained in greater detail below, rope chases  126  and rope chase openings  128  are adapted to receive and retain ropes that are integral with, or otherwise attached to the longitudinal ends of membrane  150 , as well as the longitudinal end of membrane  150  itself. 
     In the illustrated embodiment, the beams that comprise arches  110  are 5-inch by 10-inch extruded aluminum I-beams; however, it will be appreciated that other cross-sectional dimensions for the beams are possible. As well, aluminum is not the only suitable material for the production of the beams. 
     The second foot portion  116  from which the arches  110  extend is shown in  FIG. 4 . The second foot portion  116  is identical to the first foot portion  112 . The second foot portion  116  comprises a base plate  130  having bolt holes  132  formed therein, to allow the base plate  130  to be secured to the ground or other level surface using anchor bolts. The second foot portion  116  further comprises an attachment flange  134  formed integrally therewith and extending substantially perpendicularly from the base plate  130 . The base plate  130  can be made of aluminum or any other suitable material. In the illustrated embodiment, the base plate  130  is a 1-foot by 1-foot square with two corners absent. The base plate  130  is ⅝-inch thick, whereas the flange  134  is ⅜-inch thick. It will be understood, however, that other dimensions are possible. 
     Flange  134  has a pair of bolt holes extending therethrough. The bolt holes are adapted to line up with corresponding bolt holes formed in the web  120  of the arches  110 . In this manner, bolts  136  can be inserted through the bolt holes in both flange  134  and web  120 , securing the arch  110  to the second foot portion  116 . In the illustrated embodiment, a threaded bolt and nut combination are used to fasten the arch  110  to the second foot portion  116 , however other suitable fasteners can be used as well. 
     As seen in  FIG. 2 , structure  110  has a plurality of spreaders  138  extending perpendicularly between the arches  110 . Spreaders  138  provided additional structural stability to arches  110 , and serve to keep the membrane  150  in a stretched condition by urging adjacent arches  110  apart from each other. 
       FIG. 5  shows a cross sectional view of a portion of the roof of structure  100 , taken along the line V-V shown in  FIG. 3 , which shows the spreaders  138  in greater detail. Spreaders  138  are attached to arches  110  by means of spreader connectors  140 . Each spreader connector  140  includes an outwardly extending member  141 , a flange  142  perpendicular to the member  141  and a grapple portion  143 . 
     The spreaders  138  can comprise essentially any telescoping rod. In the illustrated embodiment, spreaders  138  include a hollow bar  144  having a square-shaped cross section and pi-shaped bars (pi-bars)  145  and  146 . The pi-bars  145  and  146  are slidable within the hollow bar  144 , and can extend outwardly from the hollow bar  144  to varying extents. The pi-bars  145  and  146  can be locked in place with locking mechanism  149 , or any other suitable means of locking the pi-bars  145 ,  146  in place. 
     The outwardly extending ends of the pi-bars  145  and  146  attach to members  141  of the spreader connectors  140  by means of connector assemblies  147  which in the illustrated embodiment include a ⅝-inch by 2 and ¼-inch bolt, a nut and washers; however alternative connector assemblies can have differently sized mating components. The spreader connectors  140  are in turn attached to the arches  110  by means of connector assemblies  148  which extend through apertures in the flanges  142  and the web  120 . The attaching means also includes channels in the second flange  124  that are sized to receive grapple portions  143 . 
     Membranes  150  extend between adjacent arches  110 , as shown in  FIG. 1 , to form the interior space of the structure  100 . Membranes  150  are elongate strips of a flexible, impermeable material, such as PVC-coated polyester scrim. The membranes  150  have beaded parallel longitudinal edges, adapted to be inserted into and held in place in the rope chases  126 . In the illustrated embodiment, the membranes  150  have ropes  152  integral with the beaded longitudinal edge of the membrane  150 . For reasons that will be explained below, ropes  152  extend outwardly from one end of the membrane  150  by approximately the same length of the membrane  150 . Thus, the combination of the beaded edge of membrane  150  and rope  152  is approximately twice as long as the membrane  150 . 
     Membranes  150  are attached to arches  110  by inserting the longitudinal beaded edge of the membrane  150  into the rope chases  126  of the first flange  122  on either side of the membrane  150 . Membranes  150  extend between the adjacent arches  110 , and are held in place by the rope chases  126 . Although  FIG. 4  shows only a single membrane  150  extending from each side of the arch  110 , it will be appreciated that a second membrane  150  can be attached to arch  110  by inserting the beaded edge into the rope chases  126  provided on the second flange  124 , as well as the first flange  122 . This creates a double-membrane wall having an insulating layer between the two membranes  150 . Insulation material can be provided in the space between the two membranes  150 , if desired. 
     The Door System 
     The door system  200  is shown in greater detail in  FIGS. 6, 7, and 8 . The system  200  comprises a pair of door footings  210 , one at each side of the opening  160 . The system further comprises a first door module  230  and a second door module  250 , each extending from one door footing  210  to the other door footing  210 . The first and second door modules  230  and  250  extend between the door footings  210  such that the first door module  230  covers an upper portion of the opening  160 , and the second door module  250  covers a lower portion of the opening  160 . Together, the first and second door modules  230  and  250  cover substantially all of the opening  160 . The first and second door modules  230  and  250  are hingedly attached to the door footings  210 , such that they can be selectively rotated upwardly, uncovering the opening  160 . 
     The system  200  also comprises a canopy  270 , extending from one of a pair of canopy footings  271 , adjacent to the door footings  210 , to the other of the pair of canopy footings  271 . The canopy  270  is fixedly attached to the canopy footings  271 , and does not rotate. The canopy is attached to the structure  100 , via a membrane (not shown) extending from the canopy  270  to an arch  110  of the structure  100 . As will be described in greater detail below, the first and second door modules  230  and  250  rotate to a position beneath the canopy  270  when the door system  200  is opened. However, it will be understood that the canopy  270  is not an essential component of the door system  200 , and can be omitted. 
     The door footing  210  is shown in detail in  FIG. 9 . Door footing  210  comprises a base plate  212  having bolt holes  214  formed therein, to allow the base plate  212  to be secured to the ground or other level surface using anchor bolts. The base plate  212  can be made of steel or any other suitable material. In the illustrated embodiment, the base plate  212  comprises a rectangular main portion and a flared end portion, and has openings  213  formed in the interior to reduce weight and material costs. It will be understood, however, that other dimensions are possible, and that openings  213  can be omitted, if desired. 
     Additional features of the base plate  212  are shown in greater detail in  FIG. 9A . Base plate  212  has a plurality of axle mounts  216  for supporting an axle  218 . Axle mounts  216  each comprise a rectangular tube ( 216   a  in respect of the mounts  216  adjacent to either end of the base plate  212 , and  216   b  in respect of the central mount  216 ) welded to base plate  212 , supported by triangular flanges  216   c  welded to the base plate  212  and tubes  216   a ,  216   b . In the illustrated embodiment, the tubes  216   a ,  216   b  and the flanges  216   c  are steel. Tube  216   a  is 4″×4″×7⅞″, and tube  216   b  is 6″×4″×7⅞″. Triangles  216   c  are each 7″ tall and 5″ wide. 
     Tubes  216   a  and  216   b  include pipes  217   a  and  217   b  extending through openings formed in tubes  216   a  and  216   b . Bronze bushings (not shown) are provided in pipes  217   a  and  217   b  for receiving axle  218  and allowing axle  218  to rotate about its longitudinal axis within the bushings. Pipe  217   a  is a steel pipe 3½″ in diameter and 4½″ long. Pipe  217   b  is steel, 3½″ in diameter and 7¾″ long. 
     Axle  218  is a steel rod 2″ in diameter and 44″ long. Axle  218  has a tapered end to facilitate insertion of axle  218  into the bronze bushings of the axle mounts  216 . Axle  218  is supported by, and rotatable within axle mounts  216 , as shown in  FIG. 9 . 
     Door footing  210  further comprises a pair of mounting brackets  220  for hingedly mounting the door modules  230 ,  250  to the footing  210 . Mounting bracket  220  is shown in greater detail in  FIGS. 9B and 9C . Mounting bracket  220  comprises a generally triangular assembly  222  having three facets along one side. Assembly  222  comprises two spaced-apart steel triangular plates having the faceted triangular shape of the assembly  222 , joined by a series of steel rectangular plates  223  extending between the triangular plates and joined to the triangular plates by welding. 
     Mounting bracket  220  also includes attachment brackets  224  mounted to each facet of assembly  222 , for attaching door modules  230 ,  250  to the mounting bracket  220 . Attachment brackets  224  include a steel C-shaped bracket  225  attached to each facet of assembly  222  via any suitable means (in this case, with threaded nuts and bolts). A steel flange  226  is welded to the C-shaped bracket  225  and extends perpendicularly to the facet of assembly  222 . Bolt holes are formed in the steel flange  226  to facilitate the attachment of door module  230  or  250  to the flange, in the manner described below. Triangular flanges  227  are welded to both the flange  226  and the C-shaped bracket  225  to support the flange  226 . 
     Mounting bracket  220  further includes pipe  228  extending through openings formed in assembly  222  opposite the faceted side. Pipe  228  receives axle  218  when the door footing  210  is assembled as described below. Grease fitting  229  is provided adjacent to pipe  228  to allow for lubrication. 
     A fully assembled door footing  210  is shown in  FIG. 9 . The footing  210  is assembled by providing two mounting brackets  220 , one on either side of the central tube  216   b , and aligning pipe  228  of each mounting bracket  220  with pipes  217   a  and  217   b . Axle  218  is then inserted through pipes  217   a ,  217   b  and  228 . Grease is added as required through grease fitting  229 . Finally, an encoder pack  219   a  (described in greater detail below) is mounted to the door footing  210  via flanges  219  welded to tube  216   a  adjacent to one end of the footing  210 . 
     Canopy footing  271  is shown in detail in  FIG. 10 . Canopy footing  271  comprises canopy base plate  273 , a steel plate 34″ long and 12″ wide. Canopy base plate  273  has holes formed therein to allow the canopy base plate  273  to be secured to the ground or other level surface using anchor bolts. The canopy base plate  273  also has holes to facilitate attachment to base plate  212  using attachment flanges  221  (shown in  FIG. 9 ). Anchor bolts are inserted through the bolt holes in both the attachment flanges  221  and the canopy base plate  273  to attach the canopy base plate  273  to the base plate  212 . 
     Canopy footing  271  further comprises canopy flanges  275  and  277  extending from canopy base plate  273 . Flange  275  is a 7″×6″ steel plate that is welded to canopy base plate  273  and extends from canopy base plate  273  at an angle of 70°. Flange  277  is a 9″×8″ steel plate welded to canopy base plate  273  and extending perpendicularly therefrom. Flanges  275  and  277  each have bolt holes for facilitating the attachment of other components of the canopy  270 , as described in greater detail below. 
     Canopy footing  271  further comprises triangular support plate  278 , welded to the base plate  273  as well as flange  277 , to provide structural support for flange  277 . A similar triangular support plate (not shown) is provided to support flange  275 . 
     The first and second door modules  230  and  250  are shown in  FIGS. 6, 7, and 8 . The first door module  230  comprises arches  232 ,  234  and  236  that are constructed of curved and substantially straight lengths of I-beam that are identical to that from which arches  110  are constructed. In fact, arches  232 ,  234  and  236  are essentially identical to arches  110 , with the exception of dimensions. Arches  232 ,  234  and  236  have a width W and height H that are smaller than those of arches  110 , which enables the door module  230 , made therefrom, to fit inside the interior area of the structure  100  when the door system  200  is opened. In the illustrated embodiment, arches  232 ,  234  and  236  have a width W of 86′ 8″ and a height H of 31′ 8″. It will be understood, however, that different dimensions can be used, so long as the dimensions are smaller than those of arches  110 . 
     Door module  230  is formed by mounting the ends of arches  232 ,  234  and  236  to door footings  210  positioned at either side of the opening  160  to the structure  110 , as shown in  FIG. 11 . One end of each arch  232 ,  234  and  236  is attached to one of the flanges  226  of the mounting brackets  220  by aligning bolt holes in arches  232 ,  234  and  236  with the bolt holes formed in flanges  226 , and securely bolting arches  232 ,  234  and  236  to flanges  226 . 
     Stiffening braces  242  are provided between arches  232  and  234 , and between arches  234  and  236 , to strengthen the connection of the arches  232 ,  234  and  236  to the door footing  210 , and to help ensure that the arches  232 ,  234  and  236  extend from axle  218  at an angle of 17.75° relative to each other. Stiffening braces  242  each comprise a pair of metal plates bolted to arches  232 ,  234  or  236 , with a third metal plate extending therebetween to provide structural strength. The third metal plate is dimensioned such that the arches to which brace  242  is bolted are held apart at an angle of 17.75°. 
     The result of mounting arches  232 ,  234  and  236  to door footings  210  as described above is a roughly diamond-shaped protruding frame extending outwardly from the door footing  210  at one side of the opening  160 , to the door footings  210  at the opposite side of the opening  160 . The shape of the frame formed by arches  232 ,  234  and  236  is maintained by a plurality of spreaders  238 . In the illustrated embodiment, spreaders  238  are substantially identical to spreaders  138 , and are attached to arches  232 ,  234  and  236  in substantially the same way as spreaders  138  are attached to arches  110 ; however, it will be understood that essentially any telescopic rod, and any suitable means of attachment, can be used. 
     Membranes (not shown) are stretched between arches  232  and  234 , and arches  234  and  236  to form panels that serve as upper and lower portions of the door  200 . The membranes provided between arches  232 ,  234  and  236  can be essentially identical to membranes  150 , with the exception of the shape (the membranes for the door module  230  must be roughly diamond shaped, to correspond with the space between adjacent arches  232 ,  234  and  236 ). The membranes can be held in place in the same manner that membranes  150  are held in place (i.e., by way of a beaded longitudinal edge retained in C-shaped rope chases formed by the I-beams that form arches  232 ,  234  and  236 ). 
     The second door module  250  is essentially identical to first door module  230 , with the exception of the arms  306  and stopping flanges  308  (discussed in greater detail below) and the dimensions of the arches  252 ,  254  and  256  used to create the second door module. In particular, the width W and height H of arches  252 ,  254  and  256  are smaller than those of arches  232 ,  234  and  236 . In the illustrated embodiment, the width W of arches  252 ,  254  and  256  is 83′ 4″ and the height H is 29′ 9″. It will be understood, however, that different dimensions can be used, so long as the dimensions are smaller than those of the arches  232 ,  234  and  236 . 
     An alternative embodiment of the first and second door modules  280  and  290  is shown in  FIG. 13 . Door modules  280  and  290  are essentially identical to door modules  230  and  250 , except that door modules  280  and  290  also comprise a plurality of cables  284 ,  294 , in addition to spreaders  282 ,  292 . Cables  284 ,  294  extend diagonally between the connection points between the arches  232 ,  234 ,  236 ,  252 ,  254  or  256 , and the spreaders  282 ,  292 . Cables  284 ,  294  provide additional structural support for the door modules  280 ,  290  by acting as bracing between the spreaders  282 ,  292 . Cables  284 ,  294  can be essentially any cable strong enough to providing bracing between the spreaders  282 ,  292 . By way of example, cables  284 ,  294  can be steel cables ½″ in diameter. 
     When the door  200  is closed, the second door module  250  covers the lower portion of the opening  160 . Arch  256  rests on the ground or other surface on which structure  100  is deployed, and door module  250  extends upwardly over approximately half of the opening  160 . 
     First door module  230  is held in position covering the upper half of opening  160  by a pair of cables  240 . Cables  240  are attached at one end to arch  232 , at either side of the peak of the arch. Cables  240  are attached at the other end to one of the arches  110  of the structure  100 . The length of cables  240  is selected such that, when the cables  240  are taught the first door module  230  is positioned over the upper half of the opening  160 . Cables  240  can be essentially any type of cable capable of supporting the first door module  230  over the upper half of the opening  160 . In the illustrated embodiment, cables  240  are each ½″ steel cables. 
     The canopy  270  comprises an upright arch  272  extending substantially perpendicularly from canopy footing  271  at one side of the building, to another canopy footing  271  at the other side of the building. Canopy  270  also comprises an inclined arch  274 , extending from one canopy footing  271  to the other canopy footing  271 . Inclined arch  274  is at an angle of 20° with respect to upright arch  272 , although it will be understood that other angles are possible. 
     Both upright arch  272  and inclined arch  274  are made of the same curved and substantially straight I-beams as arches  110 , and are of substantially the same dimensions. Upright arch  272  and inclined arch  274  are secured to attachment flanges  277  and  275 , respectively, in the same manner that arches  110  are secured to attachment flanges  134  of the foot portions  112 ,  116 . 
     A plurality of spreaders  276  extend between the upright arch  272  and inclined arch  274 , providing structural support to the canopy  270 . Spreaders  276  may be identical to spreaders  138  and  238 , and may be attached to the upright arch  272  and inclined arch  274  in the same manner as spreaders  138  and  238  are attached to arches  110  and  232 ,  234  and  236 , respectively. However, it will be understood that essentially any telescopic rod can be used for spreader  276 , and can be secured to the upright arch  272  and inclined arch  274  in any suitable manner. 
     A membrane (not shown) extends between the upright arch  272  and the inclined arch  274 . The membrane may be identical to the membrane  150 , with the exception of its shape. It will be understood that, in order to extend across the space between upright arch  272  and inclined arch  274 , the membrane must be roughly diamond-shaped. The membrane can be secured to the upright arch  272  and inclined arch  274  in substantially the same manner that membrane  150  is secured to arches  110  (i.e., with beaded longitudinal edges inserted in and retained by C-shaped rope chases provided along the edges of the upright arch  272  and the inclined arch  274 ). 
     Erecting a Structure Having the Door System 
     The structure  100  having the door system  200  can be erected as described herein. First, the arches  110  are stood up and spaced apart, each of the foot portions  112 ,  116  of the arches  110  are attached to the I-beams comprising the arches  110 . The foot portions  112 ,  116  are freely shiftable along the surface beneath structure  110 , by use of rollers (not shown) temporarily positioned under the foot portions  112 ,  116 , or any other suitable means. 
     The spreaders  138  are attached to the arches  110  so that there are spreaders  138  extending between each pair of adjacent arches  110 . Next, as described subsequently, the membranes  150  are attached to the arches  110 . Once this is done, the membranes  150  are down stretched with winches, and to keep them in place before the next step, bolts are put through the beaded edges of membranes  150 . Next, each of the arches  110  are spread apart by extending the length of the spreaders  138 , so that spacing of the arches  110  is increased and membranes  150  are tautened. After that, temporary rollers are removed from under the foot portions  112  and  116 , and the foot portions  112 ,  116  are secured to the surface beneath structure  110 , for example, a concrete slab, to positionally fix the arches  110 . Arches  110  are fixed to the concrete slab (or other surface) using anchor bolts inserted through bolt holes  132  in base plate  130 , and into the concrete slab, as described above. 
     The membranes  150  are connected to the arches  110  as described herein. First, the rope chases  126  of the I-beams from which arches  110  are made are lubricated, in order to reduce friction for advancement of the beaded edge of membrane  150  through the rope chase  126 . Preferably, a dry silicone lubricant can be used, and this lubricant is sold in spray canisters. Such lubricant may be conveniently sprayed into the rope chases  126 . 
     In one embodiment, known rope advancing machines, electric or hand-operated, are installed at the ends of a pair of arches  110  opposite the end into which the membrane  150  will be fed. Rope  152  extending from each side of membrane  150  is positioned in the rope chases  126  along the length of the arches  110 . This can be done in an automated manner. 
     Membranes  150  are advanced from one end of arches  110  to the other by advancing the rope  152  using known rope advancing machines. As the membrane  150  advances into the rope chases  126  of the arches  110 , two workers stand at the base of the arches  110  where the membrane  150  enters into the rope chases  126 . The workers stand at opposite edges of the membrane  150  to ensure that the membrane properly advances into the rope chases  126 . 
     A worker is also located at each of the two rope advancing machines (one rope advancing machine per arch  110  of the pair of arches  110 ). These workers can control the operation of the rope advancing machines. For example, they can slow down the advancement of the membrane  150  if instructed to do so by one of the workers at the opposite base of the pair of arches  100 . 
     In one embodiment, two additional workers can be deployed at the apex of the structure  110 , to monitor the progress of the membrane  150  through the rope chases  126  from that position. One worker can be located at the peak of each arch  110  of the pair of arches  110 , and can advise as to whether advancement of the membrane  150  should slow down (or speed up) depending on the progress of the membrane  150  through the rope chases  126 . 
     The rope advancing machines are employed again when the building structure is demounted. In particular, the rope advancing machines advance the membrane  150  half way out. At this point, the remaining portion of the membrane  150  can be simply pulled out manually. 
     The foregoing steps can be used to erect the main body of the structure  110 , having an open end  160  at which the door  200  can be deployed. Once the main body is erected, upright arch  272  can be stood up and attached to canopy footings  271  by inserting bolts through bolt holes in attachment flange  277  and corresponding bolt holes formed in upright arch  272 . Canopy footings  271  and upright arch  272  are then placed adjacent to the last arch  110  in the row of arches  110  of the structure  100 . Canopy footings  271  are freely shiftable along the surface beneath the structure  110 , by use of rollers (not shown) temporarily positioned under the canopy footings  271 , or any other suitable means. 
     Spreaders  128  are then attached to arch  110  at one end, and the upright arch  272  of the canopy  270  at the other end. A membrane  150  is then attached to the arch  110  and upright arch  272  in the same manner that other membranes  150  are installed. Membrane  150  is then stretched down with winches, and bolts are put through beaded edges of membrane  150  to hold membrane  150  in place. Upright arch  272  and canopy footings  271  are then moved away from arch  110  by extending the length of spreaders  138 , so that the membrane  150  becomes taught. Then, temporary rollers are removed from under the canopy footings  271  and the footings  271  are secured to the surface beneath the structure  100  using anchor bolts extending through bolt holes  214 . 
     Next, inclined arch  274  is erected adjacent to upright arch  272 , and a membrane similar to membrane  150  but roughly diamond shaped is installed between upright arch  272  and inclined arch  274  in substantially the same manner that membranes  150  are installed. Upright arch  272  and inclined arch  274  must be very close together to allow the narrowest portion of the diamond shaped membrane to be pulled completely over the arches  272  and  274  during installation. To allow this to occur, the diamond shaped membrane is installed before spreaders  276  are attached to the arches  272  and  274 , and before the inclined arch  274  is bolted to the canopy footings  271 . 
     Once the diamond shaped membrane is installed between arches  272  and  274 , the inclined arch  274  is fastened to canopy footings  271  by inserting bolts through bolt holes in attachment flange  275  and corresponding bolt holes formed in inclined arch  274 . Spreaders  276  are then installed extending between upright arch  272  and inclined arch  274 . Spreaders  276  are extended to tension the diamond shaped membrane. 
     Next, the first door module  230  is assembled and installed as follows. First, door footings  210  are placed adjacent to canopy footings  271  and attached to canopy footings  271  as described above. Next, arches  232 ,  234  and  236  are attached to mounting brackets  220  as described above. One of the two bolts attaching C-shaped brackets  225  to assembly  222  is removed, allowing C-shaped brackets  225 , flanges  226  and arches  232 ,  234  and  236  to pivot about the remaining bolt attaching C-shaped brackets  225  to assembly  222 . Arches  232  and  234  are then pivoted to within close proximity of each other, allowing a diamond shaped membrane to be installed between arches  232  and  234  in the same manner that a diamond shaped membrane is installed between arches  272  and  274  (described above). Similarly, a diamond shaped membrane is installed between arches  234  and  236 . 
     Once diamond shaped membranes are installed, arches  232 ,  234  and  236  are pivoted away from each other, and spreaders  238  are installed and extended between arches  232 ,  234  and  236 , tensioning the membranes. The removed bolts attaching C-shaped brackets  225  to assembly  222  are re-attached, and stiffening braces  242  are attached to arches  232 ,  234  and  236 . Finally, newly assembled first door module  230  is rotated about axle  218  to a position such that it is covering the upper half of opening  160 . At this point, cables  240  are attached to arch  232  and to arch  110 , holding the first door module  230  in place covering the upper portion of opening  160 . 
     Second door module  250  is then assembled in substantially the same manner as first door module  230 . Second door module  250  is attached to door footing  210  adjacent the first door module  230 , but not the upright arch  272  and inclined arch  274 . As well, second door module  250  is not attached to arch  110  via a cable. Rather, second door module  250  is allowed to rest on the ground (or other surface on which the structure is erected), covering the lower half of opening  160 . 
     Operating the Door System 
     Starting from the closed position, the door system  200  can be opened in the following manner. First, the second door module  250  is rotated about axles  218  towards an open position. In the illustrated embodiment, this is accomplished using a door opening system  300  (shown in  FIGS. 7 and 14 ) comprising a pair of lifting cables  302  and a pair of motorized winches  304 . The lifting cables  302  are each attached at one end to arch  252 , on either side of the apex of arch  252 . The lifting cables  302  extend from arch  252  to a pair of pulleys  303  attached to the arch  110  that is adjacent to canopy  270 . From pulleys  303 , the lifting cables  302  extend downwardly to motorized winches  304  located at the first and second foot portions  112  and  116  of arch  110 , respectively. 
     As will be explained below, the lifting cables  302  and motorized winches  304  will be used to lift both the second door module  250  to which it is attached, and the first door module  230 , and rotate said modules about the axles  218 . It will therefore be understood that any cable, pulley and motorized winch suitable for lifting the weight of both the first and second door modules  230 ,  250  and rotating said modules about axles  218  can be used. In the example embodiment, motorized winches  304  are each driven by a 10 horsepower electric motor coupled to a gearbox having a 60:1 gear ratio, which produces a relatively low amount of noise during operation. In areas where access to an electrical power grid is not available, the motorized winches  304  can be powered by a 7.5 kW electrical generator, which is known in the art. The lifting cables  302  are ¾″ in diameter, and each has an average strength of 64,000 pounds. 
     As the second door module  250  is rotated toward an open position, the leading edge of module  250  catches first door module  230  such that, continued rotation of second door module  250  causes first door module  230  to rotate as well. This is accomplished by providing a set of four arms  306  (shown in  FIGS. 6 and 8 ) that extend downwardly from arch  232  of the first door module. As second door module  250  rotates toward an open position, arch  252  of second door module makes contact with arms  306 . Continued rotation of second door module  250  therefore causes arch  252  to push on arms  206 , causing first door module  230  to rotate toward an open position. 
     It will be understood that arms  306  can be essentially any structure suitable for this purpose. In the illustrated embodiment, arms  306  are of identical construction to stopping flanges  308 , which are shown in  FIG. 15  and will be described in detail below. It also will be understood that arms  306  can be attached to arch  232  in essentially any suitable manner. In the illustrated embodiment, arms  306  are bolted to arch  232  using bolts extending through bolt holes formed in arms  306  and into corresponding bolt holes formed in the central webs of the I-beams that form arch  232 . Arms  306  also could be formed integrally with arch  232 . It will further be understood that, while the illustrated embodiment utilizes four arms  306 , two placed a distance of 10′ 8″ from the apex of arch  232 , and two placed a distance of 30′ 8″ from said apex, essentially any number of arms can be utilized, so long as the structural strength of the arm and the connection of the arm to arch  232  is sufficient to support the weight of the first door module  230 . 
     When the second door module  250  and the first door module  230  are rotated to the open position, the inclined arch  274  of the canopy  270  makes contact with a pair of stopping flanges  308  extending from arch  236  of the first door module  230 . When contact is made with the inclined arch  274 , which is not free to pivot about the axles  218 , the rotation of the first and second door modules  230 ,  250  is stopped. An embodiment of a stopping flange  308  is shown in  FIG. 15 , and consists of an I-beam extending from a mounting plate at one end, for attaching the stopping flange  308  to arch  236 , to a padded plate at the other end for making contact with inclined arch  274 . Stopping flange  308  is bolted to arch  236  using bolts extending through bolt holes formed in the mounting plate and through corresponding bolt holes formed in the I-beams that form arch  236 . As shown in  FIG. 6 , two stopping flanges  308  are provided on arch  236 , each a distance of 20′ 8″ from the apex of arch  236 . However, it will be understood that any number of stopping flanges can be used, that the stopping flanges can be of any construction that will stop the rotation of first and second door modules  230 ,  250  when the stopping flanges make contact with the inclined arch  274 , and that the stopping flanges can be attached to arch  236  in any suitable manner (or could be formed integrally with arch  236 ). 
     As an alternative to the opening method stated above, the door  200  can be moved to a partially open position by taking in lifting cable  302  until the second door module  250  has rotated approximately half way towards the open position. In this position, both the first and second door modules  230  and  250  are covering the upper portion of the opening  160 , and the lower portion of the opening  160  is left unobstructed, providing a partially open door position. 
     To move the door system  200  back to the closed position, motorized winch  304  lets out the lifting cable  302  in a controlled manner, allowing the second door module  250  to rotate back toward the closed position. It is important to let out lifting cable  302  in a controlled, as opposed to uncontrolled manner using the motorized winch  304 , so that the door module  250  does not fall uncontrollably down to the closed position, which could cause injury or property damage. 
     When the second door module  250  has rotated approximately half way towards the closed position, and is covering the upper portion of the opening  160 , the second door module  250  connects with the first door module  230 , such that continued rotation of the second door module  250  towards the closed position also rotates the first door module  230  towards the closed position. Contact is made by way of a pair of safety catches  310  and  312 , shown in  FIG. 16 . The first door safety catch  310  comprises an approximately S-shaped bracket made of ⅜″ steel. The safety catch  310  is mounted to arch  236  of the first door module  230  such that the catch  310  extends inward toward the interior of structure  100  and upward, toward the roof of structure  100 . Safety catch  310  is mounted to arch  236  with ⅝″ steel bolts  311  extending through bolt holes formed in the safety catch  310  and the second flange  124  of arch  236 . Bolts  311  are held in place using nuts and lock washers. 
     The second door safety catch  312  comprises a ⅜″ steel bracket having an F-shaped cross section, extending from a rectangular base. The safety catch  312  is mounted to arch  252  of the second door module  250  such that it extends outward toward the exterior of structure  100 , and downward toward the concrete pad on which structure  100  is erected. Safety catch  312  is mounted to arch  252  with ⅝″ steel bolts  313  extending through bolt holes formed in the base of the safety catch  312  and the first flange  122  of arch  252 . Bolts  313  are held in place using nuts and lock washers. 
     The first and second door safety catches  310  and  312  are positioned on arches  236  and  252  respectively, and dimensioned such that, when arch  252  moves past arch  236  while the second door module  250  is moving towards the closed position, the first door safety catch  310  is received within second door safety catch  312 , as shown in  FIG. 16 . The interlocking of the safety catches  310  and  312  as shown in  FIG. 16  connects the first and second door modules  230  and  250 . Thus, continued movement of the second door module  250  towards the closed position causes the first door module  230  to rotate towards the closed position. In the exemplary embodiment shown, first door and second door safety catches  310  and  312  each extend 4⅜″ from arches  236  and  252 , respectively. 
     Once the second door module  250  reaches the closed position, covering the lower portion of the opening  160 , the cables  240  will become taut and support the first door module  230  in the closed position, covering the upper portion of the opening  160 . The opening  160  will then be substantially closed. 
     Optionally, the door can be locked in the closed position using locking mechanism  320 , shown in  FIGS. 17A, 17B, and 17C . Locking the door in the closed position is important in some circumstances. In particular, in certain wind conditions the first door module  230  can rotate in an uncontrolled manner. By locking the door in the closed position, the uncontrolled movement of the first door module  230  can be prevented. 
     Locking mechanism  320  comprises an articulated arm  321  pivotally mounted to the lower arch  256  of the second door module  250  at a first end, and a lock box  328  cast into the concrete pad on which the structure  100  is erected, for receiving a second end of articulated arm  321  and locking the second door module  250  in the closed position. When the second door module  250  is locked in the closed position, and safety catches  310  and  312  are engaged (as shown in  FIG. 16 ), the first door module  230  is locked in place in the closed position as well. 
     Articulating arm  321  is shown in detail in  FIG. 17A . Articulating arm  321  comprises a first arm  322 , a second arm  324  and lock grab  326 . A first end of first arm  322  is pivotally attached to arch  256 . In particular, the first end of first arm  322  is attached to a pivot  323  that is bolted to arch  256 . First arm  322  extends 10″ from the first end to a second end, which is attached by a hinge to a first end of the second arm  324 . Second arm  324  extends 10″ from its first end to a second end that is attached to lock grab  326  by a hinge. 
     Lock grab  326  is a rectangular steel plate having a C-shaped cutout  327  formed in one side thereof. Lock grab  326  is hingedly attached to second arm  324  at one corner, and is also pivotally attached to arch  256 . In particular, lock grab  326  pivots about a pivot that is bolted to arch  256 . 
     Lock grab  326  is dimensioned to be received within lock box  328 , which is a rectangular steel box having an opening in an upper surface thereof for receiving the lock grab  326 . A cylindrical steel pin  330  extends through the interior of lock box  328  from one side thereof to the other. Cutout  327  is dimensioned to receive pin  330  when lock grab  326  moves into lock box  328 . 
     Operation of the locking mechanism  320  can be understood with reference to  FIG. 17B , which shows the locking mechanism  320  in an unlocked position, and  FIG. 17C , which shows the locking mechanism  320  in a locked position. When the door  200  moves to the closed position, arch  256  makes contact with the concrete pad on which structure  100  is erected. Articulated arm  321  is attached to arch  256  such that lock grab  326  is adjacent to lock box  328  when arch  256  makes contact with the concrete pad. A user then pushes down on the second arm  324 , causing articulated arm  321  to move into the closed position and in particular, lock grab  326  to rotate into lock box  328  and receive pin  330  within cutout  327 . In this position, locking mechanism  320  holds second door module  250  in the closed position, which in turn holds first door module  230  in the closed position by way of the safety catches  310  and  312 . 
     To open the door  200 , a user pulls upward on second arm  324 , causing lock catch  326  to rotate out of lock box  328  and release pin  330 . The second door module  250  (and first door module  230 ) is then free to move upward toward the open position. 
     It will be apparent to those of skill in the art that alternative mechanisms are available for holding the door  200  in the closed position. Such alternative locking mechanisms can be used either in conjunction with, or in alternative to the locking mechanism  320 , depending on the conditions in which the structure  100  is erected. An example alternative locking mechanism  340  is shown in  FIGS. 18A and 18B . The alternative locking mechanism  340  comprises a ½-inch steel loop ring  342  fastened to a ⅝-inch steel plate  346  using a 3/16-inch steel loop bracket  344 . The steel plate  346  is fastened to the concrete pad on which the structure  100  is erected using appropriate anchor bolts extending through bolt holes  348 . To hold the door  200  in the closed position, the loop  342  can be hooked onto a hook-shaped flange (not shown) provided on arch  256  at a point such that the hook is adjacent to the loop  342  when the door  200  is in the closed position and the arch  256  is adjacent to the concrete pad. 
     The door opening system  300  can be controlled by digital logic. The digital control logic is in communication with encoder pack  219   a , which generates an electronic signal based on the angular position of axle  218 . This signal allows the digital control logic to know the current angular position of axle  218  and thus, the current position of the door modules  230 ,  250 . 
     The digital control logic can be programmed to open the door in response to a single push of a button. While the door also can be closed with a single push of a button, this is not recommended for safety reasons. Rather, in preferred embodiments the user must be in constant contact with a button to close the door. In the event that said contact is broken, the movement of the door towards the close position stops. 
     The digital door control logic also can keep track of the number of times the door has been opened and closed. This can be useful for maintenance purposes, since the lifting cables may require replacement after a certain number of openings and closings of the door. In the illustrated embodiment, the lifting cables  302  will require replacement after approximately 5,000 cycles. The digital door control logic can provide an alert when a threshold value of cycles has been reached, and replacement of the lifting cables is required. 
     The foregoing is a description of particular example embodiments of the claimed invention. It will be understood by those of ordinary skill in the art that numerous modifications, substitutions, additions or omissions to the embodiments described above are possible, all of which are intended to fall within the scope of the claimed invention. The invention is not intended to be limited to the particular embodiments described above, but is defined by the attached claims, which are to be given the broadest possible scope consistent with this specification.

Summary:
The present invention provides a door system for a temporary, movable structure having a plurality of spaced apart frame members and a membrane stretched between adjacent frame members. The door system has multiple modules movable between open and closed positions. Each module has its own membrane, separate from the membranes of the structure, which is stretched taught over the surface of the module. In the open position, the modules do not obstruct the opening in the structure, and the membranes of the modules and the structure do not go slack. The membranes of both the structure and the modules are always under tension, thus being less susceptible to damage.