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This application claims the benefit of U.S. Provisional Application No. 61/598,194, filed Feb. 13, 2012. The entire disclosure of the provisional application is incorporated into this application by reference. 
    
    
     BACKGROUND 
     Although a number of portable shelters, tents and housing units have been suggested in the art, they all have or would have disadvantages. U.S. Pat. No. 4,945,936 (Surrendi) shows a shelter that has hinged legs 2 which pivot radially outward and downward as the top is raised like an umbrella. The legs 2 are shown in their folded position in  FIG. 2 , and in their straightened position in  FIG. 1 . In contrast to the preferred embodiment described below. Surrendi does not suggest building the top portion of a large shelter, then lifting a first side of the shelter, unhinging or unfolding legs at the first portion from under the shelter, and then lifting the other side of the shelter, and then unhinging legs at the other end from under the shelter. 
     U.S. Pat. No. 5,813,425 (Carter) shows a collapsible shelter that has an elevated canopy, where a top section is pushed up as the legs 24, 26 are telescopically extended. The Carter shelter is said to be useful at emergency sites, for temporary care and housing (column 1, lines 17-21), and is said to provide more headroom than other such shelters (column 1, lines 38-42). Carter does not, however, suggest that the top section is built first; and the legs 24, 26 of the Carter shelter may be telescoped upwardly all at the same time. There is nothing in Carter about lifting one side or portion of a partially-constructed shelter, and then lifting the other portion. Like Surrendi, and in contrast to the preferred embodiment described below, Carter does not suggest building the top portion of a shelter, then lifting a first portion of the shelter, unhinging or unfolding legs at the first portion, and then lifting another portion of the shelter, and then straightening out legs at the other end. 
     U.S. Pat. No. 6,199,572 (Rousselle) refers to military and disaster-relief situations where substantial shelters/tents need to be constructed quickly (column 8, lines 50-60), and Rousselle says that its tent can be constructed without a ladder (column 13, lines 54-57). Rousselle shows, in FIG. 1D, an umbrella-style tent frame, where hinged (312) leg tubes 310 are folded radially outward and downward (1) to raise the shelter/tent. 
     SUMMARY 
     The disadvantages of the prior art can be overcome to a great extent by a portable shelter (e.g., an emergency medical tent that is on the order of thirty-two feet long), that can be assembled rapidly by hand with just a few people. Importantly, even though the shelter may have more than eight feet (preferably at least seven feet) of headroom inside (higher than those who are setting up the shelter can reach conveniently), the shelter can be assembled without a stepladder. The shelter has improved portability (it is easy to transport and quick to set up), and the shelter may be less expensive to produce and deploy than known shelters. 
     In operation, the top portion of the shelter (e.g., the top five feet) is assembled first. At this stage, nothing is too high to be reached by the people who are doing the set-up. The supporting legs, which may be made of aluminum, are hinged together, and are folded under the top portion of the shelter. So, after the top part is assembled, one side of the shelter is lifted up the remaining three feet or so (conveniently to about waist-height of the person doing the lifting), and then the hinges in the legs at that end are straightened out, and then the other end of the shelter is lifted up, and then the hinges on the legs at the second end are straightened out, and the bottom portion (the bottom three feet or so) of the shelter is then assembled/covered. 
     According to a preferred embodiment, a portable shelter may be provided for sheltering materials or human occupants at a remote location. The shelter may have, among other things, a flexible cover and a rigid, supportive frame. The cover may be formed of canvas or the like, and a flexible liner may be provided, if desired. The frame may be made of lightweight tubes or poles, with various hinges and connections. In operation, hinged members (legs) on a first side of the shelter may be unfolded to raise the first side of the shelter, and like hinged members on the opposite side of the shelter may be unfolded to raise the opposite side of the shelter, after the cover is provided on the frame, such that the shelter can be built to an intermediate height, and then raised up to a final height. The intermediate height may be low enough for the people who assemble the shelter to easily reach all parts of the top half of the shelter, yet high enough for those people to walk through the partially constructed shelter. When the shelter is raised to its final height, the top of the shelter may be high enough to provide comfortable headroom within the shelter, and clearance space for a ventilation plenum, lighting and the like, and therefore out of reach of the people doing the construction. 
     A preferred method of constructing a shelter, which may be a tent, a home, a medical facility, etc., includes the steps of unfolding plural multi-piece frame assemblies to partially-extended configurations, then bracing the frame assemblies together, then locating a weather-proof flexible cover over the frame assemblies, then unfolding first portions of the frame assemblies to support a corresponding first portion of the cover, and then unfolding second portions of the frame assemblies to support a corresponding second portion of the cover. If desired, the step of unfolding the assemblies may include the step of extending at least four arch assemblies to partially-extended configurations. If desired, a flexible floor may be installed underneath the frame assemblies, and the floor may be connected to end portions of the frame assemblies and/or the cover, to provide desired tension within the floor, such that the floor is maintained in a stable, flat condition during use of the shelter. 
     All of the components of the shelter should be sufficiently lightweight to be easily stored and/or transported to a remote location, including by way of helicopter or air transport. The components of the structure, which may be assembled/packaged in a single crate or other container, include arch assemblies that are each made up of arch members hinged and folded together so that, when folded, the arch assemblies are not substantially longer than any one of the arch members; and braces for connecting the arch assemblies together in an essentially parallel arrangement, none of the braces being substantially longer than any of the arch members. During construction of the portable structure, the arch members are configured to be unfolded from the respective arch assemblies while raising a first portion and then a second portion of the structure. 
     According to another embodiment, two or more structures may be aligned end-to-end, and the devices (zippers, hook and loop fasteners, and the like) that are used to connect the end panels to the individual structures may be used to secure open ends of the structures together. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a shelter constructed in accordance with a preferred embodiment. 
         FIG. 2  is a front view of an arch assembly for the shelter of  FIG. 1 , in a folded configuration. 
         FIG. 3  is a top view of the arch assembly of  FIG. 2 , in the folded configuration. 
         FIG. 4  is a top view of the arch assembly of  FIG. 2 , in a partially-extended, semi-arch configuration. 
         FIG. 5  is a front view of the arch assembly of  FIG. 2 , in a fully-extended, semi-arch configuration. 
         FIG. 6  is a top view of the arch assembly of  FIG. 2 , in the fully-extended, semi-arch configuration. 
         FIG. 7  is an enlarged top view of the hinged portion identified in  FIGS. 4 and 5  by circle VII. 
         FIG. 8  is a perspective view of the shelter of  FIG. 1 , in a preliminary stage of construction. 
         FIG. 9  is a perspective view of the shelter of  FIG. 1 , in an intermediate stage of construction, subsequent to the preliminary stage of construction shown in  FIG. 8 . 
         FIG. 10  is a perspective view of the shelter of  FIG. 1 , in another intermediate stage of construction, subsequent to the stage of construction shown in  FIG. 9 . 
         FIG. 11  is a perspective view of the shelter of  FIG. 1 , in another intermediate stage of construction, subsequent to the stage of construction shown in  FIG. 10 . 
         FIG. 12  is a perspective view of the shelter of  FIG. 1 , in another intermediate stage of construction, subsequent to the stage of construction shown in  FIG. 11 . 
         FIG. 13  is a perspective view of the shelter of  FIG. 1 , in another intermediate stage of construction, subsequent to the stage of construction shown in  FIG. 12 . 
         FIG. 14  is a front view of the arch assembly of  FIG. 2 , in a fully-extended, full-arch configuration. 
         FIG. 15  is a top view of the arch assembly of  FIG. 2 , in the fully-extended, full-arch configuration. 
         FIG. 16  is a perspective view of the shelter of  FIG. 1 , in a near-finished stage of construction, subsequent to the stage of construction shown in  FIG. 13 . 
         FIG. 17  is a flowchart that illustrates a Method of making the shelter of  FIG. 1 . 
         FIG. 18  is a perspective view of the frame for the shelter of  FIG. 1 . 
         FIG. 19  is a perspective view of one of the arch hinges of the frame of  FIG. 18 . 
         FIG. 20  is a perspective view of a two-purlin connection for one of the interior arch assemblies of the frame of  FIG. 18 . 
         FIG. 21  is a perspective view of a single-purlin connection, in an un-assembled state, for the arch assemblies that are located at the ends of the frame of  FIG. 18 . 
         FIG. 22  is a perspective view of the un-assembled connection of  FIG. 21 , viewed from the opposite direction. 
         FIG. 23  is a perspective view of the connection of  FIG. 21 , in a snapped-together state. 
         FIG. 24  is a view like  FIG. 23 , showing the snapped-together connection from the other direction. 
         FIG. 25  is a partial perspective view of the shelter of  FIG. 1 , showing exemplary details of a corner thereof. 
         FIG. 26  is perspective view of a hinging base pad for the frame of  FIG. 18 . 
         FIG. 27  is a schematic side view of a combined shelter constructed according to a preferred embodiment. 
         FIG. 28  is a schematic top view of the combined shelter of  FIG. 27 . 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings, where like reference numerals designate like elements, there is shown in  FIG. 1  a shelter  10  that is constructed in accordance with a preferred embodiment of the present invention. The shelter  10  has a front wall  12 , a back wall (not shown), a roof  14 , and side walls  16 . The left side wall (not shown) is the mirror image of the right side wall  16 . The front and back walls  12 , the roof  14 , and the side walls  16  are supported by a suitable frame (not shown in  FIG. 1 ) made of lightweight aluminum (or steel) tubes, wooden poles, or the like. Depending on expected wind and other conditions, the shelter  10  may be tied to the ground by wires or ropes  18 , stakes, or the like. 
     The shelter  10  is made from components that can be assembled or packaged into a compact shipping container, and transported as such to a remote location. If desired, the shelter can be constructed at the remote location in a short period of time by people working without power tools, and without a stepladder. In the illustrated embodiment, the components may be pre-grouped into separate bags or other containers each of which can be lifted, carried and handled by a single person, for rapid positioning of the components to the approximate respective locations where they are installed into the shelter, as described below in more detail. 
       FIGS. 2 and 3  are front and top views, respectively, of an arch assembly  20 , in a folded (collapsed) configuration, that forms part of the frame for the shelter  10 . The arch assembly  20  is made up of five arch members  22 ,  24 ,  26 ,  28 ,  30  joined to each other by suitable hinges  32 . There are four such hinges  32  for each arch assembly  20 , as shown in  FIG. 3 . Each arch member  22 ,  24 ,  26 ,  28 ,  30  is curved and has essentially the same elongated, arcuate shape, so that the arch members  22 ,  24 ,  26 ,  28 ,  30  line up behind each other in the folded configuration, as shown in  FIG. 2 . Each arch member  22 ,  24 ,  26 ,  28 ,  30  extends through an arc a that is within the range of from about thirty degrees to about forty degrees, and is preferably about thirty-six degrees. The radius R of the arc α is about equal to the height (head room) H ( FIG. 1 ) of the shelter  10  along its longitudinal centerline. The height H may be at least seven feet, preferably in the range of from about eight feet to about fourteen feet, and preferably about twelve feet. 
     In operation, the arch assembly  20  can be unfolded to the partially-extended semi-arch configuration shown in  FIG. 4  (a top view of the arch assembly  20 ), and, from there, the assembly  20  can be further unfolded to the fully-extended, semi-arch configuration shown in  FIGS. 5 and 6  (front and top views, respectively). The hinge  32  between the second and third arch members  24 ,  26 , and the hinge  32  between the third and fourth arch members  26 ,  28  may then be snapped or locked in place so that the three middle arch members  24 ,  26 ,  28  remain arcuately aligned and do not easily return to the partially-extended configuration shown in  FIG. 4 . 
     In the illustrated embodiment, there are six arch assemblies  20 ,  50 ,  52 ,  54 ,  56 ,  58  that are essentially identical to each other. The first four arch assemblies  20 ,  50 ,  52 ,  54  are shown in  FIG. 8 , where the first arch assembly  20  is shown in its partially-extended semi-arch configuration, and the second through fourth arch assemblies  50 ,  52 ,  54  are in their collapsed (folded) configurations. In the preliminary stage of construction shown in  FIG. 8 , the fifth and sixth arch assemblies are located within bags  51  or other packages suitable for carrying components of the frame from a main transport container  38  to the approximate locations where the arch assemblies are assembled into the frame. 
     In a subsequent stage of construction, shown in  FIG. 9 , the first arch assembly  20  is unfolded and locked into its fully-extended, semi-arch configuration. The unfolding operation may be performed by two people  90  who pull the second and fourth members  24 ,  28  of the assembly  20  away from each other, in the direction indicated by double arrows  60 , until the three middle members  24 ,  26 ,  28  all lie in the same plane, with the hinges  32  between the middle members  24 ,  26 ,  28  snapped or locked in place, as shown in  FIGS. 5 and 6 . 
     Then, as shown in  FIGS. 10 and 11 , the second through sixth arch assemblies  50 ,  52 ,  54 ,  56 ,  58  may be successively unfolded, extended, and locked into their fully-extended, semi-arch configurations, so that they are each configured essentially identical to the first arch assembly  20  (as shown in  FIGS. 5 and 6 ). The six arch assemblies  20 ,  50 ,  52 ,  54 ,  56 ,  58  may be successively positioned upright as shown in  FIGS. 10 and 11  and braced into parallel alignment by fifteen straight brace arms, all of which may be essentially identical to each other. Five of the brace arms  70 ,  72 ,  74 ,  76 ,  78  are longitudinally aligned along a top ridge of the frame, and are connected to the six arch assemblies  20 ,  50 ,  52 ,  54 ,  56 ,  58 , respectively. 
     The top brace arms  70 ,  72 ,  74 ,  76 ,  78  may be connected to suitable connectors located at the midpoints of the six middle arch members  26 . Exemplary connections  180 ,  182  ( FIGS. 20-24 ) are described in more detail below. Five other brace arms  80 ,  82 ,  84 ,  86 ,  88  are aligned along the right side of the frame, and are also connected to the six arch assemblies  20 ,  50 ,  52 ,  54 ,  56 ,  58 . The right-side brace arms  80 ,  82 ,  84 ,  86 ,  88  may be connected to the respective second arch members  24 . The remaining five brace arms (visible in  FIG. 18 ) are aligned along the left side of the frame, where they are each connected to two of the respective fourth arch members  28 . 
     At this intermediate stage of the construction, as shown in  FIG. 11 , the first and fifth arch members  22 ,  30  of each arch assembly  20 ,  50 ,  52 ,  54 ,  56  remain folded into the configuration shown in  FIGS. 5 and 6 . That is, the first and fifth arch members  22 ,  30  remain next to the respective second and fourth arch members  24 ,  28 . As a result, the frame rests on the six hinges  32  that are located between the first and second arch members  22 ,  24 , and the six hinges  32  that are located between the fourth and fifth arch members  28 ,  30 . Those twelve hinges  32  are in contact with the ground. As a result, the top of the frame (where the top brace arms  70 ,  72 ,  74 ,  76 ,  78  are located) may be no more than about eight feet above the ground. This way, it is easy for the people who are constructing the frame to reach the highest points within the shelter  10  without using a stepladder or other means for increasing the height of their reach. 
     As shown in  FIG. 10 , for example, it is easy for a person  90  to reach the top of the frame to connect the ends of the first brace arm  70  to the center arch members  26  of the first and second arch assemblies  20 ,  50 . When the frame is in the configuration shown in  FIG. 11 , it is easy for people to walk under the frame, yet no assistance, such as a stepladder, is needed for the same people to reach the top (or any other part) of the frame. When the frame is in the configuration shown in  FIG. 11 , a ventilation plenum (not shown) may be attached to the uppermost parts of the frame. The plenum may be a flexible tube with selectively openable openings along its length. The plenum may extend from one end of the frame to the other. One end of the plenum may be connected to a source of HVAC ventilation, to distribute heated or cooled air throughout the interior of the shelter. In addition to, or instead of, installing the plenum, electrical lighting and/or electrical wiring may be connected to upper portions of the frame, if desired, while the frame is in the  FIG. 11  configuration, and before the shelter is raised up to its finished height. 
     As shown in  FIG. 12 , a liner layer  100  may be pulled over the frame. The liner layer  100  may be formed of a flexible material with a rectangular shape. The length L of the liner layer  100  may be the same as, or slightly greater than, the total length L of the frame ( FIG. 11 ), for example, within the range of from about twenty feet to about fifty feet. The length of each of the brace members  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82 ,  84 ,  86 ,  88  may be about L/5. The circumferential width W of the liner may be about the same as, or slightly wider than the arcuate length W of each arch assembly  20 ,  50 ,  52 ,  54 ,  56 ,  58  measured along its arc, as shown in  FIG. 14 . The length of each arch member  22 ,  24 ,  26 ,  28 ,  30  may be about W/5. The width W may be related to the frame height H as follows: W≈πH, where α≈180°/5. 
     After the liner layer  100  is installed over the frame, and the inner surface of the liner layer  100  is secured to the top of the frame, a cover  110  ( FIG. 13 ) may be pulled over the liner layer  100  using suitable ropes (not shown). The length L of the cover  100  may about the same as, or slightly greater than, that of the frame, and the cover  110  may have flexible semicircular portions  112  that form the front and back walls  12  of the shelter  10 . The front portion  112  of the cover  110  may have, for example, a door  114  and two windows  116 . If desired, the ventilation plenum discussed above, or another arrangement for supplying forced air into the shelter  10  from a suitable HVAC unit (not shown), may be installed while the shelter  10  is in the  FIG. 13  configuration. 
     Then, after the cover  110  is placed over the liner  100 , the first arch members  22  are folded out (away from the respective second arch members  24 ), and snapped or locked into place, so that the first arch members  22  are arcuately aligned with the respective middle arch members  24 ,  26 ,  28 . This causes the right side of the shelter  10  to be higher than the left side of the shelter  10 , as shown in  FIG. 13 . Subsequently, the fifth arch members  30  ( FIG. 16 ) are folded out (away from the respective fourth arch members  28 ), and the respective hinges  32  are snapped or locked into place, so that the six arch assemblies  20 ,  50 ,  52 ,  54 ,  56 ,  58  each assume the fully-extended, fully-assembled configuration shown in  FIGS. 14 and 15 . Unfolding the fifth arch members  30  causes the left side of the shelter  10  to reach the same height as the right side of the shelter  10 . In the  FIG. 16  configuration, the top of the shelter  10  is too high for an average person to reach points at the top of the shelter  10  without a stepladder or other means of increasing the height of his or her reach. The inside height (headroom) H of the shelter  10  ( FIGS. 1 and 16 ) may be, for example, about twelve feet. 
     The axes of rotation of the hinges  32  are perpendicular to the longitudinal extent (extending in the direction of length L) of the shelter  10 . Consequently, as first arch members  22  are rotated away from the second arch members  24 , the movement of each first arch member  22  relative to the respective second arch member  24  is through a plane that is essentially perpendicular to a line that extends from the respective hinge  32  toward the longitudinal center line of the floor of the shelter  10 . 
     There are seams  120  between the front and back portions  112  of the cover  110  and the main rectangular portion of the cover  110 . The seams  120  may be used to permit entry into the shelter  10  during assembly/construction, while the door  114  is closed. The seams  120  also make it easier for people to reach the first and fifth arch members  22 ,  30 , to lift and lock the right side, and then the left side, of the shelter  10  into the position shown in  FIG. 16 . In the  FIG. 16  configuration, the seams  120  are formed by partially unzipped zippers, as discussed in more detail in connection with  FIG. 25 . The seams  120  are shown in more detail in  FIG. 25 . 
     Before the construction of the shelter  10  is finally completed, the seams  120  may be closed by suitable hook and loop fasteners  242  ( FIG. 25 ). The bottom edges of the cover  110  all around the shelter  10 , where the cover  110  meets the ground, may be sealed to a flexible floor that is located underneath the frame. If desired, the bottom ends  122  of the first and fifth arch members  22 ,  30  may be attached to the floor (not illustrated, discussed below); and the ropes  18  shown in  FIG. 1  may then be secured to stakes located in the ground, to provide a stable, secure finished structure. 
     A flexible floor (not illustrated) may be installed within the shelter  10 . The floor may be connected to the edges of the shelter and thereby stretched tight. The floor may have a length and width respectively slightly greater than L and 2H (that is, slightly greater than the length and width L, 2H of the shelter  10 ). The extra material at the edges of the floor may be folded upwardly and then sealed (for example, by hook and loop fasteners) to inner surfaces of the cover  110 . This way, the floor has a “bath tub” configuration to help ensure that dust, insects and the like do not get into the shelter above the edges of the floor. 
     In a preferred embodiment, holes  300  in base pads  250  (discussed below in connection with  FIG. 25 ) may receive stakes to help secure the shelter to the ground. 
     In summary, the shelter  10  may be constructed in two stages. First, the arch assemblies  20 ,  50 ,  52 ,  54 ,  56  are extended to their semi-arch configurations (S 1010 ,  FIG. 17 ), and aligned and braced into the stable arrangement shown in  FIG. 11 . Then, a liner  100  and/or a cover  110  are pulled over the frame ( FIGS. 12 and 13 ; S 1012 ,  FIG. 17 ). Then, the people who are constructing the shelter  10  lift, by hand, the right side of the frame, and cause the ends  122  of all of the first arch members  22  to come into contact with the ground (S 1014 ). Then, the same people lift, by hand, the left side of the frame, and cause the ends  122  of all of the fifth arch members  30  to come into contact with the ground (S 1016 ). The floor, if desired, may be installed before or after the other components of the shelter are assembled together. 
     The illustrated shelter  10  may be constructed efficiently and quickly. All of the various parts are sized and grouped to be handled individually by a single person. All of the connections for the center top portion of the shelter, including connections made within the frame, and connection of lighting, and other devices, may be done while the frame is in the sub-assembly configuration shown in  FIG. 12 , before the first and fifth arch members  22 ,  30  are unfolded to raise the frame (and the shelter  10 ) to its finished height. 
     Moreover, all of the individual components may be sized for convenient packing in canvas bags or the like. In a preferred embodiment of the invention, the length of each arch member  22 ,  24 ,  26 ,  28 ,  30  is about the same as that of the brace arms (also called purlins)  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82 ,  84 ,  86 ,  88 . There is no piece or individual component of the shelter  10  that is substantially longer than the other pieces of the shelter. This way, the parts can be packed together in a compact volume (e.g., crate  38 ) for storage and transport. Also, the parts can be grouped together into a small number of canvas bags  51  or the like, each not too heavy for a person to carry, so that the parts can be moved into place for construction easily from the storage container  38 . If desired, the bags can be marked for separate identification, for ease of sequential construction. This way, the people who are assembling the shelter can operate efficiently as a team. Tools for assembling the shelter, including wrenches (not illustrated) for connecting the elements of the frame to each other, and a sledge hammer (not illustrated) for driving the stakes into the ground, may be provided in the crate  38  or transported separately. 
       FIG. 18  shows the frame as it would look if the liner layer  100  and the cover  110  were removed after all of the arch assemblies  20 ,  50 ,  52 ,  54 ,  56 ,  58  were fully extended, and an exemplary hinge  32  is illustrated in  FIG. 19 . The hinge  32  has a U-shaped clevis member  150  with two ears  152 ,  154  that enclose a detent member  156 . A pin  158  extends through the ears  152 ,  154  and the detent member  156 , to thereby rotatably connect the detent member  156  to the clevis member  150 . In operation, the hinge  32  permits free rotation of the detent member  156  relative to the clevis member  150 , as the first arch member  22  is moved away from the second arch member  24 , starting from the folded position shown in  FIG. 4 , toward and nearly to the extended position shown in  FIGS. 18 and 19 . 
     As the arch member  22  nears the extended position shown in  FIGS. 18 and 19 , a detent surface  170  of the detent member  156  comes into pressing contact with an inner surface  172  of the clevis member  150 . To reach the fully-extended position shown in  FIGS. 18 and 19 , the detent surface  170  presses past the inner surface  172 , causing the hinge  32  to snap into, and to tend to remain in, the extended position. In the extended position, flanges  174  are pressed against the ears  152 ,  154  to prevent further rotation of the first arch member  22  relative to the second arch member  24  in the extending direction, while the detent surface  170  and the inner surface  172  engage each other to yieldably prevent relative rotation in the opposite (folding) direction  176 . Thus, the hinges  32  for the frame can be snapped into their respective extended positions during steps S 1010 , S 1014  and S 1016  ( FIG. 17 ), and subsequently snapped back into their folded positions for disassembly, movement and/or storage of the shelter  10 . 
     Further, as shown in  FIGS. 20-24 , the connections  180 ,  182  between the brace arms (also called purlins)  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82 ,  84 ,  86 ,  88  and the respective arch assemblies  20 ,  50 ,  52 ,  54 ,  56 ,  58  may be provided with a snap-in functionality to facilitate the ease and speed with which the shelter  10  may be constructed and disassembled. In particular, each purlin ( FIG. 21 ) has a downwardly-pointing four-sided polygonal plate  184  at each end. The plate  184  may be welded or bolted, for example, to the end of the purlin. The sides  186 ,  188  of the plate  184  are angled inwardly toward the bottom  190 , so as to fit into a matching truncated-V shape of a receptacle  200 . Protruding from the exterior surface of the plate  184  may be a ball nose spring-loaded plunger  204 , the ball-shaped end of which is biased axially with respect to the purlin outwardly from the plate  184  by a coil spring (not shown). 
     The receptacle  200  has a matching hole  210  ( FIG. 22 ) that operates as a ball nose spring plunger catch. The hole  210  is positioned to be aligned with the plunger  204  when the plate  184  is fully inserted into the receptacle  200 , with the weight of the purlin being supported by the angled sides  212 ,  214  of the receptacle  200 . The width Wr of the receptacle  200  is only slightly greater than the width Wp of the plate  184 . Consequently, as the plate  184  is moved downwardly into the receptacle  200 , the plunger  204  is pushed into the plate  184 , and the spring is compressed, by a camming motion of the nose of the plunger  204  against the inner edge  218  of the receptacle  200 . Then, as the plate  184  moves downward to reach a fully-inserted position within the receptacle  200 , the nose moves axially outward and snaps into the hole  210 . 
     Exemplary details of the cover  110  are shown in  FIG. 25 . As the assembly of the shelter  10  nears completion, the openings  238  at the corners of the shelter  10  are sealed shut by a suitable contour zipper  240 , and a seam  242  formed by hook and loop fasteners. The corner of the cover  110  may be provided with a tension loop  246  that is secured onto a latch  248  ( FIG. 26 ) formed on a base pad  250  that may be located at the bottom ends  122  of each of the arch assemblies  20 . The base pads  250  are hinged with respect to the arch assemblies so as to fold compactly therewith during storage and/or transport. As explained above, the four openings  238 ,  120  ( FIG. 16 ) provide slack within the cover  110  so that the cover  110  can be placed on the frame before the end arch members  22 ,  30  are rotated into their extended positions, and the openings  238 ,  120  provide convenient access into and from the shelter  10  during assembly until the door  114  is set up for operational use. 
     The zippers  240  at opposite ends of the shelter  10  may be complementarily configured such that two or more shelters  10 ,  10 A may be connected end-to-end, as shown in  FIG. 27 , to create a combined shelter  10 ,  10 A.  FIG. 27  shows a combined shelter  10 ,  10 A that is essentially twice the size of the shelter  10  shown in  FIG. 1 . The two shelters  10 ,  10 A that make up the combined shelter  10 ,  10 A may be essentially identical to each other. In  FIG. 27 , the first shelter  10  is closed at one end by a flexible end panel  112  and open at the other end. The open end of the first shelter  10  is connected by complementary zippers  240  ( FIG. 25 ) to an open end of the second shelter  10 A ( FIG. 27 ), and the second end of the second shelter  10 A is closed by a flexible end panel  112 . The zippered connection  240  is then sealed against dust, insects and the like, along its entire extent, by a seam formed of complementary hook and loop fasteners  242 . In the illustrated embodiment, the seam  240  may be about four inches wide. 
     Referring now to  FIG. 28 , in a preferred embodiment of the invention, each zipper  240  consists of two complementary zipper parts  240 A,  204 B. Each first zipper part  240 A has a box  2402  for receiving a pin  2404  of a second zipper part  240 B. In addition, each first zipper part  240 A has a slider  2406  ( FIG. 25 ) for meshing the teeth of the mating chain  240 A,  240 B, as the slider  2406  is moved from the top of the shelter to the ground. 
     Thus, in the arrangement shown in  FIG. 28 , first and second zippered seams are formed at the right end of the shelter  10 , between the right end of the shelter  10  and an end panel  112 . First and second zippered seams are also used to connect the two shelters  10 ,  10 A together. Finally, a third set of first and second zippered seams are used to connect an end panel  112  to the left end of the second shelter  10 A. In this way, all of the panels  112  and the shelters  10 ,  10 A may be constructed identically, whether two, three or more shelters  10 ,  10 A are connected together. In each zipper seam, the closing operation starts at the top of the shelter and finishes at the ground. 
     The invention is not limited to the structures, methods and instrumentalities described above and shown in the drawings. The invention is defined by the claims set forth below.

Summary:
A portable shelter is provided for sheltering materials or human occupants at a remote location. The shelter includes, among other things, a flexible cover and a supporting frame. In operation, hinged members that are part of the frame are used to support a first portion and then a second portion of the shelter. Before the frame is raised up, the people who are constructing it can easily reach the top parts of the shelter without a stepladder or the like. By avoiding the need for a stepladder, the overall size and weight of the assembled components that have to be transported to the remote location can be reduced, and the shelter can be constructed more quickly and easily. Hinges, purlin connections, and corner closure systems for the shelter are also disclosed.