Abstract:
Precise parameters are maintained in a support structure for solar panels or other panel-like structures through use of a collapsible folding structure which is preassembled to precise tolerances at a convenient staging site before being collapsed for shipment. Easy installation with unskilled labor is facilitated by attaching one support joist to a substrate and then unfolding the entire support structure.

Description:
PRIORITY INFORMATION 
     The present invention claims priority to U.S. patent application Ser. No. 12/383,240 filed on Mar. 20, 2009, and makes reference herein to same in its entirety. 
     FIELD OF THE INVENTION 
     This invention relates to a solar energy collection system, and more particularly to a support system for an array of photovoltaic panels and method of assembling the same. The invention includes a bi-directional span of support members, including a profiled support rail having a longitudinal T-slot channel adapted to receive the head of a bolt for adjustable attachment to a support joist. A variety of panel-holding devices, such as friction clips, may also be used. 
     BACKGROUND OF THE INVENTION 
     A standard photovoltaic panel array includes a plurality of solar panels optimally arranged for converting light incident upon the panels to electricity. Various support systems are used for attachment to roofs, free-field ground racks or tracking units. Typically, these support systems are costly, labor intensive to install, heavy, structurally inferior and mechanically complicated. For example, a support system generally includes off-the-shelf metal framing channels having a C-shaped cross-section, such as those sold under the trademarks UNISTRUT™ or BLIME™, improvised for use as vertical and horizontal support members. The photovoltaic panels are directly secured to the support members and held in place by clips. The clips serve as hold-down devices to secure the panel against the corresponding top support member in spaced-relationship. The clips are positioned and attached about the panel edges once each panel is arranged in place. 
     For a free-field ground rack system as shown in  FIG. 1 , support elements, such as I-beams, are spaced and securely embedded vertically in the ground. Tilt brackets are installed at the top of each I-beam, and each tilt bracket is secured to the I-beam such that a tilt bracket flange extends above the I-beam at an angle as best seen in  FIG. 2A . As shown in this case, two UNISTRUT™ joists span the tilt brackets and are secured thereto. As seen in  FIG. 2B , UNISTRUT™ rails are positioned across and fastened to the horizontal joists. To secure each rail to the corresponding horizontal joists, a bolt through a bolt hole made in the rail sidewall attaches to a threaded opening in a transverse nut-like plate slideably mounted inside the channel of the UNISTRUT™ joist, so that the nut-like plate engages and tightly secures against the upper flange of the joist&#39;s C-channel as seen in  FIG. 2A . Importantly, the width of the plate is slightly less than the width of the channel, so that the plate can be slideably adjusted in the channel, without the plate rotating therein. 
     Once the bi-directional span is assembled, each solar panel is positioned and top and bottom clips are secured to each rail about the perimeter of each panel, to hold the panel such that the center of each panel is between two rails. 
     Another example of a support system is shown in U.S. Pat. No. 5,762,720, issued to Hanoka et al., which describes various mounting brackets used with a UNISTRUT™ channel. Notably, the Hanoka et al. patent uses a solar cell module having an integral mounting structure, i.e. a mounting bracket bonded directly to a surface of the backskin layer of a laminated solar cell module, which is then secured to the channel bracket by bolt or slidably engaging C-shaped members. Other examples are shown in U.S. Pat. No. 6,617,507, issued to Mapes et al., U.S. Pat. No. 6,370,828, issued to Genschorek, U.S. Pat. No. 4,966,631, issued to Matlin et al., and U.S. Pat. No. 7,012,188, issued to Erling. 
     Notably, existing support systems require meticulous on-site assembly of multiple parts, performed by expensive field labor. Assembly is often performed in unfavorable working conditions, i.e. in harsh weather and over difficult terrain, without the benefit of quality control safeguards and precision tooling. Misalignment of the overall support assembly often occurs. This can jeopardize the supported solar panels, or other supported devices. 
     For example, spacing of the photovoltaic panels is important to accommodate expansion and contraction due to the change of the weather. It is important, therefore, that the panels are properly spaced for maximum use of the bi-directional area of the span. Different spacing may be required on account of different temperature swings within various geographical areas. It is difficult, however, to precisely space the panels on-site using existing support structures without advanced technical assistance. For example, with the existing design described above (with reference to  FIGS. 2A and 2B ), until the rails are tightly secured to the horizontal joist, each rail is free to slide along the horizontal joists and, therefore, will need to be properly spaced and secured once mounted on-site. Further, since the distance between the two horizontal joists is fixed on account of the drilled bolt holes through the rails, it is preferred to drill the holes on-site, so that the horizontal joists can be aligned to attach through the pre-drilled attachment holes of the tilt bracket. Unfortunately, the operation of drilling the holes on-site requires skilled workers, and even with skilled installation, might still result in misalignment of the support structure and/or the solar panels supported by that structure. 
     Therefore, a need exists, for a low-cost, uncomplicated, structurally strong support system and assembly method, so as to optimally position and easily attach the plurality of photovoltaic panels, while meeting architectural and engineering requirements. 
     To accomplish the foregoing and related objectives, an improved support system would achieve a precise configuration in the field without extensive work at the installation site. The use of such an improved system would facilitate easy placement of solar panels onto the support structure. Further, a variety of different panel clips or holders could be used within the overall concept of the system. The shipping configuration of the improved support system would be such so as to be easily handled in transit while still facilitating rapid deployment. At present, none of the conventional art offers these capabilities. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to improve upon conventional photovoltaic solar panel systems, especially with regard to assembly and installation. 
     It is another object of the present invention to provide a support and installation system for solar panels in which the panels are less likely to be damaged during installation. 
     It is a further object of the present invention to provide a support system for solar panels that is easily installed on-site while still resulting in a precise configuration for purposes of mounting the solar panels. 
     It is an additional object of the present invention to provide a solar panel support system that can be assembled very quickly on site. 
     It is still another object of the present invention to provide a solar panel support system that can achieve close tolerances during field installation without the necessity of skilled labor at installation. 
     It is again a further object of the present invention to provide a solar panel support system in which specialized mounting brackets bonded to the solar panels are not necessary for the mounting of the solar panels to the support system. 
     It is still an additional object of the present invention to provide a solar panel support system which can be easily adapted to a wide variety of solar panel array sizes and shapes. 
     It is yet another object of the present invention to provide a solar panel support system which minimizes the necessity for precise measurements at the installation site. 
     It is again a further object of the present invention to provide a solar panel support system that can be arranged at a variety of different positions and exposure angles. 
     It is still an additional object of the present invention to provide a solar panel support system that can be precisely configured to a specific environment. 
     It is another object of the present invention to provide a support system for solar panels and other panel-like structures in which degradation caused by metal-to-metal contact is substantially reduced. 
     It is again another object of the present invention to provide a support system for panel-like structures in which accommodation is made for movement caused by changes in temperatures, humidity or other environmental considerations. 
     These and other goals and objects of the present invention are accomplished by a method of assembling a support structure constituted by a bi-directional array of structural members installed in a configuration of substantially perpendicular upper and lower structural members. The method includes the steps of identifying characteristics of the installed configuration of the support structure, including relevant measurements of the support structure with respect to the installation site. Then the support structure is assembled in accordance with the characteristics of the installed configuration at a staging site. Once properly assembled, the support structure is collapsed into an interconnected package appropriate for transport. After transport to the installation site, the support structure is installed according to the predetermined characteristics of the configuration as it is to be installed at the installation site. 
     In another embodiment of the present invention, the stated objects and goals of the invention are achieved by a collapsible support system, constituted by an intersecting array of structural members including a first group of lower support joists and a second group of upper support rails. These structural members are held together by adjustable connectors that facilitate rotation of the support joist and the upper support rails. Also, with this system a plurality of unique holding clips may be used to easily receive and hold each solar panel by the collapsible support structure when the collapsible structure is in its assembled and installed state. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Having generally described the nature of the invention, reference will now be made to the accompanying drawings used to illustrate and describe the preferred embodiments thereof. Further, the aforementioned advantages and others will become apparent to those skilled in this art from the following detailed description of the preferred embodiments when considered in light of these drawings, in which: 
         FIG. 1  is a perspective view of an assembled conventional field ground rack support system for securing a plurality of solar panels; 
         FIG. 2A  is a side view of a conventional tilt bracket mount with prior art C-shaped sectional channels secured back-to-back to form support joists to which upper support rails, also shown in  FIG. 2B , are secured; 
         FIG. 2B  shows an end view of prior art upper support rails, each with a C-shaped sectional channel; 
         FIG. 3  is a perspective view of a support system of the instant invention showing solar panels arranged in a column and in spaced relationship thereon wherein the support system has horizontally-aligned support joists and (relative thereto) vertically-aligned upper support rails; 
         FIG. 4A  is a top plan view of the bi-directional span of the assembly of the instant invention in the open position showing vertically-aligned upper support rails attached atop horizontally-aligned support joists; 
         FIG. 4B  is an end elevational view of the bi-directional span of the assembly shown in  FIG. 4A ; 
         FIG. 5A  is a top plan view illustrating the bi-directional span of the assembly shown in  FIG. 4A  in the folded position; 
         FIG. 5B  shows in enlarged detail the support system of the instant invention in a collapsed or folded position, and depicting, in particular, a connector for holding the support joist to a support and/or tilt bracket or similar structure held, i.e. pinched between adjacent support rails; 
         FIG. 5C  is a side view of  FIG. 5B  depicting the connector for holding the support joist to the support and/or tilt bracket or similar structure; 
         FIG. 6  is a side elevation and partial sectional view that depicts a support joist and a tubular upper support rail with a single-panel clip; 
         FIG. 7  is an end elevation and partial sectional view perpendicular to that shown in  FIG. 6 ; 
         FIG. 8  is a cross-sectional perspective view of an upper support rail; 
         FIG. 9  is an end view of the upper support rail of  FIG. 8 ; 
         FIG. 10  is a sectional elevation view showing a solar panel mounted between a two-panel clip and a single-panel clip; 
         FIG. 11  is a sectional elevation view showing a panel being fitted within a gasket of the two-panel clip and arranged to be fitted into a single-panel clip gasket; 
         FIG. 12  is a sectional elevation view showing a panel fitted within the gasket of the two-panel clip, having rearmost retaining ribs, a fulcrum ridge and a saw-tooth profile; 
         FIG. 13A  is a top plan view of the bi-directional span of the assembly of the instant invention in the open position showing upper support rails attached atop support joists; 
         FIG. 13B  is an end elevational view of the bi-directional span of the assembly shown in  FIG. 13A ; 
         FIG. 14  is a top plan view illustrating the bi-directional span of the assembly shown in  FIG. 13A  in the folded position; 
         FIG. 15  is a side elevation and partial sectional view that shows a support joist and a tubular upper support rail with a two-panel clip; 
         FIG. 16  is a perspective view of the support system similar to  FIG. 3 , but in this case using vertically-aligned support joists and horizontally-aligned support rails; 
         FIG. 17  is an end view of the a second embodiment of an I-shaped tubular upper support rail; 
         FIG. 18A  is a cross-section of the second embodiment of the upper support rail with panel framing clips; 
         FIGS. 18B and 18C  are partial sectional views showing a framed and unframed panel, respectively, fitted within pockets of adjacently spaced I-shaped tubular upper support rails; 
         FIGS. 19A and 19B  are perspective and cross-sectional views, respectively, of a panel-framing clip used with unframed panels; and 
         FIG. 20  is a partial perspective view of the support system of the instant invention depicting unframed solar panels arranged in a column and in spaced relationship thereon, wherein the support system has longituding, I-shaped tubular upper support rails. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings, a support system for a photovoltaic array of framed or unframed solar panels  12 ,  12 ′, respectively, known in the prior art includes a free ground rack structure having spaced vertical support elements  14  extending from the ground. The support system  10  of  FIG. 1  shows only two vertical support elements  14 , although multiple support elements may be used to accommodate a longer array of solar panels. Notably, the support system can also be mounted to a roof (or other structure), or tracking unit. Each of the support elements  14  for the free-field ground rack is preferably an I-beam securely embedded and vertically aligned in the ground, as is well known in the art. 
     Conventionally, a pair of lower horizontally-aligned, C-shaped support joists  11 ,  13  is mounted at the upper ends of the support elements  14  by tilt bracket mounts  16 . Thus, the vertical support elements  14  are spanned by the support joists  11 ,  13 . When there are additional arrays with additional support elements  14 , they can be spanned by multiple joists attached at their ends, or the joists  11 ,  13  can be longitudinally extended to span all of the support elements  14  in one, unbroken length. 
     Upper vertically-aligned rails  15 , arranged perpendicular to the support joists  11 ,  13 , are secured to the support joists to produce a two-dimensional span, on which the panels or other panel-like structures are supported.  FIG. 2A  illustrates conventional support joists  11 ,  13  secured to tilt bracket mounts  16  by back-to-back channels  17 ,  18 , with each channel having a C-shaped cross-section. Similarly, each conventional upper rail  15  is secured to the lower support joists  11 ,  13  by bolts through a corresponding wall of its C-channel  19 , as best seen in  FIG. 2B . 
     In accordance with one preferred embodiment of this invention,  FIG. 3  depicts a support system  10  for a photovoltaic array of solar panels  12 , attached to the same, conventional vertical support elements  14 . The support system  10  in this case, however, includes a bi-directional span of horizontally-aligned lower support joists  20  and vertically-aligned upper support rails  30 - 1  through  30 - n , as also seen in  FIGS. 4A and 4B . 
     Before proceeding with the description herein, for purposes of fully appreciating the present disclosure of the instant invention, the terminology “horizontally-aligned” refers to structural members that appear to be parallel to the horizon. “Vertically-aligned” structural members are perpendicular to the “horizontally-aligned” structural members. However, because the present invention can be mounted on almost any structural support, the terms “horizontally-aligned” and “vertically-aligned” may become inappropriate for certain situations. Accordingly, alternative terminology such as, “longitudinally extending” or “laterally extending” can be used instead. For example, in  FIG. 3 , the “horizontally-aligned” structural members are also extended longitudinally while the “vertically-aligned” members extend in a lateral direction. These various terminologies are used as a matter of convenience, and for purposes of example only. 
     As an alternative or second embodiment to that described above, the bi-directional span can have the lower support joists  20  to align along the length of tilting support brackets  16 . As a result upper support rails  30 - 1  through  30 - n  extended longitudinally, as seen in  FIGS. 13A ,  13 B and  16 . It should be understood that within the context of the present invention, either orientation in any configuration of the substantially perpendicular structural elements (lower support joist  20  and upper support rails  30 ) can be used. Further, a wide variety of different shapes, sizes and configurations is encompassed by the concept of the present invention and is not to be limited by the examples provided herein. The present invention can be adjusted to conform to any support structure or any “footprint” available for the deployment of solar panels, or any other, panel-like structure to be supported by the present invention. 
     Preferably, each upper support rail  30 - n  in this design is a hollow aluminum extrusion. However, in the alternative, the upper support rail may be made of roll-formed steel. Preferably, each support rail  30 - n  has a tubular body  31  having a generally rectangular cross-section with an upper wall section  36  and lower wall section  32  defined between spaced side walls  35  as best seen in  FIGS. 8 and 9 . The upper wall section  36  has a flat top surface  37  and upper wall of varied thickness, preferably having its thickest portion  38  in the center. This thicker center portion  38  is for added strength when fastening the single-panel clips  100 ,  100 ′ and two-panel clip  120  (described below). Strength can also be achieved for each support rail  30 - n  using a thicker lower wall section  32 . The lower wall section  32  includes a longitudinal T-slot sectional channel  33  and, preferably, a longitudinal C-slot sectional channel  34 . 
     For additional strength and/or flexibility, an alternative (i.e. second) profile of the support rail  30 ′-n shown in  FIGS. 17 ,  18 A,  18 B and  18 C can be used. This profile has an I-shaped cross-sectional tubular body  31 ′ with a flanged upper wall section  36 ′ and a flanged lower wall section  32 ′ forming a pocket  39  therebetween. More specifically, spaced side walls  35 ′ form pockets  39  with the flanged upper and lower wall sections  36 ′,  32 ′, respectively. Like the preferred upper support rail design described above, i.e. rail  30 - n , the upper wall section  36 ′ of the alternative design has a flat top surface  37  and upper wall of varied thickness, preferably having its thickest portion  38  in the center. The lower wall section  32 ′ includes a longitudinal T-slot sectional channel  33  between two longitudinal C-slot sectional channels  34 ; one on each side of the T-slot channel. The dual C-slot sectional channels  34  in the second support rail  30 ′-n allows for easier assembly, in that wires can be tucked away on either side of the rail. Notably, the pockets  39  may be used to eliminate clips  100 ,  100 ′, and/or  120  (described in detail below), i.e. to secure individual solar panels  12  (framed) or  12 ′ (unframed) by sliding the panels therein between adjacent rails  30 ′, as best seen in  FIGS. 18B ,  18 C and  20 . 
     Pockets  39  (and any clips or gaskets held therein) are especially important in that they can be configured to allow the panel (whether framed or unframed) to move therethrough along the length. This capability allows panels or panel-like structures to be slid along the lengths of the upper support rails  30 , thereby facilitating a quick and accurate installation of the panels supported by the inventive structural support system. The quick and accurate installation of the solar panels is one of the byproducts, and is a benefit coextensive with those of the present invention. With the present invention, accuracy is not sacrificed for ease of installation. 
     Referring again to the preferred embodiment, the spacing between each upper support rail  30  or  30 ′ is governed by the height of the individual solar panels  12 ,  12 ′ and the number of solar panels per column. Regarding the alternative rail and joist configuration shown in  FIGS. 13A and 16 , the spacing between each upper support rail  30  or  30 ′ is governed by the width of the individual solar panels  12 , and the number of solar panels per row. Each upper support rail  30 - 1  through  30 - n  or  30 ′- 1  through  30 ′-n, as the case may be, is attached to the lower support joists  20  by bolts  40 , wherein the head  42  of each bolt is slidably accommodated in the corresponding T-slot channel  33  of the respective upper support rail. As best seen in  FIGS. 6 ,  7  and  15 , the shank  43  of the bolt  40  passes through and is secured to the respective support joist  20  using a nut  45  or other type fastener to form the bi-directional span. 
     Notably, with the nuts  45  and bolts  40  tightened below a predetermined torque value, the bi-directional span can be easily folded to reduce space for shipping, as shown in  FIGS. 5B and 14 . Each support joist  20  is separated from the corresponding upper support rail  30 - n  or  30 ′-n by nonconductive separation washers  24 , preferably made of nylon, in order to prevent galvanic interaction between unlike materials. The nylon washer  24  is preferably about ⅛ th  inch thick, although other materials and thicknesses may be used. The use of the nylon washer  24  at the intersection of support joist  20  and a corresponding upper support rail  30  facilitates the rotation of these two elements with respect to each other. Rotation is further facilitated if the nut  45  includes a nylon insert. The nylon insert helps to prevent the nut  45  from loosening during folding and unfolding of the bi-directional span. 
     Regarding  FIGS. 6 ,  7  and  15 , it is important to notice the small differences between  FIGS. 6 and 7 , and those of  FIG. 15 .  FIGS. 6 and 7  show the alignment of the holding clip  100  attached to upper support rail  30 - n , with the length of panel  12 ,  12 ′ perpendicular the length of support rail  30 , as best seen in  FIG. 3 .  FIG. 15  shows the alignment of the holding clip attached to an upper support rail  30 - n , with the length of panel  12 ,  12 ′ parallel to the length of support rail  30 , best seen in  FIG. 16 . These two arrangements with different orientations of the length of panel  12 ,  12 ′ with respect to the length of the upper support rail  30  are illustrative of the flexibility of the present inventive system. This flexibility is facilitated by the various arrangements of the different panel holding clips  100 ,  100 ′ and  120 , as depicted throughout the drawings. The wide range of holding clips  100 ,  100 ′ and  120  complement the ability of the present invention to provide a very precise pre-arrangement of the inventive support system  10  for easy installation of the panels at the final staging site. 
     Specifically, once the upper support rail  30  or  30 ′ are secured to the support joists  20 , the solar panels (or other panel-like structures) either framed  12  or unframed  12 ′ can be fastened to the rails using holding clips  100 ,  100 ′ and  120 . Notably, as will be discussed in more detail below, upper support rail  30 ′ can also secure framed and unframed panels  12 ,  12 ′, respectively, in pockets  39  (i.e. using framing clips  150  in the case of unframed panels  12 ′). 
     Regarding panel holding clips  100 ,  100 ′ and  120 ′, as shown in  FIGS. 3 ,  10 ,  11 ,  12  and  16 , at least two types of panel holding clips are preferably used, i.e. end or single-panel clips  100 ,  100 ′ and an intermediate or two-panel clip  120 . The panel holding clips  100 ,  100 ′,  120  encompass a wide variety of devices that hold or grip panel-like structures using a number of different methods. One is simple gravity. Another is the tightness of or pressure applied by the arms of the gasket encompassing a portion of the panel-like structure. More specifically, the gasket  130 ,  131  lining the clip can create spring-like pressure through deformation of the material. One example would be rubber or nylon teeth (described below and identified as teeth  140  and  153  in  FIGS. 10 ,  11 , and  12 , as well as  FIGS. 18A ,  19 A and  19 B for panel framing clips  150 , respectively). Yet another way to grip the panel is through an adhesive material used with the gasket  130 ,  131 , to develop a bond with the portions of the panel-like structure being held. The gaskets used with holding clips  100 ,  100 ′,  120  and/or framing clips  150  can be easily changed as needed, depending upon the position of the support structure  10 , and the configuration of the panels  12 ,  12 ′ supported thereby. 
     The single-panel holding clips  100 ,  100 ′ have a generally Z-shaped profile with a base portion  110  and first wall  112 . Holding clip  100  has a first flange  114  and uses an unfulcrumed U-shaped gasket  130 . Clip  100 ′, on the other hand, has a first flange and gasket that substantially match that of flange  124  and gasket  131  described in detail below with reference to the two-panel holding clip  120 . 
     The two-panel holding clip  120  is generally U-shaped having a first extended flange  114 , a second extended flange  124 , a first wall  112 , second wall  122  and a base portion  110 , and uses two different gaskets  130 ,  131 . Generally, both gaskets  130 ,  131  have a U-shaped cross-section with a fold  138 , upper and lower contact surfaces,  132 ,  134 , respectively, with a plurality of ribs  140 , i.e. saw-tooth profiles, and a back wall  136 . 
     The fulcrumed U-shaped clip gasket  131  preferably includes resilient, rearmost retaining ribs  142 , designed to contact a top peripheral side  143  of the panel  12 ,  12 ′ to push and hold the panel downward into the clip below. Notably, there may be one retaining rib  142  extending from the upper contact surface  132  and one extending from the lower contact surface  134  as shown in  FIGS. 10 through 12 , or, in the alternative, there may be just one large rib extending from either the upper or lower contact surfaces. Still further, retaining rib  142  may extend from the back wall  136 , in which case the retaining rib  142  may be replaced with a spring to provide resiliency. 
     The lower contact surface  134  of the fulcrumed gasket  131  further includes a fulcrum point  144 , i.e. an extended elongated ridge, which forces against the solar panel  12 .  12 ′ toward the upper contact surface  132  and second clip flange  124 . 
     In use, the bottom portion of the two-panel holding clip  120  holds the top peripheral edge of the solar panel  12 ,  12 ′ therebelow, aligned with the other solar panels in the respective column of panels. As best seen in  FIGS. 10 and 11 , the bottom portion of the holding clip  120  includes a second clip flange  124 , which is longer than the opposing first clip flange  114 , which holds the bottom of an uppermost solar panel  12 ,  12 ′ in the same column. The top or first clip flange  114  of the two-panel holding clip  120  is preferably the same length as that of the flange of the bottom mounted single-panel holding clip  100 , i.e. having the same U-shaped unfulcrumed clip gasket  130  used therewith. Preferably, the length of longer clip flange  124  is at least twice the length of the shorter first flange  114 , so that the solar panel  12 ,  12 ′ can be inserted first under flange  124 , pivoted on fulcrum point  144  and then inserted under flange  114 , whereby flanges  114 ,  124 , gravity, and the resiliency, pressure and friction of the gaskets  130 ,  131 , as described, hold the panel  12 ,  12 ′ firmly in place once set in position. 
     The difference between single-panel holding clips  100  and  100 ′ is that clip  100 ′ is the first clip at the top of each upper support rail  30 - n  or  30 ′-n; while holding clip  100  is the last clip, i.e. at the bottom of each support rail  30 - n  or  30 ′-n. Since the single-panel holding clip  100 ′ is the top clip of each upper support rail, it has a fulcrumed U-shaped gasket, identical to the fulcrumed gasket  131 , to accommodate its extended flange profile (identical to flange  124 ). This is necessary since the top single-panel holding clip  100 ′ forces against the top perimeter side  143  of the uppermost solar panel  12 , aligned with the other solar panels in the respective column of panels, to push the bottom edge of the panel  12 ,  12 ′ into the top portion of the two-panel holding clip  120  therebelow. Therefore, the profile of holding clip  100 ′ substantially matches that of the bottom portion of the two-panel holding clip  120  to fit and secure the top perimeter edge of each solar panel therein. 
     Both of the clip gaskets  130 ,  131  preferably include a T-shaped engagement protuberance  137  for slidable registration and attachment via a complementary, somewhat T-shaped retaining groove  117  formed between the walls  112 ,  122  and their respective flanges  114 ,  124 . Gaskets  130 ,  131  are used with each holding clip  100 ,  100 ′,  120  to protect the front and back edges  143  of each solar panel  12 . Each gasket  130 ,  132  is preferably extruded with the T-shaped mounting protuberance  137 . 
     Preferably, the gaskets or clip liners  130 ,  131  are made of a material which is physically and chemically stable, and preferably electrically nonconductive. Furthermore, the gaskets  130 ,  131  should be of an electrically-resistant material and have good elasticity upon compression. Suitable materials, which can be employed include, but are not limited to, neoprene, butyl rubber, ethylene-propylene diene monomer (EPDM), chlorinated polyethylene (CPE) and a polytetrafluoroethylene (PTFE) material such as GORTEX® (a trademark of W. L. Gore &amp; Associates, Inc.), or TEFLON® (a trademark of E.I. DuPont de Nemours &amp; Company). 
     It is important to describe at this point, an additional structural benefit of support rails  30 ′. With reference to  FIGS. 17 through 20 , longitudinal pockets  39  of rails  30 ′ can be used as an alternative method for conveniently securing panels  30 ,  30 ′ to the bi-directional span  10 . Using framing clips  150  spaced along each panel edge  143  of unframed panels  12 ′, or simply using the frame of framed panels  12 , each panel can be inserted and slid in place along the longitudinal pockets  39  of adjacent rails  30 ′ as best seen in  FIG. 18B  (for framed panels  12 ) and  18 C (for unframed panels  12 ′). Preferably, panels  12 ,  12 ′ are slid in place and stacked in columns using the joist and rail orientation shown in  FIG. 4A . However, panels  12 ,  12 ′ may be aligned in rows using the joist and rail orientation shown in  FIG. 13A . Caps and/or extended finger stops (not shown) are used at the end of the rails  30 ′ to secure the panels within the corresponding columns or rows. 
     Regarding the framing clip  150  for unframed panels  12 ′, each framing clip  150  preferably includes a framing clip bracket  154  fitted with a gasket  152  (engaged via bracket flange  155  and gasket groove  156 ), as best seen in  FIGS. 19A and 19B . The gasket  152  further includes ribs  153  much like those of ribs  140  (discussed above with reference to panel holding clips  100 ,  100 ′ and  120 ). 
     Spacers  159  are preferably used to maintain appropriate spacing between adjacent panels in columns or rows, as shown, for example, in  FIG. 20 . The spacers  159  can be constituted by a structure similar to double-sided holding clips  120  without the base portion  110  for bolt attachment. However, any other type of appropriate structure can be used. The spacers are important since the movement permitted by the holding clips  100 ,  100 ′, and  120  or by movement in longitudinal pockets  39  may permit solar panels  12 ,  12 ′ to become misaligned with respect to each other. 
     Most notably, the support system  10  of this invention allows for off-site assembly (at a convenient staging site) to precise engineering specifications, in that, once the support members are assembled, the bi-directional span can be folded or collapsed on itself, as shown with reference to  FIGS. 5 and 14 , and then easily transported to the installation site. The support system  10  is then positioned, and secured to the roof, rack, tracking unit, or other substrate via the tilt bracket  16  (or equivalent structure) while still in the folded position. More specifically, after attaching one support joist  20  to one of the brackets  16 , using a pair of tilt bracket attachment bolts  240  (wedged between adjacent rails  30 - 2  and  30 - 3  in the folded position, as shown in  FIGS. 5B and 5C ), the assembly  10  is unfolded to the position of  FIGS. 4A and 13A , and the other support joist  20  is attached to the second bracket  16 , via a second pair of tilt bracket bolts  240 . Of course, the same procedure is followed if rails  30 ′ are used in place of rails  30 . 
     The method of assembling the inventive support system  10  for an array of photovoltaic panels  12 ,  12 ′ in columns and rows, includes the steps of building the bi-directional span by attaching support members, i.e. support joists  20  and upper support rails  30 - n  or  30 ′-n, using a plurality of attachment bolts  40  and nuts  45 . The top surface  37  of each upper support rail  30 - n  or  30 ′-n must be unobstructed for the solar panels  12 ,  12 ′ to be secure against. As previously described, each upper support rail  30 - n  or  30 ′-n has a substantial rectangular cross-section portion or an I-shaped cross-section portion, respectively. Preferably, upper support rail  30 ,  30 ′ has an upper wall section  36 ,  36 ′ and lower wall section  32 ,  32 ′. 
     Each individual support system  10  can be easily engineered, fabricated, assembled and adjusted to various specifications. For example, the longitudinal T-shaped sectional channel  33  in the lower wall section  32 ,  32 ′ is adapted to adjustably receive the heads  42  of attachment bolts  40 . Bolts  40  attach each upper support rail  30 - n ,  30 ′-n passing through one of the lower support joists  20 . The T-shaped slotted channel  33  permits the bolt  40  to be placed at any location along the length of the channel and through the lower support joist  20  as shown in  FIGS. 6 and 7  (for a first orientation of the support joist assembly) and  FIG. 15  (for the second orientation of the support joist assembly). Notably,  FIG. 18A  can be considered to show attachment of rails  30 ′ to joist  20  in either orientation. 
     Regarding holding clips  100 ,  100 ′,  120 , each clip can be pre-positioned and attached to the upper wall section  36 ,  36 ′ of the upper support rails  30 ,  30 ′, respectively, by a self-threading bolt  145  secured to the thick portion  38  and whose head engages the base portion  110  of the holding clip. The perimeter holding clips  100 ,  100 ′, and  120  can be positioned and attached to the upper wall section  36 ,  36 ′ of the upper support rails  30 ,  30 ′ off-site to adhere to the proper engineering specifications for a specific installation. The positioning and necessary spacing for the columns and rows of the photovoltaic panels  12 ,  12 ′ of the array can be optimally and easily made off-site during fabrication and assembly, without wasting space, time and materials. 
     Once the perimeter holding clips  100 ,  100 ′,  120  and upper support rails  30 - n  or  30 ′-n are attached to the support joists  20  as described above, the bi-directional span of the support system  10  can be reduced in size by folding the upper support rails  30  relative to the support joists  20 . The folded span can be easily shipped to the location for installation, then unfolded and secured to the roof, free-field ground rack, tracking unit, or other substrate for attachment of the photovoltaic panels  12 ,  12 ′ via the pre-positioned, attached and properly spaced perimeter holding clips  100 ,  100 ′,  120  (as shown in  FIGS. 3 and 16 ) or framing clips  150  and spacers  160  (shown in  FIGS. 18B ,  18 C and  20 ). 
     One preferred method to assemble the bi-directional span  10  horizontal support joists  20  is to align a first support joist  20  over tilt support bracket(s)  16  (or similar support structures), and to bolt the support joist  20  to the support bracket(s)  16  using bolts  240  secured within the folded structure, as shown in  FIGS. 5B and 5C  and previously discussed. Depending on the joist  20  to rail  30 ,  30 ′ orientation, i.e. either that shown in  FIG. 4A  or  13 A, the aligned support joist  20  will be attached first to one or two brackets. Connection is made to the tilt support bracket(s)  16  before anything else is done. Once connection is made, the bi-directional support structure  10  is unfolded sufficiently to allow a second support joist to be laid over the tilting support bracket(s)  16  so that the connection process can be repeated. It is important that the support joist  20  be provided with slots  216  (as depicted in  FIGS. 3 ,  4 A,  5 B and/or  14 ) in order to facilitate on-site adjustment. It is preferred that all of the routed or punched slots  216  be carefully made during the pre-installation operation before a support structure  10  is shipped to the permanent installation site. The alignment in which support joists  20  are laid across two or more tilt support brackets  16  is that depicted in  FIGS. 3 and 16 . Once two support joists  20  are attached to their respective tilt brackets  16  (or similar structures), then the entirety of the bi-directional support structure  10  is ready for permanent attachment of the panels  12 ,  12 ′. 
     Notably, with the aforementioned connections made to the tilt brackets  16  (or some other substrate support), installation of the overall support structure  10  is relatively simple Very little technical skill is needed for this operation, and nominal measurement or alignment is required since the overall support structure  10  was pre-engineered, fabricated and assembled off-site. This ease of installation, while maintaining engineering specifications and measurements, is one of many major advantages of the present invention. 
     This important advantage of pre-engineering, fabrication and assembly conducted off-site at a plant or other convenient fabrication area, includes the precise measurements needed to place and drill or punch the connection holes  44  for proper alignment of the lower support joist  20  and upper support rails  30 ,  30 ′. More specifically, with reference to the joist-to-rail orientation shown in  FIG. 4A , in the assembly process the support joists  20  are aligned to the position at which they will be connected to the tilt bracket  16  in the field. Next, connections are made between the first support joist  20  and the first upper support rail  30 ,  30 ′ by inserting a bolt  40  in spaced, pre-drilled holes  44  passing through the support joist  20  with the bolt head  42  at the top of the support joist and a hex nut  45  at the bottom. A separation washer  24  is included near each bolt head. The process is repeated for the other horizontal support joist(s)  20 . Next, a single upper support rail, i.e. either  30 - 1  or  30 ′- 1 , is aligned with the head  42  of the first bolt  40  located in position along the first lower support joist  20 . The bolt head  42  is lifted, separated from the separation washer  24 , and slid into the T-slot channel  33  in the upper support rail  30  or  30 ′. This step is then repeated for the other lower support joist(s)  20 . The end of the first upper support rail  30 - 1  or  30 ′- 1  is then aligned with a side wall of the first lower support joist  20 , and the hex nuts  45  are torque snug to a predetermined torque value that permits rotation between joist  20  and upper rail  30 ,  30 ′. Using a machinist square, the horizontal support joist  20  is made perpendicular to the vertical support rail  30 - 1  or  30 ′- 1 . The other vertical rails  30 - 2  through  30 - n  or  30 ′- 2  through  30 ′-n are assembled and secured in like fashion. Additionally, to reduce cost and/or to more conveniently tuck wires between rows and columns it should be noted that support rails  30  may be alternately or intermittently arranged with support rails  30 ′ in the same assembly. 
     Of course the method for assembling the bi-directional span  10  having the second orientation, i.e.  FIG. 13A , of support joists  20 , is substantially the same as that described above for support joists oriented according to  FIG. 4A . A first lower support joist  20  is aligned and bolts  40  are inserted in spaced, pre-drilled holes  44  passing through the lower support joist  20 , with the bolt head  42  at the top of the joist and a hex nut  45  at the bottom. Again, the separation washer  24  is included near each bolt head. The process is repeated for the second and any subsequent vertical support joists  20 . Next, a single upper support rail  30 , i.e. using either cross-sectional design  30  or  30 ′, is aligned with the head  42  of the first bolt  40  located in position along the first lower support joist  20 . The bolt head  42  is lifted, separated from the separation washer  24 , and slid into the T-slot channel  33 . This step is repeated for all subsequent, lower support joists  20 . Using a machinist square, the lower support joists  20  are made perpendicular to the upper support rails  30  or  30 ′ (i.e. the upper support rail  30 - 1  or  30 ′- 1  is aligned perpendicular with a side wall of each lower support joist  20 ), and all hex nuts  45  are torque to a predetermined value permitting rotation of joist and rail while maintaining the precise, intersecting position therebetween. The other rails  30 - 2  through  30 - n  or  30 ′- 2  through  30 ′-n are then assembled and secured in like fashion. 
     As previously stated, bolts  40  and hex nuts  45  are used to securely fasten the lower support joists  20 , as the case may be, to the corresponding upper support rails  30 ,  30 ′. As stated above, each hex nut  45  preferably includes a nylon insert. The nylon insert retains torque pressure of the fastener (at the predetermined value) during shipping and prevents the support rails  30  and/or  30 ′ from loosening from the support joists  20  when folded and unfolded. Notably, on account of the separation washers  24  and the nylon hex nuts  45 , the upper rails  30 ,  30 ′ can pivot relative to the support joists  20  without any significant loosening. The assembled bi-directional span can be folded and unfolded by grasping the ends of two adjacent support joists, and pushing one of the lower support joists  20  longitudinally away relative to the second support joist  20 , permitting the assembly to fold into a compacted form for shipping. Alternatively, the folding operation can be made just as easily by grasping the ends of two adjacent horizontal rails  30 ,  30 ′ and pushing one longitudinally away relative to the other. 
     It is important to note for assembly and shipping purposes, that the tubular body forms  31 ,  31 ′, having varied wall thickness  38 , and channels  33 ,  34  substantially reduces the weight of the overall support rails  30 - n  or  30 ′-n, and, therefore, the overall weight of the assembled system in comparison to the prior art. Yet, because of the aforementioned selective reinforcement, the structural strength is enhanced. 
     As previously stated, the support structure  10  of the present invention facilitates simple and quick installation. After shipping the structure assembly  10  to the field for permanent installation, it is unpackaged, and the appropriate lower support joist  20  is aligned and secured to one or both of the vertical support elements  14 , via the bottom attachment openings in the tilt bracket mounts  16 , depending on the joist-to-rail orientation. For example, in the case of the assembly  10  having the orientation of support joists  20  as shown in  FIG. 13A , either the right or left outside-most lower support joist  20  is aligned parallel, mounted and secured to the corresponding vertical support element  14 , via the tilt bracket mounts  16 . Then, the other outside-most lower support joist  20  is pushed to unfold and realign mutually parallel to the first support joist, i.e. perpendicular to the upper support rails  30 ,  30 ′, so as to align mutually parallel to the other tilt bracket. The assembly  10  is securely fixed via the top attachment openings  216  in the corresponding support joists  20  to the tilt bracket mounts  16  using bracket attachment bolts  240  as previously described. 
     Notably, the space between the support joists  20  can be adjusted (if needed) by sliding the joists along the rails via their T-slot channels, so that the spacing of the joists  20  precisely align with and attach to the tilt bracket mounts  16 . In contrast, it is not possible to easily adjust the space between the joists  11 ,  13  in the conventional design shown in  FIGS. 1 ,  2 A and  2 B along its several conventional rails  15 , since the spacing therebetween is fixed by the drilled bolt holes made in rails  15  through the side walls of channels  19 . 
     Once the assembly of this invention is unfolded and the support joists  20  are secured to the tilt bracket mounts  16 , the spacing and perpendicular relationship of the upper support rails  30  are checked relative to the side wall of the support joists  20  using a machinist square or similar setup fixture and adjusted if needed. The hex nuts  45  are also checked to assure that they continue to be snug after shipping and installation. 
     And finally, with the expanded bi-directional span properly positioned and secured to the support elements  14 , each solar panel  12 ,  12 ′ is fixed in place by sliding into rows or columns via the longitudinal pockets  39  (with reference to rails  30 ′) or by using top holding clips  100 ,  100 ′ or  120  (i.e. inserting the top of the panel into its top holding clip  100 ′ or  120 , then pivoted about the respective gasket fulcrums  144 , to fit the panel&#39;s bottom edge into corresponding bottom gravity holding clips  100 ,  120 , as best seen in  FIGS. 10 through 12 ). To finish the installation, wires are tucked away in the corresponding C-shaped slotted channels  34 . The proper spacing between panels is maintained by spacers  159 , as depicted in  FIG. 20 . 
     While the invention has been particularly shown and described with reference to the specific preferred embodiments, it should be understood by those skilled in the art that various exchanges in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.