The rig mat is laid on a substrate to provide a surface to bear and distribute weight over the substrate. The rig mat call be used individually or juxtaposed in an assembly of mats to provide the bearing surface. The rig mat has a top plate and a bottom plate disposed in parallel alignment. A spacer grate is sandwiched between and fixed to the plates. An edge binder is disposed around the perimeter edges of the plates to bind them together and to enclose the spacer grate within the space between the plates. The spacer grate is made up of a number of grating bars disposed in a parallel to each other. Cross-rods disposed in parallel to each other and perpendicular to the grating bars pass through and attach to the grating bars to fix the spacing between the grating bars.

FIELD OF THE INVENTION  
         [0001]    The present invention is in the field of surface mats for supporting a mass and distributing its weight over a surface under the mat. More specifically, the present invention relates to performed, portable mat structures for covering a relatively large area, and designed for easy transport and rapid and repeated installation, and may be juxtaposed on a surface to provide a work apron and/or travel way.  
         BACKGROUND OF THE INVENTION  
         [0002]    The field long has been motivated to provide mass supporting and weight distributing temporary surfaces over suitable substrates on which heavy equipment or vehicles can be placed and operated. In the past, such temporary surfaces over unstable substrates (e.g., soft earth) were constructed of wooden planks. However, these plank structures were found to be labor intensive, time consuming to construct and expensive. Therefore, alternatives to wooden plank structures were developed. Early alternatives to plank structures included wooden mat modules that individually could be assembled into larger surface covering structures (e.g., see Davis et al, U.S. Pat. No. 4,289,420). The field recognized advantages of modular wooden mat structures and alternative wooden mat modules were developed. See Green, U.S. Pat. No. 4,376,596; Sarver; U.S. Pat. No. 4,600,337; and Pouyer, U.S. Pat. No. 5,273,373.  
           [0003]    However wood mats had some inherent disadvantages, and recognition of these motivated tie field to develop mat modules constructed of composites and other materials as an alternative to wood. For example, Seaux, U.S. Pat. No. 5,653,551, discloses a composite mat module having partially overlapping upper and lower layers. The Seaux mats interlock with each other at their perimeter edges, where the two layers of the module do not overlap. However, this configuratoin means that a surface constricted of the Seaux mats will only be half as thick at its peripheral edge than it is everywhere else.  
           [0004]    Therefore, it would be beneficial to have an alternative performed, composite mat module that can be used either individually or in an assembly and have the same mass supporting and weight distributing properties over substantially all of its surface.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is a rig mat module for bearing and distributing weight on a surface. More specifically, one or more of the present rig mat modules is to be laid on a substrate surface that otherwise cannot bear or is to be protected from the weight of an object placed on or moved across the top surface of the rig mat module. The present rig mat module may be placed singly or juxtaposed in groups to provide a weight bearing and distributing surface such as a work apron or travel/road way. A surface assembly of juxtaposed rig mat modules can be attached together at their perimeter edges to improve the stability of the surface assembly. Such surfaces can be used as crane pads, for creating a gravel-less, field work site, and for protecting relatively soft surfaces from the effects of heavy equipment use.  
           [0006]    The present rig mat module comprises a spacer grate sandwiched between a top plate and a bottom plate. Each plate has an exposed or exterior surface, an interior surface and a perimeter edge. The plates are disposed in parallel alignment with the interior surfaces of the plates juxtaposed to define a grating space between them. The spacer grate is disposed within the grating space between the top and bottom plates and is in contact with and fixed to the interior surfaces of the top and bottom plates. An edge binder is disposed around the assemblage of the plates and spacer grate to engage the perimeter edges of the plates. The binder encloses the grating space and provides additional structural integrity to the perimeter of the assemblage.  
           [0007]    The top and bottom plates are comprised of any suitable non-metal material as is selectable by one of ordinary skill in art in view of the teachings herein. Wood plates (e.g., plywood) or plates constructed of solid laminate plastics, fiberglass or other synthetic of composite materials may be used. Light metals, such as a metal mesh, may be embedded or molded into a plate for structural purposes or to provide gripping exterior surface to the plate. The thickness of the plates is selectable The ordinary skilled artisan depending on the anticipated loading of the rig mat module and the material construction of the plates. Also, the thickness and material of the two plates may be different each from the other. Further, the outer or exposed surfaces of the top and bottom plates may be different. For example, the exposed surface of the top plate may be a traction surface (e.g., for foot or vehicular traffic) while the exposed surface of the bottom plate is stippled to help anchor or improve its engagement with fie surface on which it rests.  
           [0008]    The spacer grate comprises a plurality of grating bars disposed relative to each other in a parallel and spaced relationship. The spaced relationship may be uniform for all grating bars or it may be varied to accommodate the expected mass load on different sections of the rig mat module. Each grating bar has a length, a width and a height. The width and the height define the cross-sectional of the grating bar, and the length defines the dimension of fire grating bars that are in parallel with each other. Typically the cross-section of the grating bar is substantially oblong. The spaced relationship of the grating bars (how far one grating bar is from an adjacent grating bar) is about 2-times to 20-times the width of the grating bar. The width of the grating bar is about 0.5 to about 1.0 inches. For large mass loads, a preferred spaced relationship is 2 with the width of the grating bar being about 0.6 inches. As with the spaced relationship of the grating bars, the width slay be uniform for all grating bars or it may be varied to accommodate the mass expected mass on different sections of the rig mat module. The height of the grating bar is about 1.0 to about 2.0 inches, and preferably about 1.5 inches. The grating bars themselves may be constricted using a pultrusion process, as is known in the art (e.g., see U.S. Pat. No. 4,522,009 to Fingerson). The grating bars also each have a plurality of rod apertures passing through the surfaces of the grating bar perpendicular to its width. The rod apertures have a thickness and are disposed in alignment with adjacent grating bars. The rod apertures are for closely passing dowel-rods therethrough. The length of the grating bars is dependent on the intended use of the rig mat module and the size of the area to be covered. It is intended in the present invention that the length of the grating bars be at least about two feet.  
           [0009]    The spacer grate also comprises a plurality of dowel-rods disposed in a parallel spaced relationship to each other and in perpendicular relationship to the grating bars. The dowel-rods pass through the rod apertures and attach to the grating bars to fix the spaced relationship of the grating bars. A dowel-rod comprises two spacer-rods separated by a wedge-rod. A spacer-rod has an outer engagement surface and an inner flat surface. The outer engagement surface of the spacer rod has spaced engaging means for engaging the grate bar where the dowel-rod passes through the rod-aperture. The inner flat surface of the spacer rod slidably interfaces with the wedge-rod. Typically, the outer engagement surface of the spacer rod has a plurality of notches, for closely engaging the thickness of the rod-aperture as the dowel-rod passes through the grate bar. The notches are spaced apart to fix the spaced relationship of the grating bars. The wedge-rod has two similar opposite interface surfaces for slidably contacting the two spacer-rods and to hold the spacer-rods apart. Also, the wedge rod has two similar and opposite curvilinear surfaces, disposed to closely pass through the rod-apertures. The wedge-rod is adhered at the interface surface of each of the two spacer-rods.  
           [0010]    The rig mat module of the present invention optionally comprises a coupling means for holding the perimeter edge of the rig mat module adjacent to the perimeter edge of an adjacent rig mat module. An example of a coupling means is a simple stake passing through the rig mat module proximate its perimeter in a number of location to fix adjacent rig mat modules to the underlying surface. Alternatively, the stake can have a squared “U” configuration and be disposed to pass through two adjacent rig mat modules at the same time. If heavy anchoring of the mat modules is required, the mat modules may further comprise anchor bores distributed over the plane of the mat module, through which heavy duty stakes may be driven to anchor the mat in place on a ground surface.  
           [0011]    The overall dimensions of a rig mat module is selectable by the ordinary skilled artisan and defined by the dimensions of the component parts and the total number of grating bars used. Roughly, the width of a rig mat module is the length of the grating bars used, and the run or length of the rig mat module is the number of grating bars times the spaced relationship. Where the desired overall dimensions of a rig mat module is larger than the dimensions of the individual components (plates, gratings bars or cross-bars), a plurality of the individual components can be used (e.g., two grating bars in series) to accomplish the desire overall dimensions. If the dimensions of a module require that it be comprised of multiple plates on each side of the module, and the plates are asymmetric (i.e., have a grain line) the plates on opposite sides of the module may be disposed to have the asymmetry be perpendicular and off-setting  
           [0012]    The top and bottom plate elements of the rig mat module allow a point or focused weight to be placed on the mat and distributed over multiple grating bars, which otherwise could not be placed on an open grating. Additionally, the plate covered rig mat modules of the present invention have broader utility than open grate mats on soft surfaces such as marshy or muddy soil. The thickness of the plates can be different depending on the anticipated loading of the top plate and the surface contacting the bottom plate. The plates can be a single thickness or comprised of a laminate. Additionally, either exterior surface of a plate can be layered with a cover plate which is removable. The cover plate can be utilized to provide protection to the plate under it or to increase it loading capacity. A cover plate can be replaced when worm or not needed, or can be switched out to provide a surface on the rig mat module suitable for a specific purpose (e.g., a high friction or gripping surface in a wet environment).  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a side view of the rig mat module showing the spacer grate received in the grating space between the top and bottom plates with the grating bars seen in cross-section.  
         [0014]    [0014]FIG. 2 is a exploded perspective view showing the relationship of the grating bars to the components of the dowel-rods.  
         [0015]    [0015]FIGS. 3A to  3 C are end views of an edge section of the present rig mat module showing the top and bottom plates sandwiching a length of grating bar and an end view of cross-rods engaged in the rod apertures of the grating bar. The views  3 A to  3 C show different means of attaching an edge binder to the perimeter edge of a mat module.  
         [0016]    [0016]FIGS. 4A & 4B are side edge views of an edge section of the rig mat module showing variations on the edge binding useful for joining two separate mat modules together at an adjoining perimeter edge.  
         [0017]    [0017]FIGS. 5A & 5B are side edge views of a section of a rig mat module showing alternative embodiments of the top and bottom plates, where the plates are different thicknesses (A) or where a plate is covered with a laminate or cover plate (B).  
         [0018]    [0018]FIG. 6 is a partial schematic representation of the layout of the cross-rods of the spacer grate of a rig mat module useful in the assembly of a travel way or work apron for pedestrians and/or vehicles. Note, the top and bottom plates and the grating bars are not shown.  
         [0019]    [0019]FIG. 7 is a side view of the spacer grating viewed end-on to the grating bars (section A of FIG. 6), aid illustrates a cross-rod halving a spacer-rod made up of two shorter length pieces and end-butted together to provide an overall longer spacer-rod.  
         [0020]    [0020]FIG. 8 is a side view of the spacer grating showing a length of a grating bar (section B of FIG. 6), and illustrates a grating bar up of two shorter length pieces and end-butted together to provide the overall longer grating bar.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The rig mat of the present invention is substantially planar device for supporting a mass and distributing its weight over the surface on which it is lain. The present rig mat modules, singly or combined in an assembly, are useful for constructing temporary roadways, crane and equipment pads and foundations for other temporary structures (e.g., scaffolding and platforms).  
         [0022]    Referring now to the drawings, the details of preferred embodiments of the present invention are graphically and schematically illustrated. Like elements in the drawings are represented by like numbers, and any similar elements are represented by like numbers with a different lower case letter suffix.  
         [0023]    As shown in FIG. 1, the rig mat  10  of the present invention comprises a top plate  50  and a bottom plate  52 , each plate having an exposed surface  54 , an interior surface  56  and a perimeter edge  60 . The plates  50 ,  52  are disposed in parallel alignment with their interior surfaces  56  juxtaposed to define a grating space  14  between them. The spacer grate assembly  16  is disposed within the grating space  14 . The spacer grate  16  is in contact with and is fixed to the interior surfaces  56  of the top and bottom plates  50  &  52  at the upper and lower bearing surfaces  34 ,  36  of the spacer grate assembly  16  grating bars  22 . In the preferred embodiment, the interior surfaces  56  of the top and bottom plates  50  &  52  are fixed to the upper and lower bearing surfaces  34 ,  36  of the grating bars  22  using an adhesive  68  (see FIGS. 3 a  to  3 C).  
         [0024]    The spacer grate  16  comprises a plurality of grating bars  22  disposed in a parallel spaced relationship to each other, and a plurality of cross-rod assemblies  26  disposed in a parallel spaced relationship to each other and in perpendicular relationship to the grating bars  22 . The cross-rod assemblies  26  pass through and are attached to the grating bars  22  to fix the spaced relationship of the grating bars  22  relative to each other.  
         [0025]    As shown in FIG. 2, each grating bar  22  has a length L, a width W and a height H. The width and the height H define the cross-section of the grating bar  22 . Preferably, the grating bars  22  have a cross-section that is substantially oblong. The length L of the grating bars  22  range from at least about 2 feet and longer. The length L of the grating bars  22  defines the dimension of the grating bars  22  that are in parallel with each other. The spaced relationship D of the grating bars  22  is about 2-times to 20-times the width W of the grating bars  22 . The width W of the grating bars  22  is about 0.5 to about 1.0 inches. For large mass loads, in a preferred embodiment spaced relationship D was 2-times the width W with the width W of the grating bar  22  being about 0.6 inches. The height H of the grating bars can be about 1.0 to about 2.0 inches, and in the preferred embodiment was about 1.5 inches.  
         [0026]    The grating bars  22  further comprise a plurality of rod apertures  24  passing through a height surface  23  of the grating bar  22 , which is perpendicular to the width W of the grating bar  22 . The rod apertures  24  are disposed in alignment for closely passing therethrough the cross-rods assemblies  26 . The distance S between adjacent rod apertures  24  in a preferred embodiment was about 18 inches. However, the spacing S between rod apertures is selectable by the ordinary skilled artisan in view of the expected mass and dynamics (movement) of the load on the top plate  50  of the rig mat  10 .  
         [0027]    A cross-rod assembly  26  comprises two spacer-rods  28  separated by a wedge-rod  30 . See FIG. 2. Each spacer-rod  28  has an outer engagement surface  31  and an inner flat surface  34 . The spacer-rods  28  are shaped with a curvilinear outer engagement surface  31  to facilitate passing a relatively larger cross-section of spacer rod  28  through the cross-rod apertures  24  than without the curvilinear engagement surface  31 . The outer engagement surface  31  has a plurality of spaced engaging means  32  for engaging the grating bar  22  where the cross-rod  26  passes through the rod-aperture  24 . The inner flat surface  34  serves as a mating surface that slideably interfaces with the wedge-rod  30 . In a preferred embodiment, the outer engagement surface  31  of the spacer rod  28  includes a plurality of engagement means  32 . In the embodiment shown, the engagement means  32  were notches for closely engaging the thickness of the rod-apertures  24  as the cross-rod assemblies  26  passed through the grating bars  22 . The notches  32  were spaced apart and fixed the spaced relationship D of the grating bars  22 .  
         [0028]    The wedge-rod  30  has two similar and opposite interface surfaces  46  for slidably contacting the mating surface  34  of each of the two spacer-rods  28 , and for separating the spacer-rods  28  apart. Additionally, off-set 90 degrees from the interface surfaces  46 , the wedge-rod  30  has two other similar and opposite curvilinear surfaces  48 , the curvilinear surfaces  48  disposed to closely pass through the rod-apertures  24 . The cross bar wedge-rods  30  are preferably I-shaped, with their interface surfaces  46  indented or recessed to facilitate slidably receiving the spacer-rods  28 .  
         [0029]    As shown in FIGS. 3A to  3 C, an edge binder  66  is disposed around the perimeter of the assembled plates  50  &  52  and spacer grate  16  to engage the perimeter edges  60  of the plates  50  &  52 . The binder  66  encloses the grating space  14  (see FIG. 1) and provides additional structural integrity to the perimeter of the assembled rig mat module  10 . The binder  66  may have a cross section configured in a “T”-shape as shown in FIGS. 3A to  3 C, or may be flush with the perimeter edge (not shown), or some other configuration (e.g., see FIGS. 4A and 4B). The edge binder  66  can be fixed in place or can be removable by a choice of means known to one of ordinary skill in the art, such as a fastener  64  like a rivet or bolt and nut, a screw  67  or an adhesive  68 .  
         [0030]    To form a rig mat assembly, a plurality of rig mat modules are lain on a ground surface adjacent each other in a desired pattern and anchored to the ground surface using stakes. Alternatively, to facilitate anchoring the mat modules  10  relative to each other on a groung surface, as shown in FIGS. 4A and 4B, the rig mat module  10  of the present invention optionally comprises a coupling means for holding the perimeter edge of the rig mat module adjacent to the perimeter edge of an other rig mat module  10 . An example of a coupling means is a simple stake  70  passing through an anchor bore  72  in the plates  50  &  52  and edge binder  66  of the rig mat module  10  proximate its perimeter edge  60  in a number of location to fix adjacent rig mat modules  10  to an underlying surface. The edge binder  60  used in the coupling means can take a variety of cross sectional configurations as shown in the figures. It can be a “+”-shaped binder  66   a  or a plane block binder  66   b . In a preferred embodiment, when the binder edge  66   a  &  66   b  is utilized as a part of a coupling means, it is removable from the both of the adjacent mat modules  10 , as shown in FIG. 4A and 4B. The stake  70   a  can have a squared “U” configuration and be disposed to pass through two adjacent rig mat  10  at the same time. If heavy anchoring of the mat modules  10  is required, the mat modules  10  may further comprise anchor bores  72   a  (see FIG. 3.) distributed over the plane of the mat module, through which heavy duty stakes (not shown) may be driven to anchor the mat module  10  in place on the ground surface.  
         [0031]    As shown in FIG. 5A, the thickness of the plates  50  &  52  can be different depending on the anticipated loading of the top plate  50  and/or the condition of the surface contacting the bottom plate  52 . The plates  50  &  52  themselves can be a single thickness or comprised of a laminate. Additionally, as shown in FIG. 5B, either exterior surface  54  of a plate  50  &  52  can be layered with a cover plate  78  which is removable. The cover plate  78  can be utilized to provide protection to the plate  50  &  52  under it or to increase its loading capacity. A cover plate  78  can be replaced when worn or not needed, or can be switched out to provide an exposed surface  54   a  on the rig mat module  10  suitable for a specific purpose (e.g., a high friction or gripping surface for a slippery environment). The cover plate can be removably attached to the exposed or exterior surface of a top or bottom plate  50  &  52  by any of a variety of means known to the ordinary skilled artisan, such as the use of screw fasteners  64  as shown in FIG. 5B.  
         [0032]    Assembly of the present rig mat  10  from its component parts is readily accomplishable by one of ordinary skill in the art in view of the teachings and figures herein. Although alternative methods are known to the ordinary skilled artisan, one method of assembling the present rig mat  10  is to first assemble the spacer grating  16 . A technique useful for assembling the spacer grating  16  of the rig mat module  10  is known in the art (see U.S. Pat. No.4,522,009). Generally, the grating bars  22  are set out in the desired parallel and spaced relationship with their rod-apertures  24  in alignment. A pair of spacer rods  28  of an appropriate length, oriented with their engagement surfaces  31  in opposition, are inserted through the aligned rod-apertures  24 . A wedge-rod  30  is slid between the two spacer-rods to separate them and cause the notches  32  to engage the thickness (or width W) of the rating bars  22 .  
         [0033]    In a preferred embodiment, all of the points of contact between the various structural components and features of the rig mat module  10  are bonded together, except for the cover plate  78 , if one is utilized. In particular, this is practiced to bond the wedge-rod  30  to the two spacer-rods  28  at the two interface surfaces  46  of the wedge-rod  30 . This may be accomplished using an adhesive, epoxy resin or like bonding agent that is appropriate for the constriction materials of the surfaces to be bound. Such bonding agents are known to and selectable by the ordinary skilled artisan for practice in the present invention. Of course, attachment means other than bonding agents, such as bolt, nail or screw type fasteners  64 , may be utilized where appropriate, e.g., for attaching the binder edge  66  (see FIGS. 3A to  4 B) or a cover plate  78  (see FIG. 5B).  
         [0034]    Preferably, component parts of the present invention  10  are fiber reinforced plastic (FRP) shapes constructed using a pultrusion process as is known in fire art. Also, the present components preferably utilize an isophtalic polyester or a vinyl ester resin with flame retardant and ultra-violet (UV) inhibitor additives. After fabrication, all cut ends, holes and abrasions of the rig mat module preferably are sealed with a compatible resin to prevent fraying and intrusion of moisture. Should additional ultraviolet protection be required, a UV coating can be applied.  
         [0035]    To demonstrate the utility of the present invention, a rig mat module  10  useful in a travel way or work apron assembly was produced and fabricated into an assembled surface.  
       EXAMPLE  
     Spacer Grate for Large Rig Mat Module  
       [0036]    This embodiment of a rig mat module  10  was approximately 8 feet across and 30 feet long. FIG. 6 is a top view, partial schematic layout of the primary components of the rig mat module, without the top and bottom plates being shown. The cross-rods  26  were spaced at about 18 inches from the adjacent cross-rod  26 . Twenty cross-rods were used. In this embodiment, the wedge-rod  30  of each cross rod  26  was a single piece. However, each of the two spacer-rods  28  comprised a two-piece length (section A, FIG. 6) which is further illustrated in FIG. 7. FIG. 7 illustrates how the two separate pieces of each spacer-rod  28  were butted together and used in combination with the wedge-rod  30  to provide the wedge-rod  26  of this embodiment.  
         [0037]    The grating bars  22  (only 2 of a plurality shown) each had a length L of about 30 feet. Because grating bars of that length were not readily available, each grating bar  22  was comprised of two sub-lengths and butted together (section B, FIG. 6) to form an overall gating bar  22  of de desired length, which is further illustrated in FIG. 8.  
         [0038]    The top and bottom plates  50  &  52  were constructed of 0.5 inch thick fiberglass plating. The height H of the grating bars  22 , and hence the spacer grate  16 , was 1.5 inches, which made the overall thickness of the rig mat module  10  about 2.5 inches. The prototype rig mat module was 8 ft.×30 ft.×2.5 in. The first successful testing of the prototype rig mat module  10  was at −34° F. and with weight in excess of 185 psi. Additional testing of the prototype rig mat module  10  to 485 psi was without failure.  
         [0039]    While the above description contains many, specifics, these should not be constructed as limitations on the scope of the invention, but rather as exemplifications of one or another preferred embodiment thereof. Many other variations are possible, which would be obvious to one skilled in the art. Accordingly, the scope of the invention should be determined by the scope of the appended claims and their equivalents, and not just by the embodiments.