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CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of copending U.S. Provisional Patent Application Ser. No. 62/026,294 entitled “MONOBLOC DOOR FRAME FOR MODULAR BUILDING UNITS,” filed with the U.S. Patent and Trademark Office on Jul. 18, 2014 by the inventor herein, the specification of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to modular building construction, and more particularly to a system of interlocking door frame and wall panels in a modular building unit. 
     BACKGROUND OF THE INVENTION 
     Modular buildings typically comprise a skeletal metal (e.g., steel) frame that supports a number of elements allowing easy transportation, construction, and customization of the building unit. Those elements typically include a solid roof and floor and wall panels that are inserted into the frame. In order to provide access into the building unit, a door is typically provided on at least one of the walls of the building unit. However, installation of standard door assemblies into the modular assembly of a typical modular building unit has required that wall panels of the modular building unit be cut to receive a standard steel door frame. This often results in an unclean, unfinished appearance, a weak frame with low durability, and likewise often leaves gaps through which air will pass, making heating and cooling more difficult than if a properly sealed assembly were provided. An alternative is installation of a non-standard, custom door assembly that requires an expensive and complex aluminum design, requiring significant tooling investment and providing no flexibility. 
     Thus, there remains a need in the art for a modular building unit construction that allows fitment of a traditional door assembly, including a supporting door frame, that may be easily integrated with the modular wall panels of the modular building unit in a configuration that provides a more finished and clean appearance than previously known assemblies, and that provides a structurally reinforced frame, all while maintaining ease of transportability, installation, and customization of the modular building unit. 
     SUMMARY OF THE INVENTION 
     Disclosed is a system of interlocking door frame and wall panels for modular building units. Both the door frame panel and the wall panels are provided for easy installation into the skeletal frame of the modular building unit, and are provided with mating male and female connecting edges on opposite sides of each panel. This allows the door frame panel to mate with an adjacent wall panel in the same manner in which adjacent wall panels mate with one another, ensuring a clean, finished appearance for the connecting edges and a connection having the same physical and structural integrity as the connections between adjacent wall panel units. The door frame panel itself is particularly configured to receive a traditional, commercially available steel door frame and door assembly, but allows fitment of such commercially available steel door frame and door assembly without cutting a wall panel to receive that frame. 
     In accordance with certain aspects of an embodiment of the invention, an interlocking door frame panel and wall panel system for modular building units is disclosed, comprising: a plurality of wall panels of identical construction, each wall panel further comprising: an external sheet and an internal sheet; a vertical tongue extending along a first vertical edge of the wall panel and formed by a first edge of each of the external sheet and the internal sheet; a vertical groove extending along a second vertical edge of the wall panel and formed by a second edge of each of the external sheet and the internal sheet, wherein the vertical groove of a first panel is configured to mate with the vertical tongue of an adjacent panel to form a connected edge in which the external sheets of adjacent panels are in contact and coplanar with one another, and the internal sheets of adjacent panels are in contact and coplanar with one another; and at least one door panel comprising: a first vertical door panel frame member forming a vertical tongue extending along a first vertical edge of the door panel, the first vertical door panel frame member being configured to mate with a vertical groove of a second adjacent wall panel to form a connected edge in which the external sheet of the second adjacent wall panel is in contact and coplanar with an external side of the first vertical door panel frame member, and the internal sheet of the second adjacent wall panel is in contact and coplanar with an internal side of the first vertical door panel frame member; and a second vertical door panel frame member forming a vertical groove extending along a second vertical edge of the door panel, the second vertical door panel frame member being configured to mate with a vertical tongue of a third adjacent wall panel to form a connected edge in which the external sheet of the third adjacent wall panel is in contact and coplanar with an external side of the second vertical door panel frame member, and the internal sheet of the third adjacent wall panel is in contact and coplanar with an internal side of the second vertical door panel frame member. 
     In accordance with further aspects of an embodiment of the invention, a modular building unit is disclosed comprising: a rigid, fixed modular building unit frame having a plurality of corner support posts, a bottom rail extending between adjacent pairs of corner support posts, a top rail extending between adjacent pairs of corner support posts, a plurality of wall panels extending between the corner support posts, the top rails, and the bottom rails, and at least one door panel extending between one of the top rails and one of the bottom rails; wherein each wall panel further comprises: an external sheet and an internal sheet; a vertical tongue extending along a first vertical edge of the wall panel and formed by a first edge of each of the external sheet and the internal sheet; a vertical groove extending along a second vertical edge of the wall panel and formed by a second edge of each of the external sheet and the internal sheet, wherein the vertical groove of a first panel is configured to mate with the vertical tongue of an adjacent panel to form a connected edge in which the external sheets of adjacent panels are in contact and coplanar with one another, and the internal sheets of adjacent panels are in contact and coplanar with one another; and wherein the door panel further comprises: a first vertical door panel frame member forming a vertical tongue extending along a first vertical edge of the door panel, the first vertical door panel frame member being configured to mate with a vertical groove of a second adjacent wall panel to form a connected edge in which the external sheet of the second adjacent wall panel is in contact and coplanar with an external side of the first vertical door panel frame member, and the internal sheet of the second adjacent wall panel is in contact and coplanar with an internal side of the first vertical door panel frame member; and a second vertical door panel frame member forming a vertical groove extending along a second vertical edge of the door panel, the second vertical door panel frame member being configured to mate with a vertical tongue of a third adjacent wall panel to form a connected edge in which the external sheet of the third adjacent wall panel is in contact and coplanar with an external side of the second vertical door panel frame member, and the internal sheet of the third adjacent wall panel is in contact and coplanar with an internal side of the second vertical door panel frame member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a modular building unit in accordance with certain aspects of an embodiment of the invention. 
         FIG. 2  is a top down, sectional view of an exemplary configuration of the modular building unit of  FIG. 1 . 
         FIG. 3  is an exploded view of a wall panel for use with modular building unit of  FIG. 1 . 
         FIG. 4  is a top down, cross-sectional view of an assembled wall panel of  FIG. 3 . 
         FIG. 5  is a top view of the joining of two mated wall panels of  FIG. 4 . 
         FIG. 6  is a side view the wall panel of  FIG. 4 . 
         FIG. 7  is a cross-sectional view of wall panels of  FIG. 4  installed on the frame of the modular building unit of  FIG. 1 . 
         FIG. 8  is a front view of a door panel  200  (shown without the commercially available door and door frame assembly) for use with the modular building unit of  FIG. 1 . 
         FIG. 9  is a front view of a door panel male edge connecting profile. 
         FIG. 10  is a cross-sectional view of the door panel male edge connecting profile of  FIG. 9  along section line A-A of  FIG. 9 . 
         FIG. 11  is a front view of a door panel female edge connecting profile. 
         FIG. 12  is a cross-sectional view of the door panel female edge connecting profile of  FIG. 11  along section line B-B of  FIG. 11 . 
         FIG. 13  is a cross-sectional view of the door panel male edge connecting profile and female edge connecting profile with an installed commercially available door frame. 
         FIG. 14  is an exploded view of a transom assembly for use with the door panel of  FIG. 8 . 
         FIG. 15  is a cross-sectional view of the door panel male edge connecting profile of  FIG. 9  and the female edge connecting profile of  FIG. 11  with attached transom side members. 
         FIG. 16  is a front view of a door panel bottom profile for use with the door panel of  FIG. 8 . 
         FIG. 17  is a cross-sectional view of the bottom profile along section line C-C of  FIG. 16 . 
         FIG. 18  is a cross-sectional view of the bottom profile along section line D-D of  FIG. 16 . 
         FIG. 19  is a cross-sectional view of the door panel along section line E-E of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is of a particular embodiment of the invention, set out to enable one to practice an implementation of the invention, and is not intended to limit the preferred embodiment, but to serve as a particular example thereof. Those skilled in the art should appreciate that they may readily use the conception and specific embodiments disclosed as a basis for modifying or designing other methods and systems for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent assemblies do not depart from the spirit and scope of the invention in its broadest form. 
       FIG. 1  provides a perspective view of a modular building unit  100  in accordance with certain aspects of an embodiment of the invention. Modular building unit  100  includes a skeletal frame formed by corner support posts  112 , bottom rails  114 , and top rails  116 , wall panels  118 , windows  120 , and door panels  200 . This skeletal frame provides the key structural integrity for the modular building unit. As shown in  FIG. 1 , wall panels  118  are positioned between corner support posts  112 , bottom rails  114 , and top rails  116 , which wall panels  118  form the wall structures spanning each side of the modular building unit. 
       FIG. 2  is a top down, sectional view of the modular building unit  100 . As shown in  FIG. 2 , wall panels  118  are joined to one another, and to both corner brackets  112  and door panel  200 , by edge profiles, the construction of which is described in detail below. With continued reference to  FIG. 2  and to the exploded view of a wall panel  118  of  FIG. 3 , each wall panel  118  is comprised of an external sheet  130  which forms the exterior wall surface of the modular building unit  100 , an internal sheet  132  which forms the interior wall surface of the modular building unit  100 , and an insulation panel  134  positioned between external sheet  130  and internal sheet  132  providing added rigidity to each wall panel  118 , in addition to thermal insulation for the modular building unit  100 . With particular reference to  FIG. 3 , wall panels  118  also include a bottom profile  136  for joining wall panel  118  to bottom rails  114  of the frame of the modular building unit  100 , and a top profile  138  for joining wall panel  118  to top rails  116  of the frame of the modular building unit  100 . 
       FIG. 4  provides a top down, cross-sectional view of an assembled wall panel  118 . Each wall panel  118  has a first side edge comprising a male profile  140 , and a second, opposite side edge comprising a female profile  150 . Male profile  140  is configured to engage with female profile  150  on an adjacent wall panel  118  in a tongue-and-groove mating connection. Male profile  140  extends from both external sheet  130  and internal sheet  132 , and includes an inwardly bent leg  142  and a tongue wall  144  extending outward from leg  142  in a direction generally parallel to each of external sheet  130  and internal sheet  132  so as to form a sidewall of the tongue of male profile  140 . An inwardly extending flange  146  may also be provided extending inward from tongue wall  144 . Insulation panel  134  preferably has a complementary male profile adjacent male profile  140 , such that inwardly extending flanges  146  may overlap the edge of insulation panel  134 . Likewise, female profile  150  extends from the opposite ends of both external sheet  130  and internal sheet  132  so as to form sidewalls of female profile  150 , and is defined by two u-shaped end walls  152 . Each u-shaped end wall  152  is sized to fit within the open space between tongue wall  144  and the outer wall of the respective inner and outer sheet  130  and  132 . 
       FIG. 5  shows two such wall panels  118  mated together with the male profile  140  of the panel on the right in  FIG. 5  engaging the female profile  150  of the wall panel on the left in  FIG. 5 . As shown in  FIG. 5 , the interior face of u-shaped end wall  152  sits flush against the exterior face of tongue wall  144  of the adjacent panel, such that the outer faces of adjacent external sheets  130  and internal sheets  132  are aligned. 
       FIG. 6  shows a side view of wall panel  118 , again including external sheet  130  shown on the left side of  FIG. 6  and internal sheet  132  shown on the right side of  FIG. 6 , with insulation panel  134  positioned between them. Likewise, a bottom wall panel profile  136  is provided closing the bottom of wall panel  118 , and a top wall panel profile  138  is provided closing the top of wall panel  118 . 
     With continued reference to  FIG. 6 , bottom wall panel profile  136  includes a front wall  160  having a top portion that is positioned between external sheet  130  and insulation panel  134 , and a lower portion extending downward from the bottom of external sheet  130 . Bottom wall panel profile  136  likewise includes a back wall  162  extending between internal sheet  132  and insulation panel  134 . An internal ledge  164  extends outward from back wall  162  and sits adjacent to and supports a bottom edge of insulation panel  134 . Further, a bottom rail engaging extension  165  forms a notch  137  configured to receive a ridge  115  on bottom rail  114 , as discussed in greater detail below with respect to  FIG. 7 . 
     Likewise, with reference to both  FIGS. 3 and 6 , top wall panel profile  138  is comprised of a planar top wall and downwardly extending sidewalls that are positioned between insulation panel  134  and each of external sheet  130  and internal sheet  132 . This top wall panel profile provides a block structure that may be inserted into an open, generally rectangular space formed at the top of the frame, as discussed below with regard to  FIG. 7 . 
       FIG. 7  shows a cross-sectional view of wall panels  118  installed on the frame of the modular building unit  100 . As shown in  FIG. 7 , a ceiling tray end bracket  117  is attached to the bottom side of top rail  116 , and defines a rectangular opening that receives the top of wall panel  118  and top profile  138 . Top rail  116  and ceiling tray end bracket  117  are positioned such that when a wall panel  118  is positioned within the frame of the modular building unit  100 , an open space exists between the top of wall panel  118  and the bottom surface of ceiling tray end bracket  117 . A rubber gasket  119  may be provided between the back, top edge of wall panel  118  and ceiling tray end bracket  117  to provide a seal against, for example, water infiltration. Likewise and as mentioned briefly above, a ridge  115  extends upward from a top side of bottom rail  114 , which ridge  115  is configured to mate with notch  137  on bottom profile  136  of wall panel  118 . Likewise, a rubber gasket  121  may be provided between notch  137  and ridge  115  to provide additional sealing of the wall assembly. When so joined, ridge  115  and notch  137  serve to hold the bottom of wall panel  118  in place on the frame of the modular building unit  100 . Nonetheless, such connection allows removal and adjustment of the wall panels  118  within the frame. More particularly, the space provided between the top of the wall panel  118  and the ceiling tray end bracket  117  allows panel  118  to be lifted so that the bottom profile  136  of wall panel  118  may clear ridge  115  on bottom rail  114 , such that the bottom of the panel  118  may be pulled outward and the entire panel moved or removed from the modular building unit. 
     Next,  FIG. 8  provides a front view of a door panel  200  (shown without a commercially available door and door frame assembly for clarity) for use with the modular building unit  100  of  FIG. 1 . An important feature of the instant invention is that, as described further below, door panel  200  is provided side edge profiles having male and female connection profiles that mate with the male and female profiles  140  and  150 , respectively, described above with respect to wall panels  118 . This configuration allows ready placement of door panel  200  anywhere as may be desired within the frame of the modular building unit  100 , as it may mate with any other wall panels within the frame. Likewise, door panel  200  is provided a top profile and a bottom profile that connect to the top and bottom portions of the frame in the same manner as wall panels  118 , such that no specialized modification or construction is required in order to fit a door panel and its associated door and door frame into the modular building unit  100 . 
     Door panel  200  includes a door panel bottom profile  202  similar in configuration to wall panel bottom profile  136  and having a notch identical to notch  137  in wall panel bottom profile  136 . Likewise, door panel  200  includes a door panel male profile  204  matching the mating configuration of male profile  140  of wall panel  118 , and a door panel female profile  206  matching the mating configuration of female profile  150  of wall panel  118 . Thus, door panel male profile  204  has an edge profile configured for mating connection with wall panel female profile  150 , and door panel female profile  206  has an edge profile configured for mating connection with wall panel male profile  140 . Door panel  200  also includes a door panel top profile  208  having a rectangular exterior perimeter of substantially the same configuration as wall panel top profile  138 . 
     In order to allow fitment of a standard size, commercially available door and door frame, which typically will be of a height dimension that is smaller than the full height dimension of a wall panel  118 , a transom assembly is provided in door panel  200 , such that the bottom of the transom, along with the interior edges of door panel male profile  204 , door panel female profile  206 , and door panel bottom profile  202  define a door opening  210  configured to receive a standard door and door frame. 
       FIG. 9  provides a front view of door panel male profile  204 , and  FIG. 10  provides a cross-sectional view of door panel male profile  204  along section line A-A of  FIG. 9 . As shown in  FIGS. 9 and 10 , door panel male profile  204  comprises a single, one piece male profile bracket  240 . Bracket  240  has a side wall portion  240   a  forming an interior vertical wall of door opening  210 , external planar wall portion  240   b  forming an external surface of door panel  200 , and internal planar wall portion  240   c  forming an internal surface of door panel  200 . Inwardly bent leg  242  extends inward from the ends of each of external planar wall portion  240   b  and internal planar wall portion  240   c , and a tongue wall  244  extends outward from each inwardly bent leg  242  in a direction generally parallel to external planar wall portion  240   b  and internal planar wall portion  240   c . Each tongue wall  244  may likewise include an inwardly extending flange  246 . This configuration closely mirrors the male profile  140  of wall panel  118 , and is sized to receive u-shape end walls  152  of female profile  150  of wall panel  118 . 
     Likewise,  FIG. 11  provides a front view of door panel female profile  206 , and  FIG. 12  provides a cross-sectional view of door panel female profile  206  along section line B-B of  FIG. 11 . As shown in  FIGS. 11 and 12 , door panel female profile  206  comprises a single, one piece female profile bracket  250 . Bracket  250  has a side wall portion  250   a  forming an interior vertical wall of door opening  210 , external planar wall portion  250   b  forming an external surface of door panel  200 , and internal planar wall portion  250   c  forming an internal surface of door panel  200 . U-shaped end walls  252  are positioned at the distal ends of each of external planar wall portion  250   b  and internal planar wall portion  250   c , and are sized to fit within the open space between the tongue wall  144  and the outer wall of respective inner and outer sheets  130  and  132  of a wall panel  118 . 
       FIG. 13  is a cross-sectional view of door panel male profile  204  and door panel female profile  206  with an installed door frame  260 , such as a traditional door frame include a door stop and hinges for mounting a standard door (not shown), which door frame  260  extends from the door panel bottom profile  202  to the bottom of the transom assembly at the top of door panel  200 . Further, a reinforcement member  270  may be provided inside of the open ends of each of door panel male profile  204  and door panel female profile  206  to provide additional structural rigidity to each of profiles  204  and  206 . Reinforcement member  270  has a facing wall spanning the opening between opposing outer planar wall portions of each of door panel male profile  204  and door panel female profile  206 , and side walls for engagement with and attachment to the interior faces of those planar wall portions. Reinforcement member  270  thus forms an open space within the interior of each of door panel male profile  204  and door panel female profile  206 , which open space may optionally receive insulation to provide an additional thermal barrier to the interior of modular building  100 . 
       FIG. 14  provides an exploded view of the transom assembly at the top of door panel  200 . The transom assembly is formed by door panel top profile  208 , transom lower cross member  282 , and transom side members  284 . Each of door panel top profile  208 , transom lower cross member  282 , and transom side members  284  are preferably formed from u-shaped channels as discussed further below with regard to  FIG. 15 . Further, a solid transom panel  286  may be provided to fill the open space formed by door panel top profile  208 , transom lower cross member  282 , and transom side members  284  to fully close the door panel structure. Solid transom panel  286  may be formed of foam, a steel plate, a glass window, or any other structure as may be suitable for a particular installation. 
       FIG. 15  provides a cross-sectional view of door panel male profile  204  and female profile  206  with attached transom side members  284 . The legs of each u-shaped transom side member  284 , as well as the legs of u-shaped door panel top profile  208  and u-shaped transom lower cross member  282 , are each provided an inwardly directed hook portion at their respective distal ends, the interiors of which form an opening for receiving and holding transom panel  286  in place. 
     Further and again as mentioned above, door panel  200  includes door panel bottom profile  202  configured for attachment to bottom rails  114  in the same manner as bottom profile  136  of wall panels  118 .  FIG. 16  provides a front view of door panel bottom profile  202 , and comprises end portions  290  and a mid-section  292  extending between the two end portions  290 . Mid-section  292  is vertically aligned with door opening  210 . 
       FIG. 17  is a cross-sectional view of end portion  290  of door panel bottom profile  202  along section line C-C of  FIG. 16 , and  FIG. 18  is a cross-sectional view of mid-section  292  of door panel bottom profile  202  along section line D-D of  FIG. 16 . Each of end portions  290  and mid-section  292  comprise the same segments as wall panel bottom profile  136 , including a front wall  160 , back wall  162 , internal ledge  164 , and bottom rail-engaging extension  165  forming notch  137 . The difference between end portions  290  and mid-section  292  resides in the height of front wall  160  and back wall  162 ; namely, mid-section  292  is shortened in height from end portions  290 . As shown in the cross-sectional view of door panel  200  of  FIG. 19  (along section line E-E of  FIG. 8 ), by providing mid-section  292  with a shortened height dimension from the end portions  290 , door panel bottom profile  202  may accommodate a door stop  296 . A door frame floor insert  294  is positioned within front wall  160  and back wall  162  of the mid-section  292  of door panel bottom profile  202 , such that door stop  296  (of conventional configuration) may be positioned overtop of and affixed to door frame floor insert  294 . 
     The foregoing configuration results in a modular building construction having interlocking wall panels that mate with one another and with a door panel in the same manner, thus providing a common configuration for joining panels regardless of whether or not they serve as a wall panel or are fitted with a door. While the door panels are specially and modularly configured to receive a full, traditional, commercially available door and door frame assembly, the door frame that receives such commercially available door and door frame assembly is readily fitted into any position around the perimeter of the modular building unit, and can readily be moved, removed, or adjusted as necessary given the common configuration of mating edges between both wall panels and the door panel. This significantly simplifies installation of doors and door frames and maintains a uniform appearance at panel edges, avoids the problems experienced by traditional door installations for modular buildings that have required cutting a hole in a wall panel to fit a door and door frame into a desired spot, and provides a structurally reinforced door assembly in a flexible-use frame without requiring the investment in aluminum profile tooling required for expensive, custom door assemblies. 
     Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It should be understood, therefore, that the invention may be practiced otherwise than as specifically set forth herein.

Summary:
Disclosed is a system of interlocking door frame and wall panels for modular building units. Both the door frame panel and the wall panels are provided for easy installation into the skeletal frame of the modular building unit, and are provided with mating male and female connecting edges on opposite sides of each panel. This allows the door frame panel to mate with an adjacent wall panel in the same manner in which adjacent wall panels mate with one another, ensuring a clean, finished appearance for the connecting edges, a strong, reinforced frame having long durability, and a connection having the same physical, structural integrity as the connections between adjacent wall panel units. The door frame panel itself is particularly configured to receive a traditional, commercially available steel door frame and door assembly, but allows fitment of such commercially available steel door frame and door assembly without cutting a wall panel to receive that frame.