A modular unit connection system for joining together a plurality of box-shaped modular units to form a single or multistory building. The modular building units have elongated hollow structural framing members at their vertical corners and four substantially perpendicular vertical side walls extending between their vertical corner members. The side walls are topped with horizontal framing members extending between the vertical corner members. The vertical corner members lie within the planes formed by the side walls of the modular units, and their vertical corner members and their adjacent side walls abut with no significant space between them. The modular units may be connected at their vertical corner members with generally flat connection plates. Threaded tension rods may extend through the hollow vertical corner members and may be coupled to tension rods running through the hollow vertical corner members of vertically aligned modular units.

CROSS-REFERENCED TO RELATED APPLICATIONS 
     This non-provisional patent application claims the benefit of and priority to Provisional U.S. patent application Ser. No. 61/723,139, entitled MODULAR BUILDING UNIT CONNECTION SYSTEM, which is hereby incorporated herein in its entirety by reference thereto. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to modular buildings, and more particularly to a system for connecting modular units together to construct multiple unit modular building structures. 
     BACKGROUND OF THE INVENTION 
     Modular units have been used to construct residential and industrial structures because they can be assembled at a factory and transported via train, truck or ship to a construction site for assembly into a multi-unit building thereby avoiding much of the delay and expense of field construction. To achieve the maximum benefit from the modular building it is important that modular units be constructed and interconnected in ways which maximize the strength of an assembled modular building, while minimizing the use of external framing and bracing which requires field construction. The interconnection of the box-shaped modules which are the basic building blocks of modular construction has been found to be a critical component in achieving the goals of modular construction. 
     Various connection systems have been used to enable multi-unit modular buildings to resist external forces, such as those imposed on a building by severe weather. Examples of such systems are disclosed in U.S. Pat. No. 3,831,332 to Weese and U.S. Pat. No. 6,871,453 to Locke. Weese's system links adjacent modular units using a central support column comprised of column segments which extend diagonally from the corner of each modular unit. Tension rods extend through Weese's central columns and are secured at the opposite ends of the columns to hold the column segments and their appended modular units together. Weese's system may be effective with a modular building of a few stories, but the structural strength of Weese's connection system is limited and the geometry of his system results in adjacent modular units being separated by a distance corresponding to the aggregate lateral projections of the column segments which extend from his modular units. The present inventors have determined that avoidable separation of the exterior load bearing walls of adjacent modular units is undesirable in that it tends to make a multi-unit modular structure less rigid and less capable of resisting external forces. 
     Locke's modular unit connection system utilizes connector bars having opposed upper and lower conical sections with an axial bore to join adjacent modular units. Structural members on Locke's modular units are equipped with conical recesses which are adapted to receive the conical sections at the ends of Locke's connector bars. Tensioner run through the axial bores in Locke's connector bars to hold vertically aligned modular units together. As with Weese's system, Locke's system results in a gap between adjacent modular units. It also suffers the detriment of requiring the fabrication of complex shapes for Locke's connectors and the recesses in the structural member which receive them. 
     The present invention relates to a novel system for constructing a building with box-shaped modular units. A principal object of the invention is to provide a system which enables the quick and efficient construction of high rise modular buildings having sufficient structural strength to resist the external forces associated with tall buildings and for which the connection work can be performed from the outside of the building's modules, thereby maximizing the amount of finish work that can be completed in the factory. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a modular unit connection system for joining together a plurality of box-shaped modular units to form a single or multistory building. The modular building units of the invention have structural framing members at least their vertical corners which are tubular in nature. As used herein, the term tubular refers to elongated hollow framing members. Such framing members generally have a rectangular cross-section, but may have any hollow cross-section. Framing members with a rectangular cross-section are preferred because they are easily connected to other framing member by welding or with fasteners. However, the instant invention can also be utilized on modular building units having vertical corner framing members with any cross-section which provides a generally vertical conduit from the bottom of the modular unit to the top of the modular unit. 
     According to the invention, the vertical corner structural members of the modular units lie within the planes formed by the exterior walls of the modular units. With this construction, when two or more modular units are laid side by side with their corners aligned, their vertical corner structural members and their adjacent walls abut with no significant space between them. When four box-shaped modular units are aligned so that they share a single common corner, the four vertical structural members which comprise that common corner align with no material space between them. As used herein, no material space means within normal manufacturing tolerances for the materials of construction. In the case of framing members made of structural steel connected by welding or with fasteners the gap between adjacent vertical structural members will generally be less than one inch, and preferably one half inch or less. 
     The aforedescribed modular units are connected at their upper corners with a generally flat connection plate which overlays the adjacent corners of the modules being connected and which is fastened to the horizontal framing of the modular units adjacent to the vertical corner columns. The connection plate has apertures, which when the connection plate is mounted on an assembly of modular units, overlies the vertical tubular corner members so as to provide access to the vertical conduits formed thereby. Threaded tension rods extend through the tubular corner members and are coupled to tension rods running through the tubular corner members of vertically aligned modular units forming the successive stories of the modular building. The combination of tension rods and plates provides a connection between columns which holds the modules together, while resisting tension loads in the columns and distributing loads and stress throughout the modular building. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
         FIG. 1  is a perspective view of an eight module two story modular building section. 
         FIG. 2  is an illustrative top view of the connection plate of one embodiment of the invention for connecting four modular units on a floor at a common corner. 
         FIG. 3  is an illustrative top view of the modular building unit connection system of the invention connecting four modular units on a floor at a common corner. 
         FIG. 4  is an illustrative sectional view taken along line A-A of the connection system of  FIG. 3 . 
         FIG. 5  is an illustrative top view of the modular building unit connection system of the invention connecting two modular units on a floor at a common corner. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the invention. 
     Referring to  FIG. 1 , there is shown a perspective view of two story modular building section  10  comprised of eight modular box-shaped units  11 - 18 . Although the present invention is described with respect to modular building section  10 , it applies equally to multi-story high-rise buildings and to structures having any number of modular units on each floor. 
     Referring to  FIG. 2 , there is shown a top view of connection plate  100  for joining four adjacent box-shaped modular units at a common corner, such as modules  11 ,  12 ,  13  and  14  and modules  15 ,  16 ,  17  and  18 . Connection plate  100  includes eight pairs of unthreaded bolt holes  101  for affixing connection plate  100  to modular units  11 ,  12 ,  13  and  14  and modular units  15 ,  16 ,  17  and  18 . Connection plate  100  also has four tension rod holes  102 . Connection plate  100  has two annular setting pins  103  for ensuring the proper alignment of successive stories of modular units. The typical location of module corner columns  104  when connection plate  100  is installed on modular units  11 - 14  and modular units  15 - 18  is shown in dotted lines in  FIG. 2 . 
     Referring to  FIG. 3 , connection plate  100  is shown installed at the common corner of modular units  15 ,  16 ,  17  and  18 , which abut at matelines M 1  and M 2 . The upper story of building section  10  comprised of modular units  11 ,  12 ,  13  and  14  are mounted on top of modular units  15 ,  16 ,  17  and  18 , as shown in  FIG. 1 . Connection plate  100  is mounted to ceiling edge purlins  106  of modules  15 - 18  and separates the corner columns  104  of upper story modules  11 - 14  from corner columns  104  of lower story modules  15 - 18 . Connection plate  100  is bolted with bolts  105  through bolt holes  101  to ceiling edge purlins  106  at connector angles  107 . Connector angles  107  are preferably welded to ceiling edge purlins  106 . Tension tie rods  108  extend through each of corner columns  104  in modules  15 - 18  and tension tie rods  115 , shown in  FIG. 4 , extend through each of corner columns  104  in modules  11 - 14 . Tension tie rods  108  pass through connection plate  100  at tension rods holes  102  and are fastened to connection plate  100  with rod nuts  109  and washers  110 , which are best seen in  FIG. 4 . To ensure the precise align of upper story modules  11 - 14  with modules  15 - 18  in the lower story, modules  11  and  12  with column base caps  111  are set down over annular setting pins  103  and modules  13  and  14  are set down around washers  110  at the positions on connection plate  100  where tension rod holes  102  do not have annular setting pins  103 . Column base caps  111  are welded to module corner columns  104  on modules  11 - 14  and are sized to snuggly fit over the annular setting pins  103  to control the geometric placement of the modules. 
       FIG. 4  illustrates the abutment of modular units  12 ,  13 ,  16  and  17  along matelines M 3  and M 4  and how tension tie rods  108  and  115  connect successive vertical stories of a modular building according to the invention. Modular units  12  and  13  are placed on top of, respectively, modular units  16  and  17  with corner columns  104  of units  12  and  16  and of units  13  and  17  in overlapping alignment, but separated by connection plate  100 . The precise alignment of corner columns  104  of modular units  12  and  13  with corner columns  104  of modular units  16  and  17  is assisted by annular setting pins  103  and column base caps  111  as described above. Floor purlins  114  of modular units  12  and  13  lie on top of ceiling purlins  106  of modular units  16  and  17 , separated by connection plate  100 . Connection plate  100  is affixed to ceiling edge purlins  106  at connector angles  107 . Tension rods  108  extend through corner columns  104  of modular units  16  and  17  and through tension rod holes  102  in connect plate  100 . Tension rods  108  are threaded and are affixed to connection plate  100  using washers  110  and tie rod nuts  109 . Tension tie rods  108  extend upward beyond connection plate  100  into the interior of corner columns  104  in modular units  12  and  13 . Upper story tension tie rods  115  extend through tension rod holes  102  in a connector plate  100  which is affixed to the top of corner columns  104  of modules  11 ,  12 ,  13  and  14  and through corner columns  104  in each of modules  11 ,  12 ,  13  and  14 . As illustrated with respect to modules  16  and  17 , upper story tension rods  115  are interconnected with tension tie rods  108  in modules  15 ,  16 ,  17  and  18 , using sleeve nuts  113 . Sleeve nuts  113  are tightened to tension the connection of tension tie rods  108  and  115 . Each successive story of a modular building employing the modular unit connection system of this invention preferably has tension tie rods running through its tubular corner columns interconnected with tension tie rods extending through the corner columns of the modular building's lower and upper stories in the same way as described herein with respect to modular units  11 - 18 . Similarly, adjacent modular units are connected at their corner columns with connection plates as described herein with respect to modular units  15 - 18 . 
     In  FIG. 4 , flooring  118  in modular units  12  and  13  lies upon cement board  117 . Cement boards  117  sits upon floor decking  116 . Wall board  119  is applied to the interior surfaces of modules  12 ,  13 ,  16  and  17 , and ceiling board  120  is affixed to ceiling edge purlin  106 . 
     As illustrated in  FIG. 3  and  FIG. 4 , modular units connected with the connection system of the invention abut with minimal separation. The distance between modular units which are aligned vertically is preferably less than about 1 inch and most preferably by less than ½ inch. Adjacent modular units which are on the same floor of a modular building employing the connection system of the invention also abut with a separation which is preferably less than 1 inch, and most preferably less than ½ inch. When modular building units are connected in accordance with the teachings of the present invention, loads and stress applied to any portion of the modular building are communicated to the structure as a whole, which enables the structure to better resist the same. 
     It will be evident to those skilled in the art of the invention that the connection system of the invention is readily adapted to situations in which the number of building modules having adjacent corners is less than the aforedescribed four units. When, for example, a building floor plan calls for only two or three modules to have adjacent corners, the shape of connection plate  100  is changed so that it overlays only the modules requiring connection. Even in the case of building modules having corners which are not adjacent to any other modules, as is common for modules positioned at a structure's exterior corners, the modular unit connection system of the invention can be used to connect vertically aligned modules with tension tie rods which run through the modules isolated exterior corner columns. 
     Referring to  FIG. 5 , connection plate  200  is shown installed at the common corner of modular units  19  and  20 , which abut at mateline M 5  Connection plate  200  is bolted with bolts  105  through bolt holes  101  in connection plate  200  to ceiling edge purlins  106  of modules  19  and  20  at connector angles  107 . If an upper story of modules overlie modules  19  and  20 , connection plate  200  separates the corner columns  104  of upper story modules from corner columns  104  of lower story modules  19  and  20 . Connector angles  107  are preferably welded to ceiling edge purlins  106 . Tension tie rods  108  extend through corner columns  104  in modules  19  and  20  and pass through connection plate  200  at tension rods holes  102  and are fastened to connection plate  200  with rod nuts  109  and washers  110 . To ensure the precise align of any upper story module with module  19  in the lower story, the upper story module with column base cap  111  can be set down over setting pins  203  on connection plate  200 . Any upper story module to be mounted on top of module  20  is set down around washer  110  on tension rod  108  which runs through corner column  104  in module  20 . Column base caps  111  are welded to module corner columns  104  and are sized to snuggly fit over the setting pins  203  and around washer  110  to control the geometric placement of the modules. 
     Although the invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. It is further contemplated that various combinations and sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. It is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow. 
     It should be noted that certain objects and advantages of the invention have been described above for the purpose of describing the invention and the advantages achieved over the prior art. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.