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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a beam, that is rectangular in cress-section and in particular is made from a hollow section of an extruded light metal alloy, having at least one undercut groove running parallel to the longitudinal axis of the extruded section in at least one outer face to accommodate an element for connecting a further hollow section. The related cross-sectional faces bearing the longitudinal grooves for the connecting element of a pair of beams are of approximately the same cross-sectional dimensions. Also within the scope of the invention is a construction unit made up of such beams. 
     2. Discussion of the Prior Art 
     Known e.g. from EP 0 589 380—for the purpose of arm-like or frame-like beams for installation in machines of all kinds—are hollow sections having a quadratic cross-section and cylindrical opening along the central axis and, parallel to this axis, undercut grooves in each of the outer faces. Hollow sections of this design may be joined together e.g. by means of screws introduced transverse to their longitudinal axis and also to other items such as plates or similar flat elements. These screws are situated towards the ends in a block that can be inserted in the undercut groove. It is also possible to fit together in parallel two of the same hollow sections of quadratic cross-section, thus forming a post or the like construction unit to manufacture display stands for industrial fairs or similar constructions. 
     SUMMARY OF THE INVENTION 
     In view of this state of the art, the object of the present invention is to extend the range of application of beams of the kind mentioned above and to make it possible to fit these beams together to make a variety of construction units. 
     In accordance with the invention the beams are hollow sections with rectangular cross-section with different cross-sectional length and cross-sectional breadth, the cross-sectional length of one hollow section corresponding approximately to the cross-sectional breadth of the other related hollow section. It is possible therefore for combinations of interlocking sections to be formed from a plurality of hollow sections having the dimensions specified by the invention; it is therefore possible to fit together a large number of hollow sections with matching cross-sectional faces. It has been found particularly favourable to select a ratio of cross-sectional length to cross-sectional breadth in each of the hollow sections such that this lies between 1.2 and 2.5, preferred is to be close to the square root of 2 viz., approximately 1.414. 
     Instead of a cylindrical opening in the above mentioned hollow section of quadratic cross-section the hollow section according to the invention features an elongated rectangular central space where the ratio of cross-sectional length to cross-sectional breadth lies between 1.4 and 2.5. 
     It has been found favourable for each of at least two of facing section sides to exhibit a pair of undercut longitudinal grooves and for each longitudinal groove to neighbour onto a channel-shaped space at each corner. The pair of undercut longitudinal grooves and the channel-shaped space running between them form a corner region that is the same shape and size in all hollow sections according to the invention; this favours the possibility of joining together hollow sections of different dimensions. 
     As a result, the middle line of each of the longitudinal grooves close to the corner of the hollow section is the same distance from the corner of the section. 
     A further feature of the invention is that the narrow side of one hollow section exhibits an undercut longitudinal groove at about the middle, and a pair of undercut longitudinal grooves is provided in the broad side of the section, in each case one of which neighbours onto the one of the corners delimiting the broad faces. 
     Preferred are two different cross-sectional shapes; in one case a channel which is closed in cross-section runs in the corner, as a hollow space in the section, between the two undercut longitudinal grooves on both sides of a section corner. In another design the channel-shaped space in the section is provided in the corner region between the undercut grooves of the hollow section and opens onto a section face in the form of an undercut edge groove; such an edge groove extending out from the hollow space in the section is to be provided in each narrow face and longitudinal face of the hollow section. 
     Also within the scope of the invention is a construction unit comprising at least two hollow sections of the above mentioned shape; in each case section faces of the same shape are paired together, the cross-sectional length of one hollow section face being matched with the cross-sectional breadth of the other hollow section. Thereby the middle line of at least one longitudinal groove in the face of one hollow section is preferably in line with the middle line of a longitudinal groove in the other hollow section, and both longitudinal grooves should accommodate a common element for joining the hollow sections together by interlocking. 
     In another construction unit according to the invention, on one face of a hollow section, two faces of a pair of hollow sections are arranged such that the middle line of a longitudinal groove in each of these neighbouring hollow sections is in line with the middle lines of both longitudinal grooves in one face of the first mentioned hollow section. As a result, the smallest in cross-section of the hollow sections may be employed in duplicate. 
     In all cases interlocking connecting elements are according to the invention inserted in the aligned longitudinal grooves; these are e.g. I-shaped push-fit elements that fill the cross-section of a pair of undercut longitudinal grooves. 
     Further advantages, features and details of the invention are revealed in the following description of preferred exemplified embodiments and with the aid of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the end view of a part of a construction unit in the form of an extruded light metal section, shown here in approximately actual size; 
     FIGS. 2,  6 ,  7 : is in each case a further extrusion from the construction unit, said extrusions being of larger cross-section than that shown in FIG. 1; 
     FIG.  3 : is a perspective view of the extrusion shown in FIG. 1; 
     FIGS. 4,  5 : is the end view of two groups of sections of different size which are joined together; 
     FIG.  8 : is an end view of two sections of another construction unit shown lying next to each other; and 
     FIG. 9 to FIG.  11 : in each case shows the end view of another section of the same construction system as in FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An extruded hollow section  10  which is made of an aluminium alloy and whose outer contour is of rectangular shape, by way of example having a cross-sectional length a of 101.12 mm and a cross-sectional breadth b of 71.5 mm features a central hollow space  14 —likewise approximately rectangular in cross-section—of cross-sectional length c of 60 mm and cross-sectional breadth c 1  of 30 mm, and with corner regions  15 . Here, the ratio in section  10  of cross-sectional length a to cross-sectional breadth b may be calculated from the square root of 2 to be 1.4142. 
     Close to the corners  16  of the section  10  at each narrow face  18  and each broad face  19  is an undercut corner or edge groove  20  of width e of 9 mm. Each of the edge grooves  20  is delimited on both sides by section ribs  22  of thickness f of 5 mm and channel-shaped grooves  24  of approximately rectangular cross-section, which in turn are partly surrounded by walls of thickness f or f 1  of 6 mm; the shorter distance g of approximately 16 mm between two walls, or the smaller width g of the interior of the groove  24  runs parallel to the section ribs  22 . 
     On each narrow face  18  of the hollow section  10 , between the interior of the channel-shaped grooves  24  is an undercut space  26  of small height h in an undercut longitudinal groove  28  of width i of 9 mm; each of the narrow faces  18  of the section  10  features a pair of such longitudinal grooves  28 . Also the latter are delimited on the outside by ribs  22   a,  of thickness f 2 , here 8 mm. 
     The distance n from the middle lines M of the longitudinal grooves—running parallel to the longitudinal axis A of the section—to the corner  16 , both in the case of the hollow section  10  in FIG.  1  and in the adjoining section  10   a  in FIG. 4, is 35.75 mm. The cross-sectional length a of the first described hollow section  10  is equal to the cross-sectional breadth b of the hollow section  10   a  mounted on to it. As the middle lines M of the longitudinal grooves  28  in the abutting faces  18 ,  19  in both hollow sections  10 ,  10   a  are in line with each other, an I-shaped push-fit type of connecting element  30  can be inserted into each of the grooves  28 ; for reasons of clarity only one such connecting element  30  is shown in the drawing for each of the section pairs  10 / 10   a ;  10   a / 10   b ;  10   b / 10   c . 
     In the case of the hollow section  10   a  shown in FIG.  4 —as already indicated—the shape of the narrow face  18  of cross-sectional breadth b of 101.12 mm corresponds to that of the broad face  19  of the smaller section  10 . At the broad face  19  of the larger hollow section  10   a  the cross-sectional length a here is 143 mm, whereby the distance q between the middle lines M of the longitudinal grooves  28  close to the corner amounts to 71.5 mm. Between the inner groove walls  32  of these undercut longitudinal grooves  26 / 28  the central space  12  is extended by extensions  12   a , delimited outwards by section wall strips  34  of thickness f. 
     Here, the ratio of the inner cross-sectional length c to the inner cross-sectional breadth c 1  of section  10   a —without the recess  14   a —is 1.666. 
     Likewise in hollow section  10   b  the narrow face  18  of the section corresponds to broad face  19  of the attached hollow section  10   a , whereby the distance q in the smaller section  10   a  is the same as the distance q 1  in the larger section  10   b . In this case an additional recess  14   b  has been provided in the central space  14 ; this recess  14   b  is delimited by the strip of section wall  36 . 
     Shown in FIG. 5 are hollow sections  10   d ,  10   e ; section  10   e  is subdivided by diagonal struts  38 , forming an X in cross-section, which are of thickness t of 6 mm; both the hollow section  10   c  shown in FIG.  4  and also the approximately 60 mm narrower hollow section  10   d  in FIG. 5 may be joined onto this section  10   e.  The diagonal struts  38  are integrally joined to the base plate  27  of the undercut space  26 . 
     By matching cross-sectional dimensions, the hollow sections  10 ,  10   a  to  10   e  form a system of sections that may be joined together as shown in the following table: 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Hollow 
                   
                   
                   
                   
                   
                   
               
               
                   
                 section 
                 10 
                 10a 
                 10b 
                 10c 
                 10d 
                 10e 
               
               
                   
                   
               
             
             
               
                   
                 a 
                 101.12 
                 143.00 
                 202.23 
                 286.00 
                 286.00 
                 404.47 
               
               
                   
                 b 
                 71.50 
                 101.12 
                 143.00 
                 202.23 
                 143.00 
                 286.00 
               
               
                   
                 q 
                 29.62 
                 71.50 
                 130.73 
                 214.50 
                 214.50 
                 332.97 
               
               
                   
                 q 1   
                 — 
                 29.62 
                 71.50 
                 130.73 
                 71.50 
                 214.50 
               
               
                   
                 n 
                 35.75 
                 35.75 
                 35.75 
                 35.75 
                 35.75 
                 35.75 
               
               
                   
                 Ratio 
                 1.414 
                 1.414 
                 1.414 
                 1.414 
                 2 
                 1.414 
               
               
                   
                 a:b 
               
               
                   
                   
               
             
          
         
       
     
     It is clear that both hollow sections  10   c ,  10   d  may be mounted by their broad face  19  onto the narrow face  18  of the hollow section  10   e  featuring the diagonal struts  38  and section  10   a  onto hollow section  10   d ; on the other hand section  10   c  to section  10   b  which in turn may be connected to section  10   a . It can also be seen that e.g. two hollow sections  10  may be joined by their narrow faces  18  to the broad face  19  of section  10   a . 
     The system of sections construction purposes may be extended considerably. In the following table for the sections  10   f  and  10   g  in FIGS. 6 and 7, it can be seen that if shorter cross-sectional breadths b are desired, then these may be exchanged e.g. for the previously mentioned sections  10   a,    10   b  as they have the same outer cross-sectional length a. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Hollow section 
                 10f 
                 10g 
               
               
                   
                   
               
             
             
               
                   
                 a 
                 143.00 
                 202.23 
               
               
                   
                 b 
                 71.50 
                 101.12 
               
               
                   
                 q 
                 71.50 
                 130.73 
               
               
                   
                 ql 
                 — 
                 29.62 
               
               
                   
                 n 
                 35.75 
                 35.75 
               
               
                   
                 Ratio a:b 
                 2 
                 2 
               
               
                   
                   
               
             
          
         
       
     
     Two hollow sections  11 ,  11   a  of another construction system are shown in FIG.  8 . In these hollow sections  11 ,  11   a  of wall thickness s of 5 mm the undercut grooves  20  close to the corners  16  are missing. Instead, closed corner channels  40  featuring an undercut groove  28  on both sides are provided. 
     The broad faces  19  of the large hollow section  11   n  of this system are, in FIG. 9, integrally attached to diagonal struts  38  which in turn are integrally attached to a common axial strut  39  of thickness t of 8 mm; also the wall thickness s of the broad and narrow faces are 19.18 and 8 mm respectively. 
     The matching dimensions in this system of sections in FIGS.  8 , 9  are: 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Hollow section 
                 11 
                 11a 
                 11n 
                 * 
                 ** 
                 *** 
               
               
                   
               
             
             
               
                 a 
                 194.50 
                 275.00 
                 389.00 
                 166.00 
                 234.80 
                 332.00 
               
               
                 b 
                 137.50 
                 194.50 
                 275.00 
                 117.40 
                 166.00 
                 234.80 
               
               
                 q 
                 114.50 
                 195.00 
                 309.00 
                 91.00 
                 154.80 
                 252.00 
               
               
                 ql 
                 57.50 
                 114.50 
                 195.00 
                 37.46 
                 86.00 
                 154.80 
               
               
                 n 
                 40.00 
                 40.00 
                 40.00 
                 40.00 
                 40.00 
                 40.00 
               
               
                 Ratio a:b 
                 1.414 
                 1.414 
                 1.414 
                 1.414 
                 1.414 
                 1.414 
               
               
                   
               
               
                 */**/** by way of example, dimensions of another construction unit.  
               
             
          
         
       
     
     Two further series of sections from construction systems are shown in FIGS. 10,  11 . Both hollow sections  12 ,  13  exhibit closed corner regions  16   a  which are flanked by two of the longitudinal grooves  28 , whereby an integral groove  12  of width z of approximately 8 mm is provided along the diagonals D in each of the closed corner regions  16   a  of the hollow section  12  in FIG.  10 . In addition, the sidewalls  37  of section  13  in FIG. 11 are of approximately the same thickness m of 22.5 mm to 24 mm. 
     The same dimensions shown in table D apply to both series of hollow sections  12 ,  13 , whereby the largest of this series of hollow sections—column (5)—in FIG. 11 contains the corner struts  38   a . 
     
       
         
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE D 
               
             
             
               
                   
               
               
                 (dimensions in mm) 
               
             
          
           
               
                 Hollow sections 12, 13 
               
             
          
           
               
                 Regions 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
               
               
                 a 
                 106.07 
                 150.00 
                 212.13 
                 300.00 
                 424.26 
               
               
                 b 
                 75.00 
                 106.07 
                 150.00 
                 212.13 
                 300.00 
               
               
                 q 
                 31.67 
                 75.00 
                 137.13 
                 225.00 
                 349.26 
               
               
                 q 1   
                 — 
                 31.07 
                 75.00 
                 137.13 
                 225.00 
               
               
                 n 
                 37.5 
                 37.5 
                 37.5 
                 37.5 
                 37.5 
               
               
                 Ratio a:b 
                 1.414 
                 1.414 
                 1.414 
                 1.414 
                 1.414

Summary:
Beams, that are rectangular in cross-section and feature at least one undercut groove running parallel to the longitudinal axis of the extruded section in at least one outer face for the purpose of accommodating an element for connecting a further hollow section, are in the form of hollow sections of rectangular cross-section and different cross-sectional length (a) and cross-sectional breadth (b). The cross-sectional length (a) of one hollow section corresponds to the cross-sectional breadth (b) of the other, related hollow section.