Abstract:
A linear construction truss comprising a hollow shell with a square cross section and a core composed of linked tetrahedra, and a method of manufacturing of said linear construction truss comprising the steps of bending sheet metal strips to create a tetrahedral core such that the faces of the tetrahedral alternate between approximately 109.5 degrees and approximately 289.5 degrees throughout the length of the sheet metal strips. Strips of sheet metal are further bent to form a hollow shell with a square cross section rigidly attached to the core in a spiral pattern. The triangular faces are chamfered to better enable the connection between the core and hollow shell. The sheet metal is cut in such a way such that the perforations do not impinge on the edges of the tetrahedral.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of application Ser. No. 13/078,910, filed Apr. 1, 2011, which claims the benefit of Provisional Patent Application No. 61/460,163, filed Dec. 27, 2010. 
    
    
     BACKGROUND 
     Field of Invention 
     This invention relates to an improved design for a linear construction truss and a method of manufacturing said linear construction truss. 
     Prior Art 
     There are many designs for linear members (beams and trusses) used in construction. Such linear members have to be resistant to shear stress and bending, since the loads on them are typically perpendicular to the axis of the linear member. The I-beam is one example of such a linear member. While the I-beam is an efficient design in that it concentrates the material where the largest linear stress is bound to occur, the manufacturing process for making I-beams involves hot rolling, which is very energy-intensive and environmentally damaging; furthermore, steel I-beams are relatively heavy. 
     Another possible type of design for a linear member is a hollow sheet metal box beam, such as the one described in U.S. Pat. No. 2,007,898 to Ragsdale. Ragsdale discloses a hollow sheet metal beam with a generally rectangular cross section. While such beams are lightweight and do not require energy-intensive hot rolling methods to construct, a beam with a hollow cross-section is not very strong, and susceptible to buckling. 
     Due to the advantages offered by a sheet metal construction over an I-beam or a lumber beam, many attempts have been made to reinforce a hollow sheet metal beam from the inside. One such design for a reinforced hollow sheet metal beam is disclosed in U.S. Pat. No. 3,783,498 to Moyer (shown in  FIG. 1 ). Moyer discloses a beam with a hexagonal cross-section, made of sheet metal and welded. The beam is reinforced at two of its vertical sides with sheet metal strips welded to those two sides. These strips provide extra strength on the sides that bear the most stress. However, due to the fact that the horizontal sides are not reinforced and that there is no reinforcing elements on the interior of the beam, the resulting linear member is still not strong enough for some purposes. 
     Another method of reinforcing the interior of a hollow sheet metal beam is disclosed in U.S. Pat. No. 4,023,683 to Vargo. Vargo discloses reinforcing the interior of a hollow sheet metal beam with one or more planar webs. While that design is stronger than a simple hollow sheet metal beam, it is still not strong enough for some construction applications. One of the reasons for its relative lack of strength is that the planar webs are aligned with the axis of the beam, which increases the strain on these webs in normal operation. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a linear member (a linear construction truss) that is stronger than prior art reinforced hollow beams, and to provide a method of manufacturing the linear construction truss that does not involve hot-rolled processes. 
     A linear member designed in accordance with the present invention is a linear construction truss comprising a hollow shell with a square cross-section and a reinforcing core of linked regular tetrahedra whose edge length is approximately equal to the inner diagonal of the cross-section of the hollow shell, the core being rigidly attached to the hollow shell. Such a design offers many advantages, one of them being that rather than providing a single planar web to reinforce a hollow beam, the design provides many planar webs (i.e. the faces of the tetrahedra), which are disposed at varying oblique angles relative to the axis of the linear construction truss. This reduces the linear strain on the reinforcing material and renders the linear member stronger. Furthermore, the inter-tetrahedral webs reinforce alternate diagonals of the hollow shell, which also improves its strength and resistance to buckling. 
     The intra-tetrahedral edges, when joined to the shell, comprise two single layer struts, three double layer struts, and two triple layer struts per linear tetrahedral module. The double and triple layer struts join to form two double layer and two triple layer helices coiling in opposite directions along the length of the truss. These mutually reinforcing structures arrange in an X-form triangulation of the diametric shell corners per tetrahedral module, normal to the rectangular shell&#39;s axis. These bi-directional coiling elements impart a cable-like resilience to the structure, and render it stronger than prior art reinforced hollow beams. 
     In a preferred embodiment, both the hollow shell and the reinforcing core are made of sheet metal, though other materials may also be used. Due to its square cross-section, such a linear construction truss can be used in place of a lumber beam or an I-beam in construction applications. Furthermore, a sheet metal linear construction truss in accordance with the present invention is more lightweight than either a lumber beam or a steel I-beam and requires only cold-rolled processes to manufacture. Thus, it offers the advantages of a sheet metal design, while also offering improved strength over prior art reinforced sheet metal designs. 
     In a preferred embodiment, the intra-tetrahedral edges that attach to the hollow shell are chamfered to facilitate attachment to the hollow shell. The inter-tetrahedral edges are not chamfered. 
     Another aspect of the present invention is a method of continuous automated manufacture of a linear construction truss as described above. Both the reinforcing core and the hollow shell can be manufactured from sheet metal strips using simple folding and stamping processes. 
     In accordance with the manufacturing method of the present invention, two identical sheet metal strips are cut and bent in such a way as to result in a plurality of triangular faces with specified angles between the triangular faces. In a preferred embodiment of the invention, there are chamfers between the triangular faces. The two strips are then intertwined together to form a reinforcing core comprising a plurality of linked regular tetrahedra, and rigidly attached. A sheet metal shell with a square cross-section is then formed around the reinforcing core and rigidly attached to it. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art design of a reinforced hollow sheet metal beam. 
         FIG. 2  shows a linear construction truss of the present invention. 
         FIG. 3  shows a separate view of the hollow shell and a separate view of the reinforcing core. 
         FIG. 4  shows a view of one of the two sheet metal strips used to form the reinforcing core. 
         FIG. 5  shows a view of one of the two sheet metal strips used to form the reinforcing core after it is folded in accordance with the present invention. 
         FIG. 6  shows a view of a chamfer between two of the triangular faces of the folded sheet metal strip. 
         FIG. 7  shows a view of the reinforcing core. 
         FIG. 8  shows two possible designs for a die used to bend the sheet metal strips that comprise the reinforcing core. 
         FIG. 9  shows an embodiment of a die forming station. 
         FIG. 10  shows a view of an embodiment of a core assembly station. 
         FIG. 11  shows a side view of a web welder module of another embodiment of a core assembly station. 
         FIG. 12  shows a side view of a chamfer welder module of the same embodiment of a core assembly station. 
         FIG. 13  shows an embodiment of a shell application station. 
         FIG. 14  shows an alternate embodiment of a shell application station. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2 and 3  show the main component parts of the linear construction truss of the present invention—the reinforcing core  10  and the shell  20 .  FIG. 2  is a view of the assembled linear construction truss with a transparent view of the hollow shell.  FIG. 3  shows a separate view of the reinforcing core and a separate view of the hollow shell. As can be seen in  FIG. 2 , the reinforcing core  10  comprises a plurality of linked regular tetrahedra, situated inside a hollow shell  20  with a square cross-section such that the linked edges of the tetrahedra are approximately equal to the inner diagonal of the hollow shell. Each tetrahedron, except for the ones located on each end of the truss, is rigidly linked to its two neighboring tetrahedra along two of its edges, and rigidly linked to the inner surface of the hollow shell along its four other edges. This strengthens the structure by providing numerous planar reinforcing surfaces at oblique angles to the load. In a preferred embodiment, both the hollow shell and the reinforcing core are die-formed from continuous sheet metal strips by automated cutting and folding machines that process rolls of sheet metal; it is preferable, though not required, that the sheet metal strips that form the reinforcing core be thinner than the sheet metal strips that form the hollow shell. 
       FIG. 4  shows one of the two identical sheet metal strips  30  used to form the reinforcing core. In a preferred embodiment, the strip is die-cut with cutouts  100  to facilitate folding and chamfering, and cut with perforations  50  to reduce its weight. 
       FIG. 5  shows a view of sheet metal strip  30  after it is folded. The strip  30  is folded in such a way that the dihedral angles between successive triangular faces alternate between 109.4667 degrees and 289.4667 degrees throughout the length of the strip. For example, the dihedral angle between triangular face  70  and triangular face  71  is 109.4667 degrees; the dihedral angle between triangular face  71  and triangular face  72  is 289.4667 degrees; the dihedral angle between triangular face  72  and triangular face  73  is 109.4667 degrees; and so on. This results in a helical shape that can then be intertwined with another identical shape to form a plurality of linked tetrahedra. 
     In the preferred embodiment, there are chamfers between the triangular faces, as shown in  FIG. 6 , which is a magnified view of a section of sheet metal strip  30 . The bending angles at the edges labeled “A” are approximately 144.7333 degrees; the bending angles at the edges labeled “B” are approximately 234.7333 degrees. This pattern repeats throughout the length of the strip. The chamfers are intended to facilitate welding, both for welding the two strips together to form the tetrahedra and for welding the tetrahedra to the hollow shell. 
       FIG. 7  shows the two sheet metal strips  30  and  40  joined together to form the reinforcing core  10 . Sheet metal strip  30  is shaded while sheet metal strip  40  is left unshaded. As can be seen in the figure, each tetrahedral unit derives its parts from both of the two core strips. From each strip, each tetrahedral unit derives two of its four triangular faces, two of its four chamfered edges, and one of its two inter-tetrahedral web layers. The successive inter-tetrahedral web layers are at right angles to each other and normal to the core axis. The core is thus composed of the two helices of equilateral triangles and strips that will be precisely coiled together and, in a preferred embodiment, joined with spot or linear welds. 
     The hollow shell  20  encloses, triangulates, and is welded to, the core  10 , as is shown in  FIG. 2 . It is a linear tube with a square cross-section. Depending on the machinery deployed, it can comprise one, two, or four separate sheet metal strips. These are die-cut and folded for corners and edge joining, then welded at the seam or seams and welded to the core. 
     The linear construction truss may be manufactured by using cold-rolled processes such as stamping or bending.  FIG. 8  shows two possible designs for a die used to form the folded sheet metal strips that comprise the reinforcing core. Die  110  can achieve the folding of two facets per die strike; die  120  can achieve the folding of four facets per die strike. Alternate numbers of facets may also be used. The chamfer angles are identified on the drawing. For die  110 , the dihedral angle between faces  1  and  2  is 109.4666 degrees; the chamfer angles are 144.7333 degrees on the edges marked A and 234.7333 degrees on the edges marked B. For die  120 , the dihedral angle between faces  1  and  2  is 109.4666 degrees, as is the dihedral angle between faces  3  and  4 . The dihedral angle between faces  2  and  3  is 289.4667 degrees. The chamfer angles are 144.7333 degrees on the edges marked A and 234.7333 degrees on the edges marked B. 
       FIG. 9  shows one possible embodiment of a die forming station that continuously forms a folded sheet metal strip used to form the tetrahedral reinforcing core. A sheet metal strip  240  is continuously fed from a roll  200  into a rotary die cutter  210 , which cuts it with cutouts to facilitate folding and chamfering and perforations to reduce its weight. After it is cut, the strip is folded in a die  220 . The folded strip  250  is then ready to be welded together with a second identical strip to form the tetrahedral core. 
       FIG. 10  shows an overview of one possible embodiment of a core assembly station that assembles the two folded strips into a tetrahedral reinforcing core. Two folded strips  310  and  320  are continuously fed into a rotating assembly fixture  330 , which rotates around the axis of the reinforcing core. The rotating assembly fixture  330  holds the two strips  310  and  320  in correct angular position for welding by means of registration chute  340 . Registration chute  340  is an axially revolving rectangular chute that moves the core forward in steps to the next operation. The entire assembly moves forward in steps through the revolving fixture; at each stopping point, automatic welding, performed by a robot  350  or a weld array  360 , joins the various weld points of the core, starting with the inter-tetrahedral web and following with the tetrahedral chamfer-flanges. The reinforcing core is then ready to be assembled with the hollow shell at the shell application station. 
     Another embodiment of a core assembly station is detailed in  FIGS. 11 and 12 , and comprises two independent modules. One module—the web welder module—welds the intertetrahedral webs of the core, and moves the core forward in steps to the next operation. The other module—the chamfer welder module—welds the chamfered edges of the tetrahedra. 
       FIG. 11  shows a side view of the web welder module. Truss core  305  enters the assembly module from the right side of the drawing, and is held in the registration chute  315 . The main linear actuator  325  moves the weld frame  327 , which contains the vertical and horizontal weld arrays into position for the welding of the intertetrahedral webs. Weld array  335  is vertical and is used to weld the vertical intertetrahedral webs; weld array  345  is horizontal and is used to weld the horizontal intertetrahedral webs. Vertical linear actuators  337  move the horizontal weld arrays  345  into position for welding; horizontal linear actuators  347  move the vertical weld arrays  335  into position for welding. The main linear actuator  325  then moves the weld arrays in the direction parallel to the truss core axis and enables them to move the truss core through the module; the range of its displacement is labeled as “Dual-Tetra Module Horizontal Displacement” in  FIG. 11 . Two positions of the weld arrays—“A position” and “B position”—are shown in  FIG. 11 . 
       FIG. 12  shows a side view of the chamfer welder module. This module welds the intratetrahedral edges after the intertetrahedral webs have been welded. Truss core  305  enters this module from the right side of the drawing, and is held in the registration chute  315 . The vertical diagonal weld arrays  365  are used to weld the intratetrahedral edges that are vertical; a corresponding set of horizontal diagonal weld arrays  375  are used to weld the intratetrahedral edges that are horizontal. Vertical linear actuators  385  and horizontal linear actuators (not shown) bring the weld arrays into position. 
     One embodiment of the shell application station is shown in  FIG. 13 . The finished core  400  rotates around its axis due to the action of the core assembly station shown in  FIGS. 10-12 . Four rolls of sheet metal,  415 ,  425 ,  435 , and  445 , rotate along with the core. Rotary dies  455 ,  465 ,  475 , and  485  apply the four strips of sheet metal to the sides of the core to form the shell; spot weld arrays then weld the strips of sheet metal to the core and corner welders weld the strips of sheet metal together. For clarity,  FIG. 13  shows only one spot weld array  495  and one corner welder  497 , out of the four required spot weld arrays and four corner welders. This embodiment is not the preferred embodiment of the shell application station because it necessitates that the entire apparatus revolve around the shell axis, which is complex and expensive. 
     The preferred embodiment of the shell application station is shown in  FIG. 14 . The finished core  400  is fed into the shell application station; a roll of sheet metal  410  is fed into a diagonal corner fold die  420  and is applied to the core  400  at an angle that enables it to be wrapped around the core  400  in a spiral pattern. This method of applying the shell is more efficient because the whole apparatus can remain stationary and only the core  400  rotates. A seam welder  430  welds the successive coils of the spiral together; a spot weld array  440  welds the core assembly to the shell. A cut-off saw  450  cuts off finished lengths of the linear construction truss in modular lengths at the tetrahedral web.