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FIELD OF THE INVENTION 
   The present invention relates to connections for joining linear, structural elements capable of carrying tension and compression loads that comprise double-layer-grid, three-dimensional trussed structures and braced planar truss systems. The most common application of such connections is in double-layer-grid space frames. Therefore, the connection hereof is named after the acronym for double-layer-grid—the DLG Connector (DLGC). 
   BACKGROUND ART 
   Current connections designed for double-layer-grids receive linear, structural elements that, most commonly, are either round or square in cross-section. In such grids, the places where plural linear elements are interconnected are known as “nodes”.  FIG. 1  illustrates a conventional node connector  2  for square cross-section framing members  3 ,  4 . Note that the adjacent diagonal strut members at  4  the connection reside in different planes. That is, the non-vertical surfaces  5  of each diagonal strut member  4  lie in (or are parallel to) a plane diagonal to the common plane of the adjacent horizontal chord members  3  which is different from the diagonal plane of each other diagonal strut member associated with the connector. 
   Bolted connections are easily effected using these systems that accommodate square linear, tubular structural elements. The use of a square cross-section for the framing is advantageous since the fabrication of the framing member consists simply of drilling or punching holes at both ends after the member is cut to length. Ball-node systems are designed for the use of round cross-sections (pipes) in double-layer-grids and involve a more expensive design and fabrication process. 
   A double-layer-grid is understood to be a structure with a horizontal, square grid of framing elements that serve as the top chords and is the top “layer” of the DLG space frame. Similarly, there are the bottom chords with the same square grid that is offset horizontally by one-half the bay width in both directions. This bottom “layer” is also offset downwardly from the top “layer” by a set distance and is held in position by the use of diagonal (strut) framing elements.  FIGS. 2 ,  3  &amp;  4  show a typical double-layer-grid space frame  6 , six bays long by five bays wide; an end view of frame  6  is similar to  FIG. 4 . There are several disadvantages of the connection systems of the prior art when applied to double-layer grids as described with square cross-section framing elements. First, these systems are restricted to square, double-layer-grids only. Second, these systems can only produce flat double-layer grids. Third, contoured (free-form) footprints are difficult to design and construct. Fourth, vertical sidewall and/or end wall framing is difficult to design and construct. These disadvantages are resolved with the DLGC design. 
   SUMMARY OF THE INVENTION 
   The DLGC takes advantage of natural planes formed by the double-layer grid. When studying a double-layer-grid, the observer will see in  FIG. 4  that the diagonal members in each half-bay all lie in the same plane. As long as the diagonal framing elements have flat surfaces, such as square/rectangular tubes, angles, channels and I-beams (wide flanged sections as in steel construction), these strut framing elements can be rotated so that the non-vertical surfaces of the strut framing elements become parallel to the diagonal plane. Once the framing element is oriented so that the non-vertical flat surface(s) are in plane with the diagonals, the problem of attachment is immensely simplified. In current systems as shown in  FIGS. 1 ,  5 ,  6  and  7  ( FIGS. 5 ,  6  and  7  are simplified, and idealized, depictions of the substance of  FIG. 1 ), opposing diagonal elements have their major axes (webs) residing in vertical planes. The present DLGC system makes adjacent diagonal elements have their non-vertical surface features co-planar, as shown in  FIGS. 8-10 , e.g. When these surface features are oriented in the same plane, they become parallel to the plane defined by the row of diagonal elements. Once this is accomplished, adjacent diagonals can be connected to the joint by the same structural plates. Sharing structural plates at the connection as seen in  FIGS. 8 ,  9  and  10  creates the simplified connection. The members comprising the horizontal grid attaching to the diagonal elements can also be connected together with plates following the direction of the diagonal plane. This allows for a system of plate flanges with parallel centerlines that is ideal for fabrication as aluminum extrusions or welded steel plates that comprise a DLGC joint (see  FIGS. 11 ,  12  and  13 ). 
   The DLGC can be made of any structural material such as, but not limited to, aluminum, steel, fiber-reinforced polymers (FRP) and plastics. Fabrication of the DLGC can use any process suitable to the material used such as extruding or casting aluminum and welding steel plates. Linear members connected by the DLGC can be made of any structural material such as, but not limited to, aluminum, steel, FRP, plastic or wood. 
   Generally speaking, a node connector according to this invention is useful for interconnecting plural structural framing members at a node in a double-layer space frame which has spaced major surfaces. The major surfaces are defined by substantially orthogonally disposed chord members. The major surfaces are spaced from each other by struts. The node connector comprises an elongate body which has a substantially constant transverse cross-sectional configuration along its length. The connector body defines an elongate open-ended passage which has side walls and which is configured and sized to enable a first chord to be engaged in and along the passage between the side walls in one major surface of the frame. The connector body also defines at least first and second pairs of spaced parallel diagonal surfaces. Each pair of diagonal surfaces is associated with a respective one of two diagonal planes which intersect at a line parallel to the elongate extent of the passage. The diagonal surfaces in each pair are parallel to a respective diagonal plane. The ends of a pair of frame struts can be connected to the connector between each pair of diagonal surfaces. 
   Another aspect of this invention pertains to a double layer space frame which has longitudinal and transverse chord structural framing members which are orthogonally disposed in spaced major surfaces of the frame. The chord members are connected relative to each other by connectors at connection nodes in the frame major surfaces. The frame also includes pairs of strut structural framing members which lie in diagonal planes oblique to the frame&#39;s major surfaces and which connect a node in one major surface to four adjacent nodes in the other major surfaces. A connector at such a node has at least two pairs of struts connected to it. In that context, a connector comprises a body which has a passage through it which receives a first frame chord member. The connector has lateral surfaces which extend away from opposite sides of the passage; to each lateral surface is connected a further chord member which is disposed substantially orthogonally to the first chord member. The connector body also has at least two diagonal surfaces which extend along the body parallel to the passage and respectively parallel to a respective one of two diagonal planes associated with that node. The ends of a pair of struts in the related diagonal plane are connected to each diagonal surface to connect the node to two adjacent nodes in the other major surface of the frame. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned features and advantages of the present invention, as well as additional features and advantages thereof, will be more fully understood from the following detailed description of a preferred embodiment when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a perspective depiction of a prior art node connection using conventional connection technology where the diagonal members&#39; non-vertical surfaces lie in different planes in order to maintain their respective vertical orientations; 
       FIGS. 2 ,  3  and  4  are isometric, plan and side views, respectively, of a conventional (prior art) double layer grid that is six bays long and five bays wide; 
       FIGS. 5 ,  6  and  7  are simplified isometric, bottom and side views, respectively, of a conventional (prior art) node connection, generally like that shown in  FIG. 1 , in a double layer grid; 
       FIGS. 8 ,  9  and  10  are conceptual (simplified and idealized) isometric, bottom and plan views, respectively, of a node connection of a double layer grid according to the present invention; 
       FIGS. 11 ,  12  and  13  are isometric, bottom, and cross-section views, respectively, of a first embodiment of a DLGC node connection of the invention in which the framing members of the DLG are defined by WF (wide flange) shapes; 
       FIGS. 14 and 15  are bottom and transverse cross-section views, respectively, of a version of the invention in which the grid framing elements are defined by square tubes; 
       FIGS. 16 ,  17  and  18  are isometric, bottom and transverse cross-sectional elevation views, respectively, of a version of the invention in which the strut framing members are back-to-back angle pairs and the chord framing members are squares tubes; 
       FIG. 19  is an illustration of a connection of the invention which incorporates a mullion/rafter system for glazing with glass, polycarbonate or acrylic sheets; 
       FIG. 20  is an isometric view of the connector of  FIG. 19  with a glass sheet shown on one side only; 
       FIGS. 21 and 22  are side and isometric views, respectively, of a DLG in arch or vaulted shape; 
       FIGS. 23 ,  24  and  25  are isometric, bottom and transverse sectional elevation views, respectively, of an extruded aluminum node of a vaulted DLG in which the framing elements are defined by square tubes; and 
       FIGS. 26 ,  27 ,  28  and  29  are isometric, top, side and end views, respectively, of a DLG with variable bay spacing in one direction. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 5 through 7  depict a simplified representation of prior art node connector joint  2  (see  FIG. 1 ) and illustrate the existing convention for member orientation in conventional double-layer-grids. Wide-flanged (WF) members are shown for grid framing members  3 ,  4  in  FIGS. 5-7  to help visualize strut member orientation. As seen in  FIG. 5 , the diagonal member in the foreground has the web oriented vertically as do the other diagonal members connected to that node connection joint.  FIG. 6  shows the same joint  2  in a bottom view in which the intersections of the diagonal struts  4  trace a cruciform which indicates that the flat flange surfaces of the WF diagonals are in separate planes.  FIG. 7  is a close-up of a joint from space frame  6  (see  FIG. 4 ) which illustrates that the flanges of the strut  4  members are not parallel to the plane defined by the row of diagonals in each half-bay. 
   A simplified depiction of a DLGC joint  8  according to the present invention is shown in  FIG. 8  which reveals that the webs of WF strut members  9  in a DLG node connection of the present invention are not in a vertical orientation; chord framing members  10  are also involved in joint  8 .  FIG. 9  demonstrates that opposite pairs of diagonal strut  9  have a strut member orientation that makes their flange surfaces co-planar as evidenced by a straight line at the intersection of the four struts—two intersecting planes make a straight line.  FIG. 10  verifies that the flanges of each pair of adjacent strut members  9  on one side of the node connection are oriented so that they are now parallel to the diagonal plane. This way of orienting diagonal elements makes the DLGC possible. Working joint designs based on this innovation are presented next. 
   Shown in  FIGS. 11 through 13  is one working embodiment of the present DLGC. This node connector  12  is designed to be extruded aluminum for the joining of aluminum chords  13 ,  14  and strut  15  members; in this example, those chords and struts are WF aluminum shapes.  FIGS. 14 and 15  show a preferably extruded connector  18  with square aluminum tubes as chords  19 ,  20  and struts  21 .  FIGS. 16-18  illustrate a similar preferably extruded connection  24  with back-to-back angles used to define struts  25 ; chord members  26  and  27  are defined by square tubes. Similar constructions can be produced in steel with the DLGC consisting of welded steel plates instead of an extrusion. The DLGC also provides for using engineered wood and sawn lumber as strut members. 
     FIG. 19  illustrates that the DLGC can incorporate a mullion/rafter system  40  for glazing with panels  41  of glass, polycarbonate or acrylic.  FIG. 20  is an isometric view with glass shown only on one side for clarity. Additionally, similar attachments can be integrated with the DLGC system that would allow for the batten engagement of sheet metal panels. In all these cases, the cladding engagement mechanism can be produced integrally with the rafters of the top chords in one or both directions. Once the cladding engagement mechanism is made integrally with the rafter, no secondary framing is required to support the cladding. The top chord members can serve as mullions. 
   An inspection of  FIGS. 11-13  concerning node connector  12 , of  FIGS. 14-15  concerning node connector  18 , and of  FIGS. 16-18  concerning node connector  24  reveals that connectors  12 ,  18 , and  24  have certain structural features and properties in common with each other. Those common features and properties are discussed herein principally with reference to  FIGS. 14 and 15  and connector  18 . Connector  18  is comprised by an elongate body  30  which preferably has a constant transverse cross-sectional configuration and which, more preferably, is created by an aluminum extrusion process. Body  30  includes a pair of spaced parallel flanges  31  oriented along the length of the body to form an open ended passage  32  in which is received a chord member  19  of the pertinent space frame. Passage  32  has side walls defined by the opposing surfaces of flanges  31 . As received in the passage, the chord extends along the passage and, in some instances as shown  14 , through the passage in a continuous manner to extend beyond the opposite ends of the passage. In connector  30 , the passage has a bottom and an open side opposite that bottom so that the connector can be engaged laterally with chord  19  where desired along the length of the chord, with the chord engaging the passage bottom; the body  30  can be secured to chord  19  by pins  33 , e.g., passed through flanges  31  and the chord as shown in  FIG. 15 . 
   Also by inspection of  FIGS. 14 and 15 , body  30  of connector  18  defines surfaces  34  extending laterally in the body away from passage  32  in preferably coplanar relation, preferably from the lower ends of flanges  31  as seen in  FIG. 15 . The ends of chords  20 , disposed orthogonally to chord  19  as shown in  FIG. 14 , can be connected to surfaces  34  by pins  35 , e.g. Further, in connectors  12  and  14 , the connector body defines two pairs  36 ,  37  of diagonal flanges. The flanges  36  and  37  in each diagonal flange pair have opposing diagonal surfaces which extend along the length of passage  32  (i.e., the length of body  30 ) parallel to the length of the passage; they also extend away from the passage parallel to each other and diagonally relative to the adjacent lateral surface  34  to, in effect, define a space frame diagonal plane in which lie the two struts  21  disposed on one side of chord  19  as received in connector passage  32 . Flange pairs  36 ,  37  extend as described and shown from opposite sides of passage  32 . The ends of struts  21  which extend away from one side of chord  19  are disposed between the flanges of one flange pair  36 , and the ends of the struts which extend away from the other side of chord  19  are disposed between the flanges of the other flange pair  37 , the pairs of struts lying in their respective diagonal planes in the space frame. The end of each of strut  21  can be secured to the connector by a single pin  38  passing through aligned holes in the adjacent flanges  36 ,  37  and through the chord preferably perpendicularly to the flanges. The diagonal flange pairs  36 ,  37  preferably are located and oriented in connector  18  relative to passage  32  so that the diagonal planes defined by (associated with) flanges  36 ,  37  intersect at a line within and extending along passage  30 . The line of intersection of the diagonal planes preferably coincides with the centerline (axis) of passage  32 . The struts preferably are so located along the length of their respective diagonal flanges, in combination with the included angle between the struts in each strut pair, that the axes of the struts intersect at a common point on that line of diagonal plane intersection. Regardless of the angle of a given strut relative to connector body  30 , the strut will be in the diagonal plane defined by the diagonal flange pair (or single diagonal flange in the case of connector  24  shown in  FIGS. 16-18 ) to which the strut is connected. 
   It is apparent from the content of the proceeding two paragraphs that connectors  12 ,  18  and  24  have an aspect of directionality to them. The direction of a connector according to this invention is the direction along the connector passage and along the lengths of the diagonal flanges along the connector body. Also, the fact that a strut connected to such a connector will always lie in the diagonal plane defined by, or associated with, the diagonal flange(s) to which it is connected makes it possible to construct DLG space frames having variable bay spacing in one direction of the frame, as shown in  FIGS. 26-29  discussed more fully later in this description. 
     FIGS. 21 and 22  show a double layer grid space frame that is shaped as an arch or vault.  FIG. 21  shows the end view which demonstrates that the diagonal planes in the direction of that view are straight and uninterrupted which allows for the use of the DLGC in that direction. Note that the DLGC works easily in a direction at right angles to the direction of curvature for a vault.  FIGS. 23 through 25  show an extruded DLGC aluminum node connector  44  joint used to create the shape of the vault. The joint is designed to create curvature by orienting the lateral surfaces and diagonal flanges at the needed angle—off from the 180 plane used in a flat double-layer-grid; compare  FIG. 25  to  FIG. 15 . Again, such a joint could be produced with welded steel plates or pultruded FRP. The same joint design can be used throughout the structure with no change to the DLGC profile or to the bolt or pin patterns. 
     FIGS. 26 through 29  show a double-layer-grid space frame  46  with variable bay spacing in the longitudinal direction.  FIG. 29  shows the end view which demonstrates that the longitudinal diagonal planes are straight and uninterrupted which allows for the use of the DLGC in that longitudinal direction. However, the bays running at 90 degrees from this view, as shown in  FIG. 28 , can be set at any regular or irregular spacing. As seen in  FIG. 27 , the grid forms rectangles with the long sides in the longitudinal direction starting from either end. The bay spacing progressively changes (reduces) going towards the center of the frame until the long direction of the rectangles follows the transverse direction of the frame. This does not effect any change in the end view; the diagonals all lie in the same plane. Again, the DLGC would be oriented in this longitudinal direction without change in profile. However, the drilled bolt or pin patterns in the diagonal flanges of the node connectors would require adjustment in the bolt pattern orientation and the diagonal strut lengths would change. The last set of diagonal struts at each end of frame  46  are brought up vertically in their diagonal planes to form frame end walls as seen in  FIG. 28 . A significant advantage to this feature (variable bay spacing) is that double-layer-grid space frames are no longer forced into square bay spacing which creates modular inflexibility in the structure. This allows the width and length of the double-layer-grid frame to be independent resulting in infinitely adjustable lengths versus widths. Also, as seen in  FIG. 27 , interesting architectural effects can be achieved by varying the bay spacing in the one direction. From a structural engineering point of view, this is an easy way to increase the framing member density in high stress areas of the frame. A variation of this is a tapered 3-sided tower. 
   Having thus disclosed various preferred working and other embodiments of the present invention, it will now be apparent that many additional node connector configurations and grid and truss system configurations can be achieved by virtue of and consistent with the advantageous teaching provided herein. Accordingly, the scope hereof will be limited only by the appended claims and their equivalents.

Summary:
A connection node for a double layer grid or truss system has at least one diagonal flange receiving a pair of diagonal framing members having surfaces that lie in a single diagonal plane parallel to the flange(s). Use of co-planar diagonal members that can be at various diagonal angles or vertical, simplifies node connections and permits variations in bay spacing to produce interesting architectural effects and to provide greater member density where structural loads are greater.