STRUCTURAL ELEMENT, TETRAHEDRAL- TRUSS CONSTRUCTED THEREFROM AND METHOD OF CONSTRUCTION

BACKGROUND OF THE ^" INVENTION

This invention relates, generally to structural trusses and other articulated supporting structures and specifically to three-dimensional structural trusses, i.e. supporting structures whose primary load-bearing capacity is attributable to extension of the structure in three dimensions.

A structural truss may generally be considered to be an open, skeletal assembly of struts joined at nodes to achieve a supporting structure of high load- bearing capacity relative to its weight, i.e. high specific structural strength. Fundamentally, trusses are based on the geometric triangle to take advantage of the inherent rigidity of the skeletal-triangle in sup- porting a coplanar load. However, being based on the two dimensional triangle, conventional trusses are essentially two dimensional (2D) (planar) structures, i.e. they are not free-standing. Three dimensional (3D) stability is achieved by providing lateral support, e.g. by cords or other cross-linking members between parallel trusses. Complex, quasi-3D trusses may be built up with a grid-like network of 2D truss members; however, such complex networks are not fundamentally 3D trusses, since the base member of the network is not repeated period- ically in three dimensions.

The present invention provides a truss that is fundamentally periodic in three dimensions and therefore

has three-dimensional stability without dependence on lateral stabilizing members or complex networking. As a result of this periodicity, the truss may be built up simply in regular fashion by "repeating" a fundamental unit in the three dimensions to the extent desired.

Further, the truss design continues to take advantage of the inherent rigidity of the basic skeletal triangle. Still further, the truss design achieves these advan¬ tages with maximum geometric efficiency, i.e. the mini- mum number of struts per node (four) that is required for stability of an articulated, periodic 3D structure.

SUMMARY OF THE INVENTION In attainment of the above-mentioned advan¬ tages over conventional 2D trusses based on the 2D tri- angle, the present invention provides a 3D truss based on the "3D triangle", i.e. the equilateral tetrahedron which is the most stable elemental geometric configura¬ tion. Accordingly, the present invention provides a three-dimensional, tetrahedral truss comprising a three- dimensionally periodic skeletal array of an intercon¬ nected plurality of skeletal-tetrahedric units, said array being in the pattern of the cubic-diamond crystal- lographic structure (FIG.7). The method of the inven¬ tion provides for the assembly of such a truss. Preferably, each of the skeletal-tetrahedric units is an articulated arrangement of struts joined in the pattern of an equilateral skeletal-tetrahedron (FIG.lA). The struts may be received and joined at a male-node (FIG.2) or at a female-node (FIG.3). In combination, the struts may be of high stiffness, rela¬ tive to the node, and the node may be of high toughness, relative to the struts, thereby blending these advanta¬ geous mechanical properties in a composite structure. More preferably, each of the skeletal-tetra- hedric units is a skeletal arrangement of elongate mem¬ bers joined in the pattern formed by the face-members of a cubic-diamond unit-cell (FIG.10). The unit may be termed a "closed" skeletal-tetrahedric unit. A "face-

member", as opposed to a "corner member", is a strut that terminates on the face, rather than a corner, of the reference cube that conceptually encloses a unit- cell of the cubic-diamond structure (FIG.7). Such a skeletal-tetrahedric unit may be assembled from four hexagonic triplanar-rings (FIG.10A), each of the tri- planar-rings being of the form created by joining six bilateral-elements in a closed ring, triplanar pattern (Fig.9) wherein each of the bilateral-elements is defined as having equal sides and having an included- angle of about 109°28' (FIG.8). The triplanar-rings may in fact be constructed of the bilateral-elements, or they may be formed as jointless rings.

The truss may be a graded structure wherein the characteristic dimension of said skeletal-tetra¬ hedric units varies layer-wise within said truss by an integer power of the fraction one-half (FIG.12). The "characteristic dimension" is defined as the length of a side of the conceptual reference cube enclosing the tetrahedric unit.

Additionally, the scope of the invention broadly comprehends the above-described structural ele¬ ments per se.

BRIEF DESCRIPTION OF THE DRAWINGS Further details are given below with reference to the embodiments shown in the drawings wherein:

FIGS. 1 and lA show respectively an equila¬ teral tetrahedron and its complementary skeletal-tetra¬ hedron. FIGS. 2, 2A, and 2B show respectively an arti¬ culated skeletal-tetrahedric unit, its component struts being received onto a male-node, and the male-node.

FIGS. 3, 3A, and 3B show respectively another articulated skeletal-tetrahedric unit, its component struts being received in a female-node, and the female- node.

FIG. 4 shows an articulated skeletal-tetra¬ hedric unit enclosed in a conceptual reference cube of

characteristic dimension "a".

FIG. 5 shows the placement of an articulated skeletal-tetrahedric unit in a unit-cell of characteris¬ tic dimension "2a". FIGS. 6 and 7 show respectively placement and joining of four articulated skeletal-tetrahedric units into the pattern of cubic-diamond.

FIG. 8 begins a sequence of drawings showing another embodiment of the invention and shows a bilateral-element having equal sides about an included- angle of about 109°28'.

FIGS. 9, 9A, 9B, and 9C show respectively a hexagonic triplanar-ring element in perspective, an exploded plan view of its assembly from six bilateral- elements, a plan view, and a side view.

FIGS. 10 and 10A show respectively a closed skeletal-tetrahedric unit and its assembly from four hexagonic triplanar-rings.

FIGS. 11 and 11A show respectively a perspec- tive view and an exploded view of three closed skeletal- tetrahedric units stacked in cooperative fashion.

FIGS. 12 and 12A show respectively a perspec¬ tive view and an exploded view of a graded truss built up from a plurality of closed skeletal-tetrahedric units and having layers of different characteristic dimen¬ sions.

FIG. 13 shows an optional cross-sectional con¬ figuration at the juncture of adjacent, closed tetra- hedric units. DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring specifically to the drawings, in FIGS. 1 and 1A an equilateral tetrahedron 10 (having equal faces) and its complementary skeletal-tetrahedron 12 are shown for definitional purposes. The equilateral tetrahedron may conceptually be thought of as a three- dimensional triangle, extending spatially the excep¬ tional two-dimensional (planar) rigidity of the equi-

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lateral triangle. The skeletal-tetrahedron 12 may be thought of as consisting of four struts 14 joined at a node 16 and externally terminating at the four apexes respectively of the phantom reference tetrahedron 10 enclosing the skeletal assembly 12. The skeletal equi¬ lateral tetrahedron is the most geometrically stable articulated structure of line elements, having maximum symmetry (i.e. cubic), with the minimum number of struts per node (i.e. four) for a stable 3D articulated structure, while utilizing the rigidity of the basic triangle.

In FIGS. 2, 2A, and 2B an articulated skele¬ tal-tetrahedric unit 20 of the present invention is shown wherein four struts 14 are received and joined onto four protrusions 23 respectively of a male-node 22, the assembly forming a skeletal equilateral tetrahedron. The struts and the nodes may optionally be hollow to minimize the weight of the unit, as shown for example in the male-node 22 by channels 24 within protrusions 23. In FIGS. 3, 3A, and 3B another articulated skeletal- tetrahedric unit 30 is shown wherein four struts 14 are received into the four receptacles 33 of a female-node 32. The latter embodiment is preferred due to the increased resistance to bending loads at the joint between a strut and the node. Joining of the struts to the node may be by conventional means such as fusion joining (welding or brazing), mechanical joining (pins, clamps, and the like), or adhesive joining. Further, the unit may be formed as a continuous (jointless) element.

Conventional structural alloys, preferably those having high specific strength, may be used to con¬ struct the units. However, it is preferred to utilize complementary materials to achieve a composite with a blend of exceptional individual material properties.

For example, the struts may be tubes or rods of oriented or pyrolytic graphite, a material having exceptional specific stiffness and low thermal expansion, and the

nodes of a structural aluminum alloy having exception¬ ally toughness properties. Thus, the composite struc¬ ture would have ultra-stiff struts (though of low toughness) joined at high toughness nodes (plastically deformable upon the unit being excessively loaded).

In FIG. 4, an articulated skeletal-tetrahedri unit 42 of truss 44 and node 46 is shown enclosed in a phantom reference cube 40 of characteristic dimension "a", which as shown in FIG. 5 may. be inserted into any of the eight cubic-a (sub-cell) positions of a phantom unit-cell 50 having characteristic dimension "2a". Reference is made to these phantom volumes only to facilitate description of the invention as they do not comprise tangible structure. According to the present invention, four tetrahedric units 42 of like orientatio are joined in alternating sub-cells 40 of the unit-cell 50, as shown in the exploded view of FIG. 6, to form the completed unit-cell 50, as shown in FIG. 7. This unit- cell may be repeated simply in any or all of the three dimensions to the extent desired, thereby obtaining a three-dimensionally periodic, tetrahedric truss.

A more preferred embodiment of the tetrahedral truss of the present invention and its method of con- ^" struction is shown in FIGS. 8 to 12. In FIG. 8, a fund- amental bilateral-element 80 is shown having equal sides 82 and having an included angle 84 of about 109°28', i.e. the angle between the struts of a skeletal equi¬ lateral tetrahedron. Optional features may be included to facilitate joining of' a plurality of bilateral-ele- ments, such as a structural pin 86 at one extremity and a complementary, close fitting receptacle 88 at the other extremity. The bilateral-elements may be made of conventional alloys, preferably those having high speci¬ fic strength. Six bilateral-elements 80 are assembled into the hexagonic triplanar-ring 90 as shown in FIG. 9. In FIGS. 9A, 9B, and 9C an exploded plan view, a plan view, and a side view are shown respectively of the tri-

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planar-ring of FIG. 9. It is noted that these tri- planar-ring elements are exceptionally rigid under tor- sional loading. Joining may be secured by conventional fusion joining means or by adhesive joining means and 5 the like.

Four hexagonic triplanar-rings 90 are assem¬ bled into the closed skeletal-tetrahedric unit 100 as shown in FIGS. 10 and 10A. Rigid joining of the unit may be by conventional mechanical means such as bolting, ^* 0 riveting, strapping, clamping, and the like or by con¬ ventional fusion joining. To clarify the derivation of the unit and to emphasize that it is in fact a skeletal- tetrahedric structure, reference is again made to FIG. 7. The sixteen struts 44 making up the unit-cell 50 may 5 be classified into two categories, i.e. corner struts and face struts. A corner strut has its external extre¬ mity terminating at a corner of the unit-cell. There are four of these corner struts 72 per unit-cell. A face strut has its external extremity terminating at a 0 face of the unit-cell. There are twelve of these face struts 74 per unit-cell. The closed skeletal-tetra¬ hedric unit 100 (FIG. 10) is of the pattern formed by the face struts of the cubic-diamond unit-cell 50 shown in FIG. 7. The closed tetrahedric unit 100 is preferred 5 over the articulated tetrahedric unit 42 (FIG.7) because points of stress concentration at strut-node joints are eliminated.

A plurality of tetrahedric units 100 are co- ^• operatively stacked (nested), as shown in FIGS. 11 and 0 11A, to build up a tetrahedral truss 110. Rigid joining of neighboring tetrahedric units 100 may be accomplished by conventional means as discussed above. Note that a skeletal equilateral tetrahedron is completed at each juncture of neighboring units 100, thereby obtaining the 5 cubic-diamond structure of the first mode of the inven¬ tion (FIG.7).

In FIG. 12, the tetrahedral truss 110 of FIG. 11 is shown with further three dimensional extension

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123, i.e. repeated units 100. Additionally, the sim¬ plicity is shown with which a graded truss 120 (e.g. having layers 122 and 123) may be built up. By varying the characteristic dimension of adjacent layers by an integer power of the fraction one-half, adjacent layers may be cooperatively stacked, as shown in the exploded perspective view of FIG. 12A. Thus, a tetrahedral truss may readily be constructed having a relatively "smooth" (close) supporting surface with an open structure in the interior portions of the truss.

There are alternative methods for forming the hexagonic triplanar-ring 90 (FIG. 9), having the advan¬ tage that a jointless element is obtained. For example, the ring may be mechanically shaped from a linear member of a structural alloy and fusion joined to close the ring, with perhaps subsequent heat treatment, e.g. pre¬ cipitation hardening. As another alternative, the material may be a fiber reinforced composite. As a further alternative, the ring may be constructed of oriented graphite according to conventional methods, e.g. by pyrolyzing a shaped winding of organic fiber under orienting tension.

In FIG. 13, an optional feature is shown for promoting the rigidity at the juncture between neighbor- ing closed skeletal-tetrahedric units 100 (FIG. 11). A cross-sectional cut is taken through such a juncture. As shown, the hexagonic triplanar-rings 90 may be of hexagonal cross-section, rather than of circular cross- section as shown in the preceeding drawings. A linear, close fitting filler rod 132, also of hexagonic cross- section, is inserted into the void between neighboring rings 90. The members are shown as being hollow to minimize weight.

Although the present invention has been des- cribed in conjunction with preferred embodiments, it is to be understood that modifications and variations may be utilized without departing from the principles and scope of the invention as those skilled in the art will

readily understand. Accordingly, such modifications and variations may be practiced within the scope of the fol¬ lowing claims:

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