Isotruss Offers Amazing Strength And Material Savings
The three-dimensional composite truss structure is the lightest and most efficient structural tube available.
By Sara Black, Technical Editor | July 2003
Anyone who lives under a pitched roof can appreciate the concept of a truss — a simple network of members joined together to form a two-dimensional, rigid triangular shape that can support the weight of the roof. When the concept of a truss is combined with composite material in three dimensions, the result is an ultra-low-weight structure with a tremendous strength-to-weight advantage.
IsoTruss Structures Inc. (Brigham City, Utah, U.S.A.) is counting on the potential afforded by this unique design concept. Initially developed and tested by researchers at Brigham Young University (Provo, Utah, U.S.A.), the IsoTruss is a filament-wound, cage-like, open tubular lattice comprising a series of intersecting triangles and pyramids. IsoTruss Structures holds exclusive license on the technology in the U.S.
The composite IsoTruss is similar in design to aluminum or steel lattice radio or telecommunication towers, says Tracy Livingston, chief technology officer at IsoTruss. A lattice design is usually selected for such structures because it uses less material and thus is about half the weight of a solid tube, which enables construction of very tall towers. The open structure also reduces wind loads.
A drawback of metallic lattice towers is that they are expensive because they are constructed of individual elements that must be hand assembled and fastened.
"What we have done is taken the best of both worlds," explains Livingston. "We have the material efficiencies of a lattice structure, for less material cost, and we have combined that with automated manufacturing, so we get the labor efficiencies, as well."
Incredible strength-to-weight
The overall structure of an IsoTruss is a cage-like tubular lattice, formed by a series of intersecting triangles. When viewed in cross-section, the all-composite structure appears as a symmetrical star shape — in three dimensions, the triangles form outward-pointing pyramids along the length of the structure.
The IsoTruss structure functions like a solid tube with a constant wall thickness, but at a fraction of the weight because so much less material is involved. In bending applications, it is less than 25 percent of the weight of an aluminum tube, and less than 9 percent of a steel tube, when compared on an equal load basis (see the bar chart, this page).
The lattice elements are unidirectional fiber tows or rovings, wound over a specialized collapsible metallic mandrel. Helical and longitudinal fibers are interwoven and compacted on the mandrel to form strong, integrated nodal joints. IsoTruss is designed to accept bending, buckling, axial, torsion and combined load applications. According to Livingston, the IsoTruss is well suited for situations with loads at multiple locations around the center axis or along its length. In effect, it is analogous to the geometry of an I-beam, with the longitudinal elements functioning like the flanges of the beam, moved outward from the neutral centerline tube axis.
For example, in an axial loading case, the longitudinal elements carry approximately 95 percent of the loads, while the helical triangular elements support the longitudinals, increasing the structural stiffness and protecting them from local buckling. In a torsional situation, the helical members take the majority of the load. The radial symmetry distributes the load and resists global buckling equally around the centerline axis. Livingston says that additional stiffness benefits are possible because of the loading along the fiber axis, which is the most efficient orientation possible.
