Tension Member Design Basics
Rod vs. Cable
Many factors go into choosing whether to use cable or rod, galvanized or stainless, high strength or lower strength rod materials, flexible or more rigid cable construction. Cables tend to be more efficient when lengths are long, loads are high, or the member passes several struts or clamps, as might occur in a bow truss. Rods are often a better answer when the member length is relatively short (32 feet or less), clean and smooth appearance is important, or when stiffness rather than strength is the driving design consideration.
Rod & Cable Mechanical Properties
Different rod and cable materials and different cable constructions offer a range of mechanical properties, visual effects, and inherent costs. The table and graph at right give strength and stiffness information on the variety of rod and cable choices that TriPyramid offers.
Use the buttons above to view the assemblies which correspond to the various rod and cable sizes and materials. Order by full assembly; that is, select rod or cable, plus the fittings for both ends.
TriPyramid offers tension rods in a variety of materials and strengths, providing customers with products ideally suited for any given design parameters. TriPyramid’s stainless steel tension rods are available in three tensile strength ranges: high strength, medium strength, and LCW (Lightly Cold Worked) grade. TriPyramid’s standard carbon steel rod product has a yield strength almost twice that of A36 structural steel. Other carbon steel materials are available on a custom basis.
TriPyramid’s A03 family of high strength rods offers the highest practical strength-to-weight ratio, therefore, the smallest diameter for a given strength. This material is used to support the masts of America’s Cup sailing yachts.
TriPyramid’s medium strength range of rods (A22 and A25 series) generally provides the most cost effective combination of strength and appearance. This material has a yield strength approximately three times that of normal A36 structural steel or typical threaded stainless bar systems.
The LCW (lightly cold worked) range of rods (A35 series) is lower in strength than medium strength rod, but provides the least expensive rod assemblies when a given diameter, and not material strength, is the driving design consideration.
TriPyramid offers steel rods (A70 series) ranging in diameter from 1/2 inch [12.7 mm] to 4 inch [101.6 mm]. This standard rod material has a yield almost double that of A36 structural steel. Rods and fittings can be galvanized for exterior applications.
Cables are a construction of wire strands, laid (i.e. twisted) helically around a core, to form a tension member of symmetrical cross-section. Cables have a high strength-to-weight ratio as the wire strands are drawn to high strengths and laid to share tensile loads efficiently. There are two fundamental cable constructions: Wire rope (“flexible construction”) and structural strand (“stiff construction”). The construction and subsequent mechanical properties are determined by the wire pattern and wire size used for the core and outer strands.
For a given strength, wire rope is larger in diameter and lower in stiffness than structural strand or solid rods, so it may not be the most cost-effective product for static, tension load-carrying structural members. However, wire rope may be the appropriate choice in cases where relatively high stretch is desired or where the cable needs to turn a sharp angle. Typical constructions are 7×19 and 6×19 IWRC.
Structural strand is the cable construction most commonly used in structural applications. It offers an economical combination of strength and stiffness for static structures. As cables get larger in diameter, the number of strands increases. For instance, cables with a nominal diameter up to 3/4 inch [19 mm] have 19 strands (called 1×19), while cables of 2 inch [50.8 mm] diameter have 91 strands (called 1×91).
Full Locked Cable
A variation for larger galvanized cables is “full locked cable”. These cables have their outer strands drawn in a “Z” shape so that they interlock and form a smoother outside layer of strands. The interlocking strands also yield a denser cross section and therefore a higher effective elastic modulus.