Design of Tension Members

1. Design of Tension Members • To avoid yielding: Ag Pu / 0.9 Fy • To avoid fracture: Ae  Pu / 0.75 Fu Or : An Pu / 0.75 Fu U  • where Pu is the sum of the factored loads.

2. Design of Tension Members • If the axial load in a slender tension member is removed and small transverse loads are applied, undesirable vibrations or deflections may occur. Thus AISC recommends: • r  L/300 ( not for cables or rods) • where r is the minimum radius of gyration of the cross section and L is the length of the member. 

3. Threaded Rods and Cables • When slenderness is not a consideration, circular rods and cables are often used (hangers, suspended bridges). • Rods are solid and cables are made from individual strands wound together. • Threading the end of a rod reduces the cross sectional area (upset end prevents such reduction, but is expensive).

5. Threaded Rods and Cables • t Pn = 0.75 (0.75 Ab Fu) • Ab = nominal (unthreaded) area • It is common to use a min diameter of 5/8 in. for rods.

6. Threaded Rods and Cables • A strand consists of individual wires wound helically around a centrl core. • A wire rope is made of several strands laid helically around a core.

7. Tension Members in Roof Truss • Trusses are used where the cost and weight of a beam could be prohibitive (long spans). • A truss may be thought of as a deep beam with much of the web removed. • Tension members in roof trusses include some truss members and sag rods.

8. Sag Rods • Sag rods are used to provide lateral support for the purlins (to prevent sag in direction parallel to a sloping roof due to vertical applied loads). • They are designed to support the component of roof loads parallel to the roof.

11. Sag Rods • Each segment between purlins is assumed to support everything below it; thus the top rod is designed for the load on the roof area tributary to the rod, from the heel of the truss to the peak.

13. Sag Rods • The tie rod between ridge purlins must resist the load from all of the sag rods on either side.