Post on 17-Dec-2015
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.
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.
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).
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.
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.
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.
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.
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.
Sag Rods
• The tie rod between ridge purlins must resist the load from all of the sag rods on either side.