Tie rod
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A tie rod is a slender structural rod used as a tie and capable of carrying tensile loads only. Since the ratio of its length to the radius of gyration of its cross section is usually very large, it would buckle under the action of compressive forces. Tie rods are used for airplane structures and in steel structures such as bridges, industrial buildings, tanks, towers and cranes. Tie rods known as sag rods are sometimes used in connection with purlins to take the component of the loads which is parallel to the roof.
The working strength of a tie rod is the product of the allowable working stress and the minimum cross-sectional area. If threads are cut into a cylindrical rod, that minimum area occurs at the root of the thread. Often rods are upset (made thicker at the ends) -- then the tie rod does not become weaker when threads are cut into it.
Tie rods are connected at the ends in various ways, but it is desirable that the strength of the connection should be at least equal to the strength of the rod. The ends may be threaded and passed through drilled holes or shackles and retained by nuts screwed on the ends. If the ends are threaded right- and left-hand the length between points of loading may be altered. This furnishes a second method for prestressing the rod at will by turning it in the nuts so that the length will be changed. A turnbuckle will accomplish the same purpose. The ends may also be swaged to receive a fitting which is connected to the supports. Another way of making end connections is to forge an eye or hook on the rod.
The spokes of bicycle wheels are tie rods.
A famous incident involving tie rods is the Hyatt Regency walkway collapse in Kansas City, Missouri, on July 17, 1981. The hotel had a large atrium with three walkways crossing it suspended from tie rods. A construction flaw resulted in several of the walkways collapsing, killing 114 people and injuring over 200.
See Also: Reinforced concrete
In automobiles a tie rod is part of the steering mechanism.