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 forcess. Tie rods are used for airplane structures and in steel structures such as bridges, industrial buildings, tanks, towers and craness. 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. In a rod which is threaded at the end and not upset to allow for the reduction of area caused by the thread, the minimum area will be that which occurs at the root of the thread. 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.
