Anchor Bolts

Anchor bolts are provided to stabilize the column during erection and to prevent uplift for cases involving large moments. Anchor bolts can be cast-in-place bolts or drilled-in bolts. The latter are placed after the concrete is set and are not used often. Their design is governed by the manufacturer’s specifications before the concrete is set. Anchor rods and threaded rods shall conform to one of the following ASTM specifications: A36=A36M, A193=A193M, A354, A572=A572M, A588=A588M, or F1554. Of the three types of cast-in-place anchors shown in the figure, the hooked bars are recommended for use only in axially loaded base plates. They are not normally relied upon to carry significant tensile force.

Bolts and threaded rods with nuts can be used for both axially loaded base plates or base plates with moments. Threaded rods with nuts are used when the length and size required for the specific design exceed those of standard size bolts.

Column base plate subjected to shear.

Base plate anchors.

Cone pull-out failure.

Failure of bolts or threaded rods with nuts occurs when the tensile capacities of the bolts are reached. Failure is also considered to occur when a cone of concrete is pulled out from the pedestal. This cone pull-out type of failure is depicted schematically as shown above. The failure cone is assumed to radiate out from the bolt head or nut at an angle of 45 with tensile failure occurring along the surface of the cone at an average stress of 4 SQRT (f’c) where f’c is the compressive strength of concrete in psi. The load that will cause this cone pull-out failure is given by the product of this average stress and the projected area of the cone Ap (Marsh and Burdette 1985a,b). The design of anchor bolts is thus governed by the limit states of tensile fracture of the anchors and cone pull-out.

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