8

8.4 Base plate with Shear and Anchor bolts 8.4.1 Base plate with Shear

Figure 8.7 Column base plate subjected to shearUnder normal circumstances, the factored column base shear is adequately resisted by the frictional force developed between the plate and its support. Additional shear capacity is also provided by the anchor bolts. For cases in which exceptionally high force is expected, such as in bracing connection or in which uplift occurs which reduces the frictional resistance, the use of shear lugs may be necessary. Shear lugs can be based on the limit states of bearing on concrete and bending of the lugs. The size of the lugs should be proportioned such that the bearing stress on concrete does not exceed 0.60(0.85 fc). The thickness of the lug can be determined from equation. Mplu is the moment per unit width at the critical section of the lug. The critical section is taken to be at the junction of the lugs and the plate (Fig 8.7).8.4.2 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 in set and are not too often used. Their design is governed by the manufacturer's specifications. Cast-in-place bolts are hooked bars, bolts, or threaded rods with nuts (figure 8.8) placed before the concrete is set. Of the three types of cast-in-place anchors shown in the figure, the hooked bars are recommended for the use only in axially loaded base plates.

Figure 8.8Base plate anchorsThey are not normally relied upon to carry significant tensile force. Bolts and threaded rods with nuts can be used for either axially loaded base plates or base plates with moments. Threaded rods with nuts are used for both axial 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. Failure of bolts are threaded rods with nuts occur when their tensile capacities 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 deplicted schematically in Figure 8.9. The failure cone is assumed to radiate out from the bolt head or nut at an angle of 45 degree with tensile failure occurring along the surface of the cone at an average stress of where fc' 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 the cone Ap [23, 24]. The design of anchor bolts is thus governed by the limit states of tensile fracture of the anchors and cone pull-out. Limit State of Tensile Fracture

The area of the anchor should be such that

Where Ag is the required gross area of the anchor ,Fu is the minimum specified tensile strength, and t is the resistance factor for tensile fracture which is equal to 0.75. Limit State of Cone Pull-Out

From Figure 8.9, it is clear that the size of the cone is a function of the length of the anchor. Provided that there is sufficient edge distance and spacing between adjacent anchors, the amount of tensile force required to cause cone pull-out failure increases with the embedded length of the anchor. This concept can be used to determine the required embedded length of the anchor. Assuming that the failure can does not intersect

Figure 8.9 Cone Pullout failurewith another failure cone nor the edge of the pedestal, the required embedded length can be calculated from the equation

Where Ap is the projected area of the failure cone, Tu is the required bolt force in pounds, fc' is the compressive strength of concrete in psi and t is the resistance factored assumed to be equal to 0.75. If failure cone from adjacent anchors overlap one another or intersect with the pedestal edge, the projected area Ap must be adjusted according (see, for example [23, 24]).The length calculated using the above equation should not be less than the recommended values given by [29]. These values are reproduced in the following table. Also shown in the table are the recommended minimum edge distances for the anchors.

Bolts type (material) Minimum ambedded length Minimum edge distance

A307 (A36) 12d 5d > 4 inA325 (A449) 17d 7d > 4 in

d = nominal diameter of the anchor

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