P

FIGURE 4.31 Eccentrically loaded column base plate (moderate load eccentricity).

FIGURE 4.32 Eccentrically loaded column base plate (large load eccentricity).

FIGURE 4.32 Eccentrically loaded column base plate (large load eccentricity).

The above equations can be used to solve for A and T. The size of the plate is to be determined using a trial and error process. The size should be chosen such that fmax does not exceed the value calculated using Equation 4.124, A should be smaller than N, and T should not exceed the tensile capacity of the bolts.

Once the size of the plate is determined, the plate thickness tp is to be calculated using Equation 4.125. Note that there are two critical sections on the plate, one on the compression side of the plate and the other on the tension side of the plate. Two values of Mplu are to be calculated, and the larger value should be used to calculate tp.

4.12.1.4 Base Plates with Shear

Under 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 shear force is expected such as in a bracing connection or in which uplift occurs which reduces the frictional resistance, the use of shear lugs may be necessary. Shear lugs can be designed based on the limit states of bearing on concrete and bending of the lugs. The size of the lug should be proportioned such that the bearing stress on concrete does not exceed 0.60(0.85f'). The thickness of the lug can be determined from Equation 4.125. 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 lug and the plate (Figure 4.33).

4.12.2 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 is set and are not used often. Their design is governed by the manufacturer's specifications. Cast-in-place bolts are hooked bars, bolts, or threaded rods with nuts (Figure 4.34) placed 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.

Critical section

Critical section

Shear lug Dimension = H X W X t H = Height W = Width t = Thickness

Shear lug Dimension = H X W X t H = Height W = Width t = Thickness

FIGURE 4.33 Column base plate subjected to shear.

Hooked bar

Bolt

Threaded rod with nut

FIGURE 4.34 Base plate anchors.

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