anchor bolts calc

Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar

Indian Institute of Technology Madras

8.4 Base plate with Shear and Anchor bolts

8.4.1 Base plate with Shear

Figure 8.7 Column base plate subjected to 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 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 8.8 Base plate anchors

They 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 c4 f ' 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

ug

t u

TA

0.75F

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 failure

with another failure cone nor the edge of the pedestal, the required embedded

length can be calculated from the equation

( )u cp T / 4 f 'AL = 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 in

A325 (A449) 17d 7d > 4 in

d = nominal diameter of the anchor

anchor bolts calc

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