Forces applied across, and parallel to, the surface of an object result in a shearing stress. A shearing stress, or force per unit area, deforms the object along the direction of the applied force. For example, pressing on a block of foam along its surface. The amount of shear stress generated depends upon the area of the surface to which the force is applied, whether it's a rectangle, circle or other shape.
Measure the length of the top surface of the object in inches. For example, suppose the length is 15.0 inches.
Measure the width of the top surface of the object in inches. The width might be 8.0 inches.
Multiply the length times the width to obtain the shear area in square inches. In this example, you have 15.0 inches times 8.0 inches, or 120 square inches.
Measure the width of the circular surface by a straight line that passes through the circle's center. This is the diameter. As an illustration, suppose the diameter is 10.0 inches.
Divide the diameter by 2 to obtain the radius of the circle in inches. In this example, divide 10.0 inches by 2, which equals a radius of 5.0 inches.
Multiply the number pi times the square of the radius to arrive at the shear area in square inches. Use 3.14 for the number pi. Completing this example leads to 3.14 times (5.0 inches)^2, where the "^" symbol denotes an exponent. The shear area then is 78.5 square inches.