Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

7.8 Second Moments of Areas 189

In Chapter 9, we will look at another similarly defined property of an object, mass moment

of inertia, which provides a measure of resistance to rotational motion.

EXAMPLE 7.4 Calculate the second moment of area for a 2 4 stud with the actual dimensions shown on

Figure 7.24.

Note that the 2 4 lumber will show more than five times more resistance to bending

about they–yaxis than it does about thex–xaxis.

Iyy

1

12

hw

3

a

1

12

b11.5 in. 2 1 3.5 in. 2

3

5.36 in

4

Ixx

1

12

wh

3

a

1

12

b13.5 in. 2 1 1.5 in. 2

3

0.98 in

4

1.5 in.

3.5 in.

x x

y

y

TABLE 7.8 Examples of the Second Moment of Area of Standard Beams (continued)

(U.S. Customary version)

Designation

Depth (in.) Width (in.) Area (in

2

) Ixx(in

4

) Iyy(in

4

)

W18  76 18.21 11.03 22.3 1330 152

W16  57 16.43 7.12 16.8 758 43.1

W14  38 14.10 6.77 11.2 385 26.7

W8  31 8.00 7.95 9.1 110 37.1

y

y

x x

■Figure 7.24

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