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Statics
Chapter 10
Moment of Inertia
Eng. Iqbal Marie
iqbal@hu.edu.jo
Hibbeler, Engineering Mechanics: Statics,11e, Prentice Hall
10.1 Moment of Inertia ( MoI) Definition of Moments of Inertia for Areas: Used in formulas for Mechanics of
Materials, Fluid Mechanics, Structural Mechanics
The moment of inertia is found by integrating
2
x
Area
2
y
Area
I y dA
I x dA
2
Area
jo dA r
Jo is the polar moment of inertia about the pole O or Z axis
X
X
Yr
dA
A
Radius of Gyration has units of length. Often used in Column design
10.3
Solution
Compute the moment of inertia in the x and y planes and find the
corresponding radius of gyration.
h
b
h/2
2 2
x
Area h/2
h/23 3
h/23 12
dA bdy
I y dA y bdy
y bhb
b/2
2 2
y
Area b/2
b/23 3
b/23 12
dA hdx
I x dA x hdx
x b hh
3
x
3
y
112 12
112 12
bh
r hbh
b h
r bbh
10.2
Using the parallel axis theorem
2
x x
233
3
4
12 2 12
3
I I d A
bh hbh bh
bh
b
X’
Find I x’
Moment of Inertia for simple shapes
r
10.4 Moments of Inertia for Composite Areas
Composite area consists of group of
connected simple shapes.
If MOI of parts about common axis can be
determined, then MOI of the composite is
algebraic sum of parts.
I = I , I = I , I = IX X
i
Yi
Y
i
Zi
Z
i
i
Procedure for Analysis
Composite Area Moment of Inertia about reference axis.
1.0 Composite Parts. Divide area into composite parts. Indicate perpendicular
distance from centroid of parts to reference axis.
2.0 Apply Parallel Axis Theorem. Determine MOI of each part about centroidal
axis parallel to reference axis. Use parallel axis theorem to calculate MOI of
parts about reference axis.
3.0 Sum MOI of parts. Calculate MOI of component by summing MOI of parts. If
any part is a “hole”, subtract the MOI of hole in making summation.
2
x xi i i
4 4 460 in 144 in 204 in
I I y y A
Bodies Ai yi yi*Ai Ii di=yi-ybar di2Ai
1 18 1 18 6 -2 72
2 18 5 90 54 2 72
36 108 60 144
ybar 3 in.
I 204 in4
4
xx 2
204 in
36 in
2.38 in.
Ir
A
Find Ix and rx ( x- axis passing through the centroid
of the section
Bodies Ai yi yi*Ai Ii di=yi-ybar di2Ai
1 18 1 18
2 18 5 90
36 108
3i i
2
i
108 in3.0 in.
36 in
y Ay
A
Bodies Ai yi yi*Ai Ii di=yi-ybar di2Ai
1 21600 90 1944000 58320000 0 0
2 -9600 90 -864000 -11520000 0 0
12000 1080000 46800000 0
ybar 90 mm
I 46800000 mm4
4
xx 2
46800000 mm
12000 mm
62.45 mm
Ir
A
Find Ix and rx ( x- axis passing through the
centroid of the section
Y’
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