In: Civil Engineering
Sol: Information provided in the question:
- A quarter circle of radius (R)= 227 mm ( lies on Quadrant II of a coordinate plane )
- A rectangle is attached to the quarter circle at Quadrant I :
with base at the x-axis of width (b)= 45.6 mm
and height same as the radius of the quarter circle (h)= 227 mm
- Another rectangle with inverse dimension as the previous is attached to the quarter circle at Quadrant III :
Width (b) = 227 mm
Height (h) = 45.6 mm
(a) The moment of inertia (m4) of the composite area with respect to the x-axis at point O :
First moment of inertia for A1 = R4/16
= x2274/16 = 521354730.9 mm4
= 5.2135 x 10-4 m4
Then moment of inertia for A2 = b x h3/12
= 45.6 x 2273/12
= 44448915.4 mm4
=4.444 x 10-5m4
Then moment of inertia for A3 = b x h3/12
= 227 x 45.63/12
= 1793655.936 mm4
= 1.7936559 x 10-6m4
Hence The moment of inertia (m4) of the composite area with respect to the x-axis at point O :
= 5.2135 x 10-4 m4 + 4.444 x 10-5m4 +1.7936559 x 10-6m4
= 5.6758 x 10-4 m4
(b) The moment of inertia (m4) of the composite area with respect to the y-axis at point O :
First moment of inertia for A1 = R4/16
= x2274/16 = 521354730.9 mm4
= 5.2135 x 10-4 m4
Then moment of inertia for A2 = h x b3/12
= 227 x 45.63/12
= 1793655.936 mm4
= 1.7936559 x 10-6m4
Then moment of inertia for A3 = h x b3/12
= 45.6 x 2273/12
= 44448915.4 mm4
=4.444 x 10-5m4
Hence The moment of inertia (m4) of the composite area with respect to the y-axis at point O :
= 5.2135 x 10-4 m4 + 17936559 x 10-6m4 + 4.444 x 10-5m4
= 5.6758 x 10-4 m4
(c) The polar moment of inertia (m4) of the composite area :
First polar moment of inertia for A1 = R4/8
= x2274/8 = 1042709462 mm4
= 1.04270946 x 10-3 m4
Then polar moment of inertia for A2 = ( b x h3/12 ) + ( h x b3/12 )
= 45.6 x 2273/12 + 227 x 45.63/12
= 44448915.4 mm4 + 1793655.936 mm4
= 4.444 x 10-5m4 + 1.7936559 x 10-6m4
= 4.6233655 x 10-5m4
Then polar moment of inertia for A3 = ( b x h3/12 ) + ( h x b3/12 )
= 227 x 45.63/12 + 45.6 x 2273/12
= 1793655.936 mm4 + 44448915.4 mm4
= 1.7936559 x 10-6m4+ 4.444 x 10-5m4
= 4.6233655 x 10-5m4
The polar moment of inertia (m4) of the composite area :
= 1.04270946 x 10-3 m4 + 4.6233655 x 10-5m4 + 4.6233655 x 10-5m4
= 1.135176 10-3 m4
(d) The centroidal moment of inertia of the composite area :
For this first we have to find The Centroid of composite Area: (About X axis) with respect to origin
X' = (A1 x X1 + A2 x X2 + A3 x X3 ) / ( A1 + A2 + A3)
= ( R2/4 x (4R/3) + b x h x b/2 + bxhx b/2) / ( R2/4 + b x h + b x h)
= ( .2272/4 x (4x(-0.227)/3) + 0.0456 x 0.227 x 0.0456/2 + 0.227x0.0456x (-0.227)/2) /
( x0.2272/4 + 0.0456 x 0.227 +0.227x0.0456)
=( - 3.8990x10-3 + 2.36007x10-4 - 1.17486x10-3 ) / ( 0.061173 )
X' = - 0.0790852 m
Then we have to find The Centroid of composite Area: (About Y axis) with respect to origin
Y' = (A1 x Y1 + A2 x Y2 + A3 x Y3 ) / ( A1 + A2 + A3)
= ( R2/4 x (4R/3) + b x h x h/2 + bxhx h/2) / ( R2/4 + b x h + b x h)
= ( .2272/4 x (4x(0.227)/3) + 0.0456 x 0.227 x 0.227/2 + 0.227x0.0456x (-0.0456)/2) /
( x0.2272/4 + 0.0456 x 0.227 +0.227x0.0456)
=( + 3.8990x10-3 + 1.174859x10-3 - 2.36007x10-4 ) / ( 0.061173 )
Y' = 0.07908475 m
Now The centroidal moment of inertia of the composite area : (About X axis)
We use Parallel Axis theorem:
The centroidal moment of inertia for A1 = R4/16 + (R2/4)x((4R/3) - Y')2
= x0.2274/16 + .2272/4 x (4x(0.227)/3 -0.0790847)2
= 5.2135 x 10-4 m4 + 1.2052 x 10-5 m4
= 5.3340 x 10-4 m4
Hence The centroidal moment of inertia of the composite area : (About X axis)
= 5.3340 x 10-4 m4 + 5.67000 x 10-5m4 + 1.092427 x 10-4m4
= 6.99342 10-4 m4
Y axis 45.6 mm A1 A2 227 mm X axis A3 45.6 mm 227 mm
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Y axis 45.6 mm A1 A2 227 mm X axis A3 45.6 mm 227 mm
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