# PARALLEL-AXIS THEOREM FOR AN AREA & MOMENT OF INERTIA FOR COMPOSITE AREAS

Engineering Mechanics: Statics Theory | Parallel Axis Theorem
Engineering Mechanics: Statics Theory | Parallel Axis Theorem

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Moment of Inertia Parallel-axis Theorem MoI For Composite Areas MET 2214

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PARALLEL-AXIS THEOREM FOR AN AREA & MOMENT OF INERTIA FOR COMPOSITE AREAS
Objectives: You will be able to: Apply the parallel-axis theorem. Determine the moment of inertia (MoI) for a composite area.

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APPLICATIONS Cross-sectional areas of structural members are usually made of simple shapes or combination of simple shapes. Is there a simpler method for determining the MoI of such cross-sectional areas as compared to the integration method? If yes, can you describe the method?

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APPLICATIONS This is another example of a structural member with a composite cross-area. Design calculations typically require use of the MoI for these cross-sectional areas. Can you describe a simple method to calculate MoI?

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PARALLEL-AXIS THEOREM FOR AN AREA
This theorem relates the moment of inertia (MoI) of an area about an axis passing through the area’s centroid to the MoI of the area about a corresponding parallel axis. This theorem has many practical applications, especially when working with composite areas. Consider an area with centroid C. The x’ and y’ axes pass through C. The MoI about the x-axis, which is parallel to, and distance dy from the x ‘ axis, is found by using the parallel-axis theorem.

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PARALLEL-AXIS THEOREM FOR AN AREA
IX = A y 2 dA = A (y’ + dy)2 dA = A y’ 2 dA + 2 dy A y’ dA + dy 2 A dA y = y’ + dy Using the definition of the centroid: y’ = (A y’ dA) / (A dA) . Now since C is at the origin of the x’ – y’ axes, y’ = 0 , and hence A y’ dA = 0 . Thus IX = IX’ + A dy 2 Similarly, IY = IY’ + A dX 2 and JO = JC + A d 2

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Answer: 1. B Axis 4 d3 3 • d2 2 d1 1 CONCEPT QUIZ A C
1. For the area A, we know the centroid’s (C) location, area, distances between the four parallel axes, and the MoI about axis 1. We can determine the MoI about axis 2 by applying the parallel axis theorem ___ . A) directly between the axes 1 and 2. B) between axes 1 and 3 and then between the axes 3 and 2. C) between axes 1 and 4 and then axes 4 and 2. D) None of the above. Answer: 1. B

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Answer: 2. C Axis 4 d3 3 • d2 2 d1 1 CONCEPT QUIZ A C
smallest MoI 2. For the same case, consider the MoI about each of the four axes. About which axis will the MoI be the smallest number? A) Axis 1 B) Axis 2 C) Axis 3 D) Axis 4 E) Can not tell. Answer: 2. C

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Answers: 1. D 2. B 3 • d2 2 d1 1 d1 = d2 = 2 cm ATTENTION QUIZ
A=10 cm2 • C d2 d1 3 2 1 d1 = d2 = 2 cm 1. For the given area, the moment of inertia about axis 1 is 200 cm4 . What is the MoI about axis 3 (the centroidal axis)? A) 90 cm 4 B) cm 4 C) 60 cm 4 D) 40 cm 4 I1 = I3 + Ad2 I3 = I1 – Ad2 = (4)2 = 40 cm4 Answers: 1. D 2. B

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ATTENTION QUIZ 2cm 3cm x 2. The moment of inertia of the rectangle about the x-axis equals A) 8 cm 4 B) 56 cm 4 C) 24 cm 4 D) 26 cm 4 Ix = Ix’ + Ad2 Ix = (bh3)/12 + Ad2 = (3 . 23) / = 56cm4 x’ d

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Given: The beam’s cross-sectional area.
EXAMPLE Given: The beam’s cross-sectional area. Find: The moment of inertia of the area about the y-axis and the radius of gyration ky. [1] [2] [3] Solution 1. The cross-sectional area can be divided into three rectangles ( [1], [2], [3] ) as shown. 2. The centroids of these three rectangles are in their center. The distances from these centers to the y-axis are 0 mm, 87.5 mm, and 87.5 mm, respectively.

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EXAMPLE 3. From the inside back cover of the book, the MoI of a rectangle about its centroidal axis is (1/12) b h3. Iy[1] = (1/12) (25mm) (300mm)3 = (106) mm4 [1] [2] [3] Using the parallel-axis theorem, IY[2] = IY[3] = IY’ A (dX)2 = (1/12) (100) (25)3 + (25) (100) ( 87.5 )2 = (106) mm 4

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EXAMPLE Iy = Iy1 + Iy2 + Iy3 = ( 106) mm 4 ky = ( Iy / A) A = (25) + 25 (100) + 25 (100) = 12,500 mm 2 ky = ( 94.79) (106) / (12500) = mm

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MOMENT OF INERTIA FOR A COMPOSITE AREA
A composite area is made by adding or subtracting a series of “simple” shaped areas like rectangles, triangles, and circles. For example, the area on the left can be made from a rectangle minus a triangle and circle. The MoI of these “simpler” shaped areas about their centroidal axes are found in most engineering handbooks as well as the inside back cover of the textbook. Using these data and the parallel-axis theorem, the MoI for a composite area can easily be calculated.

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STEPS FOR ANALYSIS 1. Divide the given area into its simpler shaped parts. 2. Locate the centroid of each part and indicate the perpendicular distance from each centroid to the desired reference axis. 3. Determine the MoI of each “simpler” shaped part about the desired reference axis using the parallel-axis theorem ( IX = IX’ A ( dy )2 ) . 4. The MoI of the entire area about the reference axis is determined by performing an algebraic summation of the individual MoIs obtained in Step 3. (Please note that MoI of a hole is subtracted).

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1. The parallel-axis theorem for an area is applied between A) an axis passing through its centroid and any corresponding parallel axis. B) any two parallel axis. C) two horizontal axes only. D) two vertical axes only. 2. The moment of inertia of a composite area equals the ____ of the MoI of all of its parts. A) vector sum B) algebraic sum (addition or subtraction) C) addition D) product Answers: 1. A 2. B

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(a) (b) (c) EXAMPLE Given: The shaded area as shown in the figure.
Find: The moment of inertia for the area about the x-axis and the radius of gyration kX. Solution 1. The given area can be obtained by subtracting both the circle (b) and triangle (c) from the rectangle (a). 2. Information about the centroids of the simple shapes can be obtained from the inside back cover of the book. The perpendicular distances of the centroids from the x-axis are: da = 5 in , db = 4 in, and dc = 8in. (a) (b) (c)

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(a) (b) (c) EXAMPLE 3. IXa = (1/12) 6 (10)3 + 6 (10)(5)2 = 2000 in 4
IXb = (1/4) (2) (2)2 (4)2 = in 4 IXc = (1 /36) (3) (6)3 + (½) (3) (6) (8)2 = in 4 IX = IXa – IXb – IXc = in 4 kX = ( IX / A ) A = ( 6 ) – (2)2 – (½) (3) (6) = in 2 kX = (1192 / 38.43) = in.

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