36
Centroids
Centroids
Centroid Principles
Object’s center of gravity or center of mass.
Graphically labeled as
Centroid Principles
Point of applied force caused by
acceleration due to gravity.
Object is in state of equilibrium if
balanced along its centroid.
Centroid Principles
What is an object’s centroid location used for in statics?
Theoretical calculations regarding the
interaction of forces and members are
derived from the centroid location.
Centroid Principles
One can determine a centroid location
by utilizing the cross-section view of a
three-dimensional object.
Centroid Location
Symmetrical Objects
Centroid location is determined by an
object’s line of symmetry.
Centroid is located on
the line of symmetry.
When an object has multiple lines of symmetry,
its centroid is located at the intersection of the
lines of symmetry.
H
B
Centroid Location
The centroid of a square or rectangle is
located at a distance of 1/2 its height and 1/2
its base.
2
B
2
H
H
B
Centroid Location
The centroid of a right triangle is located at a
distance of 1/3 its height and 1/3 its base.
Centroid Location
The centroid of a ½ circle or semi-circle is
located at a distance of 4*R/3π away from the
axis on its line of symmetry
4
3
R
4 2 .
3
in
0.849 in. = 0.8in.
.849in.
Centroid Location Equations Complex Shapes
i i
i
y Ay=
A
i i
i
x Ax=
A
i i
i
z Az=
A
Centroid Location Complex Shapes
1. Divide the shape into simple shapes.
1
2
3
2. Determine a reference axis.
Centroid Location Complex Shapes
Review: Calculating area of simple shapes
Side2 Width * Height
πr2 ½ (base)(height)
Area of a
square = Area of a rectangle =
Area of
a circle =
Area of a triangle =
Centroid Location Complex Shapes
3. Calculate the area of each simple shape. Assume measurements have 3 digits.
2
Area of shape #1 =
Area of shape #2 =
Area of shape #3 =
3.00in. x 6.00in. = 18.0in.2
18in.2
½x3.00in.x3.00in. = 4.50in.2
4.5in.2
(3.00in.)2 = 9.00in.2
9in.2
side2
½ base x height
width x height
Centroid Location Complex Shapes
4. Determine the centroid of each simple shape.
1/3 b
1/3 h
Shape #1 Centroid Location
Shape #2 Centroid Location
Shape #3 Centroid Location
Centroid is located at the
intersection of the lines
of symmetry.
Centroid is located at the
intersection of the lines
of symmetry.
Centroid is located at the
intersection of 1/3 its
height and 1/3 its base.
Centroid Location Complex Shapes
5. Determine the distance from each simple shape’s
centroid to the reference axis (x and y).
4in.
4.5in.
1.5in.
3in
.
1.5
in.
4in
.
Centroid Location Complex Shapes
6. Multiply each simple shape’s area by its distance
from centroid to reference axis.
Shape Area (A) xi Axi
1 x
2 x
3 x
Shape Area (A) yi Ayi
1 18.0in.2 x
2 4.50in.2 x
3 9.00in.2 x
18.0in.2
4.50in.2
9.00in.2
1.50in.
4.00in.
4.50in.
27.0in.3
18.0in.3
40.5in.3
54.0in.3
18.0in.3
13.5in.3 1.50in.
4.00in.
3.00in.
Centroid Location Complex Shapes
7. Sum the products of each simple shape’s area and
their distances from the centroid to the reference axis.
Shape Ayi
1 54.0in.3
2 18.0in.3
3 13.5in.3
Shape Axi
1 27.0in.3
2 18.0in.3
3 40.5in.3
3
3
3
27.0in.
+ 18.0in.
+ 40.5in.
85.5in.3
Ax=
i
3
3
3
54.0in.
+ 18.0in.
+ 13.5in.
Ay=
i
85.5in.3
Centroid Location Complex Shapes
8. Sum the individual simple shape’s area to determine total shape area.
Shape A
1 18in.2
2 4.5in.2
3 9in.2
2
2
2
18.0in.
+ 4.5in.
+ 9.0in.
31.5in.2
A=
18in.2
4.5in.2
9in.2
Centroid Location Complex Shapes
9. Divide the summed product of areas and distances
by the summed object total area.
3
231.5
85.5
in.
i .=
n =
31.5in.2 A=
85.5in.3 Ax
=i
Ay=
i 85.5in.3
3
231.5
85.5
in.
i .=
n = 2.71in.
2.7in.
2.7
in.
2.71in. Does this shape have any lines
of symmetry?
Alternative Solution
• The same problem solved a different way.
– Previous method added smaller, more
manageable areas to make a more complex
part.
– Alternative Method = Subtractive Method
• Uses the exact same equations
• Uses nearly the exact same process
– Start with a bigger and simpler shape
– Treat shapes that need to be removed as
“negative” areas
Centroid Location – Subtractive Method
1. Determine reference axis and start with an area that is bigger than what is given
Square = Shape 1
2. Remove an area to get the centroid of the complex shape
Triangle = Shape 2
6 in.
6 in.
3 in.
3 in.
Centroid Location Complex Shapes
3. Calculate the area of each simple shape. Assume measurements have 3 digits.
Area of shape #1 =
6.0in. x 6.0in. = 36 in.2
-½x3.0in.x3.0in. = -4.5 in.2
-½ base x height
width x height
Area of shape #2 =
6 in.
6 in.
3 in.
3 in.
Note: Since the area is being
removed, we are going to call it
a negative area.
Centroid Location Complex Shapes
4. Determine the centroid of each simple shape.
Shape #1 Centroid Location
Centroid is located at the
intersection of the lines
of symmetry.
Middle of the square
Centroid is located at the
intersection of 1/3 its
height and 1/3 its base.
6 in.
6 in.
3 in.
3 in.
1/3 b
1/3 h
Shape #2 Centroid Location
Centroid Location Complex Shapes
5. Determine the distance from each simple shape’s
centroid to the reference axis (x and y).
6 in.
6 in.
3 in.
3 in.
5in.
3in.
3in
.
5in.
Centroid Location Complex Shapes
6. Multiply each simple shape’s area by its distance
from centroid to reference axis.
Shape Area (A) xi Axi
1 x
2 x
Shape Area (A) yi Ayi
1 36in.2 x
2 -4.5in.2 x
36in.2
-4.5in.2
3.0in.
5.0in.
108in.3
-22.5in.3
108in.3
-22.5in.3 5.0in.
3.0in.
6 in.
6 in.
3 in.
3 in.
5 in.
3 in.
3 in
.
5 in.
Centroid Location Complex Shapes
7. Sum the products of each simple shape’s area and
their distances from the centroid to the reference axis.
Shape Ayi
1 108in.3
2 22.5in.3
Shape Axi
1 108in.3
2 22.5in.3
3
3
108.0in.
+ -22.5in.
85.5in.3
Ax=
i
Ay=
i
85.5in.3
3
3
108.0in.
+ -22.5in.
Centroid Location Complex Shapes
8. Sum the individual simple shape’s area to determine total shape area.
Shape A
1 36 in.2
2 -4.5 in.2
2
2
36.0in.
+ -4.5in.
31.5in.2
A=
3 in.
6 in.
6 in.
3 in.
3 in.
3 in.
Centroid Location Complex Shapes
9. Divide the summed product of areas and distances
by the summed object total area.
3
231.5
85.5
in.
i .=
n =
31.5in.2 A=
85.5in.3 Ax
=i
Ay=
i 85.5in.3
3
231.5
85.5
in.
i .=
n = 2.71in.
2.71in. Does this shape have any lines
of symmetry? 2.7
in.
2.7in.
6 in.
6 in.
Centroid Location Equations Complex Shapes
i i
i
y Ay=
A
i i
i
x Ax=
A
i i
i
z Az=
A
Common Structural Elements
Angle Shape (L-Shape)
Channel Shape (C-Shape)
Box Shape
I-Beam
Centroid of Structural Member
Cross Section View
Neutral Plane
(Axes of symmetry)
Neutral Plane
Tension
Compression
Neutral Plane (Axes of symmetry)
