# Engineering Oasis

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# 13- A solved problem for Mohr’s circle of inertia case 4.

Last Updated on August 15, 2024 by Maged kamel

## A solved problem for Mohr’s circle of inertia case 4.

This post will introduce a solved problem for Mohr’s circle of inertia case 4. The moment of inertia about X-axis Ix is lesser than the moment of inertia about Y-axis Iy, and the product of inertia Ixy is negative.

The given solved problem is for the unequal angle of dimension 3 inches by 2 inches, and the thickness of the steel angle is (1/4) inch.

It is required to determine the principal moment of inertia. We will use Mohr’s circle of inertia to get the directions of the principal axes and later verify them with the general equation to check the estimated values. The long leg 3″ is parallel to the x-axis, while the short leg I parallel to the Y-axis.

### Area, Cg, and inertias for the angle in the solved problem for Mohr’s circle of inertia case 4.

The area of the angle-3″x2″x1/4″ equals 1.19 inch2, and the horizontal distance for the Cg from the right edge of the unequal angle is equal to 0.98″. The vertical distance for the Cg of the unequal angle to the upper edge equals 0.487 inches.

We can get the moment of inertia Ix for the given angle from Table 1-7. We will reverse the values and let the Ix value from the table is the Iy in the given solved example since the angle is rotated.

The long leg is parallel to the x-axis, unlike the data that is shown in table 1-,7 fo,r, which is the long portion parallel to the y-direction. The Area and Cg distances are given from the table as shown in the next slide image.

The data of the solved problem is shown in the next slide image.

### The product of inertia for the angle in the solved problem for Mohr’s circle of inertia case 4.

The x-axis and Y-axis are placed at the CG of the unequal angle. We want to estimate the product of inertia, Ixy. The upper leg is divided into two areas, while the short leg is considered one area.

For each area, we will estimate the product of inertia and then add the multiplication of each area by the product of x*y to the Cg axes.

the detailed estimations of the values of the product of inertia are shown in the next slide image.

The final value of the product of inertia equals (-0.3798) inch4. The values of Ix, Iy, and Ixy are collected and it is found that Ix is less Iy and Ixy is negative. This case is case 4.

### A solved problem for Mohr’s circle of inertia case 4-Drawing the circle.

For the solved problem for Mohr’s circle-Mohr’s circle of inertia, we start by drawing two intersecting axes. The horizontal axis represents the value of the moment of inertia. The vertical axis represents the value of the product of inertia, with a positive value pointing up. We could make 1 unit equal to 0.10 inch4.

We start by locating Point A, which has Ix, Ixy values as (0.39, -0.3798) inch4 and will be located below the axis of inertia by a value of negative Ixy equals 0.3978 inch4, and apart from the vertical axis Ixy by a distance of +0.39 inch4.

Similarly, we can draw point B, which has a coordinate of (Iy,+Ixy). In inch4 will be (1.39, +0.3798). We will join both two points. Line AB will intersect the horizontal axis at point O, which will be the circle’s center.

We can draw the circle by getting the middle point O, which has a coordinate of (0.50*(Ix+Iy),0) from the given data this value is equal to (0.39+1.09)*0.50=0.74 inch4.

The radius of the circle is estimated from the equation R=sqrt ((Ix-Iy)^2+Ixy^2); this has been applied since. We have Ix=0.39 inch4.Iy=1.09 inch4.Ixy=-0.3798 inch4.

calculations for the center point and the radius value of the circle are shown in the next slide image.

### A solved problem for Mohr’s circle of inertia case 4 The value of Imax and Imin.

The values of I max, and Imin are estimated as follows for Imax=(Ix+Iy)*0.50+R=(0.39+1.09)*0.50+0.5165=1.2565 inch4. The estimation of Imin is equal to =(Ix+Iy)*0.50-R=(0.39+1.09)*0.50-0.5165=0.2235 inch4.

We need to have two mirror points: the first point is A’, which has a coordinate of (0.39,+0.0.3789) inch4, and the second point is point B’, which has a coordinate of (1.09,-0.3798).

To get The U direction, join point O with point A’. For the V direction, join point O with point B’.

### Value of Angle 2ฯp2 for the Mohr’s circle of inertia case 4.

The direction of the principal axis V is making a clockwise direction angle 2ฯp2. The value of this angle can be estimated from the tan value as (2*Ixy)/(Iy-Ix), or (+*(-0.3798)/1.09-0.39)=-1.0851.

The value of the angle is 47.338 degrees.

The value of 2ฯp1 equals (180-47.338)=+132.662 degrees in the anti-clockwise direction. The angle ฯp1 value is 66.831 degrees, while the value of the angle ฯp2 equals to-23.669 degrees.

To get U and V directions in the normal view, join Point C with A’.For the V direction in the normal view, join point C with point B’.

### The solved problem for Mohr’s circle -case-4-using general expression.

We can use the general expression for Ix’ to check the value of Imax, provided that (2ฯp1), is (132.662 degrees), Ix’ value will be equal to Imax. We plug in with Ix value= 0.39 inch4,Iy value=1.09 inch4, and Ixy=-0.3798 inch4.

The value of Ix’=1.2565 inch4 is the same as Imax estimated using Mohr’s circleโthe complete details of Ix’s estimation by the general equation.

We can use the general expression for Ix’ to check the value of Imin, provided that (2ฯp1), is (132.662 degrees), Iy’ value will be equal to Imin. We plug in with Ix value= 0.39 inch4,Iy value=1.09 inch4, and Ixy=-0.3798 inch4.

The value of Iy’=0.2235 inch4 is the same as Imin estimated using Mohr’s circleโthe complete details of Iy’s estimation by the general equation.

This compares the estimated values for Imin and the value of Imin from the table of steel angles. Imin is the product of r^2*A=0.22106 inch4. There is a little difference between iv and the estimated imin by both Mohr’s circle of inertia and by the general equation.

The next post no. 14 is a solved problem-inertia value for a given direction.

This is a link to a useful external resource: a calculator for Cross-Section, Mass, Axial, and polar area moments of Inertia,ย and Section Modulus.