## FIRST MOMENT OF INERTIA

## FIRST MOMENT OF INERTIA

(OP)

I'M TRYING TO FIGURE OUT A WAY OF OBTAINING THE INERTIA AROUND X-X (Ixx, cm4) OF A COMPLEX STEEL PROFILE.

STARTING WITH THE FIRST MOMENT OF INERTIA (Kg cm2), AND KNOWING THE DENSITY OF THE MATERIAL, IS THERE A WAY OF REACHING Ixx??

THANKS.

GESQUICOR

STARTING WITH THE FIRST MOMENT OF INERTIA (Kg cm2), AND KNOWING THE DENSITY OF THE MATERIAL, IS THERE A WAY OF REACHING Ixx??

THANKS.

GESQUICOR

## RE: FIRST MOMENT OF INERTIA

## RE: FIRST MOMENT OF INERTIA

## RE: FIRST MOMENT OF INERTIA

## RE: FIRST MOMENT OF INERTIA

This equation has a similar form to that for the second moment of area, approximately calculated thus: area x distance from centroid ^2 (m^4).

I'm not sure that you can derive one from the other though. Is the only difference density (kg/m^2)? Dividing kg.m^2 by kg/m^2 would give you the correct units, m^4, although I'm unsure if this is the correct solution technically.

It's not that hard to calculate the second moment of area for most shapes. What do you call a "complex" shape?

## RE: FIRST MOMENT OF INERTIA

Dbuzz, I do have the mass moment of inertia, thanks for pointing that out. I don't think I can get to the second moment of area with the info I have in hand.

We just finished the installation of a Finnish machine that produces roof trusses made from galvanized steel. The truss chord is the section that I reffer to as complex.

It's a rollformed section with over twenty sides and round turns. If you want to see details about it you can visit www.rosettesystems.com

I've already requested this information from the machine producer in Finland, but I really appreciate your comments. I'm a new user and I think I found the best site for engineering research, wether you have a question or you want to browse through interesting topics.

Gesquicor

## RE: FIRST MOMENT OF INERTIA

Download Section Properties for an Excel spreadsheet for determining your Ixx values. It will mean you having to input the co-ordinates of the points defining the outline of the shape, in a continuous anti-clockwise direction.

## RE: FIRST MOMENT OF INERTIA

1. DRAW THE SHAPE AS A CLOSED POLYLINE( TWO IF IT IS HOLLOW)

2.FROM THE DRAW MENU SELECT REGION, AND SELECT THE SHAPE(s) TO TURN INTO A REGION.

3. IF THE SECTION IS HOLLOW YOU NEED TO SUBTRACT THE INNER REGION FROM THE OUTER. FROM THE MODIFY MENU SELECT BOOLEAN>>SUBTRACT. YOU CAN THEN SUBTRACT ONE REGION FROM ANOTHER.

4. FROM THE TOOLS MENU SELECT INQUIRY>>MASS PROPERTIES. AND SELECT THE REQION YOU WANT THE PROPERTIES OF. bE CAREFULL THE PROPERTIES ARE TAKEN ABOUT 0,0,0 IN AUTOCAD. I USUALLY MOVE THE OBJECT SO THE CENTROID IS LOCATED AT 0,0,0

TRY IT ON A SIMPLE SHAPE YOU CAN CHECK FIRST

GOOD LUCK

## RE: FIRST MOMENT OF INERTIA

corus, thanks for pointing out that site, that spreadsheet will be very usefull to me!!I'll have to sharpen up my CAD to follow

tfl's advice, but I'll keep that in mind too.Thanks again guys.

Gesquicor

## RE: FIRST MOMENT OF INERTIA

To obtain the stresses in the section you have drawn refer to the Steel Designer's Manual, IV, which gives a little formula for determing the stresses at any point given Mxx, Myy and all the calulcated values of Inertia. You can adapt the spreadsheet to show them for each of your points by inserting a column and referring to the cells that contain the values of Ixx, Iyy, and Ixy.

## RE: FIRST MOMENT OF INERTIA

The method described by tfl is the one i normally use.

But in the description is missing somethig.

The output of the inquiry is the inertia arroud de drawing axes, so you should move the drawing to fit your needs.

ajose

## RE: FIRST MOMENT OF INERTIA

that is what i was trying to point out in the second half of step #4.

thanks for aleast confiring someone out there can use autocad for more then drafting!

tfl

## RE: FIRST MOMENT OF INERTIA

---------------- REGIONS ----------------

Area: 7.0686 sq in

Perimeter: 9.4248 in

Bounding box: X: -1.5000 -- 1.5000 in

Y: -1.5000 -- 1.5000 in

Centroid: X: 0.0000 in

Y: 0.0000 in

Moments of inertia: X: 3.9761 sq in sq in

Y: 3.9761 sq in sq in

Product of inertia: XY: 0.0000 sq in sq in

Radii of gyration: X: 0.7500 in

Y: 0.7500 in

Principal moments (sq in sq in) and X-Y directions about centroid:

I: 3.9761 along [0.7071 0.7071]

J: 3.9761 along [-0.7071 0.7071]

## RE: FIRST MOMENT OF INERTIA

Am I reading the results correctly in that Autocad says that the principal axes are at 45 degrees from the global X axis?

## RE: FIRST MOMENT OF INERTIA

are rotated."

I always use the 'Moments of Inertia' because they are based on the current X,Y axes.

I am not sure though as to why it gave the axes on the circle at a 45°. The Ix and Iy are the same either way.

Here is a 3" square results:

---------------- REGIONS ----------------

Area: 9.0000 sq in

Perimeter: 12.0000 in

Bounding box: X: -1.5000 -- 1.5000 in

Y: -1.5000 -- 1.5000 in

Centroid: X: 0.0000 in

Y: 0.0000 in

Moments of inertia: X: 6.7500 sq in sq in

Y: 6.7500 sq in sq in

Product of inertia: XY: 0.0000 sq in sq in

Radii of gyration: X: 0.8660 in

Y: 0.8660 in

Principal moments (sq in sq in) and X-Y directions about centroid:

I: 6.7500 along [1.0000 0.0000]

J: 6.7500 along [0.0000 1.0000]

It gives the Principal axes along the x and y.

## RE: FIRST MOMENT OF INERTIA

A circle must just give an odd result as I checked it with another program that gave the same answer.