MASSPROP Problem

I have a drawing of an aluminum extrusion and wanted to verify some information about the section properties of it using AutoCAD. I have made the outlines of both the inner and outer surfaces of this extrusion. It is a hollow shape. Then I made each of these into a region and subtracted the inner from the outer. So far so good.

I then ran MASSPROP on this object and it reported the following data to me. Please note that I am not a structural engineer and only have a limited amount of knowledge of such things but I often need to check a beam or shape for deflection to see if it will carry a load across a given span.

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

Area: 1.7495

Perimeter: 31.4718

Bounding box: X: -1.2500 -- 1.2500

Y: -2.9630 -- 2.7870

Centroid: X: 0.0000

Y: 0.0000

Moments of inertia: X: 6.3749

Y: 1.4774

Product of inertia: XY: 0.0000

Radii of gyration: X: 1.9089

Y: 0.9189

Principal moments and X-Y directions about centroid:

I: 1.4774 along [0.0000 1.0000]

J: 6.3749 along [-1.0000 0.0000]

The particular structural property information I'm after is the Ix value so I can check this in the deflection formula. As you can see above this value seems to show up in the J: field. I assume this is the value I want because it's larger than the I: field.

The trouble is that AutoCAD reports this value as 6.3749, however the manufacturer of this extrusion reports it as being 12.84 in^4. I copied the die drawing from their website and the overall dimesions are correct. I'm pretty sure that I've got the scale of it right.

If anyone can help me clarify this I would greatly appreciate it. Like I said, I'm not a PE but I understand enough to follow most on this topic.

The extrusion I'm working with is in the attached ACAD file.

Mass_Prop(01).dwg

I don't see a mention of extruding the subtracted region into a solid body. What values to you get after you've performed the extrusion?

It appears your value is 1/2 of the manufacturer's. Does your cross-sectional area match that of manufac? Perhaps manufac was reporting I for an assembly that included 2 of these?

Well, that's what I thought at first too. That it answer seemed to be about 50% of the manufacturer's number. But I don't think the manufacture included two of these shapes in their figures. In fact I know they don't do that. There would be no reason for that as we analyze them for what they are.

I'm not sure I understand Patrick's point. The profiles of the inner and outer surfaces were downloaded from the manufacturer's website. When I dimension them they are to scale and are the size they are supposed to be. I made each into a region and then subtracted the inner from the outer. Since I am only examining the cross-sectional values, there is no need to extrude this. At least as far as I know. I would simply like to find the Ix property of shapes which will make my life so much easier as it will eliminate the need to always consult information from other sources.

If anyone else know's the secret to this riddle, I'd be glad to discuss it further. Thanks again for the input thus far.

Using your drawing, I get the same results from MASSPROP that you do.

Using a spreadsheet to calculate Momement of Inertia about the neutral axis (Ix)I with that shape (but the lower oddball shape approximated by a rectangle) I get 6.6, which is pretty close to the AutoCAD number of 6.4. So I'd say manufacturer's number of 12.84 is about 2 times the actual value.

Thanks CarlB. I was hoping someone would come along and verify what I think is this issue. The odds that the manufacturer is wrong on their number are a longshot so there must be something else in the mix.

My work depends on getting this kind of information fast. And AutoCAD provides a method for that. I think I'm going to find that AutoCAD was correct all along with the information I provided to it. And that something in the notes from the manufacturer needs revision.

Thanks.

Bill,

Could you scan the manufacturer's data sheet and post it here, or maybe provide a link..

I have a drawing of an aluminum extrusion and wanted to verify some information about the section properties of it using AutoCAD. I have made the outlines of both the inner and outer surfaces of this extrusion. It is a hollow shape. Then I made each of these into a region and subtracted the inner from the outer. So far so good.

 

The particular structural property information I'm after is the Ix value so I can check this in the deflection formula. As you can see above this value seems to show up in the J: field. I assume this is the value I want because it's larger than the I: field.

 

The extrusion I'm working with is in the attached ACAD file.

Hi Bill. The point I was trying to make is that you refer to wanting to determine the deflection of an extrusion. This would imply to me that you want a solid body to work with, not a section.

I'm not an engineer or anything like that, I only portray one.

I downloaded the drwing you posted to play around with it. Thought I would share this with you.

Firstly, you said that the manufacturer Moments of inertia of this extrusion reports it as being 12.84 in^4. That comes to 27180.66606336 (not approx double of what you came up with)

Secondly you did say you were trying to solve for the deflection of a beam across a span.

Now, extruding the profile to 36" this is the MASSPROP result:

---------------- SOLIDS ----------------

Mass: 62.9834

Volume: 62.9834

Bounding box: X: -1.2500 -- 1.2500

Y: -2.9630 -- 2.7870

Z: -36.0000 -- 0.0000

Centroid: X: 0.0000

Y: 0.0000

Z: -18.0000

Moments of inertia: X: 27438.3114

Y: 27261.9990

Z: 282.6817

Products of inertia: XY: 0.0000

YZ: -0.0383

ZX: 0.0000

Radii of gyration: X: 20.8721

Y: 20.8049

Z: 2.1185

Principal moments and X-Y-Z directions about centroid:

I: 7031.7006 along [1.0000 0.0000 0.0000]

J: 6855.3882 along [0.0000 1.0000 0.0000]

K: 282.6817 along [0.0000 0.0000 1.0000]

Hope this helps.

Patrick-

Thanks, but no that doesn't help

12.84 in^4 is not what you report, 12.84 IS the number, units are in^4. Just like an acre is 43,560 ft^2

And as Bill says the Moments of inertia are properties of a cross-sectional area, so extrusions/lengths do not come into play. Length is used in the deflection formula though, as well as load and material properties.

Carl,

If ^2 = area, ^3 = volume, what does ^4 = ?

Sorry, don't mean to sound flippant. Like I said, I'm not an engineer.

It's like the fourth dimension, time

Actually the Moment of inertia (I) is the "second moment of the area", a property that depends on area and the distance from a neutral axis squared, hence length units to the 4th power. For example the equation for Ix for a rectangle of width W and height H is I=WH^3/12.

Yeah, I was going to throw in the time reference but didn't want to be sarcastic in my reply since I'm treading in an "area" I'm not well versed in.

Thanks, I looked this up on wiki: http://en.wikipedia.org/wiki/Second_moment_of_area

Perhaps that will be of help to Bill.

Bill,

Any chance of a link to the manufacturer's details.

you have double checked your drawing for accuracy right. ?

Yes, the extrusion part is not needed. One only needs the cross sectional properties to calculate deflection....

For max deflection I use:

5wl^4

384EI

This is for a simple span with no concentrated loads, uniform load only. The number I'm after is the I value. All other variables are known.

I haven't had much luck in contacting anyone technical enough at the manufacturer's plant to verify this. So tonight I'm going to play around with some standard structural steel shapes. Those I values are published in the AISC manual as well as all over the Internet and I will report back whether MASSPROP matches those values or if there is any variance.

And Patrick, like yourself, I am no PE either...I simply portay one when needed. . All the world is a stage and we each play many parts in life.

Okay, I used the STL lisp routine to create some steel I-Beams and Channels in AutoCAD. I then made them into a region and ran MASSPROP on them. To my pleasure, the Ixx(In^4) and Iyy(In^4) matched very closely. Some of the properties were spot on, some of them varied by only a few hundredths.

What I have to do now is to approach the manufacturer of these extrusions and find out what criteria they used for their calculations.

Thanks to all who provided valuable input on this subject.

Hey Bill,

I had some free time this weekend so I decided to brush up on my moment of inertia stuff by hand.

I simplified the shapes to basic rectangles for ease of calcs, and the results get pretty close to the acads values.

I have attached a drawing with a little tutourial..Also I converted it to millimetres.. The calcs still work for inches..(I converted them back at the end.)

This might help for future, when you don't have acad around..

cheers

Paul.

Mass_Prop-PS.dwg

But you didn't describe the first step of how to find the centroid

I did something similar but broke the shape into 5 rectangular areas rather than the outer minus the inner like you did. And used Excel to crunch the numbers.

But you didn't describe the first step of how to find the centroid

.

Just for you Carl.. but I'm guessing you already know how to find the centroid..

Centroid-PS.dwg