# Thread: Moment of inertia for angle iron?

1. JD1
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## Moment of inertia for angle iron?

There's an online calculator I'm trying to use to figure out the deflection on 1/8" x 1", 1/8" x 1 1/2" and 1/8" x 2" angle iron for various spans. However, it wants to know what the moment of inertia is before it will do the calculation. I have no idea what that is. I looked online to see if there was a way to calculate that but the formulas given were way over my head. Would anyone here know what that moment of inertia is for the sizes I've given

2. ## Re: Moment of inertia for angle iron?

3. Master Welder
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## Re: Moment of inertia for angle iron?

Originally Posted by WTXBTUMOVER
Nice WTXB

According to that the values are:

1 inch = .022

1.5 inch = .078

2 inch = .19

And the deflection calculator says a 10 foot length of 1 inch angle with a 200 pound load in the center will deflect 10.9 inches

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## Re: Moment of inertia for angle iron?

Be real careful when calculating deflections (and strength) of angles under bending. Depending on the axis you are bending about, lateral restraints, and the placement of the load (relative to the shear center of the angle), an angle will experience lateral-torsional buckling way before it will reach the yield strength of the section.

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## Re: Moment of inertia for angle iron?

JD1

Originally Posted by JD1
. . . I have no idea . . . way over my head . . .
Agree - you don't know - whether your posterior
'is punched or drilled' . . .

PLS - describe your entire project, so the details
can be properly eviscerated . . .

hth

Opus

.

6. ## Re: Moment of inertia for angle iron?

I always overbuild . Depending on your use, I find people usually excede what it's rated for. Heavy is usually better.

Sent from my iPhone using Tapatalk

7. JD1
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## Re: Moment of inertia for angle iron?

Thank you all, except for Opus and his fascination with my a**. The project isn't at a critical level, if it were I'd look deeper into the concerns Ingenuity raised. I think I have the info I need now.

8. JD1
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## Re: Moment of inertia for angle iron?

Originally Posted by ronsii
Nice WTXB

According to that the values are:

1 inch = .022

1.5 inch = .078

2 inch = .19

And the deflection calculator says a 10 foot length of 1 inch angle with a 200 pound load in the center will deflect 10.9 inches
I assume that if that 200 lb load was supported with not one piece but two, like for a shelf, then the deflection would be half of that - 5.45" instead of 10.9"?

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## Re: Moment of inertia for angle iron?

Originally Posted by JD1
I assume that if that 200 lb load was supported with not one piece but two, like for a shelf, then the deflection would be half of that - 5.45" instead of 10.9"?

According to the calcs site...yep... that was also just a 'supported ends' calc not a fixed ends which would add tension to the deflection resistance... and it is a technical number because I think we all know one piece of angle will twist all over the place with 200 pounds loaded in the middle of it 2 inch was a heck of a lot less deflection btw.

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## Re: Moment of inertia for angle iron?

Originally Posted by BD1
I always overbuild . Depending on your use, I find people usually excede what it's rated for. Heavy is usually better.

Sent from my iPhone using Tapatalk

Don't say that if front of Sammmy....

11. JD1
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## Re: Moment of inertia for angle iron?

Thanks ronsii, if the project goes through the angles will be bolted on each end so that helps. Specs are changing so I'm not sure it will go through but at least I know how to deal with it now.

12. ## Re: Moment of inertia for angle iron?

Originally Posted by ronsii
Nice WTXB

According to that the values are:

1 inch = .022

1.5 inch = .078

2 inch = .19

And the deflection calculator says a 10 foot length of 1 inch angle with a 200 pound load in the center will deflect 10.9 inches
I highly doubt a 1" steel angle ten feet long is going to support 200 pounds at the center. Unless you anchor the ends and use it taunt like a piano wire.

Sincerely,

William McCormick

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## Re: Moment of inertia for angle iron?

Originally Posted by William McCormick
I highly doubt a 1" steel angle ten feet long is going to support 200 pounds at the center. Unless you anchor the ends and use it taunt like a piano wire.

Sincerely,

William McCormick
LOL!!! Yes, I just always like to put extreme values into calculators to see what the results come in at

14. ## Re: Moment of inertia for angle iron?

Originally Posted by ronsii
LOL!!! Yes, I just always like to put extreme values into calculators to see what the results come in at

I would actually just bend up sheet metal to create the shelf myself maybe 1/16" material with a 2" face 1/2" return, and a two-inch bend on the back as well.

Or you could do this. This is extremely strong. And you can load it so that it can take a lot of weight.

Sincerely,

William McCormick

15. ## Re: Moment of inertia for angle iron?

Originally Posted by OPUS FERRO

JD1

Agree - you don't know - whether your posterior
'is punched or drilled' . . .

PLS - describe your entire project, so the details
can be properly eviscerated . . .

hth

Opus

.
If your comments can't be useful in some way, you should be silent.

A member noting that he doesn't know something, and wants help, is in no way an opportunity to insult them.

16. JD1
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## Re: Moment of inertia for angle iron?

Thank you Dave. I did comment about Opus in post 7, he's quite a challenge.

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## Re: Moment of inertia for angle iron?

Just for info relative to this thread, is angle stronger with "leg up' or "leg down" or is there any difference?

18. ## Re: Moment of inertia for angle iron?

Originally Posted by cabranch47
Just for info relative to this thread, is angle stronger with "leg up' or "leg down" or is there any difference?
depends how it is suspended at the ends and depends on how exactly you plan to load it.

19. ## Re: Moment of inertia for angle iron?

Originally Posted by Oscar
depends how it is suspended at the ends and depends on how exactly you plan to load it.
The orientation doesn't change the moment of inertia (if I recall correctly).

Leg up or down is the same "strength" as long as everything else is the same (support from twisting and load applied).

Diagonal orientation is weaker though.

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## Re: Moment of inertia for angle iron?

The orientation doesn't change the moment of inertia (if I recall correctly).

Leg up or down is the same "strength" as long as everything else is the same (support from twisting and load applied).

Diagonal orientation is weaker though.
I don't know about that equal force thing. On a bender it takes less force and gives you a better bend when pulling on the web vs pushing it, probably even more so with channel. I realize that that is a controlled bend and not just a force to failure thing but it stands to reason.

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## Re: Moment of inertia for angle iron?

Moment of inertia relates to bending, not to strength in the pure sense. Sure if you have a long piece of angle it will "fail" with a relatively small weight on it and even distort/buckle from it's own weight if long enough, but moment of inertia is not used in basic strength calculations. When you pull on a short section of angle it is definitely easier to bend in one direction vs the other.

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## Re: Moment of inertia for angle iron?

The moment of inertia, I (more correctly termed the 2nd moment of area, for structural engineering) is an important structural property for ALL structural sections - for both stiffness (deflections) AND strength. For strength, the dominant structural property is section modulus (S for elastic section modulus) which is the moment of inertia divided by the distance from the section centroid to the extreme tip of the section (top and/or bottom), so S=I/y; and y is: ytop or ybot, depending on which way the section if orientated, and the stress you are looking at.

The flexural strength of single angles is complicated. There has been several PhD dissertations and many research papers on the general subject.

Depending on the orientation of the legs and the location of the load application (relative to the sections shear center), they suffer from local buckling (if ratio of the leg LENGTH to leg THICKNESS is too large), and lateral-torsional buckling if the tip of the leg is in compression and there is not adequate lateral restraints (like attachment to adjacent stiff members at close centers).

The centroid and shear center of angles do not coincide. Any load NOT applied through the shear center will cause torsion to the section.

Equal leg angles only have 1 axis of symmetry, and unequal legs have no axis of symmetry. So flexure about a X or Y geometric axis often needs to be resolved in the principal axis - the axis where the section has the maximum moment of inertia. An axis of symmetry is also a principal axis.

Structural angles fare better under axial loads, especially tension - hence why you often see diagonal bracing using angles.

If you have to use angles in flexure, double them up (back-to-back) and form a T section.

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