REG. MOMENT OF INERTIA SELECTION FOR SIMPLY SUPPORTED BEAM DEFLECTION CALCULATION 1

[sivasooriyan]

Dear all,

I have to calculate maximum deflection in simple supported beam.
I don't know how to calculate moment of inertia for the attached section.

I have two option
one is
= (12x0.0124^3) / 12

other one is

= (0.124x12^3) / 12

Can anyone help me to resolve the same?

 

[FEA way]

With a section oriented like this, the moment of inertia will be (12*0.024^3)/12.

 

[tdt108]

It sounds like you are using the formula 1/12 bh^3. Here b will be the measurement that is parallel with the x-axis I believe. See the image below.

The .124 measurement seems like it would be a thickness measurement compared to 12. Check out this link for more about the moment of inertia:

Link

 

[phamENG]

Learn to do 'gut' checks on yourself. This is a perfect opportunity:

The two options you've listed are both using bd[sup]3[/sup]/12, as tdt108 pointed out. This is the moment of inertia for a rectangular cross section. Good. You're spot on there.

Now you have two choices - what to use for b and what to use for d. You've plugged both in but haven't provided solutions. Run them both and see what they give you. One will be larger and one will be smaller. One will be the moment of inertia for the plate oriented one way, the other for other way. Of the two orientations, which would you expect to deflect more? (Forget the math here and just look at the section and how you think it will behave). Now which moment of inertia fits the reality of the behavior for the orientation of interest? (That's your answer.)

 

[sivasooriyan]

Sir.

In this case Iyy(3.54mm) is greater than Ixx(0.0000138).
What should I do?
Should I take Iyy?

 

[tdt108]

That is something you will have to decide. I think there has been more than enough information provided in this thread and we never want to take away an opportunity for someone to learn/come to a conclusion on their own. Think about what the Iyy and Ixx values measure as phamENG pointed out. Which value makes the most sense based on the figure you provided?

 

[phamENG]

Do you think a plate lying flat will be more or less flexible than a plate standing up?

Does a larger moment of inertia mean more stiff or more flexible?

Get those answers, and this turns into a Kindergarten matching game.

 

[PEinc]

Based on the diagram and the beam's dimensioning, b = 12 and d = 0.024.

http://www.PeirceEngineering.com

 

[Celt83]

Since you are a student take it back to your first principal stuff from statics and mechanics of materials:

 

[phamENG]

Celt - beautiful, but I hope you didn't do it for this guy (or gal?). Just deciding which is qualitatively more flexible seemed beyond his grasp...

 

[Celt83]

I've been drowning is first principal integrals lately for some toolbox stuff, this was a cake walk compared to some of the other ones I'm in process on.
Depending what level of schooling they are at flexible may not really have much meaning to them yet.

 

[phamENG]

Perhaps. Maybe I was just jarred by the apparent "I don't care what you have to say, just tell me the answer" implication in the followup...I don't come over here to the student forum all that often. Then again, maybe it's a language/technical vocabulary thing. Apologies to the OP if that is the case.