Bending Moment How are they getting this value?

[Saladsamurai]

Homework Statement

I have no idea how they are getting this value for M_critical

R1 and R2 are 7.5 kip and 2.5 kip, respectively. My bending diagram shows that the max moment should be (12+6)"(7.5 kip)= 135 kip-in

What on Earth are they doing?

 

[Saladsamurai]

wtf

 

[mathmate]

Sorry Saladsamurai, you'd have to wait for people to read you post and begin to respond before you get frustrated. If you read the guidelines:
https://www.physicsforums.com/showthread.php?t=94388
you will find that a wait time of 24-48 hours should be normal.

> R1 and R2 are 7.5 kip and 2.5 kip, respectively.
I believe you erred in this calculation. R1 being further away from the 10-kip load than R2, so R1 should be smaller, right?

> My bending diagram shows that the max moment should be (12+6)"(7.5 kip)= 135 kip-in
Had you not inverted the reactions, you would have got the correct maximum bending moment, which happens to be right under the 10-kip load.
You could do the calculations for R1 and R2, and calculate the bending moment under the load from R1 and from R2. They should be identical.

 

[Saladsamurai]

Sorry Saladsamurai, you'd have to wait for people to read you post and begin to respond before you get frustrated. If you read the guidelines:
https://www.physicsforums.com/showthread.php?t=94388
you will find that a wait time of 24-48 hours should be normal.

And it says all that in the guidelines Believe it or not, I have posted here on Pf before.

Sometimes I just post random things when I am venting. Now, back to business. So I reversed the reactions when I typed this. Either way, I did get 135 kip-in which is exactly my point.

I wanted to know how they got 30 000 lb-in.

 

[mathmate]

> I have posted here on Pf before.
I understand that. I was a little worried that after 1691 posts and crowned with the title "pf contributor", you don't seem to understand the rules. It doesn't give good examples to little brothers, you know? ;)

> Sometimes I just post random things when I am venting.
That sounds reasonable. I also understand that you are probably in a rush to show something to your professor.

> Either way, I did get 135 kip-in which is exactly my point.
Can you show your workings how you get 135 kip-in. working from R1 (2.5 Kips) and R2(7.5 kips)? ;)

Since you are in a rush, I am going to give you another hint:
For a uniform cross section, the most important point to consider along the span is the point under the load, namely where there is maximum bending moment.
For a variable cross section with a weaker and a stronger section, it is possible that the maximum stress be found elsewhere, right?

 

[Saladsamurai]

Mathmate: If you want to be a moderator, talk to Greg. I am not here to set the Pf archetype.

I am not in a rush to show a professor anything. I am just plain in a rush.

If you learn to use the quote button, I would not have thought that you were merely confirming my 135 lbf-in moment, I would have realized that you were quoting me.

If you think I'm an a$$hole, it's because I am.

 

[mathmate]

If you want to be a moderator...

Not at all, I'd rather spend time on mathmatical problems than behavorial ones. Thanks for the suggestion anyway.

If you learn to use the quote button...

I got carried away when I had a limited time to finish my post. Thank you for correctly reminding me about the quote format here. I usually use the square brackets format instead of the quote button, which I didn't even realize existed until you mentioned it. I am more faithful to the keyboard than the mouse.

I am not here to argue with anyone or to judge anyone. I am here to plain give help to others who could use some, hopefully in an environment where people conduct themselves with reasonable manner and language.

Back to business here. Have you found the answer to the question? This is the essential part of our conversation.

 

[Saladsamurai]

Yes, I have. Thank you. And I am sure that we will be great friends in the end

 

[Dr.D]

That was enlightening!

 

[mathmate]

We sure will be!

 

[Saladsamurai]

That was enlightening!

Glad we could oblige!