Elastic postive bending moment

In summary, the student is trying to calculate the centroid of a cross-section, but is getting incorrect results. He is told to find the first moment of the outer section and remove the inner void from it to get the centroid.
  • #1
Temoor
9
0
Hello
I just joined this forum and this is my first thread here. This is my assignment question. I'm supposed to find elastic positive bending capacities around the horizontal axes for the following cross section shown in pic. The elastic stress in tension is 10 ksi and in compression is 15 ksi. First i calculated "I" that is second moment of inertia, then i substituted in Moment = (I) x(stress) divided by c. I also have problem finding in c, because vertical length is not same. I posted my attempt as well.
 

Attachments

  • 1499547_274571629357811_824279339_n.jpg
    1499547_274571629357811_824279339_n.jpg
    14.3 KB · Views: 433
  • 1551757_542185592544175_1749738573_n.jpg
    1551757_542185592544175_1749738573_n.jpg
    14.5 KB · Views: 374
  • 1510789_274763656005275_196913646_n.jpg
    1510789_274763656005275_196913646_n.jpg
    18.7 KB · Views: 385
Last edited:
Physics news on Phys.org
  • #2
Your calculations should ALWAYS have units attached to the quantities.
 
  • #3
Sure. But what about my method? is it correct? I don't know how to fund centroid :S
 
  • #4
Since the section is not symmetrical about the horizontal axis, it looks like you have calculated the moment of inertia incorrectly. Your horizontal axis should be through the centroid of the section, which you haven't determined.

The rule at PF, and there is a template here:
https://www.physicsforums.com/showpost.php?p=3977513&postcount=2

you should try to post the entire problem as given to you without paraphrasing it if possible.
 
  • #5
Temoor said:
Sure. But what about my method? is it correct? I don't know how to fund centroid :S

If you don't know anything about centroids, no, your method is not correct.

First moment of area? Does that ring a bell?

If you've studied how to calculate moment of inertia (or the second moment of area), you must have studied what the first moment of area is used for and how it is calculated.
 
  • #6
i know how to find centroid, but in this case as you can see the lower side its thickness is 2 inches and for upper it is 1 inces, In this case i don't know how to find it.
 
  • #7
sure i will post photo of entire problem now.
 
  • #9
Please write out the text of the problem, instead of just posting a photo of it. It's OK to use a photo for diagrams that you can't simply "write", but it's not OK to use a photo for the whole thing.

Check your private messages by clicking the "Notifications" link at the top right of this page.

(Aside to other readers: this post was moved here from one of the non-homework forums, which is why it doesn't follow the usual template for the homework-help forums.)
 
  • #10
@jtbell: It won't happen next time.
 
  • #11
As people use different sign conventions, you should state yours. Does 'positive' mean 'tensile' or 'compressive'?
 
  • #12
Positive mean tensile and negative means compressive but my method above is wrong.
 
  • #13
@steam king: This is my calculation of centroid. See it i calculated it right?
 
  • #14
Please help me, i have to submite this question tomorrow. It holds 20 percent of my total grading. Please help me.
 
  • #15
I'm sorry, but you are still not understanding the correct method of calculation for the centroid.

You chose the long way to calculate the centroid by splitting up the section into four pieces. This is fine, although the long way, but then you went and subtracted the void inside the section, which is incorrect.

Instead of re-doing all this work, find the first moment of the outer section and remove the inner void from it.
 
  • #16
Formula for finding moment of inertia is I = 1/12 bh^3 ?? Am I right?
 
  • #17
Yes. But you still have to find the inertia of the section about its centroid, which means you've got to find the correct centroid location.
 
  • #18
Temoor: Your centroid (y_bar) is currently incorrect. Recompute each number having a blue slash in the attached file. And recompute y_bar.

Your formula in post 16 is correct, but you still need to add the parallel axis theorem to it.
 

Attachments

  • file04.png
    file04.png
    143.8 KB · Views: 398
Last edited:

1. What is elastic positive bending moment?

Elastic positive bending moment (EPBM) is a term used in structural engineering to describe the maximum bending stress that a material can withstand before it permanently deforms. It is a measure of the strength of a material against bending forces.

2. How is elastic positive bending moment calculated?

EPBM is calculated using the formula M = σI/y, where M is the moment, σ is the stress, I is the area moment of inertia, and y is the distance from the neutral axis to the outermost point of the material.

3. What factors affect elastic positive bending moment?

The elastic positive bending moment of a material is affected by various factors such as the type of material, its cross-sectional shape, and its dimensions. Additionally, the distribution of the load and the support conditions also play a role in determining the EPBM.

4. What is the relationship between elastic positive bending moment and yield strength?

The yield strength of a material is the maximum stress that it can withstand before it starts to deform permanently. The elastic positive bending moment is directly related to the yield strength of a material, as a higher yield strength means a higher EPBM. This is because a material with a higher yield strength can withstand higher bending stresses before it permanently deforms.

5. Why is it important to consider elastic positive bending moment in structural design?

Elastic positive bending moment is an important factor to consider in structural design as it ensures that the material used in the structure is strong enough to withstand the expected bending forces. If the EPBM is exceeded, the material may deform or fail, leading to structural instability or collapse. Therefore, it is crucial to calculate and design structures based on the expected EPBM to ensure their safety and stability.

Similar threads

Engineering Combined loading, where is the neutral axis?
    • Engineering and Comp Sci Homework Help
Replies
11
Views
1K
Engineering Analyzing Shear & Bending on Idealized Cross Section
    • Engineering and Comp Sci Homework Help
Replies
4
Views
1K
Gussets and cantilever bending
    • Mechanical Engineering
2
Replies
36
Views
3K
What is the relationship between slope and bending moment in a penstock beam?
    • Engineering and Comp Sci Homework Help
Replies
6
Views
1K
I Calculating Combined Loading Stress: Neutral Axis and Bending Moment at Point A
    • Classical Physics
Replies
3
Views
579
Moment of inertia question for two plates welded together
    • Mechanical Engineering
Replies
11
Views
1K
Determine the position and magnitude of the max bending moment
    • Engineering and Comp Sci Homework Help
2
Replies
48
Views
6K
Finding the bending moment for maximum stress
    • Engineering and Comp Sci Homework Help
Replies
6
Views
2K
Calculate the maximum vertical point force P that this beam can sustain
    • Engineering and Comp Sci Homework Help
Replies
1
Views
1K
Question related to Bending Stress and Shear Stress of a I shape beam
    • Engineering and Comp Sci Homework Help
Replies
1
Views
2K

Hot Threads

Recent Insights

Back
Top