# Math Help - Maximum stress and bending moments

1. ## Re: Maximum stress and bending moments

Originally Posted by Jock
8.33x100x10^-3/1.384x10^-4
I see the problem - in calculating the new value of I you used width = 0.12m instead of width = 0.1m. Also - again - the value for M is 8.33x10^3.

2. ## Re: Maximum stress and bending moments

so what should it look like with the correct equations plugged in? I was sure I had done it right.....

3. ## Re: Maximum stress and bending moments

I=bh^3/12, where for the new dimensions b = 0.1m and h= 0.24m. Now plug and chug.

4. ## Re: Maximum stress and bending moments

cool will do. How do I get the compressive and tensile forces? Lost the place with them now but hoping to get this wrapped up today now.....

5. ## Re: Maximum stress and bending moments

plugged and chugged and got 1807.725694 so is that 18.07x10^6??....

6. ## Re: Maximum stress and bending moments

There are no compressive and tesile forces - only tensile and compressive stresses, which you have already calculated using: stress = My/I.

7. ## Re: Maximum stress and bending moments

Originally Posted by Jock
plugged and chugged and got 1807.725694 so is that 18.07x10^6??....
Is this your value for I? That's not what I get. Show me the calculation. Also, why do you think that 1807 equals 18.07 x10^6?

8. ## Re: Maximum stress and bending moments

8.33x100x10^-3/4.608x10^-4 the 4.608x10^-4 came from working out the I=bd^3/12.......
also was my earlier effort of +6.29 tensile and -6.29 compressive correct?.....

9. ## Re: Maximum stress and bending moments

Originally Posted by Jock
8.33x100x10^-3/4.608x10^-4 the 4.608x10^-4 came from working out the I=bd^3/12.......
Your value of I is incorrect - please show me how you calculated it.

Originally Posted by Jock
also was my earlier effort of +6.29 tensile and -6.29 compressive correct?.....
As I indicated back in post #14: no, it is not correct. But I really don't understand what you're trying to do - first you say the stress is 6.29, then you say its 1807, neither of which is correct. So please clarify - what paramater is the 6.29? Compressive and tensile stresses?

10. ## Re: Maximum stress and bending moments

Ok for the new dimensions I made an error my bending moment is 8.4 not 8.33 for a start. For the new dimensions 100mmx240mm I got the I value of 1.152x10^-4. This was in turn plugged into the equation 8.4x100x10^-3/1.152x10^-4 the result was 7291.666667 which I think is 7.29x10^8
The other question I am trying to solve is this At the point of maximum stress sketch a graph of the stress
distribution through the thickness of the beam, indicating which are
tensile and compressive stresses.
It was using your equation that gave me the 6.29 results. really hoping we can solve these last two today...........

11. ## Re: Maximum stress and bending moments

Originally Posted by Jock
Ok for the new dimensions I made an error my bending moment is 8.4 not 8.33 for a start. For the new dimensions 100mmx240mm I got the I value of 1.152x10^-4.
Correct!

Originally Posted by Jock
This was in turn plugged into the equation 8.4x100x10^-3/1.152x10^-4 the result was 7291.666667 which I think is 7.29x10^8
In the numerator you should use 120 mm, not 100mm.

Originally Posted by Jock
The other question I am trying to solve is this At the point of maximum stress sketch a graph of the stress
distribution through the thickness of the beam, indicating which are
tensile and compressive stresses.
It was using your equation that gave me the 6.29 results. really hoping we can solve these last two today...........
"My equation" for max stress is Mc/I = 8333N-m x 0.12m/1.15x10^-4 m^4. Same equation you are using!

12. ## Re: Maximum stress and bending moments

why should I use 120 mm instead of 100mm as that dimension hasn't changed??
Also I see you say use the same equation therefore do I use my value and make one positive and one negative for my graph?

13. ## Re: Maximum stress and bending moments

Originally Posted by Jock
why should I use 120 mm instead of 100mm as that dimension hasn't changed??
It has indeed changed. The depth of the beam has changed from 200 to 240, so the max value of y has changed from 200/2 to 240/2.

Originally Posted by Jock
Also I see you say use the same equation therefore do I use my value and make one positive and one negative for my graph?
Once you get the correct max value, then yes - like I said before the stress distribution is linear, with max compressive stress at the top edge, max tensile stress at the bottom edge, and passing through 0 at the midpoint.

14. ## Re: Maximum stress and bending moments

changing the dimension as you said gives me 8750 which is if correct?.....

15. ## Re: Maximum stress and bending moments

Originally Posted by Jock
changing the dimension as you said gives me 8750 which is if correct?.....
Close, but two issues remain:

1. You have incorrect powers of ten.
2. You are still using M= 8.4, when in reality it's 8333.

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