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Math Help - Maximum stress and bending moments

  1. #16
    MHF Contributor ebaines's Avatar
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    Re: Maximum stress and bending moments

    Quote Originally Posted by Jock View Post
    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.
    Last edited by ebaines; September 6th 2013 at 12:27 PM.
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  2. #17
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    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.....
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  3. #18
    MHF Contributor ebaines's Avatar
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    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.
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  4. #19
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    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.....
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  5. #20
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    Re: Maximum stress and bending moments

    plugged and chugged and got 1807.725694 so is that 18.07x10^6??....
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  6. #21
    MHF Contributor ebaines's Avatar
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    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.
    Last edited by ebaines; September 6th 2013 at 12:52 PM.
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  7. #22
    MHF Contributor ebaines's Avatar
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    Re: Maximum stress and bending moments

    Quote Originally Posted by Jock View Post
    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?
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  8. #23
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    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?.....
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  9. #24
    MHF Contributor ebaines's Avatar
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    Re: Maximum stress and bending moments

    Quote Originally Posted by Jock View Post
    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.


    Quote Originally Posted by Jock View Post
    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?
    Last edited by ebaines; September 6th 2013 at 01:29 PM.
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  10. #25
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    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...........
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  11. #26
    MHF Contributor ebaines's Avatar
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    Re: Maximum stress and bending moments

    Quote Originally Posted by Jock View Post
    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!

    Quote Originally Posted by Jock View Post
    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.

    Quote Originally Posted by Jock View Post
    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!
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  12. #27
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    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?
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  13. #28
    MHF Contributor ebaines's Avatar
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    Re: Maximum stress and bending moments

    Quote Originally Posted by Jock View Post
    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.

    Quote Originally Posted by Jock View Post
    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.
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  14. #29
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    Re: Maximum stress and bending moments

    changing the dimension as you said gives me 8750 which is if correct?.....
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  15. #30
    MHF Contributor ebaines's Avatar
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    Re: Maximum stress and bending moments

    Quote Originally Posted by Jock View Post
    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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