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Need some help with finding the 2nd moment of area

  1. Dec 5, 2007 #1
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution

    hi im having trouble with findin second moment of a T-beam.

    here are the measurements of the beam:
    http://img354.imageshack.us/img354/8648/beam6xl.jpg

    I have found the neutral axic fro the beam
    and tht is 11.943 mm from the bottom rectangle
    for 3nd moment of area
    i have separated the two rectangles and calculated for each
    Ixx= (6.4*(31.7)^3)/12+6.4*31.7*(10.4)^2 for the upper rectablge
    Ixx= (38.1*(6.4)^3)/12+6.4*38.1*(8.65)^2 for the bottom rectangle

    Then i added both and i got 5.8*10^-8 m^4
    Then i had to find out the maximum stress for maxmimum load i.e for 84 Nm
    i used the formula Stree= bending moment * neutral axis maximum/ 2nd moment of area
    when i calculate it i dont get the same value as done in the practical i gues there is something wrong with my calculation in Ixx so plz can you tell me wht am i doing wron here
     
  2. jcsd
  3. Dec 5, 2007 #2

    PhanthomJay

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    I get your same results if I ignore whatever those solid filled rectangles are supposed to represent.
     
  4. Dec 6, 2007 #3
    thnnx well the another thing was that does the bending equation accurately predict the stress in the beam
     
  5. Dec 6, 2007 #4
    thnnx well the another thing was that does the bending equation accurately predict the stress in the beam
     
  6. Dec 6, 2007 #5

    PhanthomJay

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    It predicts the max bending stress at the outermost fibers (top) of the beam based on the moment in the beam at that point.
     
  7. Dec 6, 2007 #6
    I have done a practical in the beam, well i got the same neutrl axis in the practical as in the theoretical but when i go and find the maximum bending stress i get 3.4*10^8 theorretically but in practical it is 81.45*10^13 for bending moment 84 Nmm. is it alright wht am i doing wrong here
     
  8. Dec 6, 2007 #7

    PhanthomJay

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    Between mm and meters you've lost me with the decimal point. But perhaps your error is in the value of the max bending moment of 84n-m (or 84 n-mm). Where'd that number come from??
     
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