Need some help with finding the 2nd moment of area

[jsharma]

hi I am 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 don't get the same value as done in the practical i gues there is something wrong with my calculation in Ixx so please can you tell me wht am i doing wron here

 

[PhanthomJay]

I get your same results if I ignore whatever those solid filled rectangles are supposed to represent.

 

[jsharma]

thnnx well the another thing was that does the bending equation accurately predict the stress in the beam

 

[jsharma]

I get your same results if I ignore whatever those solid filled rectangles are supposed to represent.

thnnx well the another thing was that does the bending equation accurately predict the stress in the beam

 

[PhanthomJay]

thnnx well the another thing was that does the bending equation accurately predict the stress in the beam

It predicts the max bending stress at the outermost fibers (top) of the beam based on the moment in the beam at that point.

 

[jsharma]

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

 

[PhanthomJay]

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

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??