# Homework Help: Need some help with finding the 2nd moment of area

1. Dec 5, 2007

### jsharma

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 [Broken]

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

Last edited by a moderator: May 3, 2017
2. Dec 5, 2007

### PhanthomJay

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

3. Dec 6, 2007

### jsharma

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

4. Dec 6, 2007

### jsharma

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

5. Dec 6, 2007

### PhanthomJay

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

6. Dec 6, 2007

### 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

7. Dec 6, 2007

### PhanthomJay

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??