# Two free standing pillars holding beam, moment of inertia problems.

1. Apr 27, 2013

### bigandy008

Hi, I'm new here and didn't really know where to put this so decided to put it here as it is to do with my course work. I'm currently doing my Final year project on a lifter for lifting fertilizer bags its going okay and do know how to calculate moment of inertia but the problem is that I have two pillars standing beside other and I don't know weather I add the two moments of inertia or weather i multiply the depth and breath by 2 first?
1. The problem statement, all variables and given/known data
two free standing pillars connected at the bottom each pillar is 100mm x 50mm with a wall thickness of 5mm. These are holding a beam out like the way a crane looks i am assuming that it is a ridged body with no supports. to find the moment of inertia i know i have to include both pillars but i am not sure if i am writing the equation correctly.

2. Relevant equations
I={bd3}/{12}

3. The attempt at a solution
Ive tried
{(0.05x2)x((0.13)x2)}/{12}
and
({0.05x0.13}/{12})x2
and got two completely different answers
ino i have to take the inside moment of inertia away as well but i need to get my head round this first
any help would be greatly appreciated cheers Andy.

2. Apr 27, 2013

### SteamKing

Staff Emeritus
It's not clear from your description what you are trying to do. A picture of your lifting apparatus would be a big help. Are you trying to calculate the stresses in the apparatus when it is loaded?

3. Apr 27, 2013

### bigandy008

yes basically that what im trying to do im just wondering what why to go about getting the moment of inertia for the two pillars together. i think that a picture of it now. basically im trying to find the stress exerted on the two green pillars.

4. Apr 27, 2013

### FireStorm000

If you're trying to find the force exerted, or find stress, you need to find torque. You have a force m*g being exerted on a lever arm of length r. That creates a torque at the joint. If this contraption stays fixed, then the sum of the torques is 0 (taking into account direction). So each green pillar produces a restoring torque half the magnitude of the red bar.

Though honestly, I doubt this would hold together with a 500kg load and no extra supports.

I'm not sure what relevance moment of inertial has if you're trying to find the stress on the joint.