# Problems involving trusses using method of joints

1. Oct 7, 2008

### Siddy3242

While solving for problems involving trusses using method of joints, how do i know which member is in compression and which is in tension?
any help would be appreciated, this stuff's been bugging me for quite some time

thanks and god bless

2. Oct 7, 2008

### tiny-tim

Welcome to PF!

Hi Siddy3242! Welcome to PF!
You don't need to know …

the equations will come out negative one way, and positive the other …

in other words, make an intelligent guess as to whether it's in tension or compression, solve the equations, and if you guessed wrong, the force comes out negative!

If you're still worried, show us a problem that you've done.

3. Oct 7, 2008

### Siddy3242

Re: Welcome to PF!

http://img122.imageshack.us/img122/5923/88374834wb5.jpg

thats the problem i m working on

so far i have drawn the free body diagram of the truss
found the 2 horizontal forces in the 2 corner joints

right now i m starting from point A, and considering the first triangle ABC,
but like i said b4, i dont know nothing about the direction of forces
i am totally lost @ this point

4. Oct 7, 2008

### Siddy3242

Re: Trusses

nvm, i figured it out thanks a lot
i kinda have a new problem

i cant figure out where zero forces in trusses come into play, i mean how do i know which forces are zero force members or not?

5. Oct 7, 2008

### tiny-tim

hmm … I've never come across zero forces

I assume you just solve the equations, and find that some members have positive force, which is tension, some have negative force, which is compression, and some have zero force … so they can just relax!

6. Oct 7, 2008

### Siddy3242

Re: Trusses

oh well, I have it almost figured out

thanks a lot anyways, u been a great help