Problems involving trusses using method of joints

[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

 

[tiny-tim]

Welcome to PF!

Hi Siddy3242! Welcome to PF!

While solving for problems involving trusses using method of joints, how do i know which member is in compression and which is in tension?

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. ​

 

[Siddy3242]

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. ​

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 don't know nothing about the direction of forces
i am totally lost @ this point

 

[Siddy3242]

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

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

 

[tiny-tim]

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

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

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!

 

[Siddy3242]

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!

oh well, I have it almost figured out

thanks a lot anyways, u been a great help