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iviv

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Hi all, been here several times and found good help for several questions, but this time I'm out of luck, so here I go.

By taking a section through the pin-jointed frame as shown, and considering the quilibrium of the structure to either the left or right of the section, what is the force in members CD, CH and IH?

Each member of the frame has a length of 2.0 m so the height of the frame is 2 × sin 60º = 1.732 m.

http://img705.imageshack.us/img705/2225/pjfasection.gif

I'm trying to solve this using method of sections, and none of the examples I've found seem to help, though I feel I'm missing something obvious with regards to using sine and cosine to find the vertical values.

So you remove the right hand side, and have CD and IH in tension, and CH in compression.

http://img38.imageshack.us/img38/9983/pjt2.jpg

And this is where I'm getting lost, trying to resolve the forces vertically in order to find force CH, since that is the only unknown vertical force. There's the 15KN being applied upwards, and 15KN being applied downwards, but the value for CH can't be 0, because its in compression so it has to have a value.

Now, from my lectures notes I have http://img220.imageshack.us/img220/9484/pjt3.jpg but that doesn't help much since it is dealing with the unknown vertical force being directed downwards. But the other problem I'm seeing is that if I were to just use the same formula, for my example it would end up as 15 = Csin60 + 15

And that's obviously wrong, since Csin60 isn't 0!

I fear I'm getting rather confused, I used to think I understood maths pretty well until I met these trusses! Thanks in advance for any help!

## Homework Statement

By taking a section through the pin-jointed frame as shown, and considering the quilibrium of the structure to either the left or right of the section, what is the force in members CD, CH and IH?

Each member of the frame has a length of 2.0 m so the height of the frame is 2 × sin 60º = 1.732 m.

http://img705.imageshack.us/img705/2225/pjfasection.gif

I'm trying to solve this using method of sections, and none of the examples I've found seem to help, though I feel I'm missing something obvious with regards to using sine and cosine to find the vertical values.

So you remove the right hand side, and have CD and IH in tension, and CH in compression.

http://img38.imageshack.us/img38/9983/pjt2.jpg

And this is where I'm getting lost, trying to resolve the forces vertically in order to find force CH, since that is the only unknown vertical force. There's the 15KN being applied upwards, and 15KN being applied downwards, but the value for CH can't be 0, because its in compression so it has to have a value.

Now, from my lectures notes I have http://img220.imageshack.us/img220/9484/pjt3.jpg but that doesn't help much since it is dealing with the unknown vertical force being directed downwards. But the other problem I'm seeing is that if I were to just use the same formula, for my example it would end up as 15 = Csin60 + 15

And that's obviously wrong, since Csin60 isn't 0!

I fear I'm getting rather confused, I used to think I understood maths pretty well until I met these trusses! Thanks in advance for any help!

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