# Equilibrium in roof truss

#### exequor

The diagram for this quesion is at http://www.drewtemp.com/helios/physics.gif [Broken]

The first thing that i did was that i made the free body diagram. From my knowledge i would say that AB and BC are in compression and AC is in tension (i think that is correct). Now the question asked to take moments at B for finding the force in AC whether it be tension or compression, however when i do that it seems like the forces just cancel off each other, and that is where i am stuck

Here is the link for my free body diagram: http://www.drewtemp.com/helios/physics_freebody.gif [Broken]

1. What i want to find is which members are in tension and compression (AB, BC, AC)?

2. What is the force at AC? The weight of AC is negligible.

3. What force from the left (the wind) is required to make the truss pivot at C?

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#### enigma

Staff Emeritus
Gold Member
This should be in the homework help section.

What kind of restraints do the connections at points A and C have? How far up the truss are the two loads? Midpoints?

#### Bystander

Homework Helper
Gold Member
"Stresses in Framed Structures," Hool and Kinne; you might get lucky and it'll still be in your school library --- this is an old-timer's book, but does an understandable job of detailing the tension-compression question for truss members.

#### enigma

Staff Emeritus
Gold Member
Hi cipher, got your PM. It's best to respond in the thread for homework problems. That way, anyone can respond.

If the two joints are pinned at A and C, then you've got an overdetermined problem, and there is most likely no force in AC (although you can't solve it because it's statically indeterminate). If one is pinned and one has a roller, then the problem is different. What type of connections do you have?

#### exequor

the connections are unknown however it is stated that the weight of AC is negligible. there is a force of 900N because i got the question from a book and that is what they had as the answer. another part of the question goes on to say that it has to pivot at C so i dont know if they are implying that c is the only fixed which is so not real...

#### enigma

Staff Emeritus
Gold Member
Originally posted by cipher
the connections are unknown
The connections have to be known if you are going to solve the problem. Look carefully at the drawing in the book. One will be a pin, and one will be a roller, most likely.

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