# Homework Help: Tension and compression in rigid bars

1. Sep 30, 2007

### mpassman

1. Three bars each has weight W. They are pinned together at the corners to form an equilateral triangle suspended from the ceiling at point A. So points B and C (the other two vertices) lie on a horizontal line. I need to draw a diagram of each bar separately, showing all the forces acting on each one, and calculate the compressive force in BC

I don't see why there would be a compressive force in BC, because any force would be horizontal, and the only acting forces are the weights, which are vertical, so have no horizontal component. Help!
With regards to forces acting on each bar, they all have their weight, which acts half way along its length and downwards. Will the force exerted on both AB and AC (the force at A) from the ceiling be purely vertical and be equal to 3W? At the other two joins, what are the reactions or tension effects? Thanks.

2. Sep 30, 2007

### PhanthomJay

Imagine that BC was not there. The diagonals would swing together; BC is preventing them from doing so, so it must serve a purpose in preventing such a swing. You are correct about the 3W ceiling force. Draw a frree body diagram of each joint to determine the member forces. Note that the diagonals each support half the weight of the bottom chord BC.

3. Sep 30, 2007

### mpassman

OK, yes I take the point about swinging together, and instinctively I knew there was a compression, yet I can't seem to work out what it is acutally caused by. Please can you tell me what forces are acting horizontally and causing the compressive force?

4. Sep 30, 2007

### PhanthomJay

Look at joint B (isolate it). We know there is a vertical force of W/2 from BC acting down. There is also the unknown compression force in BC, and the unknown x and y components of the member force AB. Use Newton 1 in each direction to solve for these forces. You'll also have to know the value of the angle B.