1. Jan 11, 2008

So I have this problem and I can't move on until i got this assumption cleared up. Looking at this truss

If we look at the entire truss' FBD we have reactions in the x,y directions at
B and a reaction in the x direction at C.

Now I know by inspection that C_x and B_x are in the same direction (in the negative x direction). But, for the sake of consistency I will assume that all forces are in the POSITIVE direction. After solving my equations of equilibrium, I know that a negative sign will mean that the assumed direction was incorrect.

I solved for C_x directly by taking the moments about B.
$$\sum M_B=0\Rightarrow 1(10)+2(8)+2(C_x)=0\Rightarrow C_x=-13kN$$

The neg sign shows that C_x is indeed leftwards. Now to find B_x I used Sum of Forces.
this time I used the REVISED sense of C_x and the ASSUMED sense of B_x:

This gives me $$B_x-13kN=0\Rightarrow B_x=13kN$$ which seems to be a conflict. Why would B_x be positive. Clearly it should have the same sign as C_x.

Where have I gone completely awry?

2. Jan 11, 2008

Astronuc

Staff Emeritus
The x-reactions at B and C are in opposite directions. B is under tension, while C is in compression. The two load P1 and P2 pivot about B, pushing on C.

In contrast, load P1 and P2 pivot about C pulling on B.

One has to calculate the moments about B and C, due to P1, P2 and the other reaction.

There is a vertical reaction at B, which is just the sum of P1 and P2, since the roller at C provides no reaction.

3. Jan 11, 2008