**Support Reactions:**
To solve for the
support reactions the free-body diagram of the truss, as a unified
structure, is drawn first (move the mouse
over the figure to see the free-body diagram). This
free-body diagram shows the external shape of the truss with the
applied and reaction forces clearly identified. The reaction
forces, shown in green, are drawn in the positive x and y
directions. The correct direction of each reaction will be
determined from the equilibrium equations.

We begin the solution procedure by writing the moment
equilibrium equation. Moments should be summed about a joint with
the most reactions (i.e., unknowns). In this case, it would be wise
to select joint 1 as the moment center and the origin of the
coordinates xyz. Using the counter clockwise direction as positive,
we obtain the following equation with one unknown (R_{4-y})

We then write the force equilibrium equation in the x direction
to solve for R_{1-x}.

Thus, R_{1-x} is in the
-x direction because of the negative sign in front of its
magnitude.

Finally, we write the force equilibrium equation in the y
direction to solve for R_{1-y}.