Need help clarifying statics problem.

[jackspicer]

http://postimage.org/image/8i5i2dtgr/

http://postimage.org/image/8i5i2dtgr/

Homework Statement

What is the magnitude of the internal force in member BC?

Homework Equations

So I figured out the reaction force at A and D by taking the moments about point A to find the vertical force at D and then summing the x and y components to find the forces at A.

The Attempt at a Solution

So I solved for the reaction forces for A, and D already. Why can't I split the truss into three members? Why can't I take member AD and have a free body diagram for that member and solve for the Fx and Fy for Bx and By by summing the forces? Do I absolutely have to take the moment? Can I not do

Ax-Bx=0
Ay-By+Dy=0

To find the components of B? This method doesn't seem to work.
Why do I have to take the moment?

 

[PhanthomJay]

This is not a pure truss since the horizontal and vertical members are subject to bending. When taking a FBD of AD, you must include the horizontal component of force at D, found by first taking a FBD of the vertical member.