I cannot seam to calculate the magnitude and sense of the axial force on these members. The members I need to calculate are AC,AF,CD,CF,FD and FB. (see attachment, and note my paint skills are not very good).

The vertical reactions at A = 7.5kN
Horizontal reaction at A=-25kN
Vertical reaction at B= 32.5kN


can anyone help me with this question?

thanks, ross

How far have you gotten? First, have you been able to determine the horizontal and vertical components of the external forces at A and B? You can do that using the principle that the sum of forces on the structure = 0 in both x and y directions, and the sum of torques at all nodes = 0. I found it convenient to consider the torques at point A. **EDIT - I SEE THAT YOU'VE ALREADY DETERMINED THESE REACTION FORCES, SO PLEASE DISREGARD THIS FIRST PARAGRAPH**

Once you have the external forces, you can look at node A and see that the vertical force at A is resisted only by member AC - so you now have the vertical component of the compression force in AC: call that AC_y. From that you determine the total compression force in AC is AC_y/cos(theta). That force is carried up to node C - and there the vertical force component is resisted by the external 20 KN force and the tension in CF. This lets you determine the force in CF. You continue by stepping through each of the structure's components in turn, from left to right, always using the principle that at each node the sum of the horizontal forces is zero and the sum of the vertical forces is zero, and that each beam is either in simple compression or tension (no bending moments anywhere). Post back and let us kow how you're doing on this.