Solid Mechanics- Ultimate Load in truss.

I can see some errors here that I'll point out in both your method and answers.

1) Shear stress isn't involved in this problem, only normal stresses. In your equations the factor of safety relation is the same for normal stresses but here you indicate shear with tau. Just want to make sure that you are aware.

2) The allowable load is incorrect. The information you are given for the test rod is the diameter of the test rod and the load at failure. You need to convert this to stress first before you can use it. Rather than plug in the failure stress into the factor of safety relation, here you've used the failure load which is incorrect.

3) I came up with a different value for the load in AC. I'll double check my calculations but I got 111.8 kips.

Double check what you have and see if you get a better answer.

 

ledphones, Your reactions are correct, but your method of finding member forces is not. After finding reactions, isolate the simpler joints first, the ones with the least number of members framing into the joint, like B or D, which has just 2. When you look at D, you can find the x and y components of the force in AD, then solve for the force in AD. And when you look at B, you can solve the force or lack thereof in AB. Now when you go back to joint A, since you know all forces and force components acting there except AC, you can now solve for AC.