# Pin jointed truss question

1. Apr 19, 2010

### xrayzulu

Hello all.

I am give a pin jointed truss structure. (Look at attached jpeg image)

I have a worked solution of:

F2*4 + F1*3-Av*6=0

Av=1/6(3F1+4F2)=600kN
Ch=F2=600kN
Cv=Av-F1=200kN

Member forces:

S1=-5/4 Av = -750kN
S2=-CH=-600kN
S5=3/4 Av=450kN
S3=-5/4 s4=250kN
S4=-Cv=-200kN

l1=5m l2=3m l3=5m l4=4m l5=6m

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What I don't understand is how the member forces are worked out in the solution Im given, if some one could explain this.

Thanks!

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2. Apr 19, 2010

### PhanthomJay

I gather you have found the reactions at A and C, but the member forces wer given to you and you don't know how they were derived. Use the method of joints, whereby you isolate each joint one by one, starting with the simplest, and apply the law of equilibrium (sum of F_x = 0 and sum of F_y = 0) at each joint. Pure truss member forces are always axial, dircted along the longitudinal axis of the member, inward or outward from the joint. You have to use geometry and trig (pythagorean theorem) to solve for the member forces or component forces in the equilibrium equations. Start by isolating joint C to solve for s2 and s4, then isolate joint A to solve for s1 and s5, etc. Give it a try.