enter image description here

So, this is the truss I have to solve, more precisely I have to find forces in all of the members of the truss. That is why I used method of joints. I started from the joint A and realised without calculating that members attached to it (Fab and Fal) are the zero force members.

enter image description here PICTURE ON THE LEFT: After that I continued the process with joint B and calculated forces in members Fbl and Fbc. If I have made a mistake here, I will appreciate you also point that out.

PICTURE ON THE RIGHT: The only joint with 2 unknown members that has left after calculating members of joint B is joint L. But here is the problem. Since I can only use 2 equations - sum of the forces in the X and Y direction (in method of joints), I don't know how to find Flk and Flc, since those are not neither vertical nor horizontal forces. I know I should split those into their components (vertical and horizontal), but since they are both unknown members (so are the components), I don't see way to calculate any of them.


1 Answer 1


I cannot view your image clearly, but your problem is solvable:

Let me try to explain.

Since FBL is known (FBL = 45 tension), the remaining members are FLK and FLC. You need to find the inclination of FLK. Solve for angles A and B in this image,

Summing up forces along Y gives equation:

FLK (sin A) - FLC (sin B) - FBL = 0

Summing up forces along X gives equation:

FLK (cos A) + FLC (cos B) = 0

Then solve the two equations using either: Method of elimination, Method of substitution or by calculator. Please refer to your algebra notes for this :)

Let me give you the first step of Method of Substitution:

FLK = - FLC (cos B) / (cos A)

substitute in the first equation to solve FLC.

Hope this helps.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.