The two rules
- At a joint with no load and no support where only two members meet, and they are not in line, both carry no force.
- At a joint with no load and no support where three members meet and two are in line, the third carries no force.
Then take out the members you found and look again. A joint that had four members may now fit rule 2. The key lists every member with its joint, its rule and the pass it was found on. In these problems the rules find every member that carries nothing.
An example from set EX2Y2K
A Pratt roof truss with a pin at A and a roller at E carries 24 kN at the peak, G, and nothing else. Joints B and D have no load and their bottom chords are in line, so the verticals BF and DH carry no force. With those gone, joints F and H each have three members with two in line, so CF and CH carry no force. Then joint C has only BC, CD and CG left, with BC and CD in line, so CG carries no force too: that takes a third look. Only the top and bottom chords carry force. Open set EX2Y2K.
Questions
What are the rules for zero-force members?
At a joint with no load and no support: if only two members meet and they are not in line, both carry no force. If three members meet and two are in line, the third carries no force.
Why look again after finding some?
Once a member is known to carry nothing, take it out. A joint that had four members may now have three with two in line, so another member drops out. The key marks these as pass 2.
If a member carries no force, can it be left out of the truss?
No. It carries nothing only under the loads shown; move a load and it may carry force.
Do the rules apply at a support?
The rules here are used only at joints with no load and no support. At a support the reaction is one of the forces, so work that joint’s equations instead.
Can I check my own truss here?
No. Every truss here is invented for exam practice, and there is no way to enter your own. Educational practice only: not for designing or checking real structures.
How do I give my class the same problems?
Press Share with class and send the link: everyone gets the same set code and trusses. Print makes up to 30 different copies, each with its own answer key.
Sources, checked 2 November 2026: Baker and Haynes, Engineering Statics: Open and Interactive, 6.3 Trusses (simple trusses, 2j = m + r, zero-force members), 6.4 Method of Joints (unknowns drawn as tension, so a negative answer is compression) and 6.5 Method of Sections.