The forces in the members of simple trusses may be analysed by (i). Graphical method (ii). Method of joints (iii). Method of sections
- AOnly (i)
- B(i) and (ii)
- C(ii) and (iii)
- D(i), (ii) and (iii)Correct
Solution
Concept
Member forces in simple (determinate) trusses can be found by the graphical method, the method of joints and the method of sections.
Formula
ΣFx = 0, ΣFy = 0, ΣM = 0
Step-by-step solution
1. Graphical method (Maxwell diagram) gives member forces by drawing force polygons. 2. Method of joints uses ΣFx = 0 and ΣFy = 0 at each joint. 3. Method of sections cuts the truss and uses ΣFx = 0, ΣFy = 0 and ΣM = 0. 4. All three methods are valid, so the answer is (D).
Explanation
A simple truss is statically determinate, so equilibrium is enough to find member forces. Equilibrium can be applied graphically, joint by joint, or on a cut section. So all three methods can be used.
Why the other options are wrong
A, B and C each leave out at least one valid method.
Shortcut
All three standard methods apply to simple trusses, so choose 'all'.
Common mistake
Thinking the graphical method is not a valid analysis method.
Correct answer: D) (i), (ii) and (iii)