Which one of the following statements is not correct?
- AIn elastic theory of design, stresses in the structures under working load are less than the allowable working stress.
- BTheory of elasticity will be best suited for structural analysis at the time of failure.Correct
- CElastic method does not provide a uniform overload capacity for all parts of the structures.
- DThe ultimate load design method is more economical than elastic design method.
Solution
Concept
Elastic design versus ultimate (plastic) design
Step-by-step solution
1. Elastic design keeps working stresses below allowable stresses (a). 2. The elastic method gives non-uniform reserve strength in different parts (c). 3. Ultimate (plastic) design is more economical (d). 4. At collapse the material behaves inelastically, so elastic theory is not suited to analyse failure; the theory of plasticity is needed (b is incorrect).
Explanation
Failure involves yielding and redistribution of moments, which the elastic theory cannot describe. The plastic or limit state approach is used to analyse the structure at the time of failure.
Why the other options are wrong
Statements (a), (c) and (d) are accepted statements about the elastic and ultimate load methods.
Shortcut
At failure → plastic theory, not elastic theory.
Common mistake
Thinking elastic analysis can predict collapse.
Correct answer: B) Theory of elasticity will be best suited for structural analysis at the time of failure.