Which one of the following statements is not correct regarding the working stress method ?
- AStress-strain relation is considered linear till yield stress.
- BTo take care of uncertainties in the design, permissible stress is kept as a fraction of yield stress.
- CIncrease of permissible stresses by 25% is permitted when dead load, live load and wind load are considered.Correct
- DWorking stress method gives the uneconomical sections.
Solution
Concept
Working stress method basics
Formula
σpermissible = yield stress / factor of safety
Step-by-step solution
1. WSM assumes linear elastic behaviour (a), and uses permissible stresses as a fraction of yield or ultimate stress (b). 2. When wind or seismic loads act with dead and live loads, the permissible stresses can be increased by 33⅓% (not 25%). 3. WSM sections are generally larger (uneconomical) than limit-state sections (d).
Explanation
The code allowance for combinations including wind or earthquake is a one-third increase in permissible stresses, i.e. 33⅓%. So the 25% figure in statement (c) is not correct.
Why the other options are wrong
Statements (a), (b) and (d) are accepted descriptions of the working stress method.
Shortcut
WSM: permissible stress increases by 33⅓% with wind/seismic load.
Common mistake
Taking the 25% value.
Correct answer: C) Increase of permissible stresses by 25% is permitted when dead load, live load and wind load are considered.