What is the bending moment in case of continuous purlins ? (Take the effective length of the purlin as L and load intensity as w)
- AwL²/8
- BwL²/10Correct
- CwL²/32
- DwL²/24
Solution
Concept
Design moment for purlins
Formula
M = wL²/10 (continuous); M = wL²/8 (simply supported)
Step-by-step solution
1. A simply supported purlin: M = wL²/8. 2. For continuous purlins the code allows the moment wL²/10 as a design value (between the support and mid-span moments).
Explanation
Continuity reduces the bending moment compared with simple spans. The usual design value for continuous purlins is wL²/10.
Why the other options are wrong
wL²/8 is the simply supported value. wL²/24 and wL²/32 are smaller values not used for purlin design.
Shortcut
Purlins: simple wL²/8; continuous wL²/10.
Common mistake
Using wL²/12.
Correct answer: B) wL²/10