A roof of a hall measuring 8 m × 12 m consists of 100 mm thick reinforced concrete slab supported on I-beams spaced 3 m apart. The steel beam is designed considering finishing load of 1·5 kN/m² and live load of 1·5 kN/m². What is the permissible deflection for this beam as per IS 800:2007, by considering effective length of beam as 9 m ?
- A27·67 mm
- B14·5 mm
- C30 mmCorrect
- D35 mm
Solution
Concept
Serviceability deflection limit (IS 800 Table 6)
Formula
δmax = span / 300 (floor and roof elements not susceptible to cracking)
Step-by-step solution
1. For floor and roof beams supporting elements not susceptible to cracking, the deflection under live load should not exceed span/300. 2. δmax = 9000 / 300 = 30 mm.
Explanation
IS 800 Table 6 limits vertical deflection to span/300 for beams carrying floors and roofs; stricter limits (span/360) apply for brittle finishes.
Why the other options are wrong
14.5 mm and 27.67 mm do not correspond to a code limit (span/620 and span/325). 35 mm would be span/257, which is more lenient than the code.
Shortcut
Floor and roof beams: L/300 (L/360 for brittle finishes).
Common mistake
Using L/240 or L/360.
Correct answer: C) 30 mm