Beams of uniform strength are preferred to those of uniform section because these are economical for
- ALarge SpansCorrect
- BHeavy Spans
- CShorter Spans
- DLight Weight
Solution
Concept
In a beam of uniform strength, the section changes along the span so that the maximum bending stress is the same at every section. This saves material.
Formula
σ = M / Z = constant
Step-by-step solution
1. In a beam of uniform strength, the section is varied so that M/Z is constant along the length. 2. Material is reduced where the bending moment is small. 3. In long spans, bending moment varies a lot and self-weight is high, so material saving is significant. 4. So these beams are economical for large spans.
Explanation
A uniform beam is designed for the maximum moment, so most of its length is over-designed. A beam of uniform strength removes extra material where the moment is small. This saving matters most in large spans.
Why the other options are wrong
B: 'Heavy spans' is not a standard term. C: In short spans, the saving is small and the extra fabrication cost is not worth it. D: 'Light weight' is not a span condition.
Shortcut
Variable section saves material, and the saving is worth it only for large spans.
Common mistake
Choosing short spans by thinking that a varying section is easier to make for small beams.
Correct answer: A) Large Spans