Continuous floor beams supporting slabs: For reinforced concrete floor beams carrying slab loads, what is a typical recommended ratio of end span to intermediate span lengths in a multi-span continuous system?

Introduction / Context:In continuous beam-and-slab systems, the end spans often differ from interior spans to achieve favorable moment distributions, deflection control, and practical grid layouts. A conventional proportion is used in many building grids as a guide when exact architectural constraints permit.

Given Data / Assumptions:

• Reinforced concrete floor beams continuous over several supports.
• Span ratio guideline sought, not a rigid code requirement.

Concept / Approach:Keeping end spans a bit shorter than interior spans reduces negative support moments at the exterior support and balances sagging and hogging moments along the beam. A commonly adopted ratio is about 0.8 of the interior span for the end span, improving overall economy and serviceability.

Step-by-Step Reasoning:Set interior span length = L.Choose end span ≈ 0.8 L for improved moment distribution.Verify deflections and reinforcement demands with standard analysis.

Verification / Alternative check:Analytical checks using standard continuous-beam coefficients show that a slightly shorter end span reduces exterior negative moments and often leads to better utilization of section capacity.

Why Other Options Are Wrong:0.7 or 0.6: End span becomes too short relative to interior spans, which may be impractical architecturally.0.9: Very close to interior span; moment balancing benefit is reduced.

Common Pitfalls:

• Assuming a fixed mandatory ratio; this is a guideline subject to architectural grid and analysis results.

Final Answer:0.8