Continuous floor slab supported on beams: What ratio of end-span length to intermediate-span length is typically adopted for analysis and detailing assumptions?

Introduction / Context:
For continuous slabs supported on beams, simplified span assumptions are used when applying moment-coefficient methods or when distributing loads and reinforcement. End spans are commonly taken slightly shorter (or treated differently) than interior spans to reflect support conditions and cracking patterns. This question asks for the typical ratio used in many design guides and exam problems.

Given Data / Assumptions:

• Normal continuous one-way slab strips supported on beams.
• Regular framing; no large stiffness discontinuities.
• Rule-of-thumb span ratio for preliminary design and moment coefficients.

Concept / Approach:

Because end spans have only one interior support, their negative moment capacity and continuity differ from interior spans. Many textbooks adopt end-span length ≈ 0.8 of interior-span length as a practical simplifying assumption to apply standard coefficient tables consistently and to keep deflections and cracking comparable across spans.

Step-by-Step Solution:

Verification / Alternative check:

Comparative analyses (elastic frame or FEM) show end spans often develop somewhat different moment envelopes; the 0.8 ratio is a conservative simplification used in many curricula.

Why Other Options Are Wrong:

0.6 and 0.7 make the end span overly short relative to interior spans, affecting continuity assumptions; 0.9 is closer to equal spans but does not reflect the commonly adopted simplification for teaching-level design.

Common Pitfalls:

Treating end and interior spans as identical without checking support fixity; ignoring serviceability differences; not validating assumptions when significant stiffness changes occur.

Final Answer:

0.8