Effective span for design of a simply supported reinforced concrete slab: Which single expression best represents the effective span used for bending moment calculations?

Introduction / Context:

The effective span of a simply supported slab is a key parameter for determining design moments. Codes define it as the lesser of two measures to reflect support conditions and load paths.

Given Data / Assumptions:

• Simply supported RC slab spanning between bearing walls or beams.
• Standard code approach: select the lesser of two candidate spans.

Concept / Approach:

Typical provisions define effective span as the lesser of (i) centre-to-centre distance of supports and (ii) clear span plus effective depth. Since the question asks for a single expression (not the “lesser of” clause), the centre-to-centre of bearings is conventionally chosen in exam settings as the representative effective span.

Step-by-Step Solution:

Verification / Alternative check:

Where supports are narrow or slab is thin, the two values are close; using c/c supports is a widely accepted simplification unless the alternative is smaller.

Why Other Options Are Wrong:

• Clear span + 2t_wall: Overestimates span; not a standard definition.
• Clear span + d_eff: Is the other candidate, but the single best expression in many cases is c/c supports.
• None: A recognized measure exists.

Common Pitfalls:

• Forgetting the “lesser of the two” rule; designers should always check both values.

Final Answer:

distance between the centres of the bearings