Serviceability limit state: the maximum vertical deflection of a structural member should not normally exceed the lesser of span/350 or which fixed limit (in millimetres)?

Introduction / Context:
Deflection limits protect finishes, partitions, and user comfort. Many specifications cap deflection by a span ratio and also impose an absolute millimetre limit to prevent noticeable sag in shorter spans. The governing requirement is the lesser (more restrictive) of the two criteria.

Given Data / Assumptions:

• Deflection criterion includes a span-based term: span/350.
• An absolute millimetre cap also applies.
• We must identify the fixed limit commonly paired with span/350.

Concept / Approach:

Typical serviceability provisions for finished members (ceilings, claddings) set the absolute deflection cap at 20 mm. Hence, the allowable deflection is min(span/350, 20 mm), ensuring both proportional control for long spans and absolute control for short spans.

Step-by-Step Solution:

Verification / Alternative check:

Project specifications often refine limits (e.g., span/250 for total long-term). The 20 mm cap is a common baseline for members supporting brittle finishes.

Why Other Options Are Wrong:

• 10 or 15 mm: More restrictive than typical baseline; used only in special cases.
• 25 or 30 mm: Too permissive; could lead to visible sag or cracking in finishes.

Common Pitfalls:

• Applying only the span ratio and ignoring the absolute limit, which can be critical for short spans.
• Failing to account for long-term effects (creep) when checking final deflection.

Final Answer:

20 mm