Serviceability Limit – Deflection Criterion for Gantry Girders with EOT Crane For a gantry girder of span 6 m carrying an Electric Overhead Travelling (EOT) crane of 200 kN capacity, what is the maximum permissible vertical deflection under service load as per typical gantry-girder limits?

Introduction / Context:
Gantry girders support moving cranes and are sensitive to deflection because excessive vertical movement leads to rail misalignment, wheel climb, poor crane tracking, and dynamic amplification. Hence, serviceability criteria are stricter than for many building beams and are often expressed as a span ratio limit (e.g., span/750).

Given Data / Assumptions:

• Span L = 6 m (6000 mm).
• Typical deflection limit for gantry girders ≈ L/750 (commonly adopted in practice and many guides).
• Service load combination including crane loads and impact as appropriate.

Concept / Approach:

Applying the span-ratio criterion directly gives the maximum allowable deflection. This limit aims to control rail geometry for safe crane operation and to reduce fatigue effects due to repeated travel and impact.

Step-by-Step Solution:

Verification / Alternative check:

Many steel design handbooks and crane runway design recommendations cite L/750 (or even tighter) for gantry girders, corroborating the 8 mm value for a 6 m span.

Why Other Options Are Wrong:

• 10 mm, 12 mm, 18 mm: These exceed the L/750 limit for the given span and are less conservative for crane serviceability.

Common Pitfalls:

Checking only strength but ignoring serviceability; not accounting for dynamic impact and rail stiffness; applying building beam limits (e.g., L/250) inappropriately to crane runway girders.

Final Answer:

8 mm