Gantry girders — minimum web plate thickness for heavy lifting For a crane gantry plate girder carrying a lifting load exceeding 15 tonnes, the minimum thickness of the web plate shall be not less than:

Introduction / Context:
Crane runway (gantry) girders are subjected to high wheel loads, impact, fatigue, and lateral forces from crane operation. Web plate detailing must ensure adequate robustness against shear buckling, weld distortion, and handling damage, which is why a codal minimum thickness is enforced for heavier lifting classes.

Given Data / Assumptions:

• Lifting load exceeds 15 tonnes.
• Requirement sought is the minimum web plate thickness (detailing rule).
• Standard practice and teaching references for gantry girders apply.

Concept / Approach:
For heavier duty gantry girders, minimum web thickness avoids excessive panel slenderness and provides a practical welding thickness. Widely used detailing guidance sets 8 mm as the minimum web plate when lifting load exceeds 15 t, while lighter classes may permit 6 mm.

Step-by-Step Solution:
Step 1: Classify the crane load level: > 15 t.Step 2: Apply the detailing rule linking load class to a minimum web thickness.Step 3: For > 15 t, t_web ≥ 8 mm → select 8 mm.

Verification / Alternative check:
Design notes and lecture handouts on gantry girders list 8 mm minimum web thickness for > 15 t lifting, consistent with past IS practice and workshop fabrication norms.

Why Other Options Are Wrong:
4–6 mm: Too thin for heavy-duty gantry girders; excessive slenderness and poor weldability.
10 mm: More conservative than the minimum; not required by the rule as a blanket minimum.

Common Pitfalls:

• Using general plate girder limits instead of special gantry girder detailing rules.
• Ignoring impact factors that effectively increase service demands on the web.

Final Answer:
8 mm