Rolled sections for railway bridges: Which of the following dimensions corresponds to a standard plate girder section used in bridge work?

Introduction / Context:
Bridge superstructures in railways often use plate girders or built-up/rolled sections. Recognizing common dimensional designations helps in selecting members for webs and flanges. This question asks which dimensional pair corresponds to a typical large girder depth (≈ 851 mm) with a flange width around 254 mm seen in common catalogues/past specifications.

Given Data / Assumptions:

• Dimensions are listed as overall depth × flange width.
• We select the plausible combination used in bridge practice.

Concept / Approach:
Among the given sizes, 851 × 254 mm aligns with known heavy rolled/built-up girder proportions, where depth is roughly 3–4× the flange width for bending efficiency. Other listed pairs are either square or too small in depth for main girders.

Step-by-Step Solution:

Verification / Alternative check:
Typical steel catalogues and railway design examples exhibit deep plates/rolled units where depth is about 700–1000 mm with flanges around 200–300 mm; 851 × 254 mm fits that envelope.

Why Other Options Are Wrong:

• 255 × 254 mm and 551 × 254 mm: Too shallow for many bridge main girders.
• 851 × 851 mm: Impractical square proportion for a plate girder.
• 851 × 125 mm: Flange too narrow for the quoted depth.

Common Pitfalls:

• Confusing rolled universal beams with built-up plate girders; dimensions should remain realistic for bending needs.

Final Answer:
851 mm × 254 mm