In a plate girder, when must a second horizontal stiffener be placed at the neutral axis with respect to the web thickness limit relative to depth d?

Introduction / Context:
Horizontal stiffeners in plate girders are provided to improve web stability against buckling under shear and bending. A second horizontal stiffener at (or near) the neutral axis becomes necessary if the web is relatively thin for its depth.

Given Data / Assumptions:

• d = depth of web between flanges.
• t_w = web thickness.
• Material category: structural steel or high-tensile steel.

Concept / Approach:
Classic IS guidance uses depth-to-thickness limits to trigger additional stiffening. If t_w falls below d/250 (structural steel) or below d/225 (HT steel), a second horizontal stiffener is placed at the neutral axis to divide the panel and curb buckling wavelength.

Step-by-Step Solution:
Check t_w against d/250 (structural) and d/225 (HT).If either condition indicates thin web, provide second stiffener at NA.

Verification / Alternative check:
Design tables and MCQ standards adopt the same thresholds, reflecting empirical and analytical stability considerations.

Why Other Options Are Wrong:

• (a) alone or (b) alone: incomplete; both material classes are covered.
• Neither: contradicts codal practice for thin webs.

Common Pitfalls:
Using overall depth instead of clear web depth; overlooking interaction with vertical stiffeners and transverse stiffener spacing.

Final Answer:
Both (a) and (b)