In steel plate girder design, if d denotes the clear distance between the flange angles (i.e., the effective web depth), what is the maximum recommended spacing for vertical (transverse) stiffeners?

Introduction / Context:
Vertical or transverse stiffeners are provided on plate girders to stabilize the thin web against buckling under shear and bearing. Practical design rules cap the spacing to ensure the web panel aspect ratio and slenderness remain within safe limits. A commonly used limit relates the spacing to the effective web depth d (clear distance between flange angles).

Given Data / Assumptions:

• Web depth for stiffener spacing criterion: d = clear distance between flange angles.
• We seek a maximum spacing rule of thumb for vertical stiffeners.
• Plate girder web behaves as a series of panels between stiffeners.

Concept / Approach:
If stiffeners are too far apart, the web panel becomes slender and prone to elastic buckling in shear or due to concentrated loads. Limiting the panel width (stiffener spacing) to about 1.5 times the web depth provides a balanced panel aspect ratio that delays buckling and improves post-buckling reserve, while keeping fabrication economical.

Step-by-Step Solution:
Identify the effective web depth d as the governing dimension for panel proportioning. Adopt the standard spacing cap used in many design references: s ≤ 1.5 d. Select the option 1.5 d as the maximum recommended spacing.

Verification / Alternative check:
Detailed code checks may further restrict spacing based on shear buckling curves, required end/intermediate stiffeners, and serviceability. Nevertheless, the 1.5 d guideline is widely used in preliminary sizing and aligns with traditional plate girder practice.

Why Other Options Are Wrong:

• d or 1.25 d: Overly conservative for typical girders; may cause unnecessary stiffener count.
• 1.75 d: Produces wider, more slender panels that are more susceptible to buckling.

Common Pitfalls:

• Confusing clear depth d with overall section depth (including flanges).
• Ignoring special stiffener requirements near concentrated loads and supports.

Final Answer:
1.5 d.