Question

If $d$ is the distance between the flange angles, the vertical stiffeners in plate girders are spaced not greater than

• A. $d$
• B. $1.25d$
• C. $1.5d$
• D. $1.75d$

Hint:

Vertical stiffeners help increase the shear buckling resistance of the web in plate girders.

Answer 1

The Correct Option is B

The spacing limit exists to prevent the web panel between stiffeners from buckling in shear before the girder reaches its design shear capacity.

1. d: too restrictive; the code allows a small margin beyond the depth itself.
2. 1.25d: matches the standard maximum spacing recommended for plate girder web stiffeners, keeping the panel aspect ratio low enough for adequate shear-buckling resistance.
3. 1.5d: too large a spacing; the panel would become too slender in shear.
4. 1.75d: even larger and further outside the safe range.

So the maximum spacing of vertical stiffeners is 1.25d.

Answer 2

A third way to narrow this down is to contrast the rule for vertical stiffeners with the (much larger) spacing rule permitted for horizontal stiffeners, since the two exist for different reasons and codes deliberately keep them numerically distinct.

1. d: Codes generally like to give designers a slightly more generous spacing than the bare depth itself, since making the limit exactly equal to \(d\) would offer no advantage over just stating "spacing equal to depth" — the presence of a distinct multiplying factor in the code (like 1.25) signals the limit is meant to be somewhat larger than \(d\), ruling this option out.
2. 1.25d: This is a modest, specific multiple above the depth designed to remain clearly smaller than the multiples used for less critical stiffener rules (such as horizontal stiffener spacing, which is allowed to be a much larger multiple of the depth) — this distinction between "somewhat larger than d" and "much larger than d" is exactly why 1.25d is the value fixed for vertical stiffeners in plate girders.
3. 1.5d: This is a bigger jump above the depth than the code allows for the more critical vertical-stiffener case, and starts approaching the kind of margin reserved for less restrictive checks.
4. 1.75d: This is an even larger multiple, clearly outside the tighter margin fixed for vertical stiffeners.

Comparing the scale of the allowed margin against the purpose of the rule confirms the smallest, tightest multiple among the options is the one prescribed.

Therefore, the correct answer is 1.25d.