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

Step 1: Purpose of vertical stiffeners.
Vertical stiffeners in plate girders are provided to prevent buckling of the web due to shear forces.
Step 2: Code-based spacing rule.
Design recommendations specify that the spacing of vertical stiffeners should be limited relative to the depth of the web (distance between flange angles).
Step 3: Standard spacing criterion.
As per standard design practice, the maximum spacing of vertical stiffeners should not exceed:
\[ 1.25d \]
Step 4: Conclusion.
Therefore, the correct answer is (B) $1.25d$.

Answer 2

Since this is a numerical code-based limit, we can reason it out by considering the physical purpose of the spacing limit — controlling shear buckling of the web panel between two stiffeners — and then check which of the four options is consistent with this behavior and with the range recommended by steel design codes for plate girders.

A web panel between two vertical stiffeners behaves like a plate under shear, and its critical shear buckling stress falls rapidly once the panel's aspect ratio (spacing to depth) grows much beyond about 1 to 1.5. If stiffeners were spaced too far apart, the web would buckle locally under shear well before reaching its full shear capacity, defeating the purpose of providing stiffeners at all. Therefore the code intentionally keeps the maximum permissible spacing close to, but slightly larger than, the web depth \(d\), rather than allowing a large multiple of it.

1. d: Restricting spacing to exactly the web depth is overly conservative and does not match the standard code allowance, which permits a modest margin beyond \(d\) itself.
2. 1.25d: This is consistent with the shear-buckling behavior described above — it allows a small increase beyond the depth itself while still keeping the panel's aspect ratio low enough to prevent premature shear buckling, matching the limit specified in standard steel design codes for plate girders.
3. 1.5d: This spacing would push the panel's aspect ratio too far beyond 1, allowing shear buckling capacity to drop more than the code permits, so it exceeds the safe limit.
4. 1.75d: This is an even larger multiple than 1.5d, which would allow an even more slender web panel prone to premature shear buckling; this exceeds the code's permitted range further still.

Only a modest margin above the web depth keeps the panel's shear-buckling behavior within safe limits, which corresponds to the option 1.25d.

Therefore, the correct answer is 1.25d.