Minimum spacing of vertical stiffeners is limited to where d is the distance between flange angles. The correct option is 'B', which means the minimum spacing is d/3.

Explanation:

Vertical stiffeners are used in steel structures to enhance the buckling strength of the compression flanges. To ensure proper performance, these stiffeners must be placed at specific intervals. The minimum spacing of vertical stiffeners is limited to a certain value, which is calculated based on the distance between the flange angles (d).

The formula used to calculate the minimum spacing of vertical stiffeners is given as:

Minimum spacing = d/3

This means that the minimum spacing between two vertical stiffeners should be equal to one-third of the distance between the flange angles. For example, if the distance between flange angles is 900 mm, the minimum spacing between vertical stiffeners should be 300 mm (900/3).

The reason why the minimum spacing of vertical stiffeners is limited to d/3 is to ensure that the stiffeners are placed at the correct intervals to prevent buckling of the compression flanges. If the spacing is too large, it can cause the flanges to buckle under load, leading to structural failure. On the other hand, if the spacing is too small, it can increase the fabrication and installation costs, making the project economically unfeasible.

Therefore, the minimum spacing of vertical stiffeners is limited to d/3 to ensure the structural integrity and economic viability of the project.