Explanation:

In the design of steel structures, vertical stiffeners are commonly used to increase the flexural strength and stability of the members. The spacing of these stiffeners is an important consideration to ensure that they effectively enhance the structural performance.

The minimum spacing of vertical stiffeners is limited to d/3, where d is the distance between the flange angles. This means that the spacing between two consecutive stiffeners should not be less than one-third of the distance between the flange angles.

Reasoning:

1. Preventing local buckling: Stiffeners are added to prevent local buckling of the flanges. Local buckling can occur when the compressive stresses in the flange exceed the critical buckling stress. By adding stiffeners at regular intervals, the effective length of the flange is reduced, reducing the compressive stress and preventing buckling.

2. Adequate support to flanges: Stiffeners provide additional support to the flanges, preventing them from deforming or deflecting excessively under load. By spacing the stiffeners at regular intervals, the flanges are adequately supported throughout the length of the member.

3. Transferring shear forces: Stiffeners also help in transferring shear forces between the flanges. By spacing the stiffeners properly, the shear forces can be effectively distributed along the length of the member, ensuring that the member behaves as a single unit.

4. Balancing stiffness and economy: The spacing of stiffeners is a balance between providing sufficient stiffness to the member and maintaining an economical design. If the spacing is too large, the stiffeners may not provide adequate support and the member may fail prematurely. On the other hand, if the spacing is too small, it may lead to excessive material and fabrication costs.

Conclusion:

The minimum spacing of vertical stiffeners is limited to d/3 to ensure that they effectively enhance the structural performance by preventing local buckling, providing support to the flanges, transferring shear forces, and maintaining an economical design. This spacing allows for adequate stiffness and cost-effectiveness in the design of steel structures.