Thickness of the beam at the opening of the distribution box

The web thickness at the opening of a distribution box should be sufficient to maintain shear capacity, often reinforced with stiffeners, and typically ranges from 2 mm to 12 mm depending on beam size and opening dimensions.

General Guidelines

When a web opening is introduced in a steel beam, the shear capacity at the opening is reduced, while the moment capacity is less affected because the web contributes minimally to bending resistance . To compensate for this reduction, the web thickness and reinforcement around the opening must be carefully designed.

• Web thickness: For small to medium openings, typical steel beam webs range from 2 mm to 12 mm, depending on the beam depth and span . For example, a 500 mm deep beam in a case study had a web thickness of 2 mm with circular openings of 350–400 mm diameter .
• Reinforcement: Vertical and longitudinal stiffeners are often added around the opening to divide the web into panels and prevent local buckling . These stiffeners can have thicknesses of 12 mm or more, depending on the load.
• Opening placement: Openings should be centrally located in the web and avoid high shear regions, typically near supports . Rectangular openings are usually placed in the middle third of the span, while circular openings can be closer to mid-span where shear is lower .
• Stiffener extension: For wide rectangular openings, horizontal stiffeners should extend at least 150 mm beyond the opening edges to maintain structural integrity .

Practical Example

In a steel beam with a depth of 500 mm and circular web openings of 400 mm and 350 mm, the web thickness was 2 mm, with top and bottom flanges 8 mm thick. Vertical stiffeners divided the web into panels of 750 mm length, and all stiffeners had a thickness of 12 mm . This configuration maintained the beam's shear and bending capacity while allowing duct passage.

Design Considerations

• Avoid placing openings in high shear zones or closely spaced openings .
• Use regular shapes (circular or rectangular) for easier reinforcement and predictable stress distribution .
• Ensure stiffeners and web thickness are designed according to the expected shear and bending loads, following relevant codes such as EN 1993-1-1 or AISC standards . In summary, the beam web thickness at the opening should be chosen based on the beam depth, opening size, and applied loads, often reinforced with stiffeners to restore lost shear capacity. Typical web thicknesses range from 2 mm for small openings to 12 mm or more for larger openings, with stiffeners providing additional strength and stability.