Q355B Welded Box Beam and Box Column for High Rise Steel Frame Building and Bridge Construction

Square or rectangular · Q355B · Internal diaphragms · Full-penetration corners

A box section is a closed rectangle made from four plates. Its virtue is symmetry: the section modulus is nearly the same about both axes, so it carries bending in any direction without a weak side. That is why it appears where the load direction is uncertain, where a beam spans two ways, or where a column takes moment and axial force at once.

• Transfer girders. A box beam that carries several floors above and has to jump a column position sees moment in a plane that changes along its length. A box girder handles it; an H-beam needs a heavier section for the same job.
• Heavy columns. A box column in a tower takes axial load, wind moment in both directions, and sometimes crane loads. The box does it in one section, and the smallest surface area of any shape makes fire protection cheaper per tonne.
• Long-span bridge girders. The closed shape resists torsion, which an open girder does not — and a curved deck loads a girder in torsion by definition.
• Machine frames and bases. Where the frame has to be flat, stiff and square in three directions.

The section is closed. Once the last plate is welded on, the inside is gone: no inspection, no touch-up, no repair that anyone can see.

Everything that has to happen inside — the diaphragm welds, the blasting, the internal priming, the removal of weld slag — has to be done and recorded before the final plate closes. That sequence is the fabrication schedule, and it belongs on the drawing, not in the workshop’s memory.

Internal diaphragms keep the walls from bulging under load and carry the flange forces through the joint. In a heavy box column the diaphragm connects to all four walls with full penetration, and the only way to reach the inner surfaces is through cut openings that are welded back afterwards.

Where access is not possible, the standard route is electro-slag welding through the outer wall — the diaphragm and the wall are joined by a molten pool that solidifies in place. It is a box-column-specific process, and it is the reason a heavy box column costs more per tonne than the plate it is made from.

Four continuous corner welds put heat into one face at a time. Weld one side and the box bows; weld them all in the wrong order and it twists.

The controls are ordinary but they have to be deliberate: a jig that holds the squareness, a symmetrical welding sequence, back-step along each seam, and a final straightness check against the execution standard before the ends are machined. On a long box girder the sequence is planned per side, not left to the operator.

Every internal joint needs an access hole, and access holes remove material at a stress location. Code-standard cope hole shapes exist for a reason: a flame-cut corner with a sharp radius starts cracks. The holes are cut, the weld is made, and the hole is closed with a full-penetration patch — or left as a cope, depending on the design.

The closing seam runs full penetration on both faces, and it is ultrasonically tested after welding. Because it is the last chance to inspect, the internal welds are tested before it is made.

A square box can also be filled with concrete, the same way a tube can. The infill adds stiffness and fire resistance, and the flat walls make the connection to beams simpler than on a tube. The same vent and filling details are needed, and they belong on the fabrication drawing.

Send these and we return the fabrication drawing, the weight and a quotation.