By cutting the web of a rolled section along a profiled line, separating the two halves, offsetting one relative to the other and welding the high points of the web back together. A zigzag cut produces the hexagonal openings of a castellated beam; two offset curved cuts produce the circular openings of a cellular beam. No steel is added, so the finished beam is deeper for the same weight.

The shape of the openings and the flexibility that follows. Castellated beams have hexagonal openings fixed by the geometry of the zigzag cut. Cellular beams have circular openings, and because they are produced from two curved cuts the diameter and spacing can be varied independently, which allows the design to be tuned to the services passing through and to the load. Cellular is the more common modern choice.

A substantial proportion of the original section depth, with the exact increase set by the cutting geometry chosen. Because bending capacity and stiffness depend strongly on depth, that translates into a considerably stronger and stiffer beam using the same weight of steel. The trade is fabrication cost and the additional design checks the openings require.

Because services can run through the structural zone rather than in a separate zone beneath it. A conventional solid beam forces ducts, pipes and cable containment below the steel, adding their depth to every floor. Passing them through the web removes that layer, and across a multi-storey building the accumulated saving reduces overall height, with knock-on savings in cladding, cores and foundations.

Web-post buckling, where the remaining web between two openings can buckle under shear; Vierendeel bending, where the tee sections above and below an opening bend locally around it; and local effects at supports and point loads. These govern the design far more often than simple bending does, which is why cellular beams are usually designed with software from the manufacturer rather than by hand.

Yes, routinely. Near supports, where shear is highest, and at concentrated load positions, openings are commonly infilled with plate to restore web capacity. Infills are a normal part of the design rather than a remedial measure, and their positions are determined during design. Site cutting of additional openings, by contrast, is not permissible without checking, since it removes capacity the design relies on.

Long-span floors in offices and commercial buildings, roof structures, and portal frames, particularly where services integration matters or where a solid rolled section deep enough for the span would be uneconomic or unavailable. They suit repetitive long spans where the fabrication cost is spread across many identical members and the height saving is realised over several storeys.