A cellular beam is a steel beam whose web is pierced with circular — sometimes elongated — openings. It is closely related to the castellated beam, which has hexagonal openings. The Dutch name for the circular version is *patrijspoortligger*, after the porthole in a ship’s side.
How it is made
Like the castellated beam, a cellular beam is usually produced from a rolled I-section:
- The web is cut to a set profile.
- The two halves are offset against each other and welded back together.
- For circular openings, additional spacer plates are often welded in to achieve the required cell size.
The result is a beam considerably deeper than the original section — often around one and a half times — while the weight of steel per metre stays the same.
Why this shape
- More stiffness per kilogram of steel — the greater depth sharply increases the second moment of area without adding material.
- Services through the beam — pipes, ducts and cable trays run straight through the openings. On long spans that can save half a metre of storey height per floor.
- Less load on the foundations — on long spans every kilogram counts.
- Appearance — the row of circular openings is often left exposed deliberately.
Applications and points to watch
Cellular and castellated beams are used in industrial halls, office floors, car parks and long-span roof structures.
- Shear at the supports — the web is most heavily loaded in shear there. Openings are omitted near the supports or filled with an infill plate.
- Vierendeel action — local bending stresses arise in the web sections above and below each opening. This needs a specific design method; a cellular beam cannot simply be checked as a solid section.
- Point loads — a point load directly over an opening is unfavourable. Coordinate the cell positions with the load points.
- Fire protection — the web is thinner and the openings increase the section factor, so protective boarding or coating needs particular attention.
- Leave the cell layout to the engineer — burning an extra hole on site for a service run is not acceptable.
Related terms
- Castellated beam
- Beam
- I-section
- Lattice girder
- Second moment of area
- Structural steelwork
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