A cruciform column is a steel column whose cross-section has the shape of a cross. Such a section is made, for example, by welding two T-sections to the web of an I-section. The aim is to reduce the column’s tendency to buckle.
Why the cross shape?
A slender column rarely fails by crushing the steel; it fails by buckling — it moves sideways. How readily it does so depends on the second moment of area of the section, and that differs by direction.
In an ordinary I-section the difference is large: stiff in the plane of the web, much weaker at right angles to it. So the column always buckles the weak way, and that weak direction governs the entire design. Welding on two T-sections puts material exactly where the section is weak. The result is a section roughly equally stiff in both principal directions, so the steel is used far more efficiently.
Where it is used
- Heavily loaded columns on the lower floors of tall buildings, where the column stands free in two directions
- Columns without lateral restraint, for instance in atria, voids and halls, where no walls or bracing shorten the buckling length
- Composite columns, where the cruciform section is encased in concrete. The cross shape keys into the concrete, improves fire performance, and the four re-entrant corners leave room for reinforcement.
- Connections in four directions — where beams arrive from four sides, the cross shape offers a face to connect to on each side.
Points to watch
- Welding costs money. A cruciform column is a fabricated section. Weigh the material saving against the labour; at modest loads a standard UC or HE section is often simply cheaper.
- Fire resistance. A cruciform section has a large surface area relative to its mass (high section factor) and heats up quickly in a fire. Encasement, intumescent coating or concrete filling is usually needed.
- Corrosion protection. The internal corners are hard to reach with a brush or spray. Treat them before erection.
Related terms
- Column
- Buckling
- I-section
- H-section
- Steel
- Welding
- Concrete frame
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