Combined bending and direct compression

Columns are often subject to some bending in addition to compression. Two factors contribute towards the bending in a practical column; these are eccentricity of real beam connection details, and the effect of wind loading.

Bending can develop in columns due to eccentricity of beam connections; this is accounted for by assuming that the beam reaction is applied at a prescribed distance from the face of the column. The way the floor beams are connected to a column causes inevitable eccentric application of loads on the column. Beam end reactions are generally applied on the supporting brackets or cleats and consequently they are at a distance e from the centre of the column. This eccentric load would cause the column to bend towards the applied load resulting in tension on one face and compression on the other along with a uniform compression all along the section due to the direct effect of the compressive load alone. Whether both faces will be in compression, albeit unequal amounts of compression, or the outside face will be in tension would depend on the value of the beam reaction and the value of eccentricity.

The effect of connection eccentricity is negligible for internal columns supporting an approximately symmetrical arrangement of beams.

In the figure below, the column would be subjected to bending in directions opposed to each other. If W₁ and W₂ were equal and they were an equal distance away from the column centre, ie. e₁ and e₂ were equal, then the bending produced by the two beams would be equal and in the opposite directions and would cancel each other out.

If the column is an internal column with beams running in xx and yy axes the column would experience a combined effect of some or all of the following:

• Direct compression due to vertical loads imposed by the end reactions of beams 1, 2, 3 and 4.
• Compression and tension due to bending about the xx axis.
• Compression and tension due to bending about the yy axis.

In principle a simplified interaction relationship is used to assess the strength of a column in such cases, but this is beyond the scope of the material covered here.

In buildings which depend on rigid frame action for lateral stability, bending will develop in the columns due to wind loads.

All building structures need to resist horizontal loads due to wind. If the structure cannot readily transfer the wind loads to braced parts of the building such as stair towers, lift wells and shear walls, it is generally necessary to rely on rigid frame action. The effect of this is to cause bending in both the beams and columns. In such cases the bending moment distribution must be determined by analysis, and the columns designed for combined axial load and bending.