Behaviour of structural members in compression
Steel columns are often slender because of the high strength of the material.
Columns constructed in traditional materials such as stone and brick tend to be of large cross-sectional size relative to their length. A column in structural steel may also be stocky but the much greater strength of steel compared with stone enables the safe design of much more slender columns.
The load capacity of a stocky column is related to its material strength and area of cross-section
When loaded, a stocky column will shorten elastically until a point is reached when any further increase in load causes a disproportionately high reduction in length. At this stage the column is described as having reached the yield point. On further loading the column will reach its collapse load. For the column to behave in this way it must be very short in relation to the size of its cross-section. Under this condition, the failure of the column is due to the failure of steel as a material in compression and the axial load capacity of the column to carry load is determined as follows:
Axial load capacity of the column = yield stress of steel in compression x cross-sectional area of column
However, in practice such columns are rarely found since the high strength of steel requires a relatively small cross-sectional area for the column. A typical steel column is therefore likely to be slender. This contrasts with columns constructed in relatively weak materials which require a large cross-section and are therefore generally stocky.
In columns of practical proportions failure occurs well before the crushing strength of steel is reached. As the compressive stress is gradually increased, a value is reached at which the column, instead of just axially shortening in length, buckles and deforms perpendicular to its axis. This value of load is called the buckling load. When a column has reached its buckling load it has effectively failed as a structural element and is incapable of sustaining the load. The aim of column design, therefore, is to predict the load at which the column may collapse and to ensure that there is always an adequate factor of safety compared with the applied load.