Structural members subjected to axial compression are called columns or struts. Columns are normally vertical load bearing members in buildings etc., while struts may be inclined or horizontal and usually form part of a roof or bridge truss. The axial compression in columns may be accompanied with bending moments, due to external loads or due to eccentricity of loading (load being not exactly axial).
- The behaviour of a member under compressive load is more complicated than under tensile loads.
- If a column is short, that is, its lateral dimensions are not very small compared to its length, it may fail by reaching its ultimate compressive strength (yield stress) and the failure may end up in bulging of the section or its cracking up depending upon the material of the column (Figures (a) and (b)).
- However, if a column is long, that is its lateral dimensions are small compared to its length, it bends or buckles (deflects from its initial straight position) at a load smaller than that causing pure compressive failure (Figure (c)).
The element comes back to its original position when the load is removed.
It remains in the deflected position as such.
The deflection goes on increasing indefinitely causing ultimate collapse.
The ultimate compressive load depends upon :
• the material of the column which is expressed in terms of modulus of elasticity (E) and yield stress (σy).
• the effective length of the column (le) which depends upon its end conditions (whether it is pinned, fixed or free).
• its cross sectional shape and dimensions which gives its area (A), moment of inertia (I), and radius of gyration