Columns and Struts

Introduction

1. Column and strut are members which support compressive load acting along the axis.
2. A member who supports the compressive load acting along their axis, the axis of the member is may be horizontal or inclined at an angle with a horizontal plane. This member is called strut.
3. Strut makes an angle of 90 degrees with the horizontal plane is called column.

Classification of columns and struts

1. Short columns: The failure of the column is mainly due to direct compressive stress and, the effect of bending stress is considered negligibly related to column's length.
2. Medium column: the failure of the column is due to both direct compressive stress and bending stress related to the column's length.
3. Long column: the failure of the column is mainly due to bending stress and, the effect of direct compressive stress is considered negligibly related to column's length. 

Buckling load or Crippling load or Critical load or Ultimate load 

                                                           Figure 1-  Buckling of long Column 

• Buckling is the bending of columns and struts in the lateral direction of the load acting over it.
• The compressive load acting on columns or struts along the axis (longitudinal direction), due to this compressive load, column or strut will bend in the lateral direction.
• The buckling of column or strut will start at a particular load, is called buckling load or crippling load.  The column is said to have developed an elastic instability.

The assumption made in Euler’s theory for a long column

• The column or strut is initially perfectly straight.
• The cross-section of the column is uniform throughout its length.
• The shortening of the column due to direct compressive stress is neglected.
• The column is very long compared to its cross-section area.
• The material of the column is uniform throughout its length.
• Elastic limit of the material of the column is not exceeded.
• Failure of the column occurs only due to buckling.

The sign convention for bending moment in columns

• The convexity of the column to its initial condition is taken as a positive moment.
• The concavity of the column to its initial condition is taken as a negative moment.

                               Figure 2 (a) Positive Bending Moment (b) Negative bending moment

End conditions for columns and struts

1. Both ends hinged or pivoted or guided or rounded
2. One end fixed and other is free
3. Both ends are fixed
4. One end fixed and another end is hinged.