Definition of Column
A long slender bar subjected to axial compression is called a column.
The term is frequently use to describe a vertical member. Sometimes direct stresses dominate and sometimes flexural stresses dominate.
Axial Compression means the compressive forces act at the two ends of the member in the opposite direction are along the same axis .
Difference between strut and column
The difference between column and strut is that former is used to describe a vertical member whereas latter is used for the inclined members.
Strut is any inclined member subjected to compression loads.
Types of Columns
Based upon mode of failure, there are three types.
The failure initiates due to crushing of material and direct stresses are dominant. For short column, if
L < 4d and KL/r min < 30
d = least lateral dimension.
L = Unbraced length of the column.
k = effective length factor depends upon the end conditions of the column.
r min = least radius of gyration.
Slender or long Column
In these, failure initiates due to lateral buckling and flexural stresses are dominant. If
L > 30d
KL/r min > critical slenderness ratio.
Failure initiates due to lateral buckling as well as crushing of the material. Direct and flexural stresses both are dominant.
Column is intermediate when
4d < L < 30d
30 < KL /r min < 100 or Critical slenderness ratio. There is no agreed equation for intermediate column. There are many approaches that are available for design and analysis of intermediate columns such as:
- Rankine Gordon approach.
- Tangent Modulus formula.
- Straight line formula.
- Parabolic formula.
- AISC equations.
Remember all these are empirical approaches.
Type of Failure in Columns
Long compression members fails due to buckling whereas short compression member may fails due to yielding of material. Buckling of a compression member may occur even the maximum stresses in the material are less than the yield point of the material . By Buckling of a compression member it means lateral deflection of the bar.