What is unsupported length and effective length of a column?

Column is basically a vertical structure, which helps to transfer the load from top to bottom in a framed structure. Specifically columns transfer the compressive load from top floors, ceiling or beam to the foundation or to the beams. The cross section of the column is mostly rectangular , square or circular. Sometimes we modify these shape to meet the aesthetics of the building. The first step is to know the dimension (length and breadth) of the member. In the case of a column it is basically the height and the lateral dimension of the member.

The unsupported length (l) of the column is defined as the length of a particular column which is measured as the physical dimension between two lateral supports.

Effective length of column

Effective length (leff) of column is a parameter which is related to the load carrying capacity of the column member. It is not necessary that effective length and the actual physical length of column will be equal in magnitude. The value of effective length varies as per the type of support of the column. As per IS 456: 2000, the effective length is related to the unsupported length by some factor. The relation is as follows:

Sl. No. Degree of end restrainedTheoretical value of “leffRecommended value of “leff
1Restrained against rotation in both ends and effectively held in position0.50 l0.65 l
2Restrained against rotation in one end and effectively held in position0.70 l0.80 l
3Not Restrained against rotation in both ends and effectively held in position1.0 l1.0 l
4One end is restrained against rotation and held in position, other end is not held in position but restrained against rotation1.0 l1.20 l
5One end is effectively held in position and restrained against rotation, and the other end partially restrained against rotation but not held in position1.5 l
6One end is effectively held in position but not restrained against rotation, and the other end is restrained against rotation but not held in position2.0 l2.0 l
7Effectively held in position and restrained against rotation at one end but on the other end it is not held in position nor restrained. 2.0 l2.0 l

Check this post to read a brief about what is a column and what are the different types of column

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