Columns are classified as short or long depending on their slenderness ratios. Short columns usually, fail when their materials are overstressed and long columns usually fail due to buckling which produces secondary moments resulting from the P – D effect.
Columns are classified according to the way they are reinforced into tied and spirally reinforced columns. Columns are usually reinforced with longitudinal and transverse reinforcement.

When this transverse reinforcement is in the form of ties, the column is called “tied”. If the transverse reinforcement is in the form of helical hoops, the column is called “spirally reinforced”. Since the failure of columns often cause extensive damage, they are designed with a higher factor of
safety than beams.

Columns are divided into three types according to the way they are reinforced
1-Tied Columns A tied column is a column in which the longitudinal reinforcement bars are tied together with separate smaller diameter transverse bars (ties) spaced at some interval along with the column height. These ties help to hold the longitudinal reinforcement bars in place during construction and ensure the stability of these bars against local buckling. The cross-sections of such columns are usually square, rectangular, or circular in shape. A minimum of four bars is used in rectangular and circular cross-sections.

2-Spirally-Reinforced Columns They are columns in which the longitudinal bars are arranged in a circle surrounded by a closely spaced continuous spiral. These columns are usually circular or square in shape. A minimum of six bars is used for longitudinal reinforcement.
3-Composite Columns A composite column is a column made of structural steel shapes or pipes surrounded by or filled by concrete with or without longitudinal reinforcement.

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