A column is the vertical structural member which can carry compressive forces and it is responsible to transfer the loads from slabs to beam to the foundation. The column is the most important structural member of the building; without the column, you cannot construct RCC building or structure (Strong).
Additional loads from the snow, wind and other horizontal forces can cause the column to bend. Columns can also carry primary axial loads and designed for compression.
Before going in the detail of columns types lets clear the following concepts first,
Buckling is also known as a failure in the column due to compressive action on the structural member or element. The sudden large failure or deformation of the structure due to a slight increase of an existing load under which the structure has exhibited little.
The columns are classified into different types according to RCC column, According to load and based on lengths, so let’s jump into the detail.
Types of columns:
We differentiated the RCC column according to the,
Members reinforced with the longitudinal bars and lateral ties.
Member reinforced with the longitudinal bars and the continuous spirals.
Composite compression member reinforced with structural steel shapes, pipe and with or without longitudinal bars and lateral reinforcements.
The main types are,
1: Tied columns
The column in which the lateral ties are closely spaced throughout the column.
The spacing of ties is limited and close enough to prevent barrelling failures and far enough to not interface the setting of concrete.
The ties vertical spacing must be in between ACI 7.10.5: 16D of longitudinal bar (D is the diameter of the bar) and 48D from the ties diameter. If the ties spacing is too far the column then may face a shear failure.
2: Spiral columns
Are those columns in which the helical bar wrapping around the column longitudinal reinforcements.
The spirals around column provide support and prevent column for barreling.
The size of spirals shall not be less than 3/8 in diameter. ACI Code 22.214.171.124:
ACI Code 126.96.36.199: The spacing or pitch of spirals shall not exceed 3 in, nor be less than 1 in. These columns are mainly constructed in round shapes.
Types based on load condition:
1: Axially loaded column
The load is subjected to the center of the column.
The load act on the longitudinal axis.
According to IS: 456-2000 The longitudinal reinforcement cross-sectional area is not less than
0.8% or not more than 0.6% of the gross sectional area of the column. The minimum longitudinal bar not less than
4 No’s in rectangular and not less than 6 No’s in the circular column. The minimum diameter of the bar is not less than
2: Eccentric column
In this, the load is acting on the column, to the offset from the centroid of the column.
The offsets loading on the column referred to as eccentric loading.
The moment on the column was increased due to the eccentric load increases the axial load and the moment acting on the column.
With this, the bending of the column was also increased.
According to a length:
1: Short columns:
A column said to be short when its length is such that the lateral buckling need not be considered.
When the ratio of the effective length to the least lateral dimensions of the column is less than the 12, then it is called a short column.
2: Long column:
When the ratio of the effective length to the least
radius of gyration is greater than 45, then it is known as a long column. The load-carrying capacity of the long column is less than the short columns.
Its load-carrying capacity depends on the slenderness ratio (if the slenderness ratio of column increases than the load-carrying capacity of column decreases).
When the length of the column is such that the buckling need to be considered, and the column is referred to as a slender column.
3: Intermediate column:
The column is intermediate when,
4d < L < 30d
Basic requirements for column design:
a) A minimum number of longitudinal bars in the compression member shall be 4 for bars within rectangular or circular ties, three for minimum triangular ties, and 6 for bars within spirals.
b) The minimum number of longitudinal bars in the column is four within rectangular or circular ties.
c) The minimum number of longitudinal bars in the column is six numbers.
d) The clear distance between the longitudinal bars must not be less than the 1.5 times the nominal bar diameter nor 1 ½ inch.
e) 40mm concrete cover should be provided then column to avoid the rusting of steel.
f) The center to center spacing of the ties or rings must not exceed the smaller of 16 longitudinal bar diameter, 48 tie bar diameter.
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