This article provides some useful tips which can be applied to design column with adherence to ACI codes.

Column

As per ACI Code 2.2,

It is a structural component that contains a ratio of height-to-least lateral dimension greater than 3.

It withstands vertical loads and transmits them to the footings.

Generally, it can bear compressive loads with or without bending.

Based on the reinforcement provided, a column is categorized in the following types:-

Tied Columns

The longitudinal reinforced bars are attached jointly with individual smaller diameter ties which are placed keeping some distances along the column height.

These ties can retain the longitudinal reinforcement bars in proper position throughout the construction and maintain constancy of these bars from local buckling.

The cross sections of such columns come in different shapes like square, rectangular or circular.

At least four bars are utilized in rectangular and circular cross sections.

Spirally Reinforced Columns

In these columns, the longitudinal bars are placed in a circle encircled by continuous spirals arranged in narrow gaps.

The shapes of these columns are normally circular or square.

Composite Columns

These types of columns are formed with structural steel shapes or pipes encircled by or stuffed with concrete containing or not containing longitudinal reinforcement.

Reasons for collapsing of Tied and Spiral Column

Axial loading on tied and spirally reinforced columns contain the equivalent cross-sectional areas of concrete and steel reinforcement operates in the similar aspect up to the ultimate load.

Under that load connected columns collapse unexpectedly because of extreme cracking in the concrete segment accompanied by buckling of the longitudinal reinforcement among ties inside the failure area.

For spirally reinforced columns, when ultimate load is attained, the concrete shell encircling the spiral starts to strip off. After that, the spiral becomes operable by creating a restrictive force to concrete core, thus allowing the column to maintain heavy deformation prior to happening of final failure.

Necessity of ACI Code for columns:-

For Ties Column

No.3 ties for longitudinal reinforcement no. 10 bars or less, no.4 ties for no. 11 bars or larger and bundled bars.

The distance of ties should not be in excess of 16 diameters of longitudinal bars, 48 diameters of tie bars, or the minimum dimension of column.

Each corner bar and substitute bars should contain lateral tie and the angle created should not be over 135 degrees.

There should be no longitudinal bar with distance over 6 inches devoid of a lateral tie.

For Spiral Column

Minimum six longitudinal bars should be utilized inside spiral ties.

Spiral should not contain diameters under 3/8 in.

The clear spacing among spirals should not be under 1 inch or more than 3 inches.

Design considerations:

Highest and Lowest Reinforcement Ratios

ACI Code 10.9.1 indicates that a lowest reinforcement ratio of 1% should have been applied in tied or spirally reinforced columns. The objective of this lowest reinforcement is to protect against any bending, minimize the impact of shrinkage & creep as well as improve the ductility of the columns.