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Tips for that perfect column

How columns need to be designed and built in reinforced concrete buildings to withstand earthquakes as far as possible.


COLUMNS, THE vertical members in RC buildings, contain two types of steel reinforcement - long straight bars (called longitudinal bars) placed vertically along the length and closed loops of smaller diameter steel bars (called transverse ties) placed horizontally at regular intervals along its full length.

Columns can sustain two types of damage, namely axial-flexural (or combined compression bending) failure and shear failure.

Shear damage is brittle and must be avoided in columns by providing transverse ties at close spacing.

Design strategy

Designing a column involves selection of materials to be used (such as grades of concrete and steel bars), choosing shape and size of the cross-section, and calculating the amount and distribution of steel reinforcement. The first two aspects are part of the overall design strategy of the whole building. The Indian Ductile Detailing Code IS:13920-1993 requires columns to be at least 300-mm-wide. A column width of up to 200 mm is allowed if the unsupported length is less than 4 m and beam length is less than 5 m. Columns that are required to resist earthquake forces must be designed to prevent shear failure by a skilful selection of reinforcement.

Closed ties

Closely spaced horizontal closed ties help in three ways:

They carry the horizontal shear forces induced by earthquakes, and thereby resist diagonal shear cracks.

They hold together the vertical bars and prevent them from excessively bending outwards (in technical terms, this bending phenomenon is called buckling).

They contain the concrete in the column within the closed loops. The ends of the ties must be bent as 135-degree hooks. Such hook ends prevent opening of loops and consequently buckling of concrete and buckling of vertical bars.

The Indian Standard IS13920-1993 prescribes following details for earthquake-resistant columns:

Closely spaced ties must be provided at the two ends of the column over a length not less than the larger dimension of the column, one-sixth the column height or 450 mm.

Over the distance specified as above and below a beam-column junction, the vertical spacing of ties in columns should not exceed D/4 for where D is the smallest dimension of the column (e.g., in a rectangular column, D is the length of the small side). This spacing need not be less than 75 mm not more than 100 mm. At other locations, ties are spaced as per calculations but not more than D/2.

The length of the tie beyond the 135-degree bends must be at least 10 times diameter of steel bar used to make the closed tie; this extension beyond the bend should not be less than 75 mm. Construction drawings with clear details of closed ties are helpful in the effective implementation at construction site.

In columns where the spacing between the corner bars exceeds 300 mm, the Indian Standard prescribes additional links with 180-degree hook ends for ties to be effective in holding the concrete in its place and to prevent the buckling of vertical bars.

These links need to go around both vertical bars and horizontal closed ties; special care is required to implement this properly at site.

Lapping vertical bars

In the construction of RC buildings, due to the limitations in available length of bars and due to constraints in construction, there are numerous occasions when column bars have to be joined. A simple way of achieving this is by overlapping the two bars over at least a minimum specified length, called lap length. The lap length depends on types of reinforcement and concrete. For ordinary situations, it is about 50 times bar diameter. Further, IS:13920-1993 prescribes that the lap length be provided only in the middle half of column and not near its top or bottom ends. Also, only half the vertical bars in the column are to be lapped at a time in any storey. When laps are provided, ties must be provided along the length at a spacing not more than 150 mm.

C.V.R. MURTHY,

IIT, Kanpur ; Courtesy: Building Materials and Technology Promotion Council, New Delhi.

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