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Reduction of the column’s section size along its length

The reduction of the column’s size from one storey to the other is not favorable either from a theoretical or from a practical point of view. Especially in columns where earthquake resistant behavior is required it must be avoided. However, there are cases where the subjacent column is larger than the superjacent column.


Reduction of the section’s size along the height by bending rebars into the shape of a bottle
Reduction of the section’s size along the height by bending rebars into the shape of a bottle

Reduction of the section’s size along the height by terminating some of the old rebars and im-planting new ones
Reduction of the section’s size along the height by terminating some of the old rebars and im-planting new ones

The reduction of the column’s size between two successive storeys creates two basic problems. The first regards the reinforcement layout that differs between the upper and the lower column and the second regards the difficulty in the anchorage of the lower column rebars that are placed outside the peripheral stirrups of the upper column.

The first problem is faced by fitting starter bars to the proper places at the upper part of the low-er column.

The second problem is faced with two different ways:

(a) By ending the rebars that are placed outside the peripheral stirrups of the upper column and

(b) By bending the lower column rebars, into the shape of a bottle, and anchoring them inside the upper column.

This work, either done by the first or by the second way, must be done with extreme care be-cause the earthquake resistant column must have transverse reinforcement (stirrups) that will provide adequate confinement in the joint area. It is advised to form the stirrup hooks in different places between successive layers. It is reminded that this practice is necessary for the column’s strength even if the adjacent beams fail.

In order for the bent up rebars to be adequately confined in the interior part of the joint area, they have to be placed inside the corners of stirrups. This can be achieved by forming the stir-rups (by hand, one by one) with a decreasing section size.

The rebars of the lower column that continue to the upper column, is advised to be bent inside the joint area.

When ending the rebars placed outside the peripheral stirrups of the upper column, their an-chorage must be secured by one of the ways described in the previous paragraph.

The same rules apply to any case of reduced column size, e.g. in case of bilateral reduction of the column’s section size.

 


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