Reinforcement details
The rules governing the minimum and maximum amounts of reinforcement in a load bearing column are as follows.
Longitudinal steel
1. A minimum of four bars is required in a rectangular column (one bar in each corner) and six bars in a circular column. Bar diameter should not be less than 12 mm.
2. The minimum area of steel is given by
3. The maximum area of steel, at laps is given by As
where As is the total area of longitudinal steel and Ac is the cross-sectional area of the column.
Otherwise, in regions away from laps: s'""" < 0.04.
Links
1. Minimum size = j x size of the compression bar but not less than 6 mm.
2. Maximum spacing should not exceed the lesser of 20 x size of the smallest compression bar or the least lateral dimension of the column or 400 mm. This spacing should be reduced by a factor of 0.60.
(a) for a distance equal to the larger lateral dimension of the column above and below a beam or slab, and
(b) at lapped joints of longitudinal bars > 14 mm diameter.
3. Where the direction of the longitudinal reinforcement changes, the spacing of the links should be calculated, while taking account of the lateral forces involved. If the change in direction is less than or equal to I in 12 no calculation is necessary.
4. Every longitudinal bar placed in a corner should be held by transverse reinforcement.
5. No compression bar should he further than 150 mm from a restrained bar.
Although links are popular in the United Kingdom, helical reinforcement is popular in some parts of the world and provides added strength in addition to added protection against seismic loading. Sizing and spacing of helical reinforcement should be similar to links.
Figure 9.6 shows possible arrangements of reinforcing bars at the junction of two columns and a floor. In figure 9.6a the reinforcement in the lower column is cranked so that it will lit within the smaller column above. The crank in the reinforcement should, if possible, commence above the soffit of a beam so that the moment of resistance of the column is not reduced. For the same reason, the bars in the upper column should be the
"Slab
Beam Soffit
Dowel bars
Figure 9.6
Details of splices in column reinforcement
ones cranked when both columns are of the same sizes as in figure 9.6b. Links should be provided at the points where the bars are cranked in order to resist buckling due to horizontal components of force in the inclined lengths of bar. Separate dowel bars as in figure 9.6c may also be used to provide continuity between the two lengths of column. The column-beam junction should he detailed so that there is adequate space for both the column steel and the beam steel. Careful attention to detail on this point will greatly assist the fixing of the steel during construction.
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