A confinement zone in a column refers to a region where stirrup spacing must be smaller to increase ductility. Concrete is a very brittle material that can easily split under tension. The demand for reinforced concrete members increases during earthquakes more than the capacity. This is done in order to dissipate earthquake energy entering into the building in the form of hysteresis energy curves where a member goes through a series of reversible loading cycles.
The confining zone in a column is the area where the stirrup spacing is usually less and is usually near the column to beam and column to foundation junction.
The maximum moments in a column occur at its ends during earthquakes or lateral deformations. In terms of stress and strain, these end locations are subject to maximum non-linearity. Now concrete cannot carry such a large amount of strain in tension and needs a ventilator in order to continue with its work. The ventilator consists of closely spaced ties. These ties in the high moment zone contribute to the level of ductility we seek as well as enhance the performance of concrete.
As some serious failures have occurred in the past, confinement zone requirements are very stringent.
The above image illustrates how concrete spalled due to the lack of ties. As soon as the concrete is gone, the slender reinforcement buckles easily since there are no ties, too, and the column falls.
As you can see, all failures are in the end zones of a concrete column, which they call the confinement zones.
Why do the stirrups on a concrete reinforced column have a nearer spacing at the top and bottom?
The spacing of ties is determined by the column’s plastic moment capacity, as well as induced moments in the column due to slab strength or seismic stresses.
ACI has a very specified set of criteria that define the needed column strength due to probable induced moments, as well as the required column shear strength due to these produced moments.
- Check for earthquake or wind-induced moments in columns.
- Check the slab-induced probable moments as well as the column-induced probable moments. The minimum of the two will dictate, since if the slab cannot apply more than x of a moment then the column cannot be acted by any moment greater than x. Then, if the “y” moment calculated from the column’s cross section is less than the moment induced from the slab, the column will crack, and it will not bear any moments greater than its capacity.
- Column’s shear strength should be determined by the maximum moment in the two cases above.
Now, the spacing you come up with is required at the ends because the moments at the ends are greater and you have most of the cracking going on in the end zones. To reduce the chance of spalling of concrete we provide better confinement at the ends for greater ductility and performance. Our shear ties are closer at ends because of this only reason.
The shear remains constant across column height because the earthquake or wind forces are experienced at story levels and there are no forces between them. The bending moment is maximum at both ends and zero in the middle.
The above pictures illustrate how shear remains constant between story heights.
In general, shear is greater at column ends, and zero in the middle. Stirrups are designed to withstand shear loads and have closer spacing where shear is expected to be greater.
This is because the bending moment imposed by the beams is taken by the top and bottom of the columns. The bending moment reduces as it moves down the column until it hits zero in the center, at which point it changes to the lower end of the column, where it increases to a maximum at the very bottom. At the center of the column, there is no bending. Because of this, the stirrups are closer at the top and bottom to take the moments from the beams.
Additionally, in seismically prone areas, however, it is recommended by code to have closer spaced stirrups at the ends of columns, even if the shear is not required because column strength and ductility are affected by confinement, which is normally provided by the stirrups.