Behavior of Short Column During Earthquake

Short Column Effect:

The short column is stiffer as compared to the tall column, and it attracts larger earthquake force. Stiffness of a column means resistance to deformation. The larger the stiffness, larger is the force required to deform it. If a short column is not adequately designed for such a large force, it can suffer significant damage during an earthquake. This behavior is called short column effect. The damage in these short columns is often in the form of X-shaped cracking. This type of damage of columns is due to shear failure.

Result of Short Column Effect:

More damage are suffered in shorter columns compared to taller column in the same storey, is observed in previous earthquake. This situation occur when a building is rested on sloping ground. During earthquake shaking all columns along more horizontally by same amount along with floor slab at a particular level. But, if reinforced concrete frame buildings have column of different heights within one storey, the short columns attract several times larger earthquake force and suffer more damages as compared to taller ones. The short column effects also occurs in columns that supports mezzanine floors or soft slabs that are added in between two regular floors.

The Solution:

During architectural design stage the short column effect can be avoided in new structures. But when it is not possible this effect should be considered in structural design. The ACI code for ductile detailing of RC structures requires special confining reinforcement to be provided over the full height of column that are likely to sustain short column effect. The special confining reinforcement(i.e., closely spaced closed ties) must extend beyond the short column into the columns vertically above and below by a certain distance for details of special confinement reinforcement.