Short column posses high stiffness that attracts most of the force acting on the multi-story building. Inadequate design of short column to sustain these forces will result in damage and shear failure. This behaviour of the short column under excessive force is called as short column effect.
Special attention is required for short columns and hence recommended to avoid in building design especially for earthquake-prone areas. The effect of a short column is disastrous as they undergo brittle shear failure. The phenomenon of short column effect, its results and remedies are explained below.
Short Column Effect
In certain building either due to the ground conditions or due to the requirement of intermediate beam there requires the construction of short columns as shown in figure-1 below. Various structural configurations also demand the need for short columns.
Fig.1. Short Columns Used in Different Building Conditions( Alqatamin,Talposi)
The short columns are stiffer compared to long columns and hence most of the earthquake forces acting on the building are taken by the short columns (Figure-2). More the stiffness of any structural element, less will be its flexibility to undergo any sort of deformation. Earthquake design of a building is more efficient through the flexible design of the structure.
Fig.2. Short Columns Attracts more shear force compared to long columns( Alqatamin,Talposi)
More is the stiffness, more will be the force required to bring any deformation. If the short columns are not designed to take such large force, damage is caused to the column and hence the structure is affected. This is called as short column effect.
The short column fails by shear failure by means of x-shaped cracking. The shear force created in short column will be 8 times the shear force created in long columns.
Fig.3. X Shaped cracking in short columns of a multi-storey building
Behaviour of Short Columns During Earthquakes
A building constructed on a sloped ground as shown in figure-1, under earthquake action is forced to move along with the floor. If both long and short columns are present, a majority of the force is attracted by the short columns and suffer more damage compared to long columns.
Short column effect is also observed in the columns that support loft slabs or mezzanine floors (Figure-1). These elements are added between two existing floors.
Another case is the presence of short and long column on the same floor level as shown in figure-4 below. Here too the short column offers more resistance compared to long columns. Thus short columns are subjected to more force and damage.
Fig.4: Short columns and Long columns at same Floor level( Alqatamin,Talposi)
Remedies for Short Column Effect
1. The first possible solution is to avoid the use of a short column in the architectural design stage itself.
2. If short columns cannot be ignored, special design requirements are followed.As per ACI code, for those columns that have chances to undergo short column effect will require special confining reinforcement called the ductile reinforcement. The reinforcement provided must extend to the columns below and above by a certain amount as stated in standards.Figure-5 below shows the reinforcement detailing of the short column and long columns.
Fig.5: Reinforcement Details for Short Columns and Long Columns;( Alqatamin, Talposi)
3. In order to reduce the short column effect on an existing building, the short columns between the two floors are willed. The openings are closed by constructing a full height wall.
4. If wall building is not possible, the short columns have to be retrofitted by any existing methods. A quality structural engineer who has sufficient experience in this area have to implement this.