Soft story configuration in structures is a type of construction where any one story of the building is more flexible (less stiff) when compared with other floors. This may be located at the bottom, or at any intermediate points, where the floor above or below it may be stiffer compared to itself. This accounts to be a weak element in the perspective of seismic forces.
During an earthquake motion, the soft story behavior is based on the criteria that the ground motion will look for all possible weakness in the structure. This weakness may be a sharp variation in the stiffness, ductility or in the strength parameters. These variations result in the poor distribution of masses throughout the floor, which itself is undesirable.
Many structural damages recorded due to earthquake had a major problem of change in stiffness and strength along their vertical configuration. It is not only essential to have symmetry along the horizontal direction, i.e. in the plan, but also in the vertical direction. This is a factor that assures lateral stiffness. Abrupt changes in the vertical plan should be avoided to the maximum.
A typical example of the same is an open story building, where the bottom floor is constructed open for considering the parking utilities and people spaces, as shown in figure.1. This floor hence has a lower structural stiffness compared to the above floor. It represents a vertical symmetry.
Fig.1: An Open Storey Building- Bottom floor resembles soft story configuration
Different Soft Storey Configurations
The soft story configuration is possible in by different arrangement in the building. It is illustrated in figure-2. One such arrangement is the combination of open ground floor and the masonry fill at other top floors. This arrangement is shown in figure 2(a).
The presence of walls in upper stories makes them much stiffer compared to the bottom stories. This makes the upper stories to behave like a single block. Under dynamic forces, most of the horizontal displacement take place at the bottom soft story alone. This would result in the swinging of building back and forth like a pendulum, as shown in figure.3.
This irregularity if is minor in nature, the system will resist the seismic forces, as per the seismic codes. For this, it is essential to give specially characterized strength for that story to resist the dynamic force, by the structural engineer. This is carried out by extra ductile detailing and additional strength giving materials.
Another arrangement is the provision of longer columns at the bottom floor and another with smaller ones, as shown in figure 2(b). Figure 2(c) shows a soft story arrangement where the columns are arranged in a discontinuous manner. This itself have problems in a discontinuity in the load transfer, which becomes severe under the seismic forces.
Fig.2: Soft Story Arrangement
Fig.3: Upper Building Floors Moving to and forth like a Pendulum under Earthquake Motion
Structural Behavior of Soft Storey
When it comes to vertical symmetry i.e. the form of soft story construction, rather than the energy absorption by the reinforcing bars or the steel sections in their plastic zones, there is higher energy concentration at the soft story levels. This would result in serious damage to those columns that are in that zone.
Fig.4: Columns in the bottom open story of a six-story building, after undergoing plastic hinge due to the China earthquake,2008
Once it has damaged the members at the soft story, it does not have a nature to move on and damage the other members. Instead, they apply their intense pressure on the same story. Therefore, we see most of the structure above the soft story remain undamaged (Fig.5).
This is due to the sacrificial action of the soft story. The columns in the soft story lack the capacity to absorb the seismic damage under usual cases, and they are doomed for the same reason. But they continue to support the whole weight coming over it.
Fig.5. The total collapse of the bottom open floor of an apartment, caused by Northridge earthquake. The bottom floor was spaced for car parking.
Improvement in Design Strategies
The 2001 Bhuj earthquake effects on RC buildings have bought special consideration in the design of soft-story buildings in the Indian Seismic Code IS 1893 (Part 1)-2002. The initial specification is to determine when a building must be considered as a soft story and a weak story building. The second provision is to determine the higher value of forces for the soft story structure.
The design must be proceeded by determining the forces in each of the structural element ie the columns, beams and the shear walls, by neglecting the consideration of masonry walls or assume the building to be as bare frame during the calculation.
During the design of beams and the columns, the forces are equal to 2.5 times the value of forces obtained from the bare frame analysis for the respective structural elements. When it comes to the shear walls, the design is carried out for force equal to 1.5 times the calculated bare fill forces.
When it comes to a new plan for building construction, it is advised to avoid large decrease or increase of stiffness throughout the building. It is recommended to construct walls at the bottom floor also. The discontinuity in the wall construction must be minimized in the construction by the designers.
Fig.6: Provision of Shear wall in vertical direction in planned position to offer higher resistance and control the undesirable behavior of soft story
When it comes to the rehabilitation or improvement of existing soft story buildings, additional strengthening construction must be implemented under the assistance of qualified structural engineers, to prevent the building collapse from strong ground motion.
One of the methods is to fill up all the possible open story that brings the discontinuity by the masonry wall similar to the above floors.
The shear wall at plan positions, that is concluded based on analysis is another remedy for rehabilitation, which can improve the stiffness and resistance towards ground motion. When the open ground floor is essential and cannot be compromised, inside infill walls can be provided as shown in the below plan.
Fig.7: Ground Open Story with Infill inside, without interrupting the Provision of Car Parking
Fig.8: Provision of Lateral Bracing System to an already built Soft Story, to Resist against Earthquake Motion