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Soft Storey in Buildings and its Vulnerability Towards Earthquakes

Soft storey configuration in constructions refers to a kind of construction in which one level of a building is more flexible (less stiff) than the others. This can be found at the bottom or at any point in between where the floor above or below it is stiffer than it is. In the context of seismic forces, this accounts for a weak element.

The soft storey behaviour is predicated on the premise that the ground motion will look for all conceivable weaknesses in the structure during an earthquake motion. A sudden variation in stiffness, ductility, or strength characteristics could be the source of this vulnerability. These differences lead to an uneven distribution of masses throughout the floor, which is undesirable in and of itself.

Many earthquake-related structural damages had a substantial issue with changes in stiffness and strength along their vertical arrangement. Symmetry is required not just in the horizontal direction (i.e. in the plan), but also in the vertical direction. This is a determinant of lateral stiffness. To the greatest extent possible, abrupt shifts in the vertical layout should be avoided.

An open storey building, as shown in figure, is a good example of this. The bottom floor is built open to accommodate parking utilities and people spaces.1. As a result, this floor is less structurally rigid than the one above it. It symbolises vertical symmetry.

Different Soft Storey Configurations

Different arrangements of the building allow for the soft storey configuration. One such layout is the combination of open ground level with brick fill at other top stories, as shown in Upper storeys are substantially stiffer than lower stories because of the existence of walls. As a result, the top storeys act as a single block. The bottom soft storey alone experiences the majority of the horizontal movement caused by dynamic forces. As a result, the building would sway back and forth like a pendulum.

If the irregularity is modest, the system will withstand seismic forces according to seismic codes. It is critical to provide uniquely characterised strength for that storey in order to achieve this.

Soft Storey Structural Behavior There is more energy concentration at the soft storey levels, rather than energy absorption by the reinforcing bars or steel sections in their plastic zones, when it comes to vertical symmetry, i.e. the form of soft storey construction. This would cause significant damage to the columns in that zone.

Structural Behavior of Soft Storey

It is not in its nature to move on and harm the other members once it has injured the members at the soft tale. Instead, they focus all of their attention on the same storey. As a result, the majority of the building above the soft layer appears to be unharmed. This is owing to the soft story’s self-sacrificial action. In most situations, the soft storey columns lack the capacity to absorb seismic damage, and they are doomed for the same reason. However, they continue to hold the entire weight that is being placed on it.

Improvement in Design Strategies

The consequences of the Bhuj earthquake on RC structures in 2001 prompted the Indian Seismic Code IS 1893 (Part 1)-2002 to include special consideration in the construction of soft-story buildings. The first specification is to decide when a structure should be classified as a soft storey or a weak storey structure. The second requirement is to establish the greater force value for the soft tale structure.

The design must be completed by calculating the forces in each structural element, such as columns, beams, and shear walls, while ignoring masonry walls or treating the structure as a bare frame during the computation.The forces are 2.5 times the value of forces during the design of beams and columns.

It is recommended to prevent substantial decreases or increases in stiffness across the building while developing a new plan for construction. It’s also a good idea to build walls on the lowest floor. The designers must make every effort to reduce the amount of discontinuity in the wall construction.Provision of a vertically oriented shear wall Installation of a vertical shear wall in a planned position to provide increased resistance and control the unwanted behaviour of soft stories.

Soft Storey in Buildings and its Vulnerability Towards Earthquakes

Additional strengthening work must be conducted under the supervision of expert structural engineers when it comes to the rehabilitation or enhancement of existing soft storey buildings in order to prevent the building from collapsing due to strong ground motion.One method is to use all of the available space.

Another rehabilitation cure that can enhance stiffness and resistance to ground motion is the shear wall at plan positions, which is determined based on analysis. Inside infill walls can be created as illustrated in the plan below when the open ground floor is required and cannot be compromised.

A soft storey building is a multi-story structure with windows, broad doors, vast unobstructed commercial spaces, or other openings where a shear wall would ordinarily be necessary for structural stability due to earthquake engineering design. A typical soft storey building is a three- or more-story apartment building with big openings on the ground level, such as a parking garage or a succession of retail establishments with huge windows. During the Loma Prieta earthquake, a partial soft storey collapsed due to insufficient shear strength at ground level.Buildings are described as having a soft storey if the level below it is less than 70% or less than 80% as rigid as the floor immediately above it.

The floor becomes a weak point when it is subjected to disproportionate lateral stress and is unable to withstand it. As a result, the floor may suffer structural damage or complete failure, resulting in the collapse of the entire building. Soft-story collapse was responsible for over half of all homes that became uninhabitable in the 1989 Loma Prieta earthquake in California, and was predicted to cause serious damage and possibly loss of 160,000 residences in the San Francisco Bay Area in the event of a larger earthquake. As of 2009, just a few of these structures in the area had undertaken the very modest seismic retrofit required to fix the problem. In 2013, San Francisco made it mandatory to examine soft-story structures to see if retrofitting is required, and that retrofitting be completed within a year.

Soft Storey in Buildings and its Vulnerability Towards Earthquakes

During the 19 September 2017 earthquake in Morelos, Mexico (Mw = 7.1), more than 300 buildings were seriously damaged. The usage of ground level parking, which results in soft-story structures, was one of the most common structural flaws discovered. These structures were severely damaged or completely destroyed. Following the earthquake, a field visit allowed researchers to collect data on the types of damaged buildings as well as a large number of seismic ground motions. Based on data acquired in Mexico City, this study examines the seismic capacity and demands of a typical existing building with a soft-story structure, subjecting the building to a family of real accelerograms recorded in the vicinity during the earthquake. To improve the seismic capability of these structures and lessen their sensitivity to earthquakes.

Soft Storey in Buildings and its Vulnerability Towards Earthquakes

In the recent past, earthquakes have wreaked havoc on infrastructure and resulted in the deaths of many people. Pakistan has become a seismically active zone due to the continuous subduction of the Indian plate beneath the Eurasian plate. The National Disaster Management Authority of Pakistan has deemed Malakand, in Pakistan’s Khyber Pakhtunkhwa Province, to be at high earthquake risk, necessitating a seismic vulnerability assessment study for the city’s current building stock. The rapid visual screening (RVS) approach of FEMA P-154 was used to assess the vulnerability of a representative sample of diverse building use-types. The Yamane formula was used to calculate the sample size. To calculate structural scores, RVS sheets are employed, and likely seismogenic damage is shown as a function of European Macro Seismic Damage Grades.

What is soft storey in earthquake?

The term “soft-story” refers to one level of a building that is significantly more flexible or weak in lateral load resistance

How does a frame with soft storey behave under earthquake?

The soft story behavior is based on the criteria that the ground motion will look for all possible weakness in the structure.

Why should we avoid soft storey building?

Soft storey is the one of which the rigidity is lower than any other storey due to the fact that it has not got the walls with the same properties.

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