Seismic Retrofitting
BDS has the requisite expertise in seismic retrofitting of structures, having worked on several important projects.
Drawing on our experience in this field, we can ensure that optimal retrofitting is conducted in a cost effective fashion. In addition, our detailed planning and mobilisation of sufficient resources ensures that there is minimum disruption to the occupants and activities of the buildings, and the work is completed on schedule.
BDS can undertake seismic retrofitting projects for
Industrial structures
Hospital buildings
Residential and commercial buildings
Heritage structures
Earthquake Hazard in India
Based on historical occurrences, regions in India are classified into low, moderate, severe and very severe earthquake-prone zones. These zones are denoted as II, III, IV and V, respectively. More than half of the country’s population lives in moderate to very severe regions, where high-magnitude earthquake can occur.
The extent of damage to a building during an earthquake depends not only on the magnitude of the earthquake, but also on the soil, building configuration, and the quality of the design and construction. In developed countries, because of better awareness and regulation of design and construction practices, most buildings survive earthquakes, and the damage and loss of life is comparatively less. India should also achieve this standard.
Seismic Evaluation of Existing Buildings
Due to the requirement of full functionality even after an earthquake, seismic evaluation and retrofit are first undertaken for life-line buildings, such as hospitals, police stations, fire stations, telephone exchanges, broadcasting stations, television stations, railway stations, bus stations, airports (including control towers), major administrative buildings, relief co-ordination centres and other buildings for emergency operations. The next set of important buildings includes schools, educational institutions, places of worship, stadia, auditoria, shopping complexes and other places of mass congregation.
An existing building can be vulnerable to earthquake under the following situations.
The building may have been designed for seismic forces, but before the publication of the revised codes for seismic analysis and design
The building may not have been designed and detailed to resist seismic forces
The condition of the building has visibly deteriorated with time
Poor construction quality
The soil condition was not investigated properly during the design and construction
There have been additions of floors or modifications or change of use of the building, without proper analysis
Retrofit and Goals of Seismic Retrofit
Retrofit specifically aims to enhance the structural capacities (strength, stiffness, ductility, stability and integrity) of a building to mitigate the effect of a future earthquake.
The goals of seismic retrofit refer to the actions to be taken with reference to the attributes for seismic design, in qualitative terms. They can be summarized as follows.
To increase the lateral strength and stiffness of the building
To increase the ductility in the behaviour of the building. This aims to avoid the brittle modes of failure
To increase the integral action of the members and provide uninterrupted load path in a building
To enhance redundancy in the lateral load resisting system. This aims to eliminate the possibility of progressive collapse
To ensure adequate stability against overturning and sliding
To reduce damage in non-structural components for life-line and important buildings
Objectives of Seismic Retrofit
The objectives of seismic retrofit are quantitative expressions to achieve the goals of retrofit. The objectives need to be defined before designing for retrofit. For an ‘engineered building’, the objectives are based on measurement of relevant quantities. The minimum objective should be to ensure that a retrofitted building does not collapse during a severe earthquake.
Retrofit of Reinforced Concrete Buildings
It is essential to identify the deficiencies in a building before undertaking retrofit. Identification of the deficiencies is also expected to create awareness for future construction.
For convenience, the deficiencies can be broadly classified as ‘overall deficiencies’ and ‘local deficiencies’. Overall deficiencies refer to the deficiencies which are observed in the building as a whole.
Overall or Global Deficiencies
Global deficiencies are the attributes that degrade the lateral load resisting mechanism of a building subjected to an earthquake. Some of the deficiencies are caused by ‘irregularities’ in the structural configuration (IS 1893: 2002). The irregularities are broadly classified as plan irregularities and vertical irregularities. The plan irregularities can be detected by observation and simple calculations based on the plan of a building. Similarly, the vertical irregularities can be detected from the elevation of a building.
Local Deficiencies
Local deficiencies arise due to improper design, faulty detailing, poor construction and poor quality of materials. These lead to the failure of individual members of the building such as flexural and shear failures of beams, columns and shear walls, crushing or diagonal cracking of masonry walls and failure of beam-column joints or sub-beam or slab-column connections.
Retrofit Strategies
A retrofit strategy is a technical option for improving the strength and other attributes of resistance of a building or a member to seismic forces. The retrofit strategies can be classified under global and local strategies. A global retrofit strategy targets the performance of the entire building under lateral loads. A local retrofit strategy targets the seismic resistance of a member, without significantly affecting the overall resistance of the following.
Global Retrofit Strategies
When a building is found to be severely deficient for the expected seismic forces, the first step is seismic retrofit is to strengthen and stiffen the structure by providing additional lateral load resisting elements. Additions of infill walls, shear walls or braces are grouped under global retrofit strategies. A reduction of an irregularity or of the mass of a building can also be considered to be global retrofit strategies.
The analysis of a building with a trial retrofit strategy should incorporate the modelling of the additional stiffening members.
• Addition of infill walls
• Addition of shear walls or wing walls or buttress walls
• Addition of steel braces
• Addition of frames
• Reduction of irregularities
• Reduction of mass
• Energy dissipation devices and base isolation
Local Retrofit Strategies
Local retrofit strategies pertain to retrofitting of columns, beams, joints, slabs, walls and foundations. The local retrofit strategies are categorized according to the retrofitted elements.
The local retrofit strategies fall under three different types: concrete jacketing, steel jacketing (or use of steel plates) and fibre-reinforced polymer (FRP) sheet wrapping.
• Column retrofitting
• Concrete Jacketing
• Steel Jacketing
• Fibre Reinforced Polymer Sheet Wrapping.
In short, one can design the retrofitting scheme once the strategy is clearly defined and determined. The careful planning and correct application methods are two main points for a good seismic retrofit work. The understanding of a retrofit contractor is the key to quality work.