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SEISMIC PROVISIONS AND RETROFITTING

 SEISMIC EVALUATION OF BUILDINGS 

Seismic evaluation is a process of assessing the seismic resistance of a building. This evaluation is necessary for determining or planning the retrofitting scheme or techniques required for making the structure seismically safe. The estimation for seismic resistance of a building i.e., in terms of its strength and ductility is not a simple task and require a lot of effort and studies on the part of structural engineer. The methodology available so far are based on the studies and examination of various parameters of the building, which are explained below :- 

  • Architectural and structural drawing of the building. 
  • Specification of materials, soil testing reports etc. 
  • Past performance of similar type of buildings during the earthquake. 
  • Comprehensive examination/testing of load bearing members of the building. 
  • Visual inspection of the building which should include all types of weakness/deficiencies present in the building. 
The aim of these studies is to identify the weaknesses of the buildings or structures, which helps in estimating the seismic resistance, called as Available Seismic Resistance (ASR). 

The Available Seismic Resistance of a building should be least equal to Minimum Seismic Resistance (MSR) specified by IS Code, which is essentially required to withstand the effects of earthquake of any structure. Thus seismic retrofitting or strengthening of a building is required to be planned and executed properly which will increase the ASR of the building to become equal to MSR. 

The minimum seismic resistance of any building is estimated by design seismic co-efficient given by IS 1893:2016 (Part 1). These co-efficient depends upon the location of the building, its soil parameters, its geometry and weight etc. 

METHODOLOGY FOR SEISMIC RETROFITTING 

Various steps for seismic retrofitting are explained below : 
  • The seismic retrofitting and strengthening methods are recommended for buildings, based upon the detailed study and various components of buildings. 
  • Cost analysis of the various methods/techniques is done to choose the best suitable, practical and economical option. 
  • A detailed and exhaustive retrofitting and strengthening procedure is planned for the building in the best possible way. 
  • The upgraded/retrofitted building is analyzed for its seismic evaluation and is done to see the enhanced seismic performance. 

RETROFITTING OF BUILDINGS 

Various methods/techniques are used for retrofitting of buildings. These methods are broadly classified into two types : 
    (1) Local Retrofit Methods 
    (2) Global Retrofit Methods 

Local Retrofit Methods 
These are the most commonly used methods which result into the upgradation of the individual members/elements of a building which are seismically weak. These are economical as compared to global retrofit methods. These retrofit methods do not lead to any change in the design or geometry of the building. 

Global Retrofit Methods 
These methods aim at increasing the seismic resistance of the structure as a whole in terms of stiffness, strength and ductility. These are costly methods and may result in change in the geometry of the buildings. For example, adding new elements to the building like shear walls, infill walls, columns etc. 

RETROFITTING OF TRADITIONALLY BUILT CONSTRUCTIONS 

The seismic resistance of the traditionally built masonry constructions is low as compared to other type of constructions like reinforced concrete and steel. The low seismic resistance is one of the major cause of damage and casualties and is due to the following reasons : 
  • Masonry structures are heavy. 
  • Masonry show brittle behavior. 
  • Irregular/unsymmetrical geometry of the building. 
  • Stress concentration at openings (Doors/Windows)
  • In masonry construction, the wall to wall, wall to foundation, roof to wall connections are weak and fail during earthquake which results in collapse of the building. 
Despite the above weaknesses, the seismic resistance of masonry buildings can be increased by suitable retrofitting techniques, so that loss of life can be minimized and buildings can be safely used in future. The suitability of retrofitting techniques depend upon the number of factors which are listed below : 
  1. Type of structure 
  2. Material of parent construction 
  3. Type of masonry and its age 
  4. Location and extent of damage/weakness 
  5. Cost involved. 
The objective of the retrofitting measures is to increase the seismic resistance of the buildings. This can be achieved by suitably selecting the retrofitting/strengthening techniques which aim at enhancing the following very important and basic requirements of an earthquake resistant masonry building : 
  1. To increase the integral action i.e., all the members of the building must behave as a single unit. 
  2. To ensure strong and ductile connection between walls, roof and foundation of the building. 
  3. To reduce and remove any type of irregularity present in the building. 
  4. To increase the strength for in plane and out of plane bending. 
  5. To increase the ductility of the building. 
  6. To strengthen the weaker sections. 
Bureau of Indian Standards has published IS13935:2009 code for "Repair and Seismic strengthening of Masonry building" which includes various guidelines and methods regarding retrofitting and strengthening measures for a masonry building. 

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Location: Duliajan, Assam, India

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