Technologies for Retrofitting of existing buildings and structures to make them earthquake resistant
Even though, the Code of Practice on Earthquake Resistant Design of Buildings and Structures is in existence since 1962, it is being followed only by few government organisations as a result non-compliant buildings are being constructed in the country specially in private sector. Only recently, the codal provisions on Earthquake Resistant Design are made mandatory in few States and its implementation is yet to take full momentum. As a result, existing earthquake unsafe buildings are still growing to an alarming proportion.
Like other earthquakes in the past, the recent earthquakes of Killari 1993, Bhuj 2001, and Kashmir 2005 have exposed the seismic vulnerability of construction practices being followed in the country. It has clearly demonstrated that not only non-engineered rural houses are vulnerable to earthquakes; the so-called engineered multi-storeyed buildings in big cities are also mostly vulnerable due to faulty design and construction. Considering the large number of people, high fatality in RC buildings and volume of economic activities, the social risk involved in cities is also very high; the seismic retrofitting of the existing buildings has to be undertaken to make these unsafe houses safe to resist future earthquakes, thereby reducing the number of casualties significantly.
The problem of seismic retrofitting of large stock of unsafe buildings is so big that any government action is just not feasible and therefore individual house owner/builder has to undertake the retrofitting measures. However, government can take up retrofitting of its own buildings and some public utility buildings, which are of post earthquake importance.
The deficiencies in buildings and structures against earthquake may arise at (i) planning stage with faulty configuration and irregularities, (ii) design stage due to inadequate strength and ductility, and (iii) construction stage due to faulty construction practices. Revision of design codes is a continuing process world over and usually results in up-gradation of seismic hazard and increase in design forces. In India also several regions have been upgraded in terms of seismic zones thereby rendering buildings unsafe according to new code. All these factors make the retrofitting of existing structures necessary. The retrofitting may also be required if change in usage of a building takes place or there is a major alteration/extension of building.
The level of retrofitting of a building depends on the seismic zone in which building is situated and the level of performance desired from the building. Important buildings are desired to have a higher performance level during future earthquakes. The seismic zone governs the design earthquake forces and the performance level governs the permissible damage or the permissible values of member actions due to earthquake forces. Not only member forces and strength are important, the non-linear deformations and ductile capacity of members are also important for seismic safety of building and need to be evaluated and examined.
Much literature on retrofitting of building is already available including the Bureau of Indian Standards (BIS). The aim of present Guidelines is to provide an overview of the available techniques for seismic evaluation and retrofitting of existing buildings. The techniques have been presented for the type of construction prevailing in India . Emphasis has been on detailing the techniques with illustrations, so that these may be easily understood and applied by common engineers, architects and builders. A need has been felt to provide adequate information about seismic retrofit design of masonry and RC buildings, which can be easily understood and implemented.
The Guidelines deal with important aspects of seismic hazard estimation, systematic inspection of existing buildings, tests for estimation of in-situ strength and extent of damage and deterioration in masonry and RC buildings, mathematical modelling of frames, frame-tubes, shear walls and frames with infills, and various methods of analysis for earthquake forces for seismic evaluation, seismic evaluation which requires knowledge of structural behaviour, materials of construction, principles of seismic intervention and behaviour of modified structure, and various retrofitting materials. This includes performance levels of various types of buildings. The definition of these performance levels has been taken from FEMA and ATC.
Two checklists have been given for systematic inspection of masonry and RC frames. These checklists are useful in preliminary evaluation and identification of major deficiencies in existing buildings.
These Guidelines cover retrofitting of non-engineered, engineered and earthquake damaged buildings. These also cover non-engineered rural and semi-urban houses. These buildings are constructed in mud, stone or brick masonry, without any consideration to strength and ductility of the structure. The retrofitting techniques for such buildings are based on failure mode identification and behaviour of such buildings in past earthquakes.
The techniques have been tested in laboratories and field, and known to provide adequate safety intended for such buildings.
Retrofitting of RC buildings is much more systematic and rational process than that of non-engineered load bearing wall buildings. The different techniques available for retrofitting of RC buildings have been described. The principles of retrofitting of RC buildings are:
Removal of irregularities and asymmetry,
Increasing the strength and stiffness of structure,
Enhancement of deformation capacity (or ductility), and
Earthquake demand reduction by Base-isolation or Supplemental Energy Dissipation.
Different techniques based on these principles have been illustrated. The emphasis on reinforcement detailing, bond of old and new concrete, and anchorage of new reinforcement is highlighted. Outline and principle of advanced techniques (eg Base-Isolation and Supplemental Damping) has also been provided. However, a detailed description and mathematical formulation of these advanced techniques are beyond the scope of these Guidelines and references have been provided for further reference.
Evaluation and retrofitting of damaged structures is an urgent task after an earthquake, as safe shelter is under pressing demand after a damaging earthquake. This requires some quick evaluation and retrofitting techniques. The techniques for quick evaluation of need and viability of retrofitting, temporary emergency support of the damaged structures, and repair and retrofitting of structures are also covered.
Retrofitting and strengthening of existing structures require use of special materials. Bonding of old and new concrete and shrinkage are the main governing factors in selection of material. A description of materials available for this purpose, including a range from ordinary cement-sand grout, concrete to polymers and epoxy, use of Fibre Reinforced Polymers/Plastics (FRP) in strengthening and retrofitting has also been described with the points of caution. Specialized machinery and preparations required for use of different retrofitting materials are also outlined.
Three case studies on seismic retrofitting of an existing masonry building and reinforced concrete framed buildings have also been included. The first case study pertains to a masonry building of more than 150 years age. The second case study is about evaluation and retrofitting of a RC frame building constructed around 1963 and designed only for gravity loads as per the practice prevailing at that time. The third case study is about a recently constructed RC frame building with soft ground storey, and represents the typical construction going on in Indian cites.
A lot of development has taken place in the area of evaluation and retrofitting of existing structures and it is still developing. These Guidelines cover the techniques suitable for the type of construction prevailing in India and which have been widely used and accepted to be safe. However, there are some more advanced methods of evaluation and retrofitting techniques, which are still at development stage. References have been provided for further study on these techniques. It is hoped that this will provide adequate information required for effective implementation of seismic retrofitting.