Seismic Provisions of Strengthening and Retrofitting

 INTRODUCTION 

Till to date, earthquakes are one of the most unpredictable and devastating natural disaster which cause extensive damage to the buildings/structures. The damage results in loss of lives and property. Thus, it is very important on the part of the civil engineers to build structures with high seismic performance. But here the question arises, "What to do with the present old/weak or earthquake damaged structures?" It has been observed that majority of the structures being safely reused if they are made seismically strong by using some methods/techniques. It is a better and economical choice as compared to demolition and reconstruction. Thus there is a need to restore or strengthen the old and weak or damaged buildings so that they can sustain future earthquakes. It is one of the most important aspect of mitigation especially in earthquake prone areas which will reduce the earthquake hazards/damages. It is the most challenging task for a civil engineer in India as not much of experience and data is available in this field. 

Two types of buildings need to be retrofitted 

• Earthquake Damaged Buildings : The buildings which are damaged or weakened by the earthquakes thus making them unfit or unsafe for future use. 
• Weak Buildings : Buildings which have not experienced severe earthqauakes but are seismically weak and vulnerable to earthquakes. 

Various codes and standards are published by Bureau of Indian Standards to help the structural engineers in this field. This chapter explains various strengthening and retrofitting methods used for buildings as per IS 13935:2009. Code for "Seismic Evaluation" repair the strengthening of Masonry buildings. 

Need of Retrofitting 

    As discussed above the need of retrofitting and strengthening is to increase the available seismic resistance of the weak/old or earthquake damaged buildings. In addition to this retrofitting/strengthening of a building is also required to be done in the following cases : 

1. Upgradation of a Code : As the experience of the civil/structural engineers is increasing, codes/standards are also being upgraded from time to time. Thus the buildings designed by the code, which has been revised/upgraded, need to be retrofitted to fulfil the latest codal provisions. 
2. Change in use of Buildings : Whenever there are changes in the use/occupancy of a buildings, for example, public building converted to an industrial building or residential building to office building etc. there is a need to retrofit or strengthen the building to satisfy the codal provisions as per the present class of the building. 
3. Important Buildings : Important buildings such as hospitals, schools, historical monuments etc. need to be strengthened and restored from time to time to counter the effects of ageing and weathering. 
4. Retrofitting and Strengthening of a building is also needed in the case of extensions or expansion of the building. For e.g., making more stories etc. 
   
   
   

Terms associated with Retrofitting 

Retrofitting aims at increase in strength and ductility of the building/structure so that it can withstand the effects of earthquake safely. Various terms are associated with retrofitting which need to be understand carefully. These terms are explained below : 

(a) Repair : It is defined as the process of reconstructing or removing any damaged part of a building, so that it will have the same strength and ductility as it had before the damage. 

Repair includes both structural and non-structural elements. Repair work includes filling up of cracks in masonry walls, repairing damaged beams and columns, replacing or repairing damaged utility services like water to sewage pipelines, fixtures, gas and telephone lines etc. 

(b) Retrofitting : It is defined as the process of increasing the seismic resistance of damaged or weak building by appropriate techniques. Retrofitting of earthquake damaged buildings is done by repairing and strengthening the damaged portions of the building and making it reusable. 

Seismically weak buildings also need retrofitting so that they can sustain the effects of earthquakes safely thus minimizing loss of lives and property. It has been observed that cost of retrofitting is much less than constructing a new building. 

(c) Restoration : This term is commonly used with buildings of historical importance i.e., monuments. It is defined as the process by which a weak or damaged structure is strengthened to replicate the structure as originally built. 

Structure/building may damage or some parts or elements of it become weak due to weathering, ageing or any other reason. In restoration these parts are to be repaired and strengthened so that the structure become equally strong and look exactly like it was originally built. 

(d) Strengthening : It is the process of adding strength to any part of an existing building to provide higher strength and ductility than the original building. Strengthening can be done for earthquake damage buildings, seismically weak buildings etc. 

Methodology for Seismic Retrofitting 

Various steps for seismic retrofitting of a building are explained below : 

1.  Available seismic resistance of the building is estimated after detailed examination testing and studies. 
2. The seismic retrofitting and strengthening methods are recommended for the buildings based upon the detailed studies of various components of buildings and their feasibility in the present conditions. 
3. Cost analysis of the various methods/techniques is done to choose the best suitable, practical and economical option. It is suggested that the cost of retrofitting should be less than 50% of the replacement cost. 
4. A detailed and exhaustive retrofitting and strengthening procedure is planned for the building and executed in the best possible way. 
5. The upgraded/retrofitted building is analyzed as per the present code and its seismic evaluation is done to see the enhanced seismic performance.