Dealing with earthquakes: Retrofit our structures
Retrofitting is the process of changing technology or adding entities to the building that ultimately result in the addition of strength of the existing structure without its demolition. With better understanding of seismic design on structures and on the crucial experiences from the recent large earthquakes, the need of retrofitting is in high demand. It is important to keep in mind that till date there is not a single structure such as a seismic proof structure made in the history of engineering, although its performance can be highly enhanced due to proper design application and modification. Steps of retrofitting comprise assessment of structure, evaluation of seismic forces acting on the structure, selection of retrofitting strategies including their proper implementation whereas its application ranges from typical Nepali adobe residential houses to high rise multiplexes, bridges and transportation structures. Selection of right technique, material and procedure is a vital challenge whereas selection of material and technique generally depends upon requirement and availability of resources, applicability and suitability of material for the repair of damaged structures.
During my survey visits of rapid visual assessment, under NEA (Nepal Engineers’ Association), I found an amalgam of existing buildings in our society from old to newly constructed, illegally made to well designed, humble brick to gleaming new multistory structures. The main causes of damage to buildings predate modern construction practices with old decaying and less durable materials and construction of newer buildings without the application of proper seismic codes. Current earthquake codes are applicable to newer houses only and are not applicable to the buildings without earthquake resistant features. In Nepal, where a large diversity is found on the construction of houses, many houses need to be retrofitted to resist future earthquakes. In today’s scenario, multiple aspects of retrofitting can be seen as per characteristics of structures. Hence, engineers need to prepare and design the retrofitting approaches with the building codes. Some recently developed materials and techniques can play a vital role in structural repairs, seismic strengthening, and in retrofitting processes.
We need to retrofit our structures to deal with massive earthquakes in future. As retrofitting is a big arena, the proper analysis of an expert is very essential to make this process economical and reliable
Injecting GROUT (construction material used to embed rebar in masonry walls, connect sections of pre-cast concrete, fill voids, and seal joints) into the wall in order to fill the voids or cracks formed due to deterioration of a building is one of the finest and common methods applicable to all the houses with minor cracks on them. A large number of cracks can be stitched together with steel bars and mortars. That ultimately helps restore initial stiffness of the structure. Addition of shear wall and bracing is another relevant strengthening method to overcome the deficiencies in strength, stiffness, and to reduce displacement with its effectiveness, relative ease and lower cost as compared to beam and column jacketing. Engineers must keep in mind that the new loads to be carried by these elements should be efficiently delivered without deficiency in load path.
During multiple researches, it is observed that concrete shear walls and steel braced frames are common strengthening techniques and are generally used due to their low cost and familiarity in construction industry. Design of additional shear wall is performed to resist major fraction of loads likely to act on structure, which reduces the demand on beams and columns, hence increasing their safety. After performing the above activities, if the structures are still overstressed then ultimately the structure is strengthened through concrete and steel jacketing. This is more expensive and needs skilled technicians. The general problem our houses are facing is small pillars designed without application of building codes and comprising heavier loads in it. The remedial measure for this problem is the concrete jacketing of structural members that widens the area of structural member which finally enhances the strength of the whole structure. Conventional technique (jacketing) seems to be labour intensive, difficult and creates disturbance to the occupants.
One of the most common problems that almost all kinds of houses face is the presence of cracks near the openings, i.e. door and windows, rather than in any other location in the building. The general solution to these problems is installation of new lintels in these areas that helps to repair and strengthen the portion. Including earthquakes, foundation settlement and slumping due to moisture intrusion at the base can also cause cracking. After these cracks are developed, the behaviour of the building is largely
dependent on the stability of the cracked wall sections, and the design of the retrofit system should be directed towards stabilization of each of these cracked sections.
We need to retrofit our structures to deal with massive earthquakes in future. As retrofitting is a big arena, the proper analysis of an expert is very essential to make this process economical and reliable. Use of standard and innovative repair materials, appropriate technology, proper workmanship and quality control during implementation are the major preliminary factors for successful repair strengthening and restoration of damaged structures.