During an earthquake an individual could be thrown out of bed at night, be unable to stand upright and be forced to kneel on the ground. This happened during the earthquake on May 21, when the tremor measuring 5.8 on the richter scale hit Odisha at 9:52 pm.
The epicentre of the quake was 266 km off the Paradip coast in the south-east Bay of Bengal. In Bhubaneswar, over 100 people were injured and cracks developed in some high-rise buildings. People ran out of homes and the earthquake continued for 5 to 20 seconds.
An earthquake is a natural phenomenon that results from an underground movement along a fault plane. Sometimes, it is so mild that it passes unnoticed. Sometimes, it is quite strong and creates vast openings in the earth’s surface, and in its impact, buildings fall down and many lives are lost.
With careful study, geologists are slowly learning about such questions: Can earthquake be predictedIJ Can we stop earthquake before it occursIJ Can we design buildings and a city to withstand an earthquakeIJ Studies and research are going on. To know about earthquake, many clues are generally referred to such as strange animal behaviour, increase in the rate of smaller earthquakes, gap in the regular frequency of the earthquake, etc. But none has correctly predicted it.
Recently, earthquakes and the subsequent disastrous effects have established an urgent need of evolving a coordinated design system to provide earthquake-resistant structures in Odisha. Buildings can be made earthquake-resistant by incorporating methods and techniques of earthquake-resistant design and construction depending upon seismic zones.
About 54 per cent of India’s lands are vulnerable to earthquakes. The increase in demographic pressure, unplanned and ill-planned development practices and poor quality of construction techniques have contributed immensely to the proliferation of seismic risk. There are ways of making structures safer than the current ones. Researchers and engineers have mobilised their efforts to achieve that goal, working on removing shortcomings in the design of structures that have not performed well in seismic events and coming up with improved versions capable of standing up to a certain level of earthquakes.
The Bureau of Indian Standards (BIS) is a statutory organisation under the Bureau of Indian Standards Act. One of its activities is formulation of Indian standards on different subjects of engineering through various councils. Seismic codes are unique to a particular region or country. They take into account the local seismology, accepted level of seismic risk, building typologies and materials and methods used in construction. The first formal seismic code in India, namely IS 1893, was published in 1962. Today, the BIS has a number of seismic codes. IS 1893 (Part I): 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision), IS 4326: 1993, Indian Standard Code of Practice for Earthquake-Resistant Design and Construction of Buildings (2nd Revision); IS 13827: 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings; IS 13828: 1993, Indian Standard Guidelines for Improving Earthquake Resistance of low Strength Masonry Buildings; IS 13920: 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces; IS 13935: 1993, Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings. The codes cover all aspects of design including analysis, detailing and construction and strengthening of existing structures. These codes define the provisions required according to risk zone in which a structure is located.
location is a major determinant of the type and frequency of natural hazards. Before beginning construction of any building, it is very much important to have the complete knowledge about the soil. Soil tests should be conducted to know its type, bearing capacity, strata, composition, etc. There are mainly three soil types; Firm: Those soils which have an allowable bearing capacity of more than 10 t/m2, Soft: Those having allowable bearing capacity less than or equal to 10 t /m2 and Weak: Those liable to large differential settlement or liquefaction during an earthquake. Buildings can be constructed on firm and soft soils, but it will be dangerous to build them on weak soils. Hence, appropriate soil investigations should be carried out to establish the allowable bearing capacity and nature of the soil. Weak soils must be avoided or compacted to improve them so as to qualify as firm or soft.
The layout of buildings should be as simple as possible. Depending upon the seismic zones, heights of the buildings and thickness of walls are decided. The building as a whole or its various blocks should be kept symmetrical about both the axes. Asymmetry leads to torsion during earthquakes and is dangerous. In general, openings in walls of a building tend to weaken the walls, and the fewer the openings less the damage it will suffer during an earthquake. If it is necessary to have large openings through a building, special provisions should be made to ensure structural integrity.
The earthquake zones have been categorised considering its region, its risk factors, intensity and rate of damage as Zone V (Very high damage risk zone ), Zone IV (High Damage Risk Zone), Zone III (Moderate Damage Risk Zone), and Zone II (low Damage Risk Zone).
Odisha is coming under Zone II and Zone III. In Odiahs, minor to moderate earthquakes have occurred now and then at different localities. Rarely a building has collapsed or any human casualty reported ever. The maximum magnitude of these earthquakes has been reported around 4.5 to 5.3 in the Richter scale. Considering the recent earthquake of 5.8, it cannot be confidently said that a higher intensity earthquake is not unlikely in future. Besides, in the 2004 tsunami, India was the third country severely battered after Indonesia and Sri lanka. The States severely affected by tsunami are Tamil Nadu, Andhra Pradesh, Kerala and Andaman and Nicobar Island. Odisha with over 480 km of coastline may be prone to tsunami. There is a need for reassessment of the seismic status of Odisha and to devise and incorporate technologies in all structures.
land Use Zoning aims at determining the locations and the extent of areas likely to be affected by the hazards of different intensities and frequencies, and to develop such areas in a manner that the loss to the development is reduced to the minimum. Areas under seismic zones III, IV and V are based on intensities VII, VIII, IX or more. Therefore, all areas in these three zones will be considered prone to earthquake hazards. In these zones, the areas which have soil conditions including the level of water table favourable to liquefaction or settlements under earthquake vibrations will have greater risk to buildings which will be of special consideration under land Use Zoning.
The Bhubaneswar Development Plan Area (BDPA) falls under Zone III (Moderate Damage Risk Zone). Since the soil in the BDPA is laterite (bearing capacity:18-20 t/m2), buildings starting from single to multistoried may be allowed, provided earthquake-resistant measures suggested in the Indian Standards are properly adhered to.
Seismic retrofitting can be applied to the existing building constructed without following the earthquake-resistant standards. The seismic retrofitting consists in upgrading the strength of an existing structure with the aim to increase its capacity to withstand earthquakes.
The consequences of damage have to be kept in view in the design philosophy. For example, important buildings like hospitals and fire stations must remain functional immediately after the earthquake. These structures should be designed for a higher level of earthquake protection.
Seismic risk reduction demands a systematic evaluation of the hazards, vulnerability and risk mapping of the entire region. Town and Country Planning Acts, Master Plan, Development Control Rules and Building Regulations of some of the metros in India have mentioned adequately on the importance of safety requirements against natural hazards. Services of architects and structural engineers with experience in earthquake-resistant construction are highly needed for design of buildings. It should be made mandatory for all owners and developers to get their buildings certified from structural engineers for safety reasons.
It is the joint responsibility of the architectural, engineering and construction community to incorporate the code in architectural and structural design to make the building earthquake-resistant. Structural plans and details of the building submitted for approval should satisfy the structural safety requirements. However, further research will add to the design with appropriate building materials and technology most suited to earthquake zones.
(Dr Mayarani Praharaj woks in the Department of Architecture, College of Engineering and Technology,Bhubaneswar)