5 Best tips for Earthquake Resistant Building in Nepal
Nepal is geographically located in earthquake prone region and thus there is always a high risk of earthquake in Nepal. Many of us have already witnessed and experienced the earthquake of April 2015 that destroyed many buildings and claimed over 9000 lives. Since its very hard to predict the occurrence of earthquake, its fundamental for all of us to be prepared as much as possible to reduce casualties. Earthquake resistant buildings will definitely help minimize the risk of casualties from building collapse. Out of many factors here are five tips for construction of a earthquake resistant building:
- Shape and Size
- Proportion of building
- Ductile frames
- Minimized torsion
We will discuss the above mentioned factors in brief:
- Shape and Size
Building’s layout and elevation regularity, symmetry and compactness are the geometric parameters that play vital role during seismic influence. Regular shapes like square and rectangle are considered safe during earthquake as compared to shapes like V, C, L, U, H etc. These irregular shapes have variation in stiffness and hence differential motions are created among different parts of building resulting in local stress concentration at the re entrant corners. Also the twisting effect in horizontal plane (torsion) causes higher shear load on far columns and walls forcing them to fail. These effects can be avoided by sub-dividing the building into small rectangular boxes providing seismic gaps in between so that the separated blocks oscillate independently during seismic wave.
Elevations or vertical irregularities of the building consists of irregularities of structural system stiffness discontinuity i.e. soft storey, weight irregularity, discontinuity of vertical lateral force resisting elements and strength discontinuity i.e. weak storey. Such irregularities in building elevation should be avoided as much as possible.
The earthquake force developed at different floor of buildings should be transferred to foundation through shortest and direct path. Any discontinuity in the load path caused by discontinuous column, floating columns, vertical setbacks causes sudden leap in earthquake forces resulting in poor performance during earthquake. Such situation should be avoided as much as possible and should have uniform stiffness to make it earthquake resistant.
2. Proportion of Building
Proportion of buildings also plays an important role in making a building earthquake resistant and hence proper building proportion is preferred. In narrow and very long buildings, the ground imparts very different earthquake vibration to different parts of building causing severe damage. If buildings have very long horizontal dimension, differential arrival of seismic wave in different part of building might cause problems. Hence it is advised to keep the ratio of breadth to length of buildings within 1:3.
Every building has a natural period or resonance which is the number of seconds it takes for the building to naturally vibrate back and forth. As the height increases, the natural period of the building increases shifting it into the range where response amplification is lower. Height to width ratio plays crucial role during seismic wave. Slender buildings have high compressive and tensile forces on outer columns during seismic event which results in rapid strength and stiffness degradation of these columns. Hence to make a building earthquake resistant it is advised that height of building should not exceed 3 times the width of building.
Foundation is the lower part of building that transfers the load into the soil below. After the Earthquake of 2015, people of Nepal have started the practice of soil testing before construction of buildings but that too has been reducing day by day. Detailed soil investigation must be carried out before construction as nature of the soil can be different even in adjacent sites. Foundation of the building must be designed after the soil test and recommendations from Geo-technical engineers regarding nature of soil, its bearing capacity, position of ground water table, the prospects of liquefaction and so on to make it earthquake resistant.
Foundation can be designed according to the type of land. It is advised not to construct building in land more steep than 20 degrees (1:3, vertical: horizontal). There can be stepped strap footing for slope land that should follow structure analysis done by engineers. Foundation is very crucial to buildings stability during earthquake movements. The are many types of foundation like strapped footing, combined footing, isolated footing, mat foundation etc which are selected according to the site condition and soil test results.
4. Ductile frames
Ductility in material science is defined as ratio of ultimate strain to yield strain of material. The frame of the building which has adequate ductility can undergo deformation without collapsing. Masonry construction is inherently brittle hence all the elements must be tied together to act as a single unit. It is the best possible way to make a structure earthquake resistant. We must make our elements bond and bind together for strong and durable construction. In case of Reinforced Cement Concrete (RCC) buildings ductility can be improved by proper structure designs using codes of practice like IS 13920.
When a structure undergoes dynamic forces like seismic forces, the structure could not remain elastic anymore and damage is taken. It can go through plastic stage or fracture or damage, where stiffness will be decreasing and deformation will increase even when the force applied is not so strong. Hence this situation must be expected by engineers and they should ensure that the structure sustains these loads without large deformation or collapse.
For ensuring economy and safety of building structure under unexpected earthquake forces, the best method opted is to let the structure take damage either by means of plasticity, fracture, crushing etc all the while keeping its strength to carry vertical loads during deformation without collapse. Ductility of frames is very important during earthquake to minimize the damage taken and saving lives of people from collapsing buildings.
5. Minimized Torsion
If the building is irregular or asymmetric some parts of building deflect more and some less during the seismic event. This difference in deflection causes whole building to rotate leaving the corners and ends at more stress and makes them more prone to failure. This rotation of the building in known as torsion. Torsional effect of non symmetrical buildings makes design of earthquake action more complicated as compared to symmetrical buildings which has pure translation response. Causes of torsion in the buildings are:
- Non symmetric distribution of masses
- Torsional motion of ground by seismic wave and ground motion in-coherency
- Stiffness of non structural elements which are not explicitly accounted for design like brick infill walls
Torsion is created when center of the mass and center of rigidity do not coincide with each other. The distance between these two points is known as torsion eccentricity. The members resisting horizontal forces should be arranged so that the center of rigidity and center of mass in each floor lie close to each other in both major axes.
Many buildings have been destroyed due to this torsional effect during past earthquakes. If it is not completely avoided the best way is to minimize such effects by making a symmetrical building plan i.e. uniformly distributed mass and uniformly placed vertical members of the structure.
However if the twist or torsion cannot be avoided, special calculations need to be done to account additional shear forces in design of building process to ensure structure is earthquake resistant. IS 1893:2002 codes has provisions for such calculations.
Hence shape and size of the buildings play important role in making the structure earthquake resistant and minimizing damage as much as possible. It is always advisable to carry out a soil test and consult an engineer to design the building and perform a structural analysis.
Prefab homes have also come up as effective alternatives to traditional buildings to minimize the casualties from earthquake in Nepal. These are basic tips for earthquake resistant buildings that are very crucial to face seismic forces and to avoid large amount of damage to buildings as well as lives of people.