Structural Performance of Concrete Buildings
Structural Performance of Concrete Buildings in Low and High seismic zone
One of the major disasters in the world is an Earthquake in which many structures and buildings collapse and most of the damage occurs due to the improper design against the seismic motion. Earthquake also affects the economy of the country so it’s better to take preventions before construction for the development of the country.
When two tectonic plates collide then stress is produced on the lithosphere on natural calamity occur known as “Earthquake”. When there is a high level of earthquake then lithosphere breaks or shifts. The collision in the plates is of two types one is inter-plate which occurs on the boundary of plates and other intra-plate which can occur anywhere in the plate. Most earthquakes occurred in the Himalayans mountain ranges. The risk of earthquakes on current buildings is because of high seismicity due to the age of the structure and the low standard material used in the construction process. As we know that earthquakes are unpreventable and unpredictable so the only option is to design and build the structure which can resist earthquake. The amount of damage can increase in the future because the construction of buildings is increasing due increase in population. During the earthquake behavior of buildings depend upon certain factors like lateral strength, stiffness, and ductility. The buildings which are having regular geometry, stiffness, and mass suffer less damage during an earthquake. When an earthquake occurs then inertia forces generated from the building resist it. The resultant forces in building acts on the center of the mass of the building. Eccentricity develops when the center of mass and the center of gravity do not coincide then it generates torque.
To build a structure that offers resistance to earthquake the most important aspect of the design is the deformation of the structure within the inelastic range. Therefore, effective earthquake resistance in the structure can be obtained only when the design has the capacity for internal damping (energy dissipation within the materials of the structure) and for inelastic deformation. Many buildings are investigated to know the reason of failure so the major failures are due to the soft storey, floating columns, mass irregularities, poor quality of construction material, faulty practices in construction, sound and foundation effect, faulty concrete production, defects in reinforcement details, poor quality of workman ship and pounding of adjacent structures. By detailing the reinforcing steel in reinforced concrete appropriately, increase in the ductility of the R/C concrete is possible. However, this would not prevent dynamic degradation of the R/C concrete under subsequent cycles of harsh inelastic deformation. Another method is by using structural steel members with higher ductility. However, they too would undergo dynamic deteriorations and also reduction in strength due to local buckling. Also, flexible building frames that suffer post-elastic deformations are eventually expensive to fix.
While designing any structure for earthquake zones we have to take under consideration three limit state designs:
Service limit state
In this state building will not face any damage to the structure and important buildings are designed on this state like hospitals, assembly and power plants. If the earthquake takes place these buildings should be serviceable.
Damage control limit state
In this state if earthquake comes there will be less damage to structure, and we can use it after repairing. This state is only for used for permanent buildings.
Survival limit state
In this state if earthquake comes there will be damage to the structure, but support will be able to carry the load and there will be no loss of life.
Earthquakes are having large magnitude and have short duration so buildings must be safe from collapse and there should be no damage to structure elements. Concrete and masonry are the critical parts of structure so special detailing is required to ensure ductility to lateral forces. Ductility can be increased by increasing reinforcement in the structure. Ductility serves as the shock absorber in the structure. Reinforcement also effects the economy also so the correct ratio of steel to be used in the structure. Earthquakes produce forces that act horizontally on a structure and can be characterized into three types:
Earthquakes which last a longer duration and have an irregular noise like surface motions.
Earthquakes that are one of the most serve in terms of damage done to the ground.
Chandiwala, A. (2012) found that above 500,000 earthquakes occur every year. 1000 of earthquakes people feel and 100 of earthquakes cause damage. Most of the earthquake occurs at the boundary of tectonic plates. Earthquake can be resisted in two ways. First the structure should be made of smaller sections which are subject to plastic stresses. Second the structure should be made of large section which is subjected to elastics stresses. It is observed that steel structures behave well when subjected to earthquake. Steel structures are ductile, so their flexible nature provides good resistance to earthquake. Experience in past earthquake made it clear that buildings construction practice lacks resistance to earthquake. Most of the problems lie in developing countries due to lack of coordination between departments and lack of responsibilities in concerned authorities. About 70% steel structures faced brittle failure and 10% structures collapsed during earthquake. The Most essential features for earthquake are stable foundations, regularity, ductility and toughness, stiffness, redundancies are considered while designing and constructing the structure.
While designing buildings we must check the history of earthquake in the particular area. Building must be regular and foundations must be stable and earthquake resisting devices should be used in the building like base isolators and buildings not to be constructed near to each other in order to avoid the pounding affect. Building should be designed and constructed according to the codes. It is necessary that all are parts of building or structure, including non-structural components must be tied together so they can provide a continuous path that will transfer the forces to the ground. If the components of parts are not tied together so during earthquake there are chances of collapse. In 1971 San Fernando earthquake a collapse occur in the building in which the exterior walls which support structures wooden roof were not connected so during strong shaking walls moved away and roofs collapse. So, if all the components are tied together so it can save the building from collapse. Energy dissipation devices should be used like damper it helps to resist the building during earthquake.