Reinforced Concrete

Reinforced Concrete (RC) building frames are most common types of constructions in urban India. These are subjected to several types of forces during their lifetime, such as static forces due to dead and live loads and dynamic forces due to earthquake. Irregularities are not avoidable in construction of buildings. Irregular structures are those, which are having geometry discontinuity or mass discontinuity of the structure. These discontinuity will act as weakness to the structure. However a detailed study to understand structural behaviour of the buildings with irregularities under seismic loading is essential for their better performance.
The main objective of this investigation is to analyze and compare the behaviour of three dimensional reinforced concrete regular and irregular building which is subjected to earthquake load. The seismic analysis of 11 storied reinforced concrete regular and irregular building with and without infill walls due to earthquake is analyzed by computer aided analysis by using ETABS (Extended Three dimensional Analysis of Building System) software. Load consideration is based on Indian building code. Firstly, the regular and irregular buildings with external infill walls are analyzed with linear dynamic analysis using response spectrum method. Secondly, the regular and irregular shaped structure with external and internal infill walls is also analyzed. From the results it is inferred that building with severe irregularity produces more deformation than those with less irregularity.

Dynamic actions are caused on buildings by both wind and earthquakes. But, design for wind forces and for earthquake effects are distinctly different. However, in earthquake design, the building is subjected to random motion of the ground at its base, which induces inertia forces in the building that in turn cause stresses; this is displacement-type loading. The mass of the building being designed controls seismic design in addition to the building stiffness, because earthquake induces inertia forces that are proportional to the building mass. Designing buildings to behave elastically during earthquakes without damage may render the project economically unviable. There are four aspects of buildings that architects and design engineers work with to create the earthquake-resistant design of a building, namely seismic structural configuration, lateral stiffness, lateral strength and ductility, in addition to other aspects like form, aesthetics, functionality and comfort of building. Lateral stiffness, lateral strength and ductility of buildings can be ensured by strictly following most seismic design codes. All buildings are vertical cantilevers projecting out from the earth’s surface. Hence, when the earth shakes, these cantilevers experience whiplash effects, especially when the shaking is violent. Hence, special care is required to protect them from this jerky movement. Buildings intended to be earthquake-resistant have competing demands. Firstly, buildings become expensive, if designed not to sustain any damage during strong earthquake shaking. Secondly, they should be strong enough to not sustain any damage during weak earthquake shaking. Thirdly, they should be stiff enough to not swing too much, even during weak earthquakes. And, fourthly, they should not collapse during the expected strong earthquake shaking to be sustained by them even with significant structural damage. These competing demands are accommodated in buildings intended to be earthquake resistant by incorporating four desirable characteristics in them. These characteristics, called the four virtues of earthquake-resistant buildings, are:

a). Good seismic configuration, with no choices of architectural form of the building that is detrimental to good earthquake performance and that does not introduce newer complexities in the building behaviour than what the earthquake is already imposing; b). At least a minimum lateral stiffness in each of its plan directions (uniformly distributed in both plan directions of the building), so that there is no discomfort to occupants of the building and no damage to contents of the building; c). At least a minimum lateral strength in each of its plan directions (uniformly distributed in both plan directions of the building), to resist low intensity ground shaking with no damage, and not too strong to keep the cost of construction in check, along with a minimum vertical strength to be able to continue to support the gravity load and thereby prevent collapse under strong earthquake shaking; and d). Good overall ductility in it to accommodate the imposed lateral deformation between the base and the roof of the building, along with the desired mechanism of behaviour at ultimate stage. Behaviour of buildings during earthquakes depend critically on these four virtues. Even if any one of these is not ensured, the performance of the building is expected to be poor. In high rise buildings, the ordinarily occurring vertical loads, dead or live, do not pose much of a problem, but the lateral loads due to wind or earthquake tremors are a matter of great concern and need special consideration in the design of buildings. These lateral forces can produce the critical stress in a structure, set up undesirable vibrations and in addition, cause lateral sway of the structure which can reach a stage of discomfort to the occupants. In many countries situated in seismic regions, reinforced concrete frames are in filled fully or partially by brick masonry panels with or without openings. Although the infill panels significantly enhance both the stiffness and strength of the frame, their contribution is often not taken into account because of the lack of knowledge of the composite behavior of the frame and the infill. When the structure reacts with seismic wave it is subjected to displacement, after it reaches the threshold it affects the strength of the building which may cause deterioration of the building. To avoid such consequences infill walls are immensely helpful as they provide stiffness. In this study, the high rise building of different shapes are analyzed with and without infill walls by using ETABS software. Brick masonry properties are defined using standard values and assigned as a wall to a structure. The response of every structure is analyzed and differences are compared. Despite being regular or irregular reinforced concrete building with infill walls, building subjected to dynamic loads is more stable and also improves the stiffness of building.