08-02-2013, 02:00 PM
natural calamities
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INTRODUCTION
GENERAL
Among the natural calamities, earthquakes are the most destructive, in terms of loss of life and destruction of property. Often, they occur without warning, which makes them the most feared and unpredictable natural phenomena. On an average, two earthquakes occur globally every year.
Due to shaking of ground during earthquake, structure is set into motion and subjected to reversible earthquake loading, in horizontal as well as vertical directions. The behavior of the structure and earthquake loading are interrelated functions. Though earthquake load developed in the structure is mainly a function of mass and height of the structure, it primarily depends on peak ground acceleration and frequency content of the earthquake.
Compared to gravity load effects, earthquake load effects (i.e. Forces and moments in columns) on buildings are quite variable and increase rapidly with increase in height. Design of structure for gravity load is a function of mainly column-to-column span in structure. Where earthquake resistant design of structure is a function of total mass, height and location of the structure and soil characteristics. Low rise and mid-rise buildings are designed for gravity loads, it has been observed that it is very likely that structure can carry most of the earthquake loads due to infill walls. In general, the strength requirement is a dominant factor in the design of low rise structures. However, as height increases, the rigidity (i.e. Resistance to lateral deflection) and stability (i.e. Resistance to overturning moments) requirement become more important. Basically there are two ways to satisfy these requirements in a structure. The first is to increase the size of members beyond and above the strength requirements. However, this approach has its own limits, beyond which it becomes either impractical or uneconomical to increase the sizes. The second and more elegant approach is to change the form of structure into something more rigid and stable to confine the deformation and increase stability. Shear wall is one of the best structural systems, which is rigid as well as stable for building while designing for lateral loads.
Types of shear wall:-
Shear wall can be classified based on different parameters. Classification of shear wall is shown in Fig1.1.
Depending on height to width ratio shear wall may be classified as slender or squat shear wall. If height to width ratio is more than 3 then it behaves as a slender wall and if height to width ratio is less than 1 then it behaves as a squat wall.
Concrete shear wall are widely used in high rise buildings. But where force levels exceeds those permitted in all other materials or where dimensional constraint limit the width of shear walls steel plate shear walls are used. Composite shear walls consist of steel plate with reinforced concrete walls attached to one side or both sides. Sometimes wooden shear walls are used in low rise building because timber structures have a good track record of performance in major earthquakes and their low mass and good damping characteristics helps to achieve good performance during major earthquakes.
NEED OF PRESENT STUDY
In many structural walls a regular pattern of openings will be made due to various functional requirements such as to accommodate doors, windows and service ducts. Such type of openings reduces the stiffness of the shear wall to some extent depending on the size of the opening. It is seen that small opening in shear walls do not have much effect on the lateral deflections. Whereas larger openings produce larger effects. The openings causes stress concentrations in the shear wall itself.
Shape of openings also plays a significant role. It was seen that an opening with larger horizontal aspect ratio is more critical than an opening of larger vertical aspect ratio for deflections as well as column shear and moments as well as stresses within the shear wall. With the same area of the opening, square shape gives the minimum deflection. Moreover when the relative area of the opening is not small and their arrangement is rather uniform along the height of the wall, the resulting structural system is neither a solid wall nor a real frame, and is referred as a coupled wall.