Thursday, April 26, 2012

Problems Caused due to Lateral Deflection of Tall Buildings - wind load

Wind loading on a high-rise building can have a dominant influence on its structural arrangements and design. Lateral deflection due to wind load actions on tall buildings has become a major concern for the designers of today’s high-rise buildings and it is one of the key important factor that must be considered in the structural design of a tall building.

In contrast to vertical loads, lateral load effects, such as the forces exerted by wind on buildings are quite variable and increase rapidly with increase in height. Further, such lateral forces tend both to snap(shear)  and push over (bending) the tall buildings. Excessive or uncontrolled lateral deflection may cause extensive structural, non structural, constructional damages and discomfort to building occupants leading both extensive economic and social damages. It may cause cracking of partitions and external cladding, misalignment of mechanical systems and doors, and possible permanent deformations.

Lateral defections must be limited to prevent second-order P-Delata effects due to gravity loading being of a such a magnitude to precipitate collapse. Further, it must be limited or maintained at a sufficiently low level to allow the proper functioning of non structural components , to avoid distress in the structure, to prevent excessive cracking and consequent loss of stiffness, to avoid any redistribution of load to non-load-bearing components and to prevent dynamic motions becoming large enough to cause discomfort to occupants, to prevent delicate work being undertaken, or effect sensitive equipment.

In recent years the subject of tall building motion and its reduction has received considerable attention. With present trends towards taller, lighter, and more flexible structures, the importance of study and research on this topic is ever-increasing. Of particular interest are methods to reduce the lateral deflection due to wind load actions.

 Reasons for Tall Buildings

Ancient tall buildings and structures are primarily solid structures serving as monuments rather than space enclosures. By contrast, contemporary tall buildings and structures are human habitats, conceived in response to rapid urbanization and population growth.

There are various reasons for construction of tall buildings. The growth in modern tall building construction has been largely for commercial and residential purpose.   However, ego and competition still play a part in determining a building’s height. In addition, various other social and economic factors, such as increase in land value in urban areas and high density of population have lead to a great increase in the number of tall buildings all over the world. Further, the business and tourist community , with its increasing mobility, has fuelled a need for more frequently high-rise , city centre hotel accommodation. The high cost of land, the desire to avoid continuous urban sprawl, and the need to preserve important agricultural production have all contributed to the increasing number of tall buildings.

 Early Tall Building Versus Modern Tall Buildings

As mentioned in the previous section, ancient tall buildings are primarily solid structures rather than space enclosures. By contrast modern tall buildings are human habitats. The difference in the usage of buildings, from solid monumental structures to space enclosures, in itself has not changed the basic stability and strength requirements; the structural issues are still the same, the materials and methods are different.

In the design of early monuments, consideration of spatial interaction between structural sub systems was relatively unimportant, because their massiveness provided for strength and stability. Comparatively, with evolution of radically new structural systems the size and density of structural elements of modern tall building are strikingly less and continue to be diminish motivated by real-estate market, aesthetic aspects and innovative and challenging structural solutions powered by the immense analytical backup provided by computers. Early tall buildings are either prismatic, square or round. The modern tall buildings are better designed and takes more varied and irregular external architectural shapes. 

In recent decades there have been great advances in the design and construction of tall buildings throughout the world, and in the associated development of analytical techniques. The progress in reinforced concrete was slow and intermittent. The inherent advantages of the concrete which could be readily formed to simultaneously satisfy both aesthetic and load –carrying requirements, were not fully appreciated until the end of World War I. Since then significant developments in reinforced concrete occurred. Rather than bringing significant increases in height, these modern developments comprised new structural systems, improved material qualities and services and better design and construction techniques.

The constant search for more efficient solutions led to the innovative designs and new structural form of recent years. The taller and more slender a building, the more important the structural factors become, and more necessary it is to choose an appropriate structural form or system.

This chapter provides a comprehensive literature review on the research subject. It describes various important aspects that is related to the research subject and must be considered in executing the research. Further, apart from a general outline of the structural systems for concrete buildings, special attention is devoted to the structural systems those are studied under this research. These structural systems are described in detail.

Structural Concept of tall building

With regard to lateral loading, a high-rise building is essentially a vertical cantilever beam from the earth. This is the key idea in conceptualizing the structural system for a narrow tall building. This conceptualized cantilever may comprise one or more individually acting vertical cantilevers, such as shear walls or cores, each bending about its own axis and acting in unison only through the horizontal in-plan rigidity of the floor slab. Alternatively the cantilever may comprise a number of columns or walls that are mobilized to act compositely , to some degree, as the chords of a single  massive cantilever, by vertically shear resistant connections such as bracing or beams.

The laterally directed forces due to wind load actions tends to snap the building and push the building over. Hence, the building must have a system to resist both the shear and bending. In resisting the shear forces, the building must not break by shearing off and must not strain beyond the limit of elastic recovery. Also the system resisting the bending must ensure that the building is not overturned and is not broken by premature failure of columns. In addition its bending deflections should not exceed the limit of elastic recovery.

On the other hand when the structure resists to bending and shear, this may set the building in motion, creating an other engineering problem; motion perception or vibrations. As mentioned earlier, excessive or uncontrolled dynamic motions will cause discomfort to the building occupants and extensive socioeconomic damages.  

Different types of wind load on buildings

1 comment:

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