12-21-2010, 12:47 PM
The mitigation of wind-induced excitations in tall building is one of the most important design considerations that should be met with right from the conception stage of any such a building. In that respect, different options have to be considered; of which the shape of the building is one of them. Reshaping of a structure does not involve only the plan modification but and also could involve elevational modification. The shape of a building could be aerodynamically modified by changing the taper of the cross-section. For one, this modification alters the flow pattern around the building. In so doing, it does not only provide the wind front a longer distance to travel and longer time to complete its percourse (thus dilution of the effect. Remember, power is the ability to do work i.e. work done per unit time, as such the longer it takes to complete a circle of work, the less power that that force system could exercise on the receiving system), but also creates room for spin offs-thus reduces the power input-as such wind-induced vibration of tall buildings. Further, a tapered tall building that spreads the vortex-shedding over a broad range of frequencies, more effectively reduces cross-wind responses. There have been several studies into this. In one of the studies, to investigate the tapering effect on the reduction rate of wind-induced vibration and responses of differing models of tapered tall buildings to this forced vibration, models of differing sizes were tested. High-frequency force-balance test was conducted on six types of building model having differing taper ratios - 2·5%, 5%, 7·5%, 10%, 15%. The results of the analysis were compared with that for a basic building model of square cross-section. Tests were conducted under two typical atmospheric boundary layers conditions, representing suburban and urban areas. The effect of wind direction was also considered. (Copyright © 2007 John Wiley & Sons, Ltd). The results of the test show clearly that there is much to gain in shaping the structure. Tapering distributes the dynamic behavior of the structure as such it becomes difficult to achieve a particular wind speed at which all parts of the structure will attain resonance. Apart from vibration, the structure is subject to another serviceability limit condition demand- base shear and bending moment, mainly due to same lateral loading.
There are several structural configurations geared towards countering the effect of lateral loading on a structure. For low rise buildings, the moment resisting frames or the shear wall approach could proof ideal. But after a certain height, it becomes impracticable to use the moment resisting frame. This is due to the fact that despite the fact that it becomes too expensive to achieve the construction following this approach, the lateral deflection (drift) that could be verified would be so great and unacceptable that that approach would have to be abandoned. For tall buildings such as the one in consideration (the Burj), it becomes imperative and unavoidable to resort to the shear wall approach; and when this shear wall is sort of located at the core of the structure, it is referred to as the “core structure” Due to the massiveness of the core structure, its robustness, its centralization, as such the location of both the geometric center, center of mass and center of rigidity within same range, structural eccentricity is as much is possible avoided. Though every attempt would be made as much as it is structurally possible to avoid this eccentricity, certain amount of eccentricity would definitely result due construction error, due to non homogeneity of structural components etc. The effect of the resultant eccentricity is to subject the structure to twist. If a building is subject to twist, as all are (implicitly), the torsional stiffness of the core, in a “core-only” structure could constitute a significant part of the total torsional resistance of the entire building. The torsional behavior of cores is a topic that is relatively of interest to many engineers. The proportion of the height, length, and thickness of the core walls of a typical building obligates us to analytically treat the core structure as a thin-walled structure. Consequently, when the core structure twists, originally plane sections of the core warp. Because the core is restrained from warping by the foundation, and to an extent by the floor slabs/beams, warping stresses somewhat similar to axial stresses are induced throughout the height of core walls. In buildings that are predominantly dependent on a core for torsional and lateral resistance (as is the Burj), it is imperative that consideration be given to warping effects. (Due to functional necessity, the core structure is usually an open one as such is susceptible to warping). For the fact that the core structure would be subject to enormous labor in a major event, all other parts of the structure that could be employed toward countering this twisting effect –thus the accompanying stresses had to be mobilized. For this, the columns, walls etc had to be properly linked up to the core structure- the main and dominant structure that bears the brunt. Since large torques are to be transmitted, robust links between the columns and the core structure had to be used. That is one of the reasons why deep beams were aggressively deployed in the structural configuration.
Regards
Teddy
There are several structural configurations geared towards countering the effect of lateral loading on a structure. For low rise buildings, the moment resisting frames or the shear wall approach could proof ideal. But after a certain height, it becomes impracticable to use the moment resisting frame. This is due to the fact that despite the fact that it becomes too expensive to achieve the construction following this approach, the lateral deflection (drift) that could be verified would be so great and unacceptable that that approach would have to be abandoned. For tall buildings such as the one in consideration (the Burj), it becomes imperative and unavoidable to resort to the shear wall approach; and when this shear wall is sort of located at the core of the structure, it is referred to as the “core structure” Due to the massiveness of the core structure, its robustness, its centralization, as such the location of both the geometric center, center of mass and center of rigidity within same range, structural eccentricity is as much is possible avoided. Though every attempt would be made as much as it is structurally possible to avoid this eccentricity, certain amount of eccentricity would definitely result due construction error, due to non homogeneity of structural components etc. The effect of the resultant eccentricity is to subject the structure to twist. If a building is subject to twist, as all are (implicitly), the torsional stiffness of the core, in a “core-only” structure could constitute a significant part of the total torsional resistance of the entire building. The torsional behavior of cores is a topic that is relatively of interest to many engineers. The proportion of the height, length, and thickness of the core walls of a typical building obligates us to analytically treat the core structure as a thin-walled structure. Consequently, when the core structure twists, originally plane sections of the core warp. Because the core is restrained from warping by the foundation, and to an extent by the floor slabs/beams, warping stresses somewhat similar to axial stresses are induced throughout the height of core walls. In buildings that are predominantly dependent on a core for torsional and lateral resistance (as is the Burj), it is imperative that consideration be given to warping effects. (Due to functional necessity, the core structure is usually an open one as such is susceptible to warping). For the fact that the core structure would be subject to enormous labor in a major event, all other parts of the structure that could be employed toward countering this twisting effect –thus the accompanying stresses had to be mobilized. For this, the columns, walls etc had to be properly linked up to the core structure- the main and dominant structure that bears the brunt. Since large torques are to be transmitted, robust links between the columns and the core structure had to be used. That is one of the reasons why deep beams were aggressively deployed in the structural configuration.
Regards
Teddy