Transfiguration as a means of structural resistance : exploring the strength benefits of a deformable cladding system to tall buildings

Abstract

The impact of wind on tall structures is of paramount importance through all stages of design. In the past, structures have predominantly been designed to face wind-induced stresses, in their statically rigid form. Whilst past research studies have allowed for the mitigation of these stresses by developing a better understanding of what makes a structure more aerodynamic, these studies have always considered a building in its ever fixed form. However, through the application of a natural mechanism currently implemented in the Formula 1 racing car industry, structures could possibly be seen to change shape as wind incident on its surface, tries to force a more streamlined flow around the building. Obtained through the addition of a flexible exterior skin acting as a wind shield and installed over the external rain screen of the building, the wind would naturally force this less rigid skin to take up a more streamlined form, based on its direction and magnitude. This innovative wind shielding system could lead to a transfigured external building shape that ultimately transfers less stress to the structural core of the building, because of its improved ability to reduce wind pressure build up on the exterior of the building. Throughout this dissertation, it was noted that the structural efficiency of a building may be improved by means of wind shielding that seeks to lower the lateral loading caused by wind pressures acting on the facades of the building. Geometry variations in the height, width and plan shape of a building together with its external wind shield could improve the aerodynamic performance of the building. Such variations in the building geometry could be achieved by recessing the plan of the building at the points of high localised wind pressures, thereby creating a smoother transition curve at critical points on the building façade. These measures would help in reducing the magnitude of the average wind load acting on the building facades. However, there is another important wind pressure component, produced by the fluctuating manner in which wind is forced to navigate around the building, which also needs to be considered. This component, causing the across-wind load on the structure, is associated with amplified magnitudes as its frequency of oscillation approaches the natural frequency of vibration of the building. Since, nowadays structures are designed to be increasingly slender, the importance of such a wind pressure component cannot be overlooked. It was noted that twisting and tapering of a structure’s external form can greatly reduce the effect of such a component, and reduce the tendency for fluctuating wind loads to occur in tandem with structural vibrations. With such measures of aerodynamic modification, and the advent of aerodynamic optimization, there are significant performance benefits to be gained by simply changing the external shape of the building. In this way, the internal structure of the building will require less structural material due to decreased levels of lateral wind loading. This dissertation will seek to push the boundaries of applicability of such aerodynamic optimization, by considering both passive and active methods of improvement in building performance. In constantly adapting to varying wind conditions, the proposed wind shield system around the external rain screen of the building will assist the building to experience lower wind pressures. Incorporating engineering concepts currently applied in the automotive industry, most noticeably in Formula 1 racing car industry, numerical simulations will be carried out using Computational Fluid Dynamics (CFD) computer software in order to better understand how a structure may benefit from such wind shield mechanisms, and the extent to which it retains this benefit. A parametric study for buildings with wind shield systems of varying levels of air permeability will be carried out to understand how wind pressure and air flow interact with the external facades of the building, and whether this response could be manipulated to improve the structural behaviour of the building. Various stages of simulation and model development were required to achieve a final numerical model that could be used in the CFD analysis. It is also acknowledged that the realisation of such a wind shield concept requires further research, development and testing on both a global and local basis, in view of the several relevant variables involved in this investigation. The results of the CFD parametric analysis showed that the proposed wind shield was effective in reducing the magnitude of along-wind lateral load that impacts the building, thereby improving the aerodynamic performance of the structure. The extent to which this improvement occurred, was dependent upon the level of air permeability of the wind screen. There were, however, some conflicting results when considering the building performance in the across-wind direction, therefore, requiring further research work in this regard.