Aerodynamic Modifications against Wind Excitation on Tall Buildings-Shape Optimization

Aerodynamic Modifications against Wind Excitation on Tall Buildings-Shape Optimization

Neethi B.

Assistant Professor

Civil Engineering Department, College of Engineering Muttathara, Trivandrum, Kerala, India

AbstractThe structures that are man-made are bluff- bodies. During the structural analysis of a tall building, its stability under wind forces is of a major concern. Being a bluff body, the aerodynamic study plays a critical role in the determination of the principal response of a high-rise building to wind forces. Tall buildings can be susceptible to excessive motion during wind events that can cause occupant discomfort and reduce the overall appeal of the structure. Furthermore, these excessive motions can create high base loads, which can increase the cost of the structure. Structural engineers generally opt for optimizing the structural system or increasing modal mass to reduce wind-induced motion. This paper presents an overview of a proposed tall building design platform that has the aim of determining the significance of aerodynamic modifications against wind excitation, which include modifications of buildings cross-sectional shape and its corner geometry, sculptured building tops, vertical openings through-building.

Keywords Bluff body aerodynamics, shape effects, Finite Volume Analysis, Drag Coefficients, ANSYS, Computational Fluid Dynamics (CFD), Shape optimization of building

,Aerodynamic Modifications, Corner modification, Tapered building, Cross Section (C.S)

1. INTRODUCTION

   Nowadays the national development depicts the presence of tall high rise building all around the nation as a pride factor and a showcase to the prosperity of the country. Tall buildings, which are usually designed for office or commercial use, are among the most distinguished space definitions in the architectural history of American urbanism in the twentieth century. They are primarily a reaction to the rapid growth of the urban population and the demand by business activities to be as close to each other as possible. Every architects imaginative reinterpretations of the building type, the inadequacy and high cost of land in urban areas, the need to preserve significant agricultural production, the concept of skyline, cultural significance and prestige, have all contributed to force buildings upward.

   Today, it is virtually impossible to imagine a major city without tall buildings. Tall buildings are the most famous landmarks of cities, symbols of power, dominance of human ingenuity over natural world, confidence in technology and a mark of national pride; and besides these, the importance of tall buildings in the contemporary urban development is without doubt ever increasing despite their several undeniable negative effects on the quality of urban life

   Elsa Joby Structural Engineer

   Space case Architects and Designers, Erattupetta Pala, Kerala, India

   The tall buildings are designed primarily to serve the needs of the occupancy, and, in addition to the satisfied structural safety, one of the dominant design requirements is to meet the necessary standards for the comfort of the building users and the serviceability. In this context, since wind can create excessive building motion, the dynamic nature of wind is a critical issue, negatively affecting occupancy comfort and serviceability. Moreover, the human response to building motion is a very complicated phenomenon concerning both physiological and psychological features. Furthermore, excessive building motion can, create noise and crack partitions, damage non- structural elements such as curtain walls, cause glasses to break, reduce fatigue life, malfunction of the elevators and equipment, and result in structural damages or even collapse. Different design methods and modifications are possible, ranging from alternative structural systems to the addition of damping systems in order to ensure the functional performance of flexible structures and control the wind induced motion of tall buildings

   An extremely important and effective design approach among these methods is aerodynamic modifications in architecture, including, all possible modifications of buildings cross-sectional shape and its corner geometry, sculptured building tops, and horizontal and vertical openings through building. By changing the flow pattern around the building, aerodynamic modifications in building shape i.e., an appropriate choice of building form, could moderate wind responses when compared to original building shape.

2. AERODYNAMIC MODIFICATIONS OF TALL BUILDING

   The wind is the most powerful and unpredictable force affecting tall buildings. Tall building can be defined as a mast anchored in the ground, bending and swaying in the wind. This movement, known as wind drift, should be kept within acceptable limits. Moreover, for a well-designed tall building, the wind drift should not surpass the height of the building divided by 500. Wind loads on buildings increase considerably with the increase in building heights. Furthermore, the speed of wind increases with height, and the wind pressures increase as the square of the wind speed. Thus, wind effects on a tall building are compounded as its height increases. Besides this, with innovations in architectural treatment, increase in the strengths of materials, and advances in methods of analysis, tall building

   have become more efficient and lighter, and so, more vulnerable to deflection, and even to swaying under wind loading.

   Despite all the engineering sophistication performed with computers, wind is still a complex phenomenon, mainly owing to two major problems. Unlike dead loads and live loads, wind loads change rapidly and even abruptly, creating effects much larger than when the same loads were applied gradually, and that they limit building accelerations below human perception. Although the true complexity of the wind and the acceptable human tolerance to it have just begun to be understood, there is still a need to understand more the nature of wind and its interaction with a tall building, with particular reference to allowable defections and comfort of occupants.

   The wind induced motion of a tall building can be controlled either by reducing the wind loads or by reducing the response. A proper selection of building shape and architectural modifications can result in the reduction of motion by altering the flow pattern around the building. A building can be designed with smooth lines and curves so that it, like a plane, is highly aerodynamic, and that the wind will just move smoothly over it, without pushing too much. Therefore, aerodynamic modifications, including modifications of cross-sectional shape of the building, its corner geometry, sculptured building tops, openings through building are also an extremely important and effective design tool to mitigate wind induced motion

   III MODELLING

   Finite volume method is considered to be the best tool for analyzing the structures recently many softwares uses this method for analyzing and designing. The most popular one is ANSYS (FLUENT) software. The physical aspects of fluid flow are governed by the conservation principles of mass, momentum and energy. Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. The fundamental basis of any CFD problem is the Navier- Stokes Equation, which define any single phase fluid flow.

   The cross-section area of each of the buildings considered were same however, the length and width of the model was varied. The ful scale dimensions and coordinate origins for each of the model are shown in Figure 1. The height of the buildings was taken to be 150 m at full scale. The dimensions shown in Figure 1 were chosen to minimize the geometric correction factors and provide the same cross- sectional area for each model. The north arrow, in the center of Figure 1, can be used to identify the wind direction.

   Fig.1. Dimensions of basic building models

   The domain constructed around the building model is of rectangular shape. It has 150 m length in the upstream direction and a 250 m in downstream direction. The domain has 150 m width sideways and a height of 300 m from the base of the building model. The wind velocity applied is 165 kmph.

   The boundary condition applied for the inlet of the domain is velocity inlet and for outlet is out flow condition. All other four faces of the rectangular domain are assigned a wall condition.

   IV. AERODYNAMIC MODIFICATIONS OF RECTANGULAR SHAPED BUILDING AGAINST WIND

   EXCITATION

   An appropriate choice of building shape and architectural modifications are also extremely mportant and effective design approaches to reduce wind induced motion by altering the flow pattern around the building. These aerodynamic modifications can be classified into three main groups:

   • Modifications to building shape including effect of tapered cross section, setback and sculptured top, and efficient building shapes;

   • Modifications to corner geometry;

   • Addition of openings.

Since the analysis of basic cross section shapes shows that building with rectangular shape has the highest drag, further analysis is concentrated on the building with rectangular C.S. All the building models with aerodynamic modification considered for further analysis has a height of 150 m. The CFD analysis of modified shapes are conducted at 46.6 m/s (165 kmph).

1. Building with Chamfered Corner

   C.S Dimension Length is 60.60 m, Breadth is 20.2 m with Corner chamfered at a distance of 5 m from edge at 45 deg angle.

   Fig. 2 Plan and isometric view of rectangular model with chamfered corner

   Fig. 3 Pressure Contour front view of rectangular model with chamfered corner

2. Building With Rounded Corner

   C.S Dimension Length is 60.60 m, breadth is 20.2 m with corner rounded at a radius of 5 m

   Fig. 4 Plan and isometric view of rectangular model with rounded corner

   Fig. 5 Pressure Contour front view of model with rounded corner

3. Building with Tapered faces

   C.S Dimension Length is 60.60 m (at base), breadth is

   20.2 m (at base), tapering angle 2 degrees in front and rear faces and 6 degrees on other two faces

   Fig. 6 Plan and isometric view tapered rectangular model

   Fig. 7 Pressure Contour front view tapered rectangular model

4. Building with Stepped Cross section

   C.S Dimension For Length 60.60 m, breadth is 20.2 m (at base and up to 50 m height), Length is 45.45 m with breadth is 15.15 m (from 50 m to 100 m height) and Length is 30.30 m with breadth is 10.1 m (from 100 m to 150 m height)

   Fig. 8 Plan and isometric view of Stepped rectangular model

   Fig. 9 Pressure Contour front view of Stepped rectangular model

5. Building With Single Step at Corner

   C.S Dimension Length is 60.60 m, breadth is 20.2 m and has a single step of 2.5 m × 2.5 m on vertical edges

   Fig. 10 Plan and isometric view of rectangular model with Single Step at Corner

   Fig. 11Pressure Contour front view of rectangular model with Single Step at Corner

6. Building With Two Steps at Corner

   C.S Dimension Length is 60.60 m, breadth is 20.2 m, and Corner has two steps of 2.5 m × 2.5 m dimensions

   Fig. 12 Plan and isometric view of two steps at corner rectangular model

   Fig. 13 Pressure Contour front view of rectangular model with two steps at Corner

7. Building With Opening

C.S Dimension Length is 60.60 m, breadth is 20.2 m, opening of dimension 27 m × 27 m at a height of 95 m from base.

Fig. 14 Plan and isometric view of rectangular model with opening

Fig. 15 Pressure Contour front view of model with opening TABLE 1 Summary of analysis of building models with different

V CONCLUSIONS

An appropriate choice of building shape can result in a significant reduction of aerodynamic forces by changing the flow pattern around the building. This way of treatment can moderate wind responses when compared to the original building shape.

• From the wind engineers point of view, architectural modifications such as setback, tapering and sculptured building tops are very effective design methods of controlling wind excitation and many of the most elegant and notable buildings.

• Architectural modifications to corner geometry, such as chamfered corners, rounded corners, tapered corner can also significantly reduce wind induced response of buildings.

• Addition of openings completely through the building, particularly near the top, is another very useful way of improving the aerodynamic response of that structure against wind.

• From the analysis carried out in the aerodynamic modification of building with rectangular C.S with rounded corners edges have the least drag.

ACKNOWLEDGMENT

First of all, I would like to thank Almighty God, the source of all. Then my parents and my husband whose constant support is with me on my every single step I take in my life. And my good friend, class mate and well-wisher Mrs. Elsa Joby who helped in publishing the journal papers and was at my need each and every time with all her co- operation and valuable suggestions.

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