Wind Load Effects on Skyscrapers - Essay Example

?TUNED MASS DAMPER IN SKYSCRAPER Department of Civil, Architectural and Environmental Engineering Advisor: The construction of skyscrapers involves numerous considerations for loading effects upon the building. While many sources of loading exist, the most prevalent source remains wind loading, which results from continued presence of wind. Tuned mass dampers are devices utilised by engineers and architects to ensure minimisation of the adverse effects of wind loading. These devices are utilised in high-rise buildings reaching above 50 storeys. The TMDs enhance strength of the building while increasing comfort of residents. The Taipei 101 structure remains the building with the largest TMD weighing over six tons. This massive structure was constructed using advanced technology providing a significant breakthrough for the engineering industry. The massive structure has contributed towards the capacity for the building to withstand numerous typhoons, common in Asia. TABLE OF CONTENTS ABSTRACT 2 TABLE OF CONTENTS 3 TABLE OF FIGURES 4 INTRODUCTION 4 WIND AND TUNED MASS DAMPER 5 THE TAIPEI 101 TUNED MASS DAMPER 8 Operation principles of the damper 11 Tuning frame 12 Steel cables 12 Primary Hydraulic Viscous Damper 13 Bumper System 13 Structural design for the TMD 13 Installation & safety 14 Potential mechanical problems of the TMD 15 Manufacturing of the TMD 15 CONCLUSION 15 REFERENCES 17 TABLE OF FIGURES Figure 1 tuned mass damper of the Taipei 101 9 Figure 2 Location of the Taipei 101's tuned mass damper(Kourakis, 2007) 10 Figure 3 simple tuned mass damper 11 Figure 4 engineers installing the mass damper atop the building 14 INTRODUCTION In many modern cities around the world, skyscrapers have continued to dominate the skylines. The construction of these structures continues to define the element of modernisation and evolvement of modern city designs. These magnificent structures decorating the skylines of major cities require a considerable amount of planning before commencing construction. The planning for construction of skyscrapers begins with consideration of various elements among them cost, economic efficiency and strength of the structure(Chew, 2001). The element of strength remains most important as it relates to safety of the tenants and other occupants of the building. The strength of a skyscraper remains the determinant element for the successful construction of these high rise structures. These structures remain highly susceptible to natural calamities like earth quakes and hurricanes, among others. During the planning phase for the construction of these high rise structures, those designing components seek to ensure the structure becomes sufficiently resistant to any forces that might arise. The consideration of these wind forces makes the designing phase most important. The forces exerted by winds on the structures remain fundamental for designing of high-rise structures. WIND AND TUNED MASS DAMPER Wind remains a complex phenomenon because of the variations in wind speeds resulting from interaction with the structures surrounding the wind. Wind comprises of numerous particles moving in different directions. The speed and direction of wind contributes immensely towards the effects produced by the wind(Holmes, 2007). During construction of tall structures, engineers and architects must consider the expected wind effects upon the intended structure. While normal wind force could be resisted by many structures, extreme cases like tornadoes, hurricanes present great challenges as they create massive forces, capable of collapsing entire buildings. The resulting effects of wind damage can cause massive destruction as have been recorded. Hurricanes, for example, are some of the most feared winds across the globe. Some hurricanes have enough strength to even uproot trees and destroy even the shortest of structures. During these extreme wind cases, like winds measuring 10 or higher on the Beaufort scale, the wind load affecting tall buildings is normally higher than that affecting the short buildings. The effects of wind load on skyscrapers result in continuous vibrations movements and continuous vibrations among many skyscrapers. This effect becomes effectively reduced through the introduction of a harmonic absorber called the tuned mass damper. This device is commonly mounted on almost all buildings above 50 storeys tall –skyscrapers, to reduce the loading effects of blowing winds. The utilization of this device provides the structures with protection from strong winds and even natural calamities like earthquakes and hurricanes. The application of tuned mass dampers remains essential in ensuring comfort while within the skyscraper, preventing damage and structural failure. The tuned mass damper acts to stabilize violent motions of structures resulting from harmonic vibrations caused by movements around a structure. The prevention of these movements remains essential in eliminating the possibility of structural failure and ultimate destruction(Zhihong, 1994). The device reduces system vibrations through application of a relatively lightweight component, ensuring vibrations become less intense. Consequently the effects of these scenarios become minimized to a non-destructive level. The application of tuned mass dampers has enabled construction of extremely tall structure in different parts of the world. The tuned mass damper systems commonly work in two different ways. They can either move the structure away from troubling frequency or add damping to difficult resonance. TUNED MASS DAMPERS IN SKYSCRAPERS Almost all skyscrapers are constructed with tuned mass dampers located inside the structures(Zhou, Kijewski, & Kareem, 2002). Typical tuned mass dampers for skyscrapers consist of huge concrete or steel blocks, commonly mounted within the structures. These blocks are moved in opposite direction of the structural resonance frequency by means of springs, pendulums or fluids. These devices enable buildings to withstand unwanted vibrations, resulting from external forces acting upon the structures(Chien-Liang, Chen, Chung, & Wang, 2006). The main danger of these external forces remains the resulting resonance which could be unpleasant, inconveniencing and sometimes destructive. Many natural disasters cause resonance and oscillation of buildings in different directions, and this ultimately results in destruction through collapsing. These effects and resulting oscillations are the major reasons for collapsing of buildings during instances of earthquakes and extreme winds(Ungerer, 1998). Tuned mass dampers could be identified as important components of skyscrapers which have a fundamental protective function of the structures. The effects of flowing wind upon skyscrapers can result in movement of the building in a distance of more than a meter(Roffel, Lourenco, Narasimhan, & Yarusevych, 2011). This movement commonly occur in form of sway or twisting. The resulting effects of the movement could present destructive results upon skyscrapers. Aerodynamic properties of skyscrapers could possibly accentuate the movement of the building causing motion sickness and discomfort among individuals within the building(Haskett, Breukelman, Robinson, & Kottelenberg, 2004). A TMD remains tuned to desired building frequencies to provide protection against such wind effects. Natural frequency changes, resulting from wind speed changes, necessitate robust TMD design. These designs enable the structure to withstand winds from different direction and having different strengths. Horizontal movements of skyscrapers, resulting from external forces, could cause destruction of the building. While these movements are inevitable owing to the effects of external forces, appropriate methods must be applied in construction of skyscrapers to eliminate the possible effects of the movements. TMDs installed within a building act to reduce the movement to a safe distance. Through the damping effect of the TMD, the movement is relatively reduced eliminating the resulting destructive effects. Mass dampers act to reduce horizontal movement ultimately providing protection against damage which might result from these movements Another source of loading upon skyscrapers is Seismic waves caused by earthquakes. Earthquakes cause seismic oscillations and vibrations upon the surface holding the buildings resulting in unwanted movements. Technological advancements have enhanced the seismic performance on buildings through utilisation of TMD. Vibration control technologies are commonly employed in making assessments regarding structural seismic performance(Kourakis, 2007). TMD are utilized in ensuring a buildings seismic performance can withstand a considerable vibration from earthquake. Through damping devices have been utilized in construction for almost a century, damping devices for earthquake related effects remain a relatively new technology. The TMD installed within a structure absorbs the earthquake oscillations and minimises their effects to a defined level, which the structure can withstand. The sustained oscillation remains similar to that experienced during wind loading. THE TAIPEI 101 TUNED MASS DAMPER The Taipei 101remains one of the most popular skyscrapers located in Xinyi District in the Republic of China. The building was the world’s tallest building since 2004 until the opening on Burj Khalifa in 2010. The building consists of 101 floors above the ground floor and five floor underground. The major striking element of the building remains the structural design, designed to withstand typhoons and earthquakes. The structural design of the building enhances strength through flexibility in strong winds and rigid structures preventing lateral drift. The height and environmental conditions, including winds exceeding speeds of 36km/hr, in Taiwan required increased innovation in designing of the building. Strength and flexibility have been achieved through utilization of high performance steel in constructing the skyscraper(Li et al., 2010). Figure 1 tuned mass damper of the Taipei 101 The building contains the world’s heaviest and largest TMD, which weighs approximately 660 metric tons(Kourakis, 2007). The mass block consists of 41 steel layers, each plate being approximately 12.5 centimetres thick, all closely welded to form the 5.5 metre diameter sphere. The device is a spherical shaped massive steel block suspended by eight steel cables from the 92nd floor of the building. Each of these cables holding the block, have been designed to enhance durability and flexibility over time(Fan, Li, Tuan, & Xu, 2009). The TMD is uniquely open for public viewing on 88th and 89th floors of the building. This amazing device continues to attract tourists and many other persons who can take photos of the massive spherical steel structure hanging atop the building. The figure below is a cross-section indicating the location of the damper on the building Figure 2 Location of the Taipei 101's tuned mass damper(Kourakis, 2007) The tuned mass damper is specifically designed to withstand heavy typhoons and earth quakes, which remain common occurrences within the region. The damper normally performs its functions through reducing vibration of the building by about 40%(Fan et al., 2009). This vibration reduction is accompanied by an oscillation movement by the damper. Under normal circumstances, the damper moves at a speed of one oscillation every seven second, making a movement of up to 35 centimetres. These oscillation movements remain invisible as the distance remains relatively short for a massive object the size of the spherical damper unit. During typhoons and earth quakes, the damper can move 150 centimetres. This movement is mitigated by the bumper system specifically designed to mitigate massive movement of the tuned mass damper. Operation principles of the damper The tuned mass damper can be defined as vibration absorber utilised in reducing high frequency vibrations in structure. Many dampers are commonly set to required operating frequencies, though adjustments can be performed to suit prevailing operating conditions. The tuned mass damper weight remains entirely dependent on the weight of system being damped. The damper operates by receiving resonance vibration, which become broken down into different vibration. The peaks of the vibrations then become eliminated, leaving only vibrations with small deflective characteristics. Structures commonly develop considerable amplitude vibration resulting from loads acting close to resonance frequency. The attachment of a secondary mass through a relatively suitable spring is the principle utilised in mitigating the vibrations generated by the loads. The tuning of the spring to ensure optimal reduction remains the fundamental feature of tuned mass dampers. A simple mass damper can be represented by the figure below. Figure 3 simple tuned mass damper The structural mass is represented by m1, supported by a spring of stiffness k1. The functions m2 and k2 represent the damper mass and the supporting sprig frequency, respectively. C1 and C2 represent the different constants for the air damping effect created following exposure to vibrations. These parameters of the structure are essential in determining the corresponding natural frequency(Datta, 2010). The equation for determining the natural frequency can be presented as indicated below. The resulting frequency also represents the angular frequency for structural vibration, experienced during movements. Tuning frame The tuning frame for the damper is situated on the 91st floor of the building, and performs the essential function of monitoring building vibrations. The tuning frame holds the suspending cables for the large damper mass. The tuning frame adjusts the natural frequency, with adjustable air dampers adjusting damping effect. The adjustment of the natural frequency contributes to relative adjustment in the stiffness of the suspending material. This process ultimately results in consequent adjustment in the structural vibrations(Roffel et al., 2011). The damping process is optimised through the utilisation of adjustable air dampers. Damping compensation within the tuned mass damper is achieved through assistance of an automated microcontroller and a system of stepper motors(Xiudong & Zuo, 2011). These two systems enhance the monitoring of structural vibration and optimise the damping effect of the TMD. Steel cables The tuning frame holds the tuned mass damper through 8 eight large steel cables each measuring 42 metres and 9 centimetres in length and width respectively. The massive cables remain strong enough to suspend the weight of the damper. Each cable comprises of more than 2000 smaller cables. The smaller cables enhance flexibility of the large cables as well as durability. The cables can be maintained through changing the small cables while the damper is still in place. This has continuously enhanced the durability of the suspending cables over time. The strength of the cables remains based on an index score of 9, giving a single cable the capacity to support the entire weight of the TMD(Li et al., 2010). Primary Hydraulic Viscous Damper The TMD consists of 8 primary hydraulic viscous dampers utilised in the absorption and dissipating of vibration impacts experienced by the building. These hydraulic dampers manage the movement of the tuned mass damper by ensuring the equipment does not oscillate beyond required movement. Since they operate on a hydraulic system, they slow down the movement of the mass damper, consequently causing reduction of the vibrations resulting from the movements and oscillations(Ayothiraman & Hazarika, 2008). The hydraulic dampers also serve to provide support for the mass damper and reduce the risk of extreme movement during earthquakes and typhoons. Bumper System Beneath the viscous hydraulic dampers lies a bumper system which offers support for the entire unit. The bumper is circular in shape and is located beneath the entire system. Functions of the bumper remains similar to those of the viscous hydraulic dampers, however, the bumper is not in constant contact with the mass damper. During cases of extreme vibrations, when the mass damper oscillation might exceed 150cm, the bumper ensures absorption of the resulting vibration(Ames, Droessler, & Hoit, 2011). The bumper system also contains a system of 8-hydraulic viscous dampers operating similar to the other dampers. These are however fixed on the circular ring and serve the purpose of absorbing vibrations during extreme conditions(Infanti, Robinson, & Smith, 2008). They provide mass damping support to the other damping system under excessive vibrations. This ensures the entire system functions well during earthquakes and typhoons. The bumper system is specifically designed to enhance damping effect during extreme conditions. Structural design for the TMD The general structure of any TMD consists of a large oscillating mass suspended from a certain distance. The main purpose of the mass remains damping of vibrations resulting from loading effects experienced by tall structures. These loading effects result from numerous elements including the structural weight of the material utilised in construction of the structure. Uncontrolled loading effects, however, cause the greatest danger upon tall structures. These include uncontrollable sources of loading like wind and other natural calamities. The extreme conditions present increased vibrations, which could produce catastrophic effect, like structural failure. The TMD is specifically installed upon these structures to handle such vibrations and enhance structural resistance to such events(Halis & Ilgin, 2007). Installation & safety The mass damper of the building was constructed on site during the construction of the building and it was divided into 41 separate pieces and each piece was lifted by the crane to the 87th floor. All the steel units of the TMD were welded together atop the building. This minimised the risk of lifting the heavy mass for the entire height. The mass block is welded to a steel cradle suspended from the 92nd floor of the building. Figure 4 engineers installing the mass damper atop the building Potential mechanical problems of the TMD The tuned mass damper of the Taipei 101 has been constructed through simple physical and mechanical designs. The TMD can therefore, withstand majority of the resulting extreme conditions in the Asian region. This mechanical principle minimises the possibility of mechanical problems occurring upon the structure. When mechanical problems occur upon the equipment, the effects would be felt upon the occupants of the building. Individuals within the building would experience discomfort resulting from the minimal changes caused by mechanical failure of the TMD. Mechanical problems occurring upon the damper cannot result in destruction of the building as the damper remains suspended and entirely supported by the building’s piles(Li et al., 2010). Among the 8 cables suspending the mass block, each cable has enough strength to support the mass block alone. This ensures that the block cannot fall from its current location. Though it might be impossible for the spherical mass block to fall out of position, human sabotage can cause such failure. The 87th floor has been enhanced to hold more than double the weight of the damper. This would ensure the building cannot collapse following falling of the mass block. Manufacturing of the TMD The TMD was constructed by a Canadian engineering firm called Motioneering Inc, with structural design being developed by Evergreen Consulting Engineering inc. these engineering firms worked in close relationship with other engineering firms involved in construction of different sections of the skyscraper. The overall cost for constructing the tuned mass damper was USD 4.4 million, with the entire building costing $700 million CONCLUSION The construction of skyscrapers has significantly improved the construction and engineering industry. These structure present magnificent technological designs utilised to minimise structural failure. Natural calamities present the largest amount of risk for structural failures among high rise buildings. The 21st century saw surmountable development in the construction of the Taipei 101 building in Taiwan. The building became the first skyscraper to surpass the half kilometre mark in height. The principle behind reaching such immense construction heights remains the capacity to construct structure which can withstand the effects of natural conditions. Wind poses a continuous amount of loading upon the structures creating a challenge for engineers designing skyscrapers. The vibrations produced by seismic waves, however, present an even greater challenge as they are intense than those presented by wind(Fan et al., 2009). The introduction of tuned mass dampers within the buildings ensures they become capable of withstanding prevailing environmental conditions. The Taipei 101 building houses the largest and heaviest TMD among all skyscrapers. The mechanical principles utilised in the installation and construction of the building’s TMD has continuously enabled the building to withstand many typhoons and earthquakes, occurring in Asia and surrounding regions. The building’s TMD performs an essential protective function upon the building, by enhancing resistance to vibrations resulting from activities within the vicinity. Since construction, the building has endured and survived numerous typhoons and earthquakes over time(Fan et al., 2009). Safety of the building occupants remains sufficiently enhanced by the utilisation of a tuned mass damper. The weight of the damper meant that the component had to be constructed partly and assembled atop the building. This is because no crane could lift the 6.6 metric ton mass block to the 87th floor of the building. The TMD remains one of the most magnificent engineering components of the 21st century, offering sufficient support for the entire building(Ames et al., 2011). REFERENCES Ames, D., Droessler, T. L., & Hoit, M. I. (2011). Structures Congress. Reston: American Association of Civil Engineers. Ayothiraman, R., & Hazarika, H. (2008). Earthquake Hazards and Mitigation. New Delhi: I K International. Chew, M. Y. L. (2001). Construction Technology for Tall Buildings (2nd ed.). Singapore: Singapore University Press. Chien-Liang, L., Chen, Y.-T., Chung, L.-L., & Wang, Y.-P. (2006). Optimal design theories and applications of tuned mass dampers. Engineering structures, 28(1), 43–53. Datta, T. K. (2010). Seismic Analysis of Structures. Singapore: Willey. Fan, H., Li, Q. S., Tuan, A. Y., & Xu, and L. (2009). Seismic analysis of the world’s tallest building. Journal of Constructional Steel Research, 65(5), 1206–1215. Halis, G. M., & Ilgin, H. E. (2007). A proposal for the classification of structural systems of tall buildings. Building and environment, 42(7), 2667–2675. Haskett, T., Breukelman, B., Robinson, J., & Kottelenberg, J. (2004). Tuned mass dampers under excessive structural excitation. Toronto. Holmes, J. D. (2007). Wind Loading of Structures (2nd ed.). New York: Taylor & Francis. Infanti, S., Robinson, J., & Smith, R. (2008). Viscous dampers for high-rise buildings. In The 14th World Conference on Earthquake Engineering. Beijing: China. Kourakis, I. (2007). Structural systems and tuned mass dampers of super-tall buildings?: case study of Taipei 101. Boston. Li, Q. S., Zhi, L.-H., Tuan, A. Y., Kao, C.-S., Su, S.-C., & Wu, C.-F. (2010). Dynamic behavior of Taipei 101 tower: field measurement and numerical analysis. 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