Modern Approach Towards Controlling the Response of Structures Towards Winds and Earthquakes - Coursework Example

Contents Page No. Introduction 3 Content 4 ification of the modern structural control mechanism 5 Conclusion 9 References 11 Modern approach towards controlling the response of structures towards winds and earthquakes: Introduction: Achieving control over the response of a structure to the stresses induced in it by the large natural forces of earthquakes and wind storms and designing various vibration control techniques are some of the most fundamental areas which are studied in the Structural Dynamics. One of the biggest concerns while designing a multi storey building, a bridge or a skyscraper is to estimate the deformations the structure is likely undergo as a result of taking natural and unpredictable loads brought by high speed windstorms and earthquakes, given the poor performance of tall structures noticed in the past under such circumstances. Mega-structures and skyscrapers have become a fundamental need of the modern world because of the unique advantages they offer in terms of minimized space consumption on the earth and increased vertical accommodation. However, these structures have been known to exhibit unexpectedly large sways especially in the higher stories because of the wind forces and the earthquakes. “In extreme cases, vibration may cause damage to the structure as a result of loosening of connections, brittle fracture of welds, etc”. (Taranath, 2005: 830). Owing to the growing trend of the dependency of a vast majority of population on the high rise buildings all over the world, the aim of much of the research and development made in the past few decades in the field of Civil Engineering has been to gain increased control and command over the dynamic behavior of a structure when it is subjected to any kind of load in general and when it encounters forces because of winds and earthquakes in particular. This paper offers an introduction to a structure’s response to natural forces such as earthquakes and wind forces. Moreover, in this paper, advancements made in the way of exercising increased command over the structures’ response to naturally unpredictable and large forces are unveiled and all foreseeable achievements made to date in this area of Civil Engineering are recognized. Content: Structures constructed in regions prone to large scale earthquakes and windstorms may experience loads equal to their ultimate load-bearing capacity and sometimes even beyond that. In other words, the loads a structure is subjected to in its lifetime may be either insignificant or unexpectedly large. The danger to the integrity of a structure is much pronounced in the latter than in the former intensity of load. Conventionally, the tendency of a structure to resist the earthquakes is enhanced by enlarging the size of the various constituent structural members because of the fact that larger structural members are equipped with added stiffness in comparison to their shorter alternatives. However, the negative aspect of this change is that the larger structural members also add to the total seismic load because of their increased self-weight. Although not capable of doing so completely, increased weight of the enlarged structural components tends to somehow balance out the additional benefits brought by the extra stiffness induced in the structure by the practice. Thus, the little addition achieved in the structural safety is by no means comparable to the huge costs incurred in the process of enlarging the structural members. In addition to that, the conventional way of achieving the structural safety calls for upsizing the structural members which might prove to be a compromise on the utility and aesthetics of the structure. Purposeful buildings like hospitals, schools and halls do not offer room for a modification in the design which may limit their utility in an attempt to enhance their safety. Thus, the need for approaching more promising methods of achieving the structural safety is indispensable. Although it is never possible to actually design a structure that would not develop any stresses in its whole life when loads are applied on it, yet the utmost efforts of the designers should be towards making the structure as stable against the vibrations induced in it by periodic earthquake and wind forces as possible. In order to achieve this without having to limit the structural utility and aesthetics, Civil Engineers have devised some structural control mechanisms that rely on the installation of certain devices, tools, mechanisms or systems in the structure which serve to minimize the vibrations, and hence the deflections caused in it in case of the occurrence of earthquakes and heavy storms. Classification of the modern structural control mechanism: Depending upon the type of device employed in a certain structural control method, these methods are known to offer the following four types of control over the dynamic behavior of a structure: 1. The passive control system. 2. The active control system. 3. The hybrid control system. 4. The semi-active control system. (Li and Huo, 2009). Today, innovative theoretical approaches towards achieving structural stability against vibrations have been put to practical use in the construction industry. These approaches are explained with respect to the above mentioned structural control types as follows: 1. The passive control system: The passive control type of structural control aims at developing an isolated effect so that the structure remains intact against the impacts of geological movements caused as a result of earthquakes. Four basic techniques are employed in this type of structural control mechanism as identified by (Li and Huo, 2009), which are “base isolation, energy dissipation, tuned mass damper and tuned liquid damper”. These techniques are described briefly as follows: 1. Base isolation: Different types of isolators are in use in different parts of the world. For example, in China, five types of isolators, namely the sand layer, slide friction layer, graphite lime mortar layer, rubber bearing and roller are generally used to isolate the structure for achieving safety. (Li and Huo, 2009). The effect of structural isolation is achieved by constructing bearings which result from the effect of rubber sheets bonded with the steel. The rubber bearing serves to elongate the period of vibration while minimizing its magnitude, and hence, the structure’s response to the earthquakes becomes minimal. 2. Energy dissipation: This method tends to dissipate the excessive vibration with the help of certain dissipaters installed in the structure. Thus, a damping effect is achieved which keeps the structure safe against the effect of earthquakes and windstorms. Friction dampers, viscous fluid dampers, visco-elastic dampers, lead dampers and metallic dampers are some types of energy dissipaters which are currently in use in China. (Li and Huo, 2009). 3. Tuned mass damper: In this method, the damping effect is achieved by hysteresis that is influenced by the frequency which is supplied by the movement of a mass attached with a spring. Construction of a few rubber bearing supported floors on the top of a building serves the purpose. 4. Tuned liquid damper: Chiefly meant for providing the structure with resistance against windstorms, the tuned liquid damper system achieves structural damping using the qualities of viscous liquids as they loose their head, and thus, the energy. The liquid is placed in longitudinal tubes placed on the top of building as shown in the figure below: “Eccentric structure with liquid dampers.”. (Li and Huo, 2009). 2. The active control system: Unlike the passive control system in which the devices are installed anywhere inside the structure, the active control system controls the structural vibrations with the help of external mechanisms or devices. It is also known as the non resonant active variable stiffness (AVS) control system. (Kobori et al, 1993). It monitors the structural stiffness which serves to maintain a non resonant condition, which minimizes the response of the structure towards the earthquakes. The concept of active control systems is based on reducing the structural deformations and sways caused by the vibrations with the help of some external sources that exert forces on the structure in a certain designated manner that counteract the vibrations induced in the structure by the occurrence of such natural phenomena as earthquakes and wind storms. An active control system is composed of a mass that moves along the external face of the higher storeys of a structure within certain limits and inside a certain medium which can be any fluid (air or water). Electric powered hydraulic pistons are one of the most employed means of controlling the structural vibrations. 3. The hybrid control system: The hybrid system of achieving structural control is, by far, the most modern among all vibration control systems. (Smith, 2010). This system is indeed, influenced by both the active structural control system and the passive structural control system. In fact, a hybrid control system makes use of the active control system where there is a room for improvement in the performance of the passive control system and it can also be reversed to lessen the energy requirements of an active control system. It is because of the ability of a hybrid control system to modify the active and passive control systems by adding one’s qualities to the other in order to achieve the ultimate system that it has been identified as the most modern of all the existing structural control systems. Thus the limitations of either of the two systems are obviated. The tuned liquid damper (TLD) or the tuned mass damper (TMD) can be combined with an active control system to form a hybrid control system in which the former is used to monitor small structural responses and the latter serves to limit the increasingly severe structural responses. As a result of the combined effect of the TLD / TMD and the active control system, there occurs a substantial reduction in the energy needed to drive the hybrid structural control system. Therefore, this system is considered to be highly reliable and is widely employed in buildings subjected to large seismic forces throughout the world. An example of the use of hybrid control system is in the 610 meters tall Guangzhou TV and Sightseeing Tower in China with a flexible structure in which the TMD hybrid structural control system based on two water tanks has been employed to limit the vibrations induced in the tower by the earthquakes and the wind forces. (Li and Huo, 2009). 4. The semi-active control system: It is one of the most distinguishing characteristic features of a semi-active structural control system that it requires considerably lesser external energy to be applied to function in comparison to all other structural control mechanisms in general and the active control system in particular. This system is easier to operate and its ability to control the deflections in a structure outshines that of an ordinary passive control system. Some semi-active control tools are variable stiffness devices, controllable friction devices, orifice fluid dampers and semi-active tuned liquid dampers. (Li and Huo, 2009). Conclusion: The chief objective of anti-seismic development of structures is to achieve as much of the structural safety as possible since it is not possible for the constructors to make a building completely damage proof. In spite of the fact that all structures undergo certain deflections in their lifetime irrespective of whether or not they experience an earthquake, measures can be taken to cause noticeable reduction in the amount of deflection they undergo to the extent that they can be considered safe. In the event of occurrence of an earthquake, the geological forces induce shear stresses in the structure that serve to weaken the structure’s supports. Hence, the need to make the structure stronger and shock proof is vital. Excessive vibrations caused by wind storms come as a challenge in designing a skyscraper. (Chai and Feng, 1997). Traditionally, this is achieved by enlarging the structural components which may come as a compromise on the internal room and utility of the structure. A generic approach towards achieving structural safety is to provide it with sufficient reinforcement so that its overall stiffness and stability is enhanced. There are four fundamental modern structural control techniques, namely the active, passive, hybrid and the semi-active control techniques. These structural control techniques rely on the application of certain internal or external mechanisms or devices that serve to either isolate the structure or create a damping effect to absorb the vibrations and dissipate the excessive energy dangerous for the stability of a structure. The modern approach towards controlling the structural vibrations provides a cost effective solution for the dangers brought to the structure by the occurrence of such natural phenomena as earthquakes and wind storms. References: Chai, W. and Feng, M. Q., 1997. “Vibration control of super tall buildings subjected to wind loads”. Elsevier Science Ltd. doi:10.1016/S0020-7462(96)00094-7. [Accessed: 12 July 2010]. Kobori, T., Takahashi, M., Nasu, T., Niwa, N. and Ogasawara, K., 1993. “Seismic response controlled structure with Active Variable Stiffness system”. doi: 10.1002/eqe.4290221102. [Accessed: 12 July 2010]. Li, H. and Huo, L., 2009. “Advances in Structural Control in Civil Engineering in China”. Available at: http://www.hindawi.com/journals/mpe/2010/936081.html. [Accessed: 12 July 2010]. Smith, B., 2010. “The principles of anti-seismic building”. Available at: http://www.helium.com/items/268285-the-principles-of-anti-seismic-building. [Accessed: 12 July 2010]. Taranath, B. S., 2005. “Wind and earthquake resistant buildings: structural analysis and design”. USA: Marcel Dekker. Available at: http://books.google.com.pk/books?id=mHofQq9M2g4C&pg=PA830&lpg=PA830&dq=structures+vibration+deflection+winds+earthquakes&source=bl&ots=XBgenpXKhs&sig=G8a_Uzb8X4IlE0VkI9-WbbtMEtg&hl=en&ei=ZAZCTMPtOoqgvgOOgrmBDQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CCcQ6AEwAw#v=onepage&q&f=false. 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