Sharif Digital Repository

Base-isolated structures are known for their effectiveness in performance improved of structures subjected to earthquake excitations. The limitations on the base displacements of these isolations systems, however, limit the amount of performance improvement that can be practically obtained. In fact, this is among the most important design parameters of base isolation systems. In this study, tuned liquid dampers have been employed to reduce the displacement demands at the isolated bases of structures, while maintaining performance.
For this purpose, structural models including three-, six-, and nine-story buildings are considered. Furthermore, “Housner Model” is employed in order to... 

In this paper, a methodology has been proposed for design of passive energy dissipation systems through active control theory. The passive control system is designed to achieve a structural response as close to that of active control system. It may be needed to reduce the stiffness of the original structure at some locations to reduce the acceleration response. Then, to compensate for the resulting increased interstory drifts, viscous fluid dampers are added to the structure at their optimum locations. The optimal distribution of dampers and the locations with reduced stiffness are obtained using the LQR active control algorithm. It is shown that the response of a properly-designed... 

In this study, an effective method is developed for design of passive control systems using viscous fluid dampers, considering the flexibility of braces used for installation of dampers. Using control design spectra, this approach provides a way to easily determine the optimum properties of the control system without complicated calculations or iterations.In the fist part of this study, it is attempted to find the optimum damping ratio of system by using the input energy spectra of ground motions. First, the relations among the dissipated energy in viscous dampers the strucutre and control system properties and structural response is evaluated.A theorically exact relation is established... 

Since stock markets play an important role in economy, reflect expectation about macroeconomic situation and affect monetary policy effects on macroeconomic variables, are special interest of monetary policymakers. Hence, this dissertation studies relationship between monetary policy and market of equities. We study the effects of monetary policy of federal reserve on New York Stock Market Exchange return at firms level during 1999-2007 in an event study. Results confirm the conventional relationship, which is monetary policy affects firms’ return negatively. However these effects are not same for all firms. We find that market capital as proxy for size of firm and the industry which firm... 

In this study, an active control strategy is used for design of passive energy dissipation systems using viscous fluid dampers and structural stiffness modification. The combination of supplemental damping and modification (usually reduction) of stiffness is used to minimize the two main structural response parameters, namely the maximum inter-story drifts and maximum absolute accelerations. In this manner, both structural and nonstructural damages can be controlled.
While active control systems are expected to improve the structural esponse beyond what can be achieved in passive control, the practical implementation of control systems are mostly of passive type.... 

Real time hybrid simulation is considered as one of the most efficient experimental methods for investigation of the behavior of structures and their components during seismic loading. In pseudo-dynamic approach of hybrid simulation considered in this study, the majority of the structural mass is modelled numerically and inertia forces are calculated in the numerical model.Using this approach, loading can be carried out on a real-time basis, and component scaling requirements will be reduced. As a result, in addition to lower costs, this testing method is sometimes considered to be more realistic compared to shaking table tests. However, several challenges are currently faced in... 

Friction drilling or flow drilling is a method of making holes in metal sheets, in which the material is pushed with the aid of heat from friction. This procedure allows making holes in sheet metal, with a sleeve formed as the hole is created. This sleeve is then threaded and used as the nut, thus eliminating the need for access to the interior faces of hollow steel sections. On the other hand, with the development of advanced manufacturing techniques, larger and thicker hollow structural sections are becoming more popular in mid-rise and multistory structures. onsequently, flow drilling can be considered as a viable alternative for traditional drilling techniques, where the material is... 

Bridges are among the crucial elements of transportation networks, and play an important role in facilitating emergency relief efforts following earthquakes. For this reason, it is emparative to monitor their damages to ensure about their performace in earthquakes. The structure of a bridge is divided into two parts, namely the superstructure and substructure. Due to the relatively high rigidity and strength of superstructures, most of structural damage is usually concentrated in substructures, which may ultimately affect the overall stability of the bridge. Therefore, assessing the level of damage in the substructure is of utmost concern. This research attempts to evaluate the damage in the... 

As more structures are equipped with supplemental energy dissipation devices, the assumption of classical damping becomes less accurate. For this reason, development of a general identification method that is accurate enough for response simulation is emparative for such structures. Modal identification of structures can be carried out using either modal or state space approach. In modal methods (such as wavelet transform method), the damping matrix needs to be identified for response simulation. This is sometimes difficult, and may result in simulation inaccuracies. In contrast, direct identification of damping matrix for simulation is not required in state space approach. Hence, the... 

A methodology is developed for designing optimum passive energy dissipation systems using active control algorithms. A combination of stiffness reduction and increase of damping is utilized to reduce both acceleration and displacement response. In this method, first an active control system is designed using pole assignment and sliding mode control algorithm. Here, the methods to use the actives algorithms aremodified such that the resulting properties can be achieved by a passive control system using viscous fluid dampers. Next, a passive control system is designed to result in the structural properties close to those extracted from the actively controlled system. It is shown that this... 

Considerable research work has been dedicated to the development of structural control systems to improve the performance of structures and reduce the casualties resulting from earthquakes. It is generally accepted that semi-active control systems result in better structural performance compared to what can be achieved using passive systems. However, it has been observed that the majority of the performance improvement obtained from semi-active control systems stems from the reduction of brace flexibility effects, rather than the control algorithm used. In this study, it is attempted to utilize this fact to improve the performance of passive control systems, or develop simple semi-active... 

This study is an investigation of the effects of structural nonlinearities on the performance of different structural vibration control systems, including active control using LQR algorithm and passive control using viscous fluid dampers and hysteretic dampers. By designing the control systems for linear structurs and comparing their performances in nonlinear response range, the abilities of these control systems to handle structural nonlinearities are compared. In other words, the dependency of these control systems on the properties of the structure (which may be altered by nonlinearities or uncertainties in estimations) is assessed. It is demonstrated that as a general trend, the ability... 

By constraining the steel members against lateral buckling, buckling-restrained braces (BRB’s) show similar load-deformation behaviors and energy absorption capacities in both tension and compression. As a result, BRB’s demonstrate significant energy dissipation capacity compared to ordinary braces. On the other hand, the relatively small post-yield stiffness of BRB’s usually leads to significant residual drifts, which may render the structure unusable. In recent decades, significant research has been put into improving the performance of structures equipped with BRB’s. In this project, the current methods to enhance the performance of the BRB’s are explored, and novel approaches are... 

This study presents the advantages of using unsymmetrical buckling restrained braces in zipper frames for simultaneous improvement of peak and residual response of low- and mid-rise buildings. It is demonstrated that such combination allows for the usage of benefits of each of these systems, namely the significant energy dissipation capacity of buckling restrained braces and better distribution of hysteretic energy dissipation demands through the elevation observed in zipper frames. It has already been shown that using buckling restrained braces of varying yield strengths within each story of a building can improve the residual response of buckling-restrained-braced frames without severely... 

The increased need for experimental verification of the seismic performance of conventional and novel structural systems has resulted in highly sophisticated dynamic test procedures. Hybrid simulation is one of the most efficient experimental methods for assessment of dynamic and rate-dependent behavior of large-scale structural systems under earthquake excitation. Compared to earthquake simulations using shake tables, hybrid simulation may have significant advantages in terms of cost, scale, geometry, and required physical mass of structures. However, recent hybrid simulations have been limited to simplified structural models with only a few degrees of freedom. Currently, the major... 

By comparing the control systems, researcherstypically believe that the active systems of control have better performance than passive ones. However, the high costs and the required technology for their erection and maintenance have limited the use of active control systems. In order to improve the control system performance at lower costs, semi-active control systems were also considered. Their performance is based on creating the desired control force by changing the instrument characteristics.Moreover, recent researches have shown that the improved performance of these instruments in structures compared to passive control systems, are typically due to reducing the flexibility of the... 

Application of convolution-based transforms such as Fourier transform and wavelet transform are among the widely-used approaches in structural identification problems. These methods, however, are mostly based on a classical damping model for the structures. Hence, such methods cannot be used to accurately identify the modal properties of non-classically damped systems, such as structures equipped with energy dissipated devices, base isolated and multi-segment structures. In this study, complex-valued wavelet transform is used to identify the dynamic properties of structures with arbitrary damping models. This method is known to be able to identify the modal properties of highly damped...