• Earthquakes and Structures
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• v.4 no.4
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• pp.397-428
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• 2013

This paper examines whether the "effective period" of bilinear isolation systems, as defined invariably in most current design codes, expresses in reality the period of vibration that appears in the horizontal axis of the design response spectrum. Starting with the free vibration response, the study proceeds with a comprehensive parametric analysis of the forced vibration response of a wide collection of bilinear isolation systems subjected to pulse and seismic excitations. The study employs Fourier and Wavelet analysis together with a powerful time domain identification method for linear systems known as the Prediction Error Method. When the response history of the bilinear system exhibits a coherent oscillatory trace with a narrow frequency band as in the case of free vibration or forced vibration response from most pulselike excitations, the paper shows that the "effective period" = $T_{eff}$ of the bilinear isolation system is a dependable estimate of its vibration period; nevertheless, the period associated with the second slope of the bilinear system = $T_2$ is an even better approximation regardless the value of the dimensionless strength,$Q/(K_2u_y)=1/{\alpha}-1$, of the system. As the frequency content of the excitation widens and the intensity of the acceleration response history fluctuates more randomly, the paper reveals that the computed vibration period of the systems exhibits appreciably scattering from the computed mean value. This suggests that for several earthquake excitations the mild nonlinearities of the bilinear isolation system dominate the response and the expectation of the design codes to identify a "linear" vibration period has a marginal engineering merit.

• Smart Structures and Systems
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• v.19 no.4
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• pp.351-360
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• 2017

Protection of structures against natural hazards such as earthquakes has always been a major concern. Semi-active control combines the reliability of passive control and versatility and adaptability of active control. So it has recently become a preferred control method. This paper proposes an algorithm based on Uniform Deformation Theory to mitigate vulnerable buildings using magneto-rheological (MR) damper. Due to the successful performance of fuzzy logic in control of systems and its simplicity and intrinsically robustness, it is used here to regulate MR dampers. The particle swarm optimization (PSO) algorithm is also used as an adaptive method to develop a fuzzy control algorithm that is able to create uniform inter-story drifts. Results show that the proposed algorithm exhibited a desirable performance in reducing both linear and nonlinear seismic responses of structures. Performance of the presented method is indicated in compare with passive-on and passive-off control algorithms.

• Smart Structures and Systems
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• v.24 no.1
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• pp.37-52
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• 2019

Despite its success and wide application, base isolation system has been challenged for its passive nature, i.e., incapable of working with versatile external loadings. This is particularly exaggerated during near-source earthquakes and earthquakes with dominate low-frequency components. To address this issue, many efforts have been explored, including active base isolation system and hybrid base isolation system (with added controllable damping). Active base isolation system requires extra energy input which is not economical and the power supply may not be available during earthquakes. Although with tunable energy dissipation ability, hybrid base isolation systems are not able to alter its fundamental natural frequency to cope with varying external loadings. This paper reports an overview of new adventure with aim to develop adaptive base isolation system with controllable stiffness (thus adaptive natural frequency). With assistance of the feedback control system and the use of smart material technology, the proposed smart base isolation system is able to realize real-time decoupling of external loading and hence provides effective seismic protection against different types of earthquakes.

• Earthquakes and Structures
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• v.10 no.1
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• pp.239-260
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• 2016

Tuned mass dampers (TMDs) have been frequently proposed to mitigate the detrimental effects of dynamic loadings in structural systems. The effectiveness of this protection strategy has been demonstrated for wind-induced vibrations and, to some extent, for seismic loadings. Within this framework, recent numerical studies have shown that beneficial effects can be achieved by placing a linear TMD on the roof of linear elastic structural systems subjected to pulse-like ground motions. Motivated by these positive outcomes, closed-form design formulations have been also proposed to optimize the device's parameters. For structural systems that undergo a near-fault pulse-like ground motion, however, it is unlikely that their dynamic response be linear elastic. Hence, it is very important to understand whether such strategy is effective for inelastic structural systems. In order to provide new useful insights about this issue, the paper presents statistical results obtained from a numerical study conducted for three shear-type hysteretic (softening-type) systems having 4, 8 and 16 stories equipped with a linear elastic TMD. The effectiveness of two design procedures is discussed by examining the performances of the protected systems subjected to 124 natural pulse-like earthquakes.

• Journal of Korean Society of Steel Construction
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• v.22 no.1
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• pp.43-53
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• 2010

Seismic isolation is one of the most widely implemented and accepted seismic protection systems to limit or avoid damages from unforeseeable earthquakes. As an energy absorption device, however, the supplemental lead itself tends to pollute the environment. Consequently, it is predicted that the use of lead would be controlled. Considering the pollution caused by lead, several researchers are interested in the viability of using steel in place of lead. In this study, first, based on the results of a non-linear finite element analysis, the excellent deformation capacity of a very tough steel damper was demonstrated by comparing it with that of the SS400 damper and determining the effects of main parameters (the aspect ratio, thickness, and width) on the deformation capacity. Second, an optimum shape and design equation for a U-shaped damper with an opening based on stress distribution was suggested.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.1
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• pp.426-433
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• 2019

After an earthquake, fire and gas explosions are more likely to cause more casualties in cities with many apartment buildings and large complex buildings. In order to prevent this, seismic design is necessary for the fire protection sprinkler system. However, most systems currently use stainless-steel pipes, although synthetic resin pipes are used in some special places. These materials are susceptible to vibration and earthquakes. This study evaluated the displacement absorption flexibility of polyethylene (PE) and aluminum (Al) multi-layered composite pipes to increase the seismic performance in a vibration environment and during earthquakes. The seismic performance was compared with that of a stainless-steel and PE pipes. The seismic characteristics can be measured by measuring the amount and extent of vibration transmitted by the sprinkler pipe. This method can be used to judge the seismic characteristics to attenuate the vibration during an earthquake. The seismic characteristics of the pipe were verified by comparing the logarithmic attenuation rate to the initial response displacement of the vibration generated by the transverse vibration measurement method.

• Structural Monitoring and Maintenance
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• v.3 no.1
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• pp.51-69
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• 2016

Monitoring the performance and estimating the remaining useful life of aging civil infrastructure in the United States has been identified as a major objective in the civil engineering community. Structural health monitoring has emerged as a central tool to fulfill this objective. This paper presents a review of the major structural monitoring programs that have been recently implemented in the United States, focusing on the integrity and performance assessment of large-scale structural systems. Applications where response data from a monitoring program have been used to detect and correct structural deficiencies are highlighted. These applications include (but are not limited to): i) Post-earthquake damage assessment of buildings and bridges; ii) Monitoring of cables vibration in cable-stayed bridges; iii) Evaluation of the effectiveness of technologies for retrofit and seismic protection, such as base isolation systems; and iv) Structural damage assessment of bridges after impact loads resulting from ship collisions. These and many other applications show that a structural health monitoring program is a powerful tool for structural damage and condition assessment, that can be used as part of a comprehensive decision-making process about possible actions that can be undertaken in a large-scale civil infrastructure system after potentially damaging events.

• Smart Structures and Systems
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• v.4 no.2
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• pp.221-232
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• 2008

This paper represents the final results of a research program sponsored by the European Commission through project WIND-CHIME ($\underline{W}$ide Range Non-$\underline{IN}$trusive $\underline{D}$evices toward $\underline{C}$onservation of $\underline{HI}$storical Monuments in the $\underline{ME}$diterranean Area), in which the possibility of using advanced seismic protection technologies to preserve historical monuments in the Mediterranean area is investigated. In the current research, the dynamic characteristics of two outstanding Mamluk-Style minarets, which similar minarets were reported to experience extensive damage during Dahshur 1992 earthquake, are investigated. The first minaret is the Qusun minaret (1337 A.D, 736 Hijri Date (H.D)) located in El-Suyuti cemetery on the southern side of the Salah El-Din citadel. The minaret is currently separated from the surrounding building and is directly resting on the ground (no vaults underneath). The total height of the minaret is 40.28 meters with a base rectangular shaft of about 5.42 ${\times}$ 5.20 m. The second minaret is the southern minaret of Al-Sultaniya (1340 A.D, 739 H.D). It is located about 30.0 meters from Qusun minaret, and it is now standing alone but it seems that it used to be attached to a huge unidentified structure. The style of the minaret and its size attribute it to the first half of the fourteenth century. The minaret total height is 36.69 meters and has a 4.48 ${\times}$ 4.48 m rectangular base. Field investigations were conducted to obtain: (a) geometrical description of the minarets, (b) material properties of the minarets' stones, and (c) soil conditions at the minarets' location. Ambient vibration tests were performed to determine the modal parameters of the minarets such as natural frequencies and mode shapes. A $1/16^{th}$ scale model of Qusun minaret was constructed at Cairo University Concrete Research Laboratory and tested under free vibration with and without SMA wire dampers. The contribution of SMA wire dampers to the structural damping coefficient was evaluated under different vertical loads and vibration amplitudes. Experimental results were used along with the field investigation data to develop a realistic 3-D finite element model that can be used for seismic risk evaluation of the minarets. Examining the updated finite element models under different seismic excitations indicated the vulnerability of such structures to earthquakes with medium to high a/v ratio. The use of SMA wire dampers was found feasible for reducing the seismic risk for this type of structures.

• Smart Structures and Systems
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• v.21 no.3
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• pp.359-371
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• 2018

Strong seismic events commonly cause large drift and deformation, and functionality failures in the superstructures. One way to prevent functionality failures is to design structures which are ductile and flexible through yielding when subjected to strong ground excitations. By developing forces that assist motion as "negative stiffness forces", yielding can be achieved. In this paper, we adopt the weakening and damping method to achieve a new approach to reduce all of the structural responses by further adjusting damping phase. A semi-active control system is adopted to perform the experiments. In this adaptation, negative stiffness forces through certain devices are used in weakening phase to reduce structural strength. Magneto-rheological (MR) dampers are then added to preserve stability of the structure. To adjust the voltage in MR dampers, an inverse model is employed in the control system to command MR dampers and generate the desired control forces, where a velocity control algorithm produces initial required control force. An extensive numerical study is conducted to evaluate proposed methodology by using the smart base-isolated benchmark building. Totally, nine control systems are examined to study proposed strategy. Based on the numerical results of seven earthquakes, the use of proposed strategy not only reduces base displacements, base accelerations and base shear but also leads to reduction of accelerations and inter story drifts of the superstructure. Numerical results shows that the usage of inverse model produces the desired regulated damping, thus improving the stability of the structure.

• Smart Structures and Systems
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• v.24 no.1
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• pp.53-65
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• 2019

Misaligned wind-wave and seismic loading render offshore wind turbines suffering from excessive bi-directional vibration. However, most of existing research in this field focused on unidirectional vibration mitigation, which is insufficient for research and real application. Based on the authors' previous work (Sun and Jahangiri 2018), the present study uses a three dimensional pendulum tuned mass damper (3d-PTMD) to mitigate the nacelle structural response in the fore-aft and side-side directions under wind, wave and near-fault ground motions. An analytical model of the offshore wind turbine coupled with the 3d-PTMD is established wherein the interaction between the blades and the tower is modelled. Aerodynamic loading is computed using the Blade Element Momentum (BEM) method where the Prandtl's tip loss factor and the Glauert correction are considered. Wave loading is computed using Morison equation in collaboration with the strip theory. Performance of the 3d-PTMD is examined on a National Renewable Energy Lab (NREL) monopile 5 MW baseline wind turbine under misaligned wind-wave and near-fault ground motions. The robustness of the mitigation performance of the 3d-PTMD under system variations is studied. Dual linear TMDs are used for comparison. Research results show that the 3d-PTMD responds more rapidly and provides better mitigation of the bi-directional response caused by misaligned wind, wave and near-fault ground motions. Under system variations, the 3d-PTMD is found to be more robust than the dual linear TMDs to overcome the detuning effect. Moreover, the 3d-PTMD with a mass ratio of 2% can mitigate the short-term fatigue damage of the offshore wind turbine tower by up to 90%.