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CURATOR
A pinboard by
Jingzhe Wu

PhD candidate, University of Maryland

PINBOARD SUMMARY

Economical testing and control approaches to enhance structural seismic performance with SSI effect

Structural seismic behavior can be significantly different when includes the effect of soil structure interaction (SSI), which potentially decreases the effectiveness of seismic control design with traditional fixed-base assumption. Therefore, it is of great necessity to include SSI effect into analysis and design consideration to enhance structural seismic performance. My research focuses on developing economical control approach to enhance structural performance under earthquake excitation with SSI effect. Such approaches include passive or with on-off feature, are targeted for superstructure or foundation soil, based on geometric nonlinearity of elastic buckling mode jump or magnetorheological mechanism. Further, the inherent nonlinear and complex behavior of soil still remains as a challenge for seismic control analysis and design including SSI effect. My research also works on developing a real-time hybrid simulation approach to capture the foundation-soil-structural interaction behavior. With the most complex and nonlinear foundation-soil substructure under physical testing and interface interaction communicate between physical model and superstructure in numerical simulation, it can serve as a more accurate combined testing and analysis tool for seismic performance control design including SSI effect.

7 ITEMS PINNED

Centrifuge modeling to evaluate natural frequency and seismic behavior of offshore wind turbine considering SFSI

Abstract: Understanding of dynamic response of offshore wind turbine is important to reduce vibration of offshore wind turbine induced by structural and environmental loadings. Although dynamic characteristics of the offshore wind turbine such as natural frequency and seismic behavior are affected by foundation and soil conditions, there are little experimental studies about the dynamic behavior of offshore wind turbine with consideration of proper soil–foundation–structure interaction (SFSI). The goal of this research is to evaluate the natural frequency and seismic behavior of offshore wind turbine with a monopod foundation considering SFSI. Scaled model of offshore wind turbine and monopod foundation is produced for this research. Geotechnical centrifuge tests in fixed-based and SFSI condition were performed to measure natural frequency in each case. Also, a series of seismic loadings with different intensities are applied to observe seismic behaviors of the offshore wind turbine during the earthquake and permanent changes after the earthquake. Experimental results show apparent natural frequency reduction in SFSI condition compared with the fixed-based condition, non-linear changes in dynamic response during a series of earthquakes and permanent changes occurred in natural frequency and rotational displacement after earthquakes. Copyright © 2017 John Wiley & Sons, Ltd.

Pub.: 28 Jun '17, Pinned: 05 Jul '17

Hybrid simulation theory for a classical nonlinear dynamical system

Abstract: Hybrid simulation is an experimental and computational technique which allows one to study the time evolution of a system by physically testing a subset of it while the remainder is represented by a numerical model that is attached to the physical portion via sensors and actuators. The technique allows one to study large or complicated mechanical systems while only requiring a subset of the complete system to be present in the laboratory. This results in vast cost savings as well as the ability to study systems that simply can not be tested due to scale. However, the errors that arise from splitting the system in two requires careful attention, if a valid simulation is to be guaranteed. To date, efforts to understand the theoretical limitations of hybrid simulation have been restricted to linear dynamical systems. In this work we consider the behavior of hybrid simulation when applied to nonlinear dynamical systems. As a model problem, we focus on the damped, harmonically-driven nonlinear pendulum. This system offers complex nonlinear characteristics, in particular periodic and chaotic motions. We are able to show that the application of hybrid simulation to nonlinear systems requires a careful understanding of what one expects from such an experiment. In particular, when system response is chaotic we advocate the need for the use of multiple metrics to characterize the difference between two chaotic systems via Lyapunov exponents and Lyapunov dimensions, as well as correlation exponents. When system response is periodic we advocate the use of L2 norms. Further, we are able to show that hybrid simulation can falsely predict chaotic or periodic response when the true system has the opposite characteristic. In certain cases, we are able to show that control system parameters can mitigate this issue.

Pub.: 28 Dec '16, Pinned: 05 Jul '17

Rocking damage-free steel column base with friction devices: design procedure and numerical evaluation

Abstract: Earthquake-resilient steel frames, such as self-centering frames or frames with passive energy dissipation devices, have been extensively studied during the past decade, but little attention has been paid to their column bases. The paper presents a rocking damage-free steel column base, which uses post-tensioned high-strength steel bars to control rocking behavior and friction devices to dissipate seismic energy. Contrary to conventional steel column bases, the rocking column base exhibits monotonic and cyclic moment–rotation behaviors that are easily described using simple analytical equations. Analytical equations are provided for different cases including structural limit states that involve yielding or loss of post-tensioning in the post-tensioned bars. A step-by-step design procedure is presented, which ensures damage-free behavior, self-centering capability, and adequate energy dissipation capacity for a predefined target rotation. A 3D nonlinear finite element (FE) model of the column base is developed in abaqus. The results of the FE simulations validate the accuracy of the moment–rotation analytical equations and demonstrate the efficiency of the design procedure. Moreover, a simplified model for the column base is developed in OpenSees. Comparisons among the OpenSees and abaqus models demonstrate the efficiency of the former and its adequacy to be used in nonlinear dynamic analysis. A prototype steel building is designed as a self-centering moment-resisting frame with conventional or rocking column bases. Nonlinear dynamic analyses show that the rocking column base fully protects the first story columns from yielding and eliminates the first story residual drift without any detrimental effect on peak interstory drifts. The study focuses on the 2D rocking motion and, thus, ignores 3D rocking effects such as biaxial bending deformations in the friction devices. The FE models, the analytical equations, and the design procedure will be updated and validated to cover 3D rocking motion effects after forthcoming experimental tests on the column base. Copyright © 2017 John Wiley & Sons, Ltd.

Pub.: 02 May '17, Pinned: 05 Jul '17

Theory and implementation of switch-based hybrid simulation technology for earthquake engineering applications

Abstract: Hybrid simulation (HS) is a novel technique to combine analytical and experimental sub-assemblies to examine the dynamic responses of a structure during an earthquake shaking. Traditionally, HS uses displacement-based control where the finite element program calculates trial displacements and applies them to both the analytical and experimental sub-assemblies. Displacement-based HS (DHS) has been proven to work well for most structural sub-assemblies. However, for specimens with high stiffness, traditional DHS does not work because it is difficult to precisely control hydraulic actuators in small displacement. A small control error in displacement will result in large force response fluctuations for stiff specimens. This paper resolves this challenge by proposing a force-based HS (FHS) algorithm that directly calculates trial forces instead of trial displacements. The proposed FHS is finite element based and applicable to both linear and nonlinear systems. For specimens with drastic changes in stiffness, such as yielding, a switch-based HS (SHS) algorithm is proposed. A stiffness-based switching criterion between the DHS and FHS algorithms is presented in this paper. All the developed algorithms are applied to a simple one-story one-bay concentrically braced moment frame. The result shows that SHS outperforms DHS and FHS. SHS is then utilized to validate the seismic performance of an innovative earthquake resilient fused structure. The result shows that SHS works in switching between the DHS and FHS modes for a highly nonlinear and highly indeterminate structural system. Copyright © 2017 John Wiley & Sons, Ltd.

Pub.: 15 Jun '17, Pinned: 05 Jul '17

Effects of nonlinear soil–structure interaction on the seismic response of structure-TMD systems subjected to near-field earthquakes

Abstract: Abstract The present investigation illustrates the performance of structure-Tuned Mass Damper (TMD) system to suppress the excessive vibration of structure buildings subjected to near-field ground motions involving the nonlinear effects of three dimensional soil–structure interaction (SSI). Accordingly, three medium-to-high-rise controlled structures based on a shallow mat foundation located on soft to very dense soil are examined. The ground motion database compiled for nonlinear time history (NTH) analyses of the soil–structure-TMD systems consists of an ensemble of 52 near-field ground motions. Comparisons are made in terms of maximum inter-story drift ratio as well as maximum inter-story acceleration ratio for the three possible conditions of the foundation: fixed-base structure, linear SSI (LSSI) and nonlinear SSI (NLSSI). The seismic responses of building structures are studied under the variation of key parameters such as peak ground velocity, factor of safety against vertical load bearing of the foundation (FS), non-dimensional frequency (a 0), ground motion characteristic and number of stories. On the one hand, the results indicate that the nonlinear effects of SSI significantly modify the structural responses in comparison with the LSSI counterpart. On the other hand, soil failure decreases the effectiveness of TMD. In a more precise view, it can be demonstrated that installing TMD can suppress the response of structures with linear SSI and without SSI (fixed-base structure) more significantly than that of structures considering NLSSI. Consequently, the responses would generally be underestimated if a linear behavior of the soil is assumed.AbstractThe present investigation illustrates the performance of structure-Tuned Mass Damper (TMD) system to suppress the excessive vibration of structure buildings subjected to near-field ground motions involving the nonlinear effects of three dimensional soil–structure interaction (SSI). Accordingly, three medium-to-high-rise controlled structures based on a shallow mat foundation located on soft to very dense soil are examined. The ground motion database compiled for nonlinear time history (NTH) analyses of the soil–structure-TMD systems consists of an ensemble of 52 near-field ground motions. Comparisons are made in terms of maximum inter-story drift ratio as well as maximum inter-story acceleration ratio for the three possible conditions of the foundation: fixed-base structure, linear SSI (LSSI) and nonlinear SSI (NLSSI). The seismic responses of building structures are studied under the variation of key parameters such as peak ground velocity, factor of safety against vertical load bearing of the foundation (FS), non-dimensional frequency (a 0), ground motion characteristic and number of stories. On the one hand, the results indicate that the nonlinear effects of SSI significantly modify the structural responses in comparison with the LSSI counterpart. On the other hand, soil failure decreases the effectiveness of TMD. In a more precise view, it can be demonstrated that installing TMD can suppress the response of structures with linear SSI and without SSI (fixed-base structure) more significantly than that of structures considering NLSSI. Consequently, the responses would generally be underestimated if a linear behavior of the soil is assumed.Sa0

Pub.: 01 Jan '17, Pinned: 05 Jul '17