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This paper focuses on the ductility demand of self-centering structures with flag-shaped (FS) hysteretic behavior under near-fault (NF) pulse-like ground motions. To this end, the general features ...

The objective of this paper is to propose a novel fragility analysis method pertinent to the combination of the least squares support vector machine (LS-SVM) algorithm and JC method for the vulnerability estimation of deep-water high piers located in reservoirs under near-field ground motions. First, the uncertainties associated with the design parameters are considered and the LS-SVM algorithm is adopted to predict the cross-sectional critical curvature values of each damage state of the example pier and determine the probability distributions. Then, a group of finite element models of the example pier is developed using the OpenSees platform for considering the uncertainties related to the design parameters and near-field seismic waves; the Morrison equation is employed to calculate the added mass to simulate the hydrodynamic pressure. Meanwhile, the dynamical responses of the example pier with seven different water depth conditions are investigated using increment dynamic nonlinear analysis along the longitudinal and transverse directions under seismic excitations. Finally, the fragility functions are derived based on the JC method to consider the probability distributions of the cross-sectional critical curvature values and dynamical responses simultaneously; these functions are applied for assessing the damage probability at different damage states of the example pier. It can be concluded that the hydrodynamic pressure plays an important role in influencing the natural vibration period and mode shape of the deep-water high pier. In the slight, moderate, and extensive damage states, the damage probability of the middle-upper and bottom areas of the example pier is relatively large under longitudinal near-field seismic excitation; nevertheless, only the bottom is easily damaged under transverse near-field seismic excitation.

Vibration measurements have been widely used for structural health monitoring (SHM). Usually, wired sensors are required to attach on the testing structure, which may be arduous, costly and sometimes impossible to install those sensors on the remote and inaccessible part of the structure to be monitored. To overcome the limitations of contact sensors based vibration measurement methods, computer vision and digital image processing based methods have been proposed recently to measure the dynamic displacement of structures. Real-life structure subjected to bi-directional dynamic forces is susceptible to significant out-of-plane movement. Measuring the vibrations of structures under the out-of-plane movements using target-free vision-based methods have not been well studied. This paper proposes a target-free vision-based approach to obtain the vibration displacement and acceleration of structures subjected to out-of-plane movements from minor level excitations. The proposed approach consists of the selection of a region of interest (ROI), key-feature detection and feature extraction, tracking and matching of the features along the entire video, while there is no artificial target attached on the structure. The accuracy of the proposed approach is verified by conducting a number of experimental tests on a reinforced concrete structural column subjected to bi-directional ground motions with peak ground accelerations (PGA) ranging from 0.01 g to 1.0 g. The results obtained by the proposed approach are compared with those measured by using the conventional accelerometer and laser displacement sensor (LDS). It is found that the proposed approach accurately measures the displacement and acceleration time histories of the tested structure. Modal identification is conducted using the measured vibration responses, and natural frequencies can be identified accurately. The results demonstrate that the proposed approach is reliable and accurate to measure the dynamic responses and perform the system modal identification for structural health monitoring.

In current practice the shear load-slip behaviour of bolted steel connections is estimated based on the friction coefficient and bolt preload. The friction coefficient is approximated by the slip factor, which is determined experimentally through testing of standard specimens. A linear load-slip (F-d) behaviour is typically assumed, with an arbitrary initial stiffness depending on the slip load and an assumed slip displacement. However, even for the standard slip factor test specimens, the F-d behaviour in the initial stage is not necessarily linear. To enable the development of more accurate models for bolted steel connections, it is essential to understand the F-d behaviour of standard specimens constructed using AS/NZS 3678-350 steel, known as G350, a common Australian steel with limited data in the current literature. An experimental program was performed to investigate the effect of surface finish on the shear behaviour. In addition, numerical simulation was carried out and the results were compared with the experimental results including the F-d behaviour. An empirical formula was proposed for the initial slip behaviour and a good fit was demonstrated. The improved understanding of the shear behaviour could be applied to the design of connections for modular buildings which provide suitable site assembly tolerances while controlling the possible cumulative slip displacement.

Reynolds-Averaged Navier-Stokes (RANS) method equipped with the shear stress transport ( S S T ) K − ω model is widely used to simulate the vortex-induced vibration (VIV) of elastically-mounted rigid cylinders subjected to fluid flow. Previous studies show that this method is very difficult to capture the maximum response of the cylinder occurring in the upper regime. Moreover, previous numerical studies by using this method mainly focus on the two-dimensional (2-D) simulations. In reality, VIV is a three-dimensional (3-D) phenomenon, the 3-D effect must then be incorporated in the numerical simulations. To improve the accuracy of the RANS method with S S T K − ω , the low-Reynolds numbers ( R e ) correction technique is applied to the numerical model in the present study and the 3-D VIV responses of an elastically-mounted cylinder subjected to the subcritical fluid flow regime are numerically investigated. The two-way coupled Fluid-Structure Interaction (FSI) framework is developed and computational fluid dynamics (CFD) analyses are performed for a range of Reynolds numbers R e = 2000 − 12000 . Numerical results show that the present method can lead to more accurate VIV response estimations compared to the previous numerical studies. Moreover, the formation of the wake vortex shedding modes and its transition are well captured by this method.

Offshore wind turbines (OWTs) are more and more widely used to produce electrical energy nowadays. Besides the constant wind and wave loads, earthquake excitation can be another important vibration source to the OWTs since many OWTs have been/will be constructed in the seismic prone areas. Extensive research works have been carried out to understand the dynamic behaviours of OWTs when they are subjected to multi-hazards (e.g. the simultaneous wind, wave and/or earthquake loadings). However, when seismic excitations are considered, the onshore earthquake records are normally used as inputs in the analyses due to the lack of offshore data and the difficulty in synthesizing the offshore seismic motions. This practice may lead to inaccurate structural response estimations since it is well known that the seawater can significantly suppress the seafloor vertical motions near the P wave resonant frequencies of the seawater layer, which in turn results in the different characteristics of onshore and offshore earthquake recordings. Moreover, the earthquake motions along the pile of OWTs are different from those at the ground surface, i.e. the earthquake motions vary with the soil depth. Recently, a method to stochastically simulate the earthquake ground motions on the offshore site was proposed, in which the influence of seawater layer was considered and the earthquake motions at any soil depth could be obtained. This paper carries out numerical simulations on the dynamic behaviours of OWTs subjected to the combined wind, wave and earthquake loadings, and the depth-varying offshore seismic motions are used as inputs in the analyses. The seismic responses of OWTs obtained from the onshore and offshore earthquake motions are calculated and compared. The influence of depth-varying ground motions on the dynamic responses of OWTs is discussed.

Precast segmental concrete column, as one of the prefabricated structures used to accelerate the construction speed in the urban areas with heavy traffic, has been more and more widely used recently. Current applications of such structures are mainly limited to the low seismicity areas due to the lack of knowledge on its seismic performances. Recently, extensive research efforts have been made to understand the force-displacement relationship of such columns under quasi-static cyclic loading through experimental and numerical investigations. The behaviours of the column under real dynamic earthquake motions were, however, rarely investigated. Moreover, only the uniaxial motion was considered in the limited shake table tests of such columns despite earthquake excitations have three components in reality. This study carries out shake table tests on the seismic performances of precast segmental concrete column, and bidirectional earthquake motions were used as inputs. For comparison, the behaviours of the traditional cast-in-situ monolithic column were also experimentally investigated and compared with the segmental column. Interestingly, the experimental results showed that significant twisting occurred to the segmental column under the bidirectional earthquake motions. To prevent the adverse twisting response, shear keys between the segments are proposed. The effectiveness of the proposed method is demonstrated through numerical simulations after the model is validated by the experimental results.

The undesirable vortex-induced vibration (VIV) may seriously influence the fatigue life and serviceability of bridge structures. It is important to take countermeasures to suppress the adverse VIV of long-span bridges. In the present study, a novel inerter-based system, namely the tuned mass damper inerter (TMDI), is proposed to control the VIV of the main deck of long-span bridges. In this system, an inerter device, which is able to transform the linear motion into the high-speed rotational motion and thus significantly amplifies the physical mass of the system, is incorporated into the conventional tuned mass damper (TMD) system to further improve the performance of TMD. An applicable layout of the TMDI inside the bridge deck is introduced and the governing equations of the structure-TMDI system subjected to VIV are established. The optimization of the TMDI parameters with the consideration of nonlinear aeroelastic effect is derived. The control performance and robustness of the proposed system are investigated through an analytical case study in both the time and frequency domains. It is observed that the TMDI system can obviously reduce the VIV responses of the bridge deck. Moreover, compared to the conventional TMD system, the static stretching of the spring due to gravity and the oscillation amplitude of the mass block in the TMDI system are significantly reduced. These properties make the proposed TMDI system an attractive alternative for the VIV control of long-span bridges.

The composite rigid-frame bridge presented in this study is a new type of structural solution that combines the steel–concrete composite box girder and the concrete-filled double-skin stee...

Offshore wind turbines are vulnerable to external vibration sources such as wind and wave excitations due to the increasing size and flexibility. It is necessary to mitigate the excessive vibration...

The seismic response characteristics of underground structures are very different from those of aboveground structures. Great efforts have been made in investigating the influence factors on the seismic responses of rectangular underground structures, while the theoretical analyses and comprehensive understandings remain insufficient in the current research. In order to shed light on the seismic response characteristics of the shallow buried rectangular underground structure, an extended parametric study, based on a real underground structure, the Daikai subway station in Japan was conducted in the present paper. In particular, a two-dimensional soil-structure system was adopted for the dynamic time-history analysis. The equivalent linear model was used to consider the nonlinear behaviors of the soil elements, and the elastic model was used to simulate the structure elements. A variety of critical factors that may significantly affect the seismic responses, such as: (i) earthquake load condition, (ii) structure inertia effect, (iii) soil inertia effect, (iv) soil-structure relative stiffness, and (v) soil-structure interface properties were comprehensively investigated in this study. The variations of the horizontal relative deformation between the top and bottom slabs of the structure and the internal forces of four representative sections that were most concerned in seismic design were calculated and compared in different cases, and the soil pressures and shear forces around the structure were also systematically discussed. The numerical results indicated that the seismic responses were obviously affected by the soil inertia effect and soil-structure relative stiffness, and more attentions should be paid to these critical factors during the seismic design of the underground structure. The presented results can lead to better understandings on the seismic responses of the shallow buried rectangular underground structure and can provide some improvements to the existing simplified methods and guide the seismic design of the underground structure.

The undesirable motions resulting from wave loading can lead to the long-term fatigue damage or even catastrophic sinking of offshore semi-submersible platforms (SSP). It is therefore by all means necessary to suppress the excessive vibrations of SSP. Many methods have been proposed to mitigate the heave motion of offshore platforms, such as using a fixed heave plate (FHP) to increase the draft and damping of the system, or adopting a tuned heave plate (THP) to form a tuned mass damper (TMD) system. In this paper, a novel inerter-based control system, namely a tuned heave plate inerter (THPI), is proposed for control of SSP heave vibrations. In this system, an inerter device, which can transform the linear motion into the high-speed rotational motion and thus significantly amplifies the physical mass of the system, is added to the THP to further improve the performance of conventional THP. Analytical studies are performed to investigate the effectiveness of the proposed method. The mean square heave motions of SSP without control device and with FHP, THP and THPI are stochastically formulated, and the optimal design parameters for THP and THPI are derived. Parametric studies are conducted to investigate the influences of the size and original depth of heave plate on the optimal performances of FHP, THP and THPI. Finally, a novel waterwheel inerter is developed to realize the suggested device. The analytical results show that THPI is more effective to mitigate the heave motion of SSP compared to the conventional methods, and the novel waterwheel inerter is capable of generating a large apparent mass by using a small waterwheel.

This paper presents a novel approach to model and simulate the multi-support depth-varying seismic motions (MDSMs) within heterogeneous offshore and onshore sites. Based on 1D wave propagation theory, the three-dimensional ground motion transfer functions on the surface or within an offshore or onshore site are derived by considering the effects of seawater and porous soils on the propagation of seismic P waves. Moreover, the depth-varying and spatial variation properties of seismic ground motions are considered in the ground motion simulation. Using the obtained transfer functions at any locations within a site, the offshore or onshore depth-varying seismic motions are stochastically simulated based on the spectral representation method (SRM). The traditional approaches for simulating spatially varying ground motions are improved and extended to generate MDSMs within multiple offshore and onshore sites. The simulation results show that the PSD functions and coherency losses of the generated MDSMs are compatible with respective target values, which fully validates the effectiveness of the proposed simulation method. The synthesized MDSMs can provide strong support for the precise seismic response prediction and performance-based design of both offshore and onshore large-span engineering structures.

As key components in the transportation networks at coastal areas, sea-crossing cable-stayed bridges play a very important role in the development of regional economy. These bridges may be subjected to severe earthquakes during their life-cycles. Owing to the lack of actual seafloor earthquake recordings and approaches in synthesizing offshore seismic motions, the onshore seismic motions are commonly utilized as inputs in the seismic design of sea-crossing cable-stayed bridges. However, this approach may lead to erroneous structural response predictions since the characteristics of onshore and offshore seismic motions are different. In this paper, the seismic performance of a sea-crossing cable-stayed bridge is comprehensively evaluated based on the fragility function methodology. A novel approach is presented to theoretically calculate the ground motion transfer function at any location within an offshore site and stochastically synthesize the offshore multi-support ground motions at different depths (MGMDDs). The OpenSees analysis platform is employed to develop the three-dimensional finite element model of the example bridge, in which the p-y, t-z and q-z elements are installed at the pile nodes to simulate the interaction between the bridge piles and surrounding soils. Moreover, the effect of seawater on the bridge seismic responses is modeled using the hydrodynamic added mass method. The seismic fragility curves of the example bridge are generated by using the synthesized MGMDDs as inputs. The influences of spatial and depth varying offshore seismic motions, soil-structure interaction (SSI) and seawater added mass on the bridge component and system fragilities are investigated and discussed. Numerical results show that the seismic fragility of the example sea-crossing cable-stayed bridge is affected by the above mentioned influencing factors with different extents. The proposed approach can rationally and effectively assess the seismic fragilities of sea-crossing cable-stayed bridges.

Contemporary bridge codes recommend adjusting the fundamental frequencies of adjacent segments close to each other to mitigate relative displacement induced damages arising during the strong seismic events. Previous studies revealed that such recommendation leads to effective mitigation of damages on the bridge structures subjected to uniform ground motions. However, in an elongated bridge structure spatial variations of earthquake ground motions at different supports are inevitable which can result in larger relative displacements. This study presents experimental results from a large-scale (1/6) shake table testing of bridge models with two bridge frames having a total length of 16.67 m, subjected to spatially varying ground motions. Experiments were also carried out with bridge model with superelastic Shape Memory Alloy (SMA) restrainers to evaluate its effectiveness on mitigating bridge responses. It is revealed that even the adjacent bridge frames with fundamental frequencies close to each other are susceptible to the localized damages at the joints due to poundings, which could lead to delayed access to the affected sites after an earthquake. Superelastic SMA restrainers could effectively reduce the opening relative displacement and pounding intensity. Moreover, owing to its superelastic behaviour the restrainers would not require replacement even after strong seismic events. Finally, numerical models of the bridge were developed and parametric studies were performed to comprehend the results of the experiment.

A combined method of finite element reliability analysis and multiplicative dimensional reduction method (M-DRM) is proposed for systems reliability analysis of practical bridge structures....

Prefabrication by off-site manufacturing leads to a reduced overall construction schedule, improved quality, and reduced resource wastage. Modular building is therefore increasingly popular and promoted. With the recent promotion a number of relevant studies have been completed, however, a review of the design, construction, and performance of modular buildings under different loading conditions is lacking. This paper presents a state-of-the-art review of modular building structures. First, structural forms and construction materials are presented as a brief introduction to the modular structures. Modular building is shown to refer not to a single structure type, but a variety of structural systems and materials. These modular structures might perform differently to similar traditional structures and the structural performance is highly dependent on inter- and intra-module connections. The structural response of modules to different hazards is then considered, followed by the current design practice and methodology. As a currently developing area there is great potential for innovation in modular structures and several key research areas are identified for further work.

This study proposes replacing the spring and dashpot elements in the conventional tuned mass damper (TMD) by shape memory alloy (SMA) wires to form an SMA-based TMD to control the seismic responses of wind turbine tower. An equivalent linearization method is adopted to optimize the initial stiffness of SMA-based TMD by minimizing the root mean square (RMS) displacement of the tower. To have an insight into the control performance of SMA-based TMDs, a three-dimensional (3D) finite element model of a typical wind turbine tower is developed in ABAQUS, and tower responses without and with SMA-based TMDs when subjected to simulated and recorded ground motions are systematically analysed. For comparison, seismic responses of the tower controlled by the linear TMDs are also simulated. Numerical results show that the SMA-based TMDs can substantially mitigate seismic responses of the tower with almost the same reduction ratios as the linear TMDs, while the strokes of the SMA-based TMDs are much smaller than those of the linear TMDs. This merit makes the proposed method more practical compared to the conventional TMD-based method since the space in the nacelle and tower is normally very limited.

Pipe-in-pipe (PIP) systems have been increasingly used in offshore applications because of their favourable thermal insulation capacity. Very recently, the conventional PIP system was slightly revised by using carefully designed springs and dashpots to connect the inner and outer pipes. This revised PIP system can be considered as a structure-Tuned Mass Damper (TMD) system. It therefore has the potential to mitigate the offshore structural vibrations induced by various sources such as earthquake excitation and/or vortex shedding. This paper carries out three-dimensional (3D) numerical simulations to investigate the effectiveness of the proposed method. The cross-flow oscillation of the conventional and optimized PIP systems are numerically investigated by developing a two-way coupled Fluid-Structure Interaction (FSI) framework for computational fluid dynamics (CFD) analysis. The developed FSI model is validated with the available experimental and numerical benchmark data on a single cylinder. This validated model is then extended to the PIP system to study its efficiency for Vortex-Induced Vibration (VIV) suppression. Numerical results show that the optimized PIP system can noticeably reduce VIV.

This paper proposes using a tuned inerter damper (TID) system to mitigate the potential pounding and unseating damages between the adjacent bridge structures under severe earthquakes. The control effectiveness of the proposed method is investigated both in the frequency and time domains. For comparison, the models and responses of the adjacent bridge structures without control and controlled by the traditional viscous dampers that are modelled by a Kelvin system or a Maxwell system are also developed and calculated. The analytical results reveal that the TID system can achieve almost the same or even better control effectiveness compared to the conventional viscous dampers with a much smaller additional damping. The proposed method can provide a good alternative to control the excessive relative motions between the adjacent bridge structures.

Structural vibration control has received considerable research attentions in the past few decades, with special emphasis on developing effective, affordable and applicable control systems to protect the structures against natural or man-made hazards. In 2002, a two-terminal mechanical element, dubbed inerter, was proposed based on the force-current analogy, offering many potentials for upgrading conventional structural vibration control systems. Over the past two decades especially in the recent five years, extensive research efforts have been made for the development of inerter-based vibration control systems. This paper aims to provide a state-of-the-art review on the research and development of inerter-based passive vibration control systems and their applications. It begins with the concept and physical realizations of inerter. The mechanical models of inerter are then presented. Subsequently, this paper reviews the applications of inerter in civil engineering and discusses its benefits comparing to conventional vibration control systems. Finally, some discussions are made on the unresolved problems and the possible topics for future studies.

Many civil engineering structures are supported by tall slender columns, which are susceptible to seismic excitations. Inspired by the tuned mass damper (TMD) concept, these conventional columns are proposed to be divided into two parts in the present study, i.e. to form a column-in-column (CIC) system, to mitigate seismic induced vibrations of these structures without compromising their loading capacities and increasing the structural materials. In particular, the outer column serves as the primary structure in the structure-TMD system, and the inner column performs as the TMD. The inner and outer columns are connected together by the connecting devices that are selected based on the TMD concept. To examine the effectiveness of the proposed method, comprehensive numerical simulations are carried out by using the commercial finite element code ABAQUS. Numerical results show that, without changing too much of the conventional design, the proposed method is effective to reduce seismic induced vibrations. This system is believed has great application potentials in engineering practices.

This article presents a statistical study on strength reduction factors for seismic-isolated bridges in far-fault areas. 1410 ground motions are selected and modified to be compatible with the reco...

Recently, extensive experimental and numerical studies have been carried out to understand the seismic behaviors of segmental columns. Very limited studies, however, focused on the seismic performances of a whole bridge system with precast segmental columns. This paper carries out numerical studies on the seismic responses of bridge structures with precast segmental columns. For comparison, the seismic responses of the bridge with conventional monolithic columns are also calculated. The two-dimensional (2 D) finite element (FE) models of these two bridge types are developed by using the FE code OpenSEES. The segmental column and monolithic column are simulated by the simplified lumped-mass model and fiber-based model respectively and validated by the previous experimental studies. The calibrated column models are then incorporated into the whole bridge structures to calculate the structural responses. The influences of pounding, frequency ratio and gap size on the structural responses are investigated and discussed. Numerical results show that the bridges supported by the segmental columns or monolithic columns have very different seismic responses.

To accelerate construction speed, precast segmental column is becoming more and more popular in recent years due to its obvious advantages in saving construction time, reducing site disruption and controlling construction quality. However, the applications are still limited primarily in low seismic areas because its performance under earthquake loading is not well known yet. Many experimental studies have been carried out to investigate the seismic performance of segmental columns under cyclic loading. Due to the complexity in modelling such structures, numerical study of precast segmental columns subjected to seismic loads is limited. In this paper, three dimensional (3D) finite element (FE) models for two precast segmental columns are developed to predict the responses of such columns under lateral cyclic loading. The numerical models are first validated against the cyclic test results and then used to perform parametric studies. The influences of five parameters including bonding condition of the tendon, total initial axial forces level, confinement of the segments, number of segments, and energy dissipation (ED) bars on the performance of segmental columns are systematically investigated. Moreover, columns with shape memory alloy (SMA) bars are also investigated to increase the energy dissipation capacity and reduce the residual drift of the segmental columns. It is found that both mild steel and SMA bars can increase the energy absorption capacity of the column, but the SMA bars can minimize the residual drift due to its innate mechanical property. This study clearly identifies the influences of different factors on the performance of segmental columns. The developed numerical model can be used in the future studies to predict the seismic responses of structures with segmental columns.

Buckling restrained braces (BRBs) are normally incorporated in the beam-column structures to serve as energy dissipation members due to their stable hysteretic behavior. However, the structures equipped with BRBs may suffer excessive residual deformations when they are subjected to large earthquakes. To minimize the residual deformations of the structures with traditional BRBs, a novel self-centering buckling restrained brace (SC-BRB) consisting of a self-centering system and a traditional BRB system is developed in the present study. Large-scale experimental studies are carried out and the hysteretic behavior of the proposed system is compared with the traditional BRB and self-centering brace (SCB). Experimental results show that the SC-BRB exhibits flag-shaped hysteresis response with a small residual deformation and a moderate energy dissipation capability. The proposed SC-BRB is applied to a reinforced concrete (RC) double-column bridge pier for seismic retrofitting. Nonlinear dynamic analyses are carried out to examine its effect on the seismic behavior of the bridge. Numerical results demonstrate that the bridge equipped with SC-BRB system shows much smaller residual displacement compared to the ones equipped with traditional BRB and SCB systems. Numerical results also indicate that SC-BRB system tends to amplify the peak acceleration of the bridge.

This paper aims to propose a design of using the revised PIP system to control vortex induced vibrations (VIV) of cylindrical structural components. Analytical studies are carried out to examine the effectiveness of the proposed method. To this end, the fluid-induced vibration of a single pipe is investigated and the equation of motion of the system is solved and validated by the experimental data. This single pipe system is then extended to the proposed PIP system and its dynamic behaviour under the excitation of vortex shedding is simplified as a Two-Degree-of-Freedom (2DoF) system. The optimal damping ratio and tuning frequency of the revised PIP system are obtained through a series of numerical searching technique and sensitivity analyses. Explicit formulae are also derived for practical ease of use. The governing equation of the system under VIV is solved in the time domain using the MATLAB/Simulink program. The responses of the single pipe system and the proposed PIP system due to vortex shedding are calculated and compared. Analytical results demonstrate that the proposed PIP system can significantly suppress the VIV of offshore cylindrical components. It could be then a cost-effective passive solution to suppress vibration of offshore cylindrical components.

Precast segmental column has attracted a lot of interests over the past decade. It has become more and more popular in construction industry to achieve fast construction, improve construction quali...

Offshore wind turbines can be built larger and lighter than they used to be due to the application of new materials. These large and flexible structures are vulnerable to external vibration sources such as wind, sea wave and earthquake excitations. It is necessary to mitigate the dynamic responses of offshore wind turbines to ensure the safety of these structures. Extensive research works have been carried out to mitigate the vibrations of the tower and/or blades of offshore wind turbines. Almost all the previous studies on the offshore wind turbine tower vibration control propose installing the control device at the top of the tower, i.e. in the nacelle. This method is effective to suppress the fundamental vibration mode of the tower, in which the maximum displacement occurs at the top of the tower. This practice is reasonable when wind and/or sea wave loadings are of interest since the energies of these vibration sources are concentrated in the low frequency range, and normally only the fundamental vibration mode of the tower is excited. On the other hand, offshore wind turbines may locate in the seismic prone areas, earthquake loading can be another vibration source during their lifetimes. When offshore wind turbines are subjected to earthquake excitation, higher vibration modes might be also excited. These higher vibration modes can further contribute to the structural responses and in certain circumstances they may even dominate the structural responses. In this case, installing the control device only in the nacelle will not be effective and more control devices should be installed at certain locations along the tower. In other words, one single control device will not be effective to control the tower vibrations if both the fundamental and higher vibration modes are of interest. This paper proposes using multiple tuned mass dampers (MTMDs) to control vibrations from the fundamental and higher modes of offshore wind turbine tower under multiple hazards, i.e. under the combined wind, sea wave and earthquake excitations. The effectiveness of the proposed method is numerically investigated. It should be noted that only the vibration of the tower is of interest in the present study. The vibration control of the blades is out of the scope of this paper, which will be further investigated.

In this study, three rapid repair techniques are proposed to retrofit circular bridge piers that are severely damaged by the flexural failure mode in major earthquakes. The quasi-static tests on three 1:2.5 scaled circular pier specimens are conducted to evaluate the efficiency of the proposed repair techniques. For the purpose of rapid repair, the repair procedure for all the specimens is conducted within four days, and the behavior of the repaired specimens is evaluated and compared with the original ones. A finite element model is developed to predict the cyclic behavior of the repaired specimens and the numerical results are compared with the test data. It is found that all the repaired specimens exhibit similar or larger lateral strength and deformation capacity than the original ones. The initial lateral stiffness of all the repaired specimens is lower than that of the original ones, while they show a higher lateral stiffness at the later stage of the test. No noticeable difference is observed for the energy dissipation capacity between the original and repaired pier specimens. It is suggested that the repair technique using the early-strength concrete jacket confined by carbon fiber reinforced polymer (CFRP) sheets can be an optimal method for the rapid repair of severely earthquake-damaged circular bridge piers with flexural failure mode.

A dual-meta panel functioning as a sacrificial cladding is proposed and its blast mitigation capacity is investigated in this study. The proposed panel possesses the potential to generate bandgaps that target at a specific range of frequencies to stop stress wave propagating through the panel, leading to the favourable stress wave mitigation for structural protection. Aside from the unique stress wave manipulation capability, more energy can be absorbed by a combination of plastic deformation and local resonance. The effectiveness of the proposed panel is validated through numerical simulations. An analytical solution of wave propagation in an ideal meta truss bar is derived to validate the numerical model with good agreement. It is found that the proposed dual-meta panel exhibits an increase in energy absorption, a reduction in transmitted reaction force (up to 30%), and the back plate central displacements (up to 20%) compared to other conventional sandwich panels, e.g. sandwich panel with hollow trusses and solid trusses, in resisting blast loadings. In pursuit of optimizing the performance of the proposed panel, parametric investigations are also conducted to examine the influences of the plate thickness, boundary condition, and the blast load profiles including duration and intensity on the transient response of the proposed dual-meta panel.

For the structural response estimations of modular steel buildings, the shear, axial and moment-rotation behaviours of the inter-module connections are normally simplified and input into the overall numerical model. While linear models have traditionally been used for the simplification, they do not reflect the true non-linear responses of the connections. Through recent studies, improved models have been developed for the structural behaviours of the newly proposed post-tensioned (PT) and interlocking (IL) inter-module connections (IMCs). In this paper, the newly proposed models are implemented in the numerical simulation of a selected modular steel building. The effect of the inter-module connections on the global building responses to simulated time-varying wind loads and earthquake ground accelerations is examined. Hence, the application of the new simplified models is demonstrated, while the performances of the building incorporating the two different connections are compared. The results demonstrate that the new simplified models are well-suited for use in global numerical simulations. The analytical basis of the models allows application to varied connection geometries, and the more accurate connection behaviours contribute to an improved understanding of the global building responses. Inter-storey drift ratio limits, for example, which are not yet defined for modular buildings, are proposed based on the simplified connection behaviours.

Bridges may suffer large residual displacements after a severe earthquake, which significantly impedes the post-quake rescue activities. It is imperative to minimize the residual deformations of bridge structures. In the present study, a self-centering slip friction (SCSF) brace is developed to dissipate seismic energy and reduce the residual displacement of RC double-column bridge bents. The design and working mechanism of the brace are firstly introduced, and parametric studies are performed to examine the influences of the key parameters of the brace. Experimental studies on the components of the brace and the whole brace were carried out to examine its behaviors and to validate the analytical results derived according to the working mechanism. This brace is then applied to a RC double-column bridge bent to enhance its cyclic behaviors, and its effectiveness is examined through numerical simulations. In particular, the influences of the location and type of the connection between the brace and bridge bent are investigated. Experimental and numerical results show that the brace has prominent energy dissipation and self-centering capabilities, and it can significantly enhance the cyclic behaviors of RC a double-column bridge bent.

Pipeline systems are commonly used to transport oil, natural gas, water, sewage and other materials. They are normally regarded as important lifeline structures. Ensuring the safety of these pipeline systems is crucial to the economy and environment. There are many reasons that may result in the damages to pipelines and these damages are often associated with pipeline vibrations. Therefore it is important to control pipeline vibrations to reduce the possibility of catastrophic damages. This paper carries out numerical investigations on the effectiveness of using viscoelastic materials to mitigate the seismic induced vibrations of above-ground pipelines. The numerical analyses are carried out by using the commercial software package ANSYS. The numerical model of the viscoelastic material is firstly calibrated based on the experimental data obtained from vibration tests of a 1.6 m long tubular sandwich structure. The calibrated material model is then applied to the above-ground pipeline system. The effectiveness of using viscoelastic materials as the seismic vibration control solution is investigated. The influences of various parameters, including the constraining arrangement scenarios, the constraining length and angle, the thicknesses of the viscoelastic material and constraining layer are discussed in detail. The influence of earthquake frequency content is discussed as well. Numerical results show that with properly selected viscoelastic materials and constraining layers, the proposed method can be used to effectively mitigate seismic induced vibrations of above-ground pipelines.

Pounding and unseating damages to bridge superstructures have been commonly observed in many previous major earthquakes. These damages can essentially attribute to the large closing or opening relative displacement between adjacent structures. This article carries out an experimental study on the pounding responses of adjacent bridge structures considering spatially varying ground motions using a shaking table array system. Two sets of large-scale (1:6) bridge models involving two bridge frames were constructed. The bridge models were subjected to the stochastically simulated ground motions in bi-direction based on the response spectra of Chinese Guideline for Seismic Design of Highway Bridge for three different site conditions, considering three coherency levels. Two types of boundary conditions, that is, the fixed foundation and rocking foundation, were applied to investigate the influence of the foundation type. In addition, a detailed three-dimensional finite element model was constructed to simulate an experimental case. The nonlinear material behavior including strain rate effects of concrete and steel reinforcement is included. The applicability and accuracy of the finite element model in simulating bridge pounding responses subjected to spatially varying ground motions are discussed. The experimental and numerical results demonstrate that non-uniform excitations and foundation rocking can affect the relative displacements and pounding responses significantly.

Base isolation techniques have been extensively used to improve the seismic performance of the bridge structures. The decoupling of the bridge decks from the piers and abutments using rubber isolator could result in significant reduction in seismic forces transmitted to the bridge substructures. However, the isolation devices could also increase the deck displacement and thus enhance the possibility of pounding and unseating damage of bridge decks. Moreover, previous investigations have shown that pounding and unseating damages on isolated bridges exacerbate due to the spatial variation in earthquake ground motions. Recent earthquakes revealed that isolation bearing could also be damaged due to the excessive movements of decks during large earthquake events. This study proposes the use of rotational friction hinge dampers to mitigate the damages that could be induced by large displacement of bridge decks, particularly focusing on pounding and unseating damages and bearing damages. The device is capable of providing large hysteretic damping and the cost of installing the devices is relatively economical. This article presents numerical investigations on the effectiveness of these devices on a typical Nepalese simply supported bridge subjected to spatially varying ground motions. The results indicate that rotational friction hinge dampers are very effective in mitigating the relative displacement and pounding force, as well as controlling the bearing deformation and pier drift. It is also revealed that the effectiveness of the device is not significantly affected by small changes in the slip forces; thus, small variations in the optimum slip forces during the lifetime of the bridge do not warrant any adjustment or replacement of the device.

A simulation method of spatially correlated seafloor motions is proposed by considering the influences of seawater on the seafloor motions and their spatial variations at different subsea sites. The offshore site transfer functions are theoretically derived using the fundamental hydrodynamics and one-dimensional wave propagation theory. Three-dimensional spatially varying ground motions on the surfaces of multiple onshore and offshore sites are synthesized based on the spectral representation method and the calculated site transfer functions. A pair of onshore and seafloor recordings from the same earthquake event is employed to examine the basic characteristics of simulated onshore and seafloor motions.

Previous earthquakes have highlighted the seismic vulnerability of bridges due to excessive movements at expansion joints. This movement could lead to the catastrophic unseating failure if the provided seat width is inadequate. Moreover, seismic pounding is inevitable during a strong earthquake due to the limited gap size normally provided at the expansion joints. Various types of restrainers, dampers and other devices have been proposed to limit the joint movement or to accommodate the joint movement so that the damages caused by excessive relative displacements could be mitigated. To select and design appropriate devices to mitigate the relative displacement-induced damages to bridge structures during earthquake shaking, it is important that results from the previous studies are well understood. This paper presents an overview on various pounding and unseating mitigation devices that have been proposed by various researchers. Based on an extensive review of up-to-date literatures, the merits and...

Inter-module connections (IMCs) are the key component which transforms individual building modules into complete modular buildings. Structural design of these connections is, however, reported to be a major challenge, due to a lack of knowledge of the structural performance. Therefore, in this study, the structural behaviours of the newly proposed interlocking (IL) IMC, which incorporates interlocking components to improve the constructability and shear force–displacement (V-x) behaviour, are investigated. The simplified model previously proposed for the shear behaviour of the experimental IL connection specimens is reviewed, and numerical simulations are undertaken using ABAQUS to investigate the effect of the loading and boundary conditions on the V-x behaviour. Then, extended numerical models are developed based on a more realistic geometry which has two bolts per column to improve the resistance to in-plane rotation. A simplified empirical model is established, in which the V-x behaviour of the more realistic connections is represented by exponential functions, allowing for the effect of the axial load. Next, the numerical model is extended, and the axial force–displacement and bending moment-rotation behaviours are evaluated, after which semi-empirical models are derived. In this way, simplified models are developed which can predict the structural behaviours of the interlocking IMC.

Tuned mass dampers (TMDs) are widely adopted to control the adverse vibrations of engineering structures. To further improve the effectiveness of TMD, inerter was introduced into TMD recently to form inerter-based TMD systems. Similar to TMD, inerter-based TMDs should be carefully designed (optimized) in order to get their best control performances. In the previous studies on using inerter-based devices for seismic induced vibration control, the external excitation was normally simplified as a white noise and the inherent structural damping was ignored. However, it is well known that seismic excitation cannot be simply assumed as a white noise and damping always exists in the structure. The parameters obtained by the previous optimization procedures thus do not necessarily result in the best performance of the device. In the present study, the equations of motion of a single-degree-of-freedom (SDOF) structure equipped with three types of inerter-based TMDs subjected to seismic excitation are firstly developed. Instead of using a white noise as input, the filtered Kanai-Tajimi spectrum, which is characterized by the site damping and frequency, is adopted to model seismic ground motion. Then the effects of site damping, site frequency and structural damping on the inerter-based TMDs are comprehensively investigated and formulas are proposed to estimate the optimal parameters. Lastly, the responses of a structure without control and controlled by an inerter-based TMD under simulated and recorded earthquake ground motions are analysed by using MATLAB/Simulink. Numerical results show that the optimal parameters of inerter-based TMDs are significantly dependent on the site frequency and structural damping, while the site damping has little influence. Moreover, the accuracy of the proposed formulas is validated, and the control effectiveness of the inerter-based TMD is confirmed.

When designing modular buildings, the structural behaviour of the inter-module connections (IMCs) must be simplified so they can be incorporated in the structural model. However, the existing models for the shear, axial and moment-rotation behaviour are limited, and further study is required to develop improved models. In this study, the shear, axial and moment-rotation behaviours of a post-tensioned IMC are investigated. A numerical model is developed and calibrated based on the shear behaviour obtained from experiments. Following the experiments, a simplified empirical model is proposed for the shear behaviour. The numerical model is then extended to determine the axial and moment-rotation behaviour. Simplified analytical models are derived and compared with the numerical results. Further, the numerical results are compared with the behaviour of other existing IMCs, for which experimental data are available in the literature. In this way, simplified models which can predict the shear, axial and moment-rotation behaviour of the post-tensioned IMC are developed. The developed simplified models can be used to predict the behaviour of modular structures with the newly proposed post-tensioned IMC.

This paper develops an inerter-based vibration isolation system (IVIS) for heave motion mitigation of semi-submersible platforms (SSPs) subjected to sea waves. An analytical model of a classical SSP equipped with IVIS is first established and validated by comparing the heave response amplitude operator (RAO) with the previous experimental and numerical results, and the corresponding equations of motion are derived. Optimization analyses are performed to search for the optimum inertance-to-mass ratio of IVIS by minimizing the heave motion standard deviation of the SSP. A case study is carried out to demonstrate the control performance of IVIS in the frequency and time domains. The results indicate that the proposed IVIS is more effective and has a wider effective frequency range compared to the conventional vibration isolation system (VIS). By introducing a parallel-connected inerter, the reduction ratio of the receiving body can be further improved by more than 23%. More importantly, the IVIS can achieve the best control performance under different wave conditions by adjusting the inertance in the system. The proposed IVIS is an attractive alternative to the conventional vibration isolation systems for offshore platforms.

Despite the increasing popularity of modular buildings, there are relatively few published case studies considering the overall structural responses to wind and earthquake load. As a result, there is limited structural performance data for the overall behaviour of modular buildings, which in turn impedes the development of appropriate design standards. At the same time, different inter-module connections exhibit different structural behaviours, and the overall response of a building assembled from interconnected modules may be influenced by the inter-module connection. However, the structural behaviours of inter-module connections in modular building structures subjected to lateral dynamic loads are not well known in the existing literature. This paper, therefore, presents a numerical investigation into the overall structural behaviour of a selected case study building. In particular, the moment-rotation and force-displacement behaviours of the inter-module connection were simplified, and incorporated in the overall structural model. The numerical technique was verified by comparison with a previous study, and the verified technique was then extended to the selected six storey case study building. The results showed that the overall response was affected by the inter-module connection stiffness, especially the translational stiffness in the along load direction.

Semi-submersible platforms (SSPs) are widely applied for energy mining in the ocean. During its service life, SSP may be subjected to excessive heave and pitch motions induced by sea waves, which in turn may result in fatigue problems of structural components or even catastrophic capsizing of the platform. It is therefore important to mitigate the heave and pitch motions of SSP by all means. In the present study, a novel inerter-based control system, rotational inertia damper (RID), is proposed to simultaneously mitigate the heave and pitch motions of SSP in the shallow sea. The responses of SSP equipped with RID systems subjected to six typical wave conditions are calculated in the frequency and time domains respectively based on the developed analytical models and Simulink models. For comparison, the models and responses of SSP without and with traditional control devices, i.e. fixed heave plate (FHP) and tuned heave plate (THP), are also developed and calculated. The results reveal that the RID system can achieve almost the same or even better control effectiveness compared to the conventional systems with a much smaller additional mass to the SSP especially when the SSP is under harsher waves.

Compared to the seismic motions recorded on the onshore sites, the quantity of offshore earthquake recordings is very limited. This paper presents a novel method to simulate spatially varyi...

Pipe-in-pipe (PIP) systems are increasingly used in subsea pipeline applications due to their favourable thermal insulation capacity. Pipe-in-pipe systems consist of concentric inner and outer pipes, the inner pipe carries hydrocarbons and the outer pipe provides mechanical protection to withstand the external hydrostatic pressure. The annulus between the inner and outer pipes is either empty or filled with non-structural insulation material. Due to the special structural layout, optimized springs and dashpots can be installed in the annulus and the system can be made as a structure-tuned mass damper (TMD) system, which therefore has the potential to mitigate the pipeline vibrations induced by various sources. This paper proposes using pipe-in-pipe systems for the subsea pipeline vibration control. The simplification of the pipe-in-pipe system as a non-conventional structure-TMD system is firstly presented. The effectiveness of using pipe-in-pipe system to mitigate seismic induced vibration of a subsea pipeline with a free span is investigated through numerical simulations by examining the seismic responses of both the traditional and proposed pipe-in-pipe systems based on the detailed three dimensional (3D) numerical analyses. Two possible design options and the robustness of the proposed system for the pipeline vibration control are discussed. Numerical results show that the proposed pipe-in-pipe system can effectively suppress seismic induced vibrations of subsea pipelines without changing too much of the traditional design. Therefore it could be a cost-effective solution to mitigate pipe vibrations subjected to external dynamic loadings.

In the performance-based seismic bridge design, piers are expected to undergo large inelastic deformations during severe earthquakes, which in turn can result in large residual drift and concrete crack in the bridge piers. In this paper, longitudinal unbonded prestressing strands are used to minimize residual drift and residual concrete crack width in reinforced concrete (RC) bridge piers. Seven pier specimens were designed and tested quasi-statically and the numerical simulations were carried out. The effectiveness of using vertical unbonded prestressing strands to mitigate the residual drift and concrete crack width of RC bridge piers are examined and discussed in detail. It is found that the residual drift and residual concrete crack width of the piers can be reduced significantly by using the prestressing strands. Moreover, the strands can increase the lateral strength of the piers while have little influence on the ductility capacity of the piers. The hysteretic curves, residual drifts and strand stress of the piers predicted by the numerical model agree well with the testing data and can be used to assess the cyclic behavior of the piers.

Vertical earthquake loading is normally regarded not as important as its horizontal components and are not explicitly considered in many seismic design codes. However, some previous severe near-fault earthquakes reveal that the vertical ground motion component can be much larger than the horizontal components and may cause serious damage to the bridge structures. This paper theoretically investigates the vertical pounding responses of a two-span continuous bridge subjected to the severe near-fault vertical ground motions. The bridge is simplified as a continuous beam-spring-rod model. The structural wave effect and the vertical pounding between the bridge girder and the supporting bearing are considered, and the theoretical solutions of bridge seismic responses are derived from the expansion of transient wave functions as a series of eigenfunctions. The effects of vertical earthquake and vertical pounding on the bridge bearing, girder and pier are investigated. The numerical results show that the severe vertical earthquake loading may cause the bridge girder to separate from the supporting bearing and hence result in vertical poundings between them when they are in contact again. These vertical poundings can significantly alter the seismic responses of the bridge structure and may cause severe damage to the bridge components such as bridge girder, supporting bearing and bridge pier. Neglecting the influence of vertical earthquake loading may lead to inaccurate estimation of seismic responses of bridge structures, especially when they are subjected to near-fault earthquake with relatively large vertical motion.

A compound passive vortex-induced vibration (VIV) control technique, namely the textured pipe-in-pipe (Textured-PIP) system, is introduced in the present study for VIV suppression. This novel system, which is modified from the conventional PIP, includes a smooth inner pipe to transport the natural resources and a textured outer pipe to protect the inner pipe, and the inner and outer pipes are connected together by the optimized springs and dashpots. With such a design, the VIV responses are mitigated by appreciating the suppression mechanisms of tuned mass damper (TMD) and surface roughness. Three-dimensional (3-D) computational fluid dynamics (CFD) analyses are carried out to examine the effectiveness of the proposed method. For comparison and demonstration of the vibration mitigation effectiveness, the responses of a smooth pipe and a textured pipe of the same mechanical properties and mass are also investigated. The results show that the textured-PIP technique can more evidently suppress the VIV compared to the textured pipe.