SUMMARY
This paper presents a study on multi‐degree‐of‐freedom (MDOF) structures equipped with a negative stiffness amplifying damper (NSAD). The NSAD not only preserves the negative stiffness ...feature of negative stiffness devices (NSDs) but also achieves prominent damping magnification effect, substantially reducing a NSD's requirement for high additional damping, which is used to contain the increased displacements resulting from the reduction in overall stiffness of a system. The dynamic equations of MDOF systems with NSADs are described in state–space representation, and the effective damping and frequencies are parametrically studied. Then, a simple optimization method is proposed. A study of the 20‐storey benchmark building shows that NSADs are the most efficient of supplemental devices in reducing interstorey drifts compared with viscous dampers (VDs) and viscoelastic dampers (VEDs) with the same supplemental damping coefficient. For instance, when compared with that of VEDs, the maximal peak interstorey of NSADs can be further reduced by about 30%. In terms of reducing acceleration responses, NSADs perform much better than VDs and VEDs owing to their negative stiffness feature. Partially arranged, NSADs are best implemented at the storeys that have smaller interstorey drift responses. This is because the negative stiffness preserved by NSADs significantly reduces the interstorey drift of storeys without NSADs.
Summary
This study presents a new crosswind mitigation strategy by using negative stiffness damped outrigger (NSDO) system for tall buildings. Using the “assisting motion” feature of negative ...stiffness, NSDO amplifies the motion of viscous damper resulting in and significant improvement of energy dissipation for tall buildings. Systematical evaluation and comparison on crosswind performance are carried out for tall buildings using (a) conventional outrigger (CO), (b) conventional damped outrigger (CDO), and (c) NSDO. It is shown that the proposed NSDO is able to achieve a damping amplification factor larger than a unit, which would never be achieved by CDO in practical due to the actual deformation of perimeter columns. In this way, NSDO has the most satisfactory crosswind reduction effect, especially when perimeter column stiffness is insufficient. For example, replacing CDO with NSDO not only attenuates crosswind‐induced harmful drift and structural acceleration by 22% and 36%, respectively, but also further saves about 75% of the size of viscous dampers. In other words, NSDO adopts less outrigger damping coefficient but reduces more crosswind‐induced vibration.
•Building direct connection between NSDO parameters and seismic responses.•Proposing an optimization method on harmful drift, acceleration and damping cost.•Retrofitting existing CO structure with ...supplemental NSDO in resisting earthquakes.
In this paper, the seismic performance of conventional damped outrigger (CDO) is improved by incorporating negative stiffness (NS) in parallel with the viscous damper. The applied negative stiffness spring upgrades the classical Maxwell damping element, which is used to describe the assembly of outrigger viscous damper and perimeter columns, to negative stiffness amplifying damper with higher damping effect. Tall buildings with such negative stiffness damped outrigger (NSDO) is described in the state space expression of the structure-outrigger-excitation augmented assembly, and the root-mean-square (RMS) responses of NSDO structures are parametrically studied to develop a direct connection between outrigger parameters and structural seismic responses. Based on these results, a multi-objective optimization method with both performance and economic objectives—reducing harmful inter-story drift and structural acceleration simultaneously with less outrigger damping cost—is proposed. Finally, this optimally designed NSDO is used to retrofit existing conventional outrigger (CO) structures as a supplemental energy dissipation outrigger. The performance of this retrofitted structure is studied and compared with that retrofitted by optimal CDO under numerous real earthquake records. Result indicates the CO structure retrofitted by NSDO substantially saves the outrigger damping cost of CDO by 85.9%, and further decreases the harmful drift and structural acceleration responses over 51% and 29%, respectively. Although optimal design of NSDO for strong earthquakes alone is the focus of this study, it is shown that NSDO is able to reduce wind-induced maximum acceleration by 33% without amplifying wind-induced maximum harmful drift.
Gut microbial dysbiosis and alteration of microbial metabolites in Parkinson's disease (PD) have been increasingly reported. Dysbiosis in the composition and abundance of gut microbiota can affect ...both the enteric nervous system and the central nervous system (CNS), indicating the existence of a microbiota-gut-brain axis and thereby causing CNS diseases. Disturbance of the microbiota-gut-brain axis has been linked to specific microbial products that are related to gut inflammation and neuroinflammation. Future directions should therefore focus on the exploration of specific gut microbes or microbial metabolites that contribute to the development of PD. Microbiota-targeted interventions, such as antibiotics, probiotics and fecal microbiota transplantation, have been shown to favorably affect host health. In this review, recent findings regarding alterations and the role of gut microbiota and microbial metabolites in PD are summarized, and potential molecular mechanisms and microbiota-targeted interventions in PD are discussed.
This paper deals with the multi-hazard vibration reduction of high-rise buildings in resisting earthquakes and winds, emphasizing on the optimization of supplemental negative stiffness damped ...outrigger (NSDO) system. Since the risks of wind and earthquake are both inevitable, the competing relationship between two performance indexes—harmful inter-story drift and structural acceleration—of tall buildings with supplemental NSDO are carefully studied concerning along wind, cross wind, and earthquake loading. As an energy dissipation outrigger system, NSDO amplifies the deformation of viscous damper at the outrigger ends, thus achieving higher efficiency as compared to conventional damped outrigger (CDO) systems. The resultant superiority of NSDO in resisting multi-hazard is then verified. For example, NSDO provides a further reduction of 25.6% in seismic harmful drift and an extra 22.3% decrease in cross wind acceleration response. In the meantime, NSDO only uses about 1/3 of the outrigger damping coefficient as compared to CDO systems. Moreover, NSDO optimized for multi-hazard can achieve a good tradeoff design without compromising too much for one specific hazard.
•Optimizing negative stiffness damped outrigger (NSDO) systems in high-rise buildings concerning winds and earthquakes.•Competing relationships between harmful drift and acceleration subjected to multi-hazard are investigated.•Verifying the high efficiency of NSDO as compared with conventional damped outrigger in resisting multi-hazard.
Damped outrigger system is effective for improving energy dissipation for tall buildings. However, conventional damped outrigger (CDO) system with viscous damping has two limitations: (i) its maximum ...damping ratio cannot be improved when outrigger/column stiffness is inadequate; (ii) different modes achieve their maximum damping ratios at different outrigger damping values, and thus the dampers cannot be optimized to simultaneously reduce vibrations of multiple modes of concern to their minimum. In this paper, a purely frequency‐independent negative stiffness damped outrigger (FI‐NSDO) system is proposed by combining frequency‐independent damper (FID) and negative stiffness device (NSD). The damped outrigger with FID can achieve the maximum damping ratio for all modes as compared to frequency‐dependent damper like viscous damper. As the NSD has the features of assisting and enhancing motion and frequency‐independence, the utilization of NSD will considerably improve the maximum damping ratios when outrigger/column stiffness is inadequate and maintain the frequency‐independent feature of the whole system. Therefore, the FI‐NSDO has the capability of simultaneously increasing the damping ratios of all target modes to their maximum values. Analysis in frequency domain and time domain, demonstrate that the proposed FI‐NSDO performs better in controlling the multi‐mode vibration of seismic responses.
•Proposing a new device with negative stiffness and damping magnification effect.•Deriving optimization method for undamped SDOF systems with NSAD.•Evaluating NSAD’s performance under pulse-like ...earthquake.
A new passive device named negative stiffness amplifying damper (NSAD) is proposed in this paper by introducing a negative stiffness (NS) spring into to the flexibly-supported-viscous-damper systems (represented by classical Maxwell damping element, MDE).. The NS spring is combined with the dashpot of the MDE, amplifying the stroke of the dashpot; therefore, lead to significant damping magnification effect. The proposed NSAD not only achieves significant damping magnification effect, but also preserves the property of negative stiffness. This feature is attractive for reducing both displacement and structural acceleration when subjected to earthquakes. The closed-form expressions of optimal parameters for an undamped SDOF system with a NSAD is also proposed by modifying the ‘fixed point’ method of tuned mass damper. Then, the performance of NSAD is investigated and evaluated under stochastic excitations, pulse excitations, and real earthquakes. Result shows that the optimal NSAD can substantially reduce displacement and acceleration responses simultaneously. For instance, even using the same small additional damping ratio (i.e., 2.8%), the optimal NSAD reduces the resonance response of MDE by 76.9%. Also with that small additional damping, the NSAD improves the energy dissipation capability by 5–16 times, causing 40–60% of seismic response reduction for most structural period range. Moreover, the NSAD is also effective for both far-field and near-fault earthquakes. Especially for near-fault pulse-like earthquakes which may potentially cause larger seismic responses for long-period structures, the NSAD provides an extra improvement of 15–20% in energy dissipation capability for long-period structures.
•Experimental validation of frequency dependent/independent features of ID/NSD.•Highlighting the influence of frequency variation caused by SSI on the damping magnification of TVMD/NSAD.•Proposing ...unified optimal design method for TVMD/NSAD considering SSI.
As passive control techniques, both inerter device (ID) and negative stiffness device (NSD) produce forces that have a phase delay of π from common springs, i.e., negative stiffness behavior, when subjected to harmonic excitations. Thus, these two are both able to assist the motion of viscous damper in typical tuned inerter-based or negative stiffness-based absorbers, like tuned viscous mass damper (TVMD) or negative stiffness amplifying damper (NSAD), leading to a damping magnification effect. However, the negative stiffness value of NSD is frequency-independent; while that value of ID is frequency-dependent. In this regard, ID and NSD will perform differently due to structural frequency variation caused by soil-structure-interaction (SSI), which in turn further affects the energy dissipation capabilities and the seismic performance of TVMD and NSAD. Based on these effects, this paper systematically compares and discusses the optimal design and seismic performance of TVMD and NSAD from the point of SSI.
Turbine discs in gas-turbine engines always experience high-temperature and high-stress environment. Extensive investigations have been performed towards achieving high performance. Deformation ...behaviors and degradation factors above normal service temperature are important to be comprehensively understood for further development and optimization of turbine disc superalloys. In this study, the low cycle fatigue behaviors were investigated under total strain amplitude control in U720Li disc superalloys. The results revealed that with the increasing strain, the number of dislocation increased and dislocation configuration transferred from pile-up at the matrix/γ′ precipitates interfaces to penetrating into γ′ precipitates. The great extent of recrystallization was found above the normal service temperature through transmission-EBSD combined with TEM analysis. The low cycle fatigue property above the normal service temperature was weakened to a large extent after recrystallization of small-angle grains formed by the dislocation substructures and movements. Large extent of local recrystallization could also provide much more sites for the crack initiation and propagation.
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•T-EBSD combined with TEM was performed to analyze detailed microstructure exactly.•Deformation and degradation behaviors above service temperature were investigated.•Recrystallization caused LCF property degradation significantly at 725 °C.
Introduction
Brain metastasis is common in relapsed neuroblastoma patients, but the characteristics of brain metastasis remain largely unknown. This study aimed to investigate the status of brain ...metastasis with neuroblastoma in South China.
Methods
In this retrospective case‐based study, 106 patients with stage 4 neuroblastoma from the Department of Pediatric Oncology in Sun Yat‐sen University Cancer Center between January 2004 and May 2013 were included. The incidence, risk factors, and survival status of these patients were reviewed and analyzed.
Results
Of the 106 patients, 11 (10.4%) developed brain metastasis, accounting for 20.0% of 55 patients with relapse or progression. The age at initial diagnosis of the 11 patients ranged from 2 to 10 years (median 4 years), which was younger than that of the patients without brain metastasis (median 5 years, range 1–10 years, P = 0.073). The male to female ratio of the 11 patients was 8:3, which was not significantly different from that of the patients without brain metastasis (P = 0.86). Patients with brain metastasis had higher lactate dehydrogenase levels than those without brain metastasis, but the differences were not significant (P = 0.076). Eight patients died, and 3 patients survived. The median interval from the initial diagnosis to the development of brain metastasis was 18 months (range 6–32 months). The median survival was 4 months (range 1 day to 29 months) after the diagnosis of brain metastasis. The median interval from the manifestation of brain metastasis to death was 3 months (range 1 day to 11 months).
Conclusions
High‐risk factors for brain metastasis in cases of neuroblastoma include bone marrow involvement and a younger age at initial diagnosis. Nevertheless, multiple treatment modalities can improve disease‐free survival.
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