地基-非线性结构相互作用体系的 超谐和亚谐振动

本文对上部结构的以线型恢复力进行最小二乘法拟合，得到类似于Duffing振子的恢复力曲线模型，地基上简化为集中参数模型，运用多尺度法研究地基－结构相互作用体系在强激励下当频率接近结构的三分之一固有频率（超谐共振），三倍固有频率（亚谐共振）时的非线性动力行为，并通过算例得到了异频振动时一些有益的结论。     

结构-地基体系水平与扭转位移耦合效应研究

采用动力理论对地基-结构非线性相互作用体系的振动方程进行了定性分析.基于多线性随动强化模型,采用非线性有限元法求解了基础和地基土之间的水平刚度与摇摆刚度,建立了结构-地基非线性相互作用体系的力学模型.利用拉格朗日能量法推导了结构水平位移和扭转相耦合的振动方程.采用多尺度法研究了结构-地基相互作用体系的主共振.通过分析不...     

地基-非线性结构相互作用空间效应的超谐和亚谐共振研究

建立了考虑空间效应的无质量基础-地基-结构相互作用模型,将模型简化为三个自由度体系,得到了考虑结构水平侧移和扭转位移相耦合的振动方程,用多尺度法研究了体系在强激励下的超谐共振和亚谐共振行为,通过算例分析得到了一些不同于线性体系的结论,为结构非线性分析提供了更深刻的理论依据。     

平行索面悬索桥多阶段有限元模型修正

根据平行索面悬索桥的结构体系特征,可以将其划分为几个相对独立的子结构。充分利用振动测试得到的子结构局部振动信息和结构整体振动信息,可以分阶段进行有限元模型修正,从而提高优化效率。本文以克拉玛依市龙门桥为例,结合该桥设计的有限元模型和环境激励振动试验结果,进行了整桥模型修正和子结构多阶段模型修正。首先通过振动法计算得到边悬索的索力近似替代主缆索力,将其施加在整桥有限元模型中,运用基于动力特性的响应面法完成传统的模型修正,得到修正后的土-结构相互作用的弹簧刚度系数和吊杆索力;其次利用主塔横向模态相对独立的特点,通过对主塔子结构模型的修正得到合理的弹性支承参数;再将其施加在初始的整体有限元模型中,完成该桥的子结构分阶段有限元模型修正;最后比较子结构多阶段模型修正与传统整桥模型修正方法的效果,可以看出相比传统模型修正,所提方法不仅提高了修正精度,而且大幅度提高了优化效率。     

利用ARX模型识别土-结构体系的低阶模态频率和阻尼

利用美国Alaska-14层的办公大楼及周围场地上记录到的地震动，对此结构进行了低阶模态频率和阻尼的识别。和考虑土-结构动力相互作用后的土-结体系的低阶模态的频率和阻尼的识别。提供了一种ARX参数模型辨识方法，并与非参数模型辨识比较分析，发现两种模型得到的低阶模态频率和阻尼基本一致，但在高阶模态上会出现明显的差异。通过分析还发现考虑土-结相互作用后，体系的传递函数幅值有所降低。并编制了相应的Matlab计算程序。     

考虑空间效应的非线性结构主共振分析

本文针对考虑空间效应非对称刚度的单层结构，利用能量法得到了结构水平侧移和扭转相耦合的振动方程，采用多尺度法研究了侧移刚度和扭转刚度之间有某一关系时的主共振，分析了结构主共振的定常解并且讨论了其稳定性，通过算例分析得到了考虑空间效应时结构的一些有益的结论。     

重力波非共振激发的数值研究

应用二阶算子分裂格式，采用全非线性模式，对可压大气中重力波波包非共振激发进行数值模拟和分析.研究表明：两个向上传播的重力波波包，通过非共振相互作用，也能激发出一个向下传播的重力波波包.生成波的能量几乎正比于主波的能量，也随次波的能量增大而增大.能量传递方向主要表现为从高频的主波向次波和生成波转移，这与共振相互作用不完全相同.非共振作用的特征时间由主次波的初始位置和波长频率决定，与主次波的初始振幅无关.生成波的波矢量并不严格满足弱非线性相互作用理论预言的波矢量匹配条件，而且生成波的波长和频率在传播的过程中会随时间发生变化，这表明了弱非线性相互作用理论的局限性.     

桩土相互作用分析中的动力Winkler模型研究评述

Winkler地基梁集中参数模型在桩一土动力相互作用分析中得到了普遍应用,在现有研究成果的基础上,对桩一土相互作用分析中的动力Winkler模型进行了较全面的综述,指出了各种模型的优缺点,并对有待于深入探讨的问题及发展动向进行了简要评述,为今后深入研究提供了基础。     

遥感反演连续植被叶面积指数的空间尺度效应

叶面积指数（LAI）是描述植被冠层几何结构的最基本的参数,是气候模型、地-气相互作用等过程的重要输入参数．近年来遥感反演LAI的尺度效应问题得到广泛关注．在作物冠层反射率模型基础上,讨论了LAI尺度效应的产生机制,并获得不同尺度之间LAI的转换公式,数值模拟试验和实际遥感图像试验都证明了该尺度转换公式的可靠性．     

Winkler地基梁在温度场中受简谐激励的主共振分析

研究W inkler地基梁在温度场中受简谐激励作用的主共振问题。应用弹性理论和Galerk in方法建立W inkler地基梁在温度场中受简谐激励作用的非线性动力方程。应用非线性振动的多尺度法,求得系统主共振的近似解。分析不同参数对主共振响应曲线的影响。     

Hydrodynamic effects in a reservoir with semicircular cross-section and absorptive bottom

A three-dimensional dam-reservoir system under seismic load is analysed. The dam is assumed to be rigid. The reservoir is an infinite channel with semi-circular cross-section. The exact analytical solution, based on the assumption of potential fluid motion is presented, as well as numerical results for selected parameters.The most significant parameters are: the direction and frequency content of the seismic input; the radiation damping at the reservoir bottom; and the compressibility of the fluid. The response of the system depends strongly on the direction of the input ground motion. This is shown by the transfer functions as well as by the pressure time histories due to two earthquakes with different frequency content. The energy absorption at the reservoir bottom is important. A simple plane-wave model shows, that even for a rock foundation, the amount of transmitted energy can reach up to 80%. For comparison the case without bottom absorption is also shown. Compressbility has to be included to capture the resonance effects. The exact analytical solution is also used to verify numerical results obtained by a new method that combines a finite element model with a rigorous radiation boundary for the infinite channel in the time domain.     

Seismic response of nonstructural components in case of nonlinear structures based on floor response spectra method

This paper investigates the response of nonstructural components in the presence of nonlinear behavior of the primary structure using floor response spectra method (FRS). The effect of several parameters such as initial natural frequency of the primary structure, natural frequency of the nonstructural components (subsystem), strength reduction factor and hysteretic model have been studied. A database of 164 registered ground acceleration time histories from the European Strong-Motion Database is used. Results are presented in terms of amplification factor and resonance factor. Amplification factor quantifies the effect of inelastic deformations of the primary structure on subsystem response. Resonance factor quantifies the variation of the subsystem response considering the primary structure acceleration. Obtained results differed from precedent studies, particularly for higher primary structure periods. Values of amplification factor are improved. Obtained results of resonance factor highlight an underestimation of peak values according to current design codes such as Eurocode 8. Therefore a new formulation is proposed.     

Winkler地基上有限长梁联合共振分析

为进一步完善弹性地基上梁的非线性振动理论以及满足实际工程的需要,本文对三种联合共振情况进行了研究。文中基于已建立的Winkler地基上有限长梁的非线性振动方程,运用Galerkin离散法以及多尺度法推导其稳态运动方程组,分析了主/次联合共振时,结构参数及初始条件对系统幅频响应曲线、调谐-相位曲线的影响,初步探讨了主/超联合共振、次/超联合共振的基本性质。研究结果表明:当且仅当两个激励频率是可公度关系时,联合共振才存在稳态响应;对于一个给定的调谐参数,系统最多存在七个解,而解的最终形式是由初始条件决定;系统参数及初始条件的微小变动,可能会激烈地改变系统的响应;工程中为了较好地抑制结构的振动,必须选择合适的方法提高系统的刚度。     

Coupling between waveguide modes and field line resonances

WKBJ methods are often used to study the excitation of waveguide modes and field line resonances in the magnetosphere. In this paper, these methods are used to study the nature of the reflection and transmission process at a turning point when conditions are favourable for strong excitation of the field line resonance. The accuracy and region of validity of such WKBJ solutions are discussed. In regions where they are not valid, a series solution of the relevant equations is given. The wave fields can be found to great accuracy by matching WKBJ solutions across the turning point and resonance using this series solution. This allows the calculation of the Stokes constant which does not show Airy function behaviour as often assumed. The Stokes constant is effectively a reflection coefficient referred to the turning point. Its phase, when the transmission to the resonance is strong, is nearer to 180° than 90° as often assumed. Its magnitude allows the fraction of incident energy which is transmitted to the resonance to be calculated. These results are expected to be useful in studying the behaviour of waveguide modes.     

Groundwater flow to a system of drainage canals

A classical model of groundwater flow to a system of drainage canals is considered within a framework of a two-dimensional steady-state problem. The solution to this problem is derived based on the Riemann-Schwarts principle of symmetry. This solution yields simple analytical relationships expressed in terms of special or elementary functions. Numerical calculations are used to analyze in detail the effect of all physical characteristics of the model on the flow pattern. In particular, it was established that the presence of water in channels has a significant effect on the flow regime. The limiting cases of the scheme are considered, and simple approximated formulas are derived in these cases for the flow and discharge components.     

A novel analytical solution for a slug test conducted in a well with a finite-thickness skin

An aquifer containing a skin zone is considered as a two-zone system. A mathematical model describing the head distribution is presented for a slug test performed in a two-zone confined aquifer system. A closed-form solution for the model is derived by Laplace transforms and Bromwich integral. This new solution is used to investigate the effects of skin type, skin thickness, and the contrast of skin transmissivity to formation transmissivity on the distributions of dimensionless hydraulic head. The results indicate that the effect of skin type is marked if the slug-test data is obtained from a radial two-zone aquifer system. The dimensionless well water level increases with the dimensionless positive skin thickness and decreases as the dimensionless negative skin thickness increases. In addition, the distribution of dimensionless well water level due to the slug test depends on the hydraulic properties of both the wellbore skin and formation zones.     

Extraction of hysteretic properties of seismically isolated bridges from quick‐release field tests

A simple procedure for identifying hysteretic properties of seismically isolated bridges from full‐scale quick‐release tests is presented in this paper. An analytical solution for the quick‐release response of a SDOF system with a bilinear spring is derived. Based on the solution, some characteristics of such systems are obtained. A time domain optimization method is employed to identify the hysteretic properties of the lead–rubber bearings installed in seismically isolated bridges. The total damping effects of the isolation system are expressed as a combination of the rate‐independent (hysteretic) damping and the linear viscous damping. The Menegotto–Pinto (MP) model and bilinear model are used to represent the force–displacement relation of the lead–rubber bearings. In both the longitudinal and transverse directions the bridges have been idealized as single degree of freedom (SDOF) systems. Time histories recorded from the field quick‐release tests on two bridges are used for the examples presented herein. The hysteretic loops of the isolators obtained from laboratory tests are compared with those obtained using the optimization method, and they agree well. In conclusion, the procedure shown in this paper can be used to identify the essential in situ hysteretic characteristics of isolation bearings from quick‐release field testing. Copyright © 2001 John Wiley & Sons, Ltd.