基于超级单元的钢-混凝土混合结构动力特性及地震反应研究

本文提出了基于超级单元的钢框架－混凝土剪力墙结构体系动力特性及地震反应分析的一种方法，该方法考虑了混合结构体系中钢框架部分的受力和变形特点，即节点柔性（半刚性）、梁柱效应（p-δ效应）及P－△效应（以下合称P－Delta效应），推导了考虑三种因素情况下，钢框架的层间侧移刚度及钢框架－混凝土剪力墙结构体系的超级单元刚度矩阵，采用刚度法求解钢框架－混凝土混合结构周期和振型。该方法物理概念清楚，计算简便。     

土-结构动力相互作用对基础隔震的影响

本文研究土-结构动力相互作用对基础隔震的影响。文中根据间接边界元方法，推导了空间域中的格林函数公式，并建立了地基土的动力刚度矩阵；进而在频域内采用子结构法，建立了考虑土-结构动力相互作用的隔震结构的运动方程；通过数值仿真某具有埋置刚性基础的剪切型基础隔震结构的地震反应，分析了地基土的刚度对隔震效果以及结构地震反应的影响，得到了一些有意义的结论。     

场地动力特性对土-基础相互作用的影响

采用边界元法结合斜面分布荷载格林函数计算了水平层状半空间中的埋置刚性基础在入射P波,SV波和SH波作用下的动力响应,研究了场地动力特性对土-基础动力相互作用的影响。分析表明,层状半空间中土-基础相互作用的机理和均匀半空间有很大区别,且层状半空间中的土-基础相互作用大于均匀半空间中的土-基础相互作用;基础动力特性和场地动力特性较为相似,且土层厚度和刚度对基础动力响应的影响较大,而基础质量的影响则较小;土-基础相互作用的基本规律和土-基础-地上结构相互作用的基本规律相差较大,而与土-地下隧道相互作用则较为相似。     

分层土-基础-高层框架结构相互作用体系振动台模型试验研究

本文设计实现了分层土－基础－高层框架结构相互作用体系的振动台模型试验，再现了地震动激励下上部结构和基础的震害现象和砂质粉土的液化现象。通过试验，研究了相互作用体系地震动反应的主要规律：由于动力相互作用的影响，软土地基中相互作用体系的频率小于不考虑结构－地基相互作用的结构频率，而阻尼比则大于结构材料阻尼比；体系的振型曲线与刚性地基上结构的振型曲线明显不同，基础处存在平动和转动。土层传递振动的放大或减振作用与土层性质、激励大小等因素有关，砂土层一般起放大作用，砂质粉土层一般起减振隔振作用；由于土体的隔震作用，上部结构接受的振动能量较小，各层反应均较小。上部结构顶层加速度反应组成取决于基础转动刚度、平动刚度和上部结构刚度的相对大小。     

软土地层中地下隧道结构地震反应分析

本文从土与结构动力相互作用观点出发,同样条有限元和半解析无限元对地下隧道结构进行抗震分析,该方法具有降低维数,减少自由度,避免人为边界条件处理等特点,并可考虑复杂成层地基的情况。本文为探讨地下隧道结构－土相互作用内在机理提供了一个有效途径。     

确定结构基底等效输入地震动的简化方法

本文用理论和实例计算分析了土与结构间的动力相互作用。根据基底等效输入的地震动相对入射地震动的传递函数特点研究出了一种等效输入的方法，该方法比较好地反映了场地和结构的动力特性对基底等效输入的影响。为利用刚性基底假设理论来分析土-结构动力相互作用提供了一种方法。     

上部结构与地基相对刚度比对土-结构体系基频影响试验研究

本文进行了室内刚性地基上和土槽中1:4钢框架模型的顶部牵引释放试验。对采集的信号进行频谱分析,得到不同刚度时刚性地基上钢框架模型的基频和土槽中土-结构体系的基频。比较钢框架在土槽中的基频和在刚性地基上的基频,发现土-结构动力相互作用(SSI)使土-结构体系基频降低,基频折减率最大为29.37%,且基频折减率与上部结构与地基相对刚度比有关,相对刚度比越大,折减率越大。根据试验结果得出钢框架模型基频折减率随上部结构与地基相对刚度比变化拟合公式。本文试验结果还表明即使在Ⅱ类场地上的结构,当上部结构与地基刚度比较大时有必要考虑土-结构动力相互作用。     

软土地基核电厂房动力响应振动台试验

为了分析软土地基－筏基础核电厂房结构地震反应规律和特征,利用地震模拟振动台开展了软土地基－筏基础－核电厂房动力相互作用问题的试验研究。分别进行了表面水平土体模型和表面凹陷土体模型的运动相互作用试验、地基土－筏基础－核电厂房振动台相互作用试验、核电厂房直接固定在振动台面上的刚性基底振动台试验。试验采用圆形叠层剪切模型箱,地基土模型为某工程场地的均匀粉质粘土,其剪切波速为213 m/s;核电厂房简化为3层框架剪力墙结构模型。试验输入波形为美国核电规范常用的RG1.60反应谱合成得到的人工地震动时程。振动台试验结果对比分析表明:土－结构体系中系统的振动周期和阻尼明显大于刚性基底下结构的振动周期和阻尼;相同地震作用下在土－结构动力相互作用体系中结构加速度明显小于刚性基底下的结构加速度反应;而位移明显大于刚性基底下结构的位移。本文的研究成果可为软土地基建立核岛厂房的适应研究提供参考。     

隔震板柱结构-基础-地基相互作用地震反应分析

对广州地区一例地基-基础-隔震板柱结构动力相互作用体系进行了计算分析.通过与常规设计方法及非隔震体系的比较,研究了该体系地震反应的变化规律,并分析了阻尼比、地基土特性、基础刚度、基础型式、基础埋深、土体深度、上部结构刚度和地震波等因素对相互作用体系动力特性及地震反应的影响.     

大型基础等效输入地震波研究

提出了一种考虑土与结构相互作用的"等效输入"概念,所谓等效输入就是采用合理的方法获得基础顶面的地震动,认为此地震动已充分考虑土与结构相互作用,可以作为抗震设计中刚性地基假定下的输入地震动.这种方法可以把开放体系中的动力问题转换到封闭系统中,这样既可以利用传统的刚性地基假设,又考虑了土与结构相互作用.用此方法对带箱形基础的框架结构模型进行计算,验证了等效输入的计算精度.     

Strain effects on kinematic pile bending in layered soil

The kinematic bending of single piles in two-layer soil is explored to account for soil stiffness degradation and associated damping increase with increasing levels of shear strain, a fundamental aspect of soil behaviour which is not incorporated in current simplified seismic design methodologies for pile foundations.A parametric study of a vertical cylindrical pile embedded in a two-layer soil profile to vertically-propagating S waves, carried out in the time domain by a pertinent beam-on-dynamic-Winkler-foundation (BDWF) model, is reported. Strain effects are treated by means of the equivalent-linear procedure which provides soil stiffness and damping ratio as function of shear strain level. Whereas the approach still represents a crude representation of the actual soil behaviour to dynamic loading, it is more realistic than elementary solutions based on linear visco-elasticity adopted in earlier studies.The paper highlights that soil nonlinearity may have either a detrimental or a beneficial effect on kinematic pile bending depending on the circumstances. The predictive equations for kinematic pile bending in visco-elastic soil recently developed by the Authors are extended to encompass strain effects. Numerical examples and comparisons against experimental data from case histories and shaking table tests are presented.     

Nonlinear lateral interaction in pile dynamics

A model for pile lateral response to transient dynamic loading and to harmonic loading is presented allowing for nonlinear soil behaviour, discontinuity conditions at the pile-soil interface and energy dissipation through different types of damping. The approach is used to establish equivalent linear stiffness and damping parameters of single piles as well as dynamic interaction factors for approximate nonlinear analysis of pile groups. The applicability of these parameters to the pile-group analysis was examined, and a reasonable agreement with the direct analysis was found. The superposition technique may be used to analyze the response of small pile groups. Also, the dynamic stiffness of pile groups is greatly affected by both the nonlinear behavior of the soil and the slippage and gapping between the pile and soil. For a basic range of soil and pile parameters, equivalent linear stiffness and damping parameters of single piles and interaction factors for approximate nonlinear analysis are provided.     

Dynamic response of flexible strip-foundations by boundary and finite elements

A time domain Boundary Element-Finite method is employed to determine the dynamic response of flexible surface two-dimensional foundations under conditions of plane strain placed on an elastic soil medium and subjected either to transient external forces or to obliquely incident seismic waves. The elastic, isotropic, and homogeneous soil medium is treated by the time domain Direct Boundary Element Method, while the flexible foundation is treated by the Finite Element Method. The two methods are appropriately combined through equilibrium and compatibility considerations at the soil-foundation interface. Parametric studies examining the effect of the relative stiffness between the foundation and the soil and the spatial distribution of the dynamic disturbances on the foundation response are presented.     

Dynamic stiffness and kinematic response of single piles in inhomogeneous soil

The effect of soil inhomogeneity on dynamic stiffness and kinematic response of single flexural elastic piles to vertically-propagating seismic SH waves is explored. A generalized parabolic function is employed to describe the variable shear wave propagation velocity in the inhomogeneous stratum. A layered soil with piece-wise homogeneous properties is introduced to approximate the continuous inhomogeneity in the realm of a Beam-on-Dynamic-Winkler-Foundation model. The problem is treated numerically by means of a layer transfer-matrix (Haskell–Thompson) formulation, and validated using available theoretical solutions and finite-element analyses. The role of salient model parameters such as pile-head fixity conditions, pile-to-soil stiffness ratio, surface-to-base shear wave velocity ratio and rate of inhomogeneity is elucidated. A new normalization scheme for inertial and kinematic response of such systems is presented based on an average Winkler wavenumber. With reference to long piles in moderately inhomogeneous soils, results indicate that: (a) kinematic pile response is essentially governed by a single dimensionless frequency parameter accounting for pile-to-soil stiffness ratio, pile slenderness and soil inhomogeneity and (b) definition of a characteristic pile wavelength allows an approximate estimation of pile elastodynamic response for preliminary design or analysis. Issues related to domain discretization and Winkler moduli are discussed.     

First-order doubly-asymptotic formulation of the direct stiffness method for elastodynamic problems

A first-order formulation to analyze the dynamic response of layered soil profiles is presented as an alternative to the widely used second-order thin-layer method by the direct stiffness approach, including an efficient simulation of the underlaying elastic half-space. In contrast to the thin-layer method where response is expressed through a combination of second-order propagation modes, the proposed procedure uses first-order modal parameters that have the capacity to provide a good approximation in the complete wave number domain k, including the exact stiffness values for k=0 and k→∞, thus justifying its designation of doubly-asymptotic. This feature allows obtaining the exact soil profile response for static loads, while the proposed treatment of the elastic half-space reproduces naturally the radiation condition without a need of artificial damping. The capacity of the proposed formulation to solve elastodynamic problems is assessed by comparing its results with those of exact solutions available in the literature, and numerical solutions of rigid disks supported on the surface of different soil profiles.     

Kinematic response functions and dynamic stiffnesses of bridge embankments

Recognizing that soil–structure interaction affects appreciably the earthquake response of highway overcrossings, this paper compares approximate analytical solutions and finite element results to conclude on a simple procedure that allows for the estimation of the kinematic response functions and dynamic stiffnesses of approach embankments. It is shown that the shear‐wedge model yields realistic estimates for the amplification functions of typical embankments and reveals the appropriate levels of dynamic strains which are subsequently used to estimate the stiffness and damping coefficients of embankments. The shear‐wedge model is extended to a two‐dimensional model in order to calculate the transverse static stiffness of an approach embankment loaded at one end. The formulation leads to a sound closed‐form expression for the critical length, L_c, that is the ratio of the transverse static stiffness of an approach embankment and the transverse static stiffness of a unit‐width wedge. It is shown through two case studies that the transverse dynamic stiffness (‘spring’ and ‘dashpot’) of the approach embankment can be estimated with confidence by multiplying the dynamic stiffness of the unit‐width wedge with the critical length, L_c. The paper concludes that the values obtained for the transverse kinematic response function and dynamic stiffness can also be used with confidence to represent the longitudinal kinematic response function and dynamic stiffness, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Dynamic finite element analysis of three-dimensional soil models with a transmitting element

A method for the dynamic finite element analysis of a non-axisymmetric soil model with an axisymmetric boundary is presented. In the non-axisymmetric soil domain an arbitrary discretization with three-dimensional isoparametric solid elements is used. At the boundary a transmitting element is arranged. It is based on the semi-analytical element of Waas and Kausel. The transformation of the stiffness matrix of the Waas/Kausel element with cyclic symmetric displacements to general displacement fields is presented. For earthquake excitation the forces acting on the discretized domain are given. The method is illustrated by the dynamic analysis of an embedded box-type building. The distribution and magnitude of significant section forces are discussed.     

Soil–structure interaction in resonant railway bridges

This paper explores dynamic soil–bridge interaction in high speed railway lines. The analysis was conducted using a general and fully three-dimensional multi-body finite element–boundary element model formulated in the time domain to predict vibrations caused by trains passing over the bridge. The vehicle was modelled as a multi-body system, the track and the bridge were modelled using finite elements and the soil was considered as a half-space by the boundary element method. The dynamic response of bridges to vehicle passage is usually studied using moving force and moving mass models. However, the multi-body system allows to consider the quasi-static and dynamic excitation mechanisms. Soil–structure interaction was taken into account by coupling finite elements and boundary elements. The paper presents the results obtained for a simply supported short span bridge in a resonant regime under different soil stiffness conditions.     

Response of unbounded soil in scaled boundary finite‐element method

The scaled boundary finite‐element method is a powerful semi‐analytical computational procedure to calculate the dynamic stiffness of the unbounded soil at the structure–soil interface. This permits the analysis of dynamic soil–structure interaction using the substructure method. The response in the neighbouring soil can also be determined analytically. The method is extended to calculate numerically the response throughout the unbounded soil including the far field. The three‐dimensional vector‐wave equation of elasto‐dynamics is addressed. The radiation condition at infinity is satisfied exactly. By solving an eigenvalue problem, the high‐frequency limit of the dynamic stiffness is constructed to be positive definite. However, a direct determination using impedances is also possible. Solving two first‐order ordinary differential equations numerically permits the radiation condition and the boundary condition of the structure–soil interface to be satisfied sequentially, leading to the displacements in the unbounded soil. A generalization to viscoelastic material using the correspondence principle is straightforward. Alternatively, the displacements can also be calculated analytically in the far field. Good agreement of displacements along the free surface and below a prism foundation embedded in a half‐space with the results of the boundary‐element method is observed. Copyright © 2001 John Wiley & Sons, Ltd.     

Investigation of the dynamic response of rigid footings on tensionless Winkler foundation

The dynamic response of a rigid footing resting on an elastic tensionless Winkler foundation is examined. A parametric investigation, concerning the effect of the main parameters on the response, is performed for harmonic excitation. The parameters examined include the stiffness and the damping of the foundation, the excitation frequency and the superstructure characteristics and loads. The maximum rocking response, the minimum length of contact after uplift, the maximum stress developed at the soil and the factor of safety with respect to the bearing capacity of the soil are used to measure the effect of each dimensionless parameter. An example for earthquake excitation is also given for a plane frame. The results are compared to the ones of a simplified static approach based on the maximum values of the applied loads, similarly to the procedure that is usually applied in practice. The results show that the static approach can predict the response satisfactorily if resonance does not happen, if the stiffness of the foundation is not large compared to the stiffness of the superstructure and if the dynamic part of the axial force of the column is not large; in these cases, it may underestimate or overestimate the response significantly, depending on the sign of the dynamic axial force that is considered.