桩-土-结构动力相互作用的线弹性地震反应分析

采用集中质量法(简化模型)，用ANSYS软件作为桩—土—结构动力相互作用分析的工具，建立了小震下钢筋混凝土剪切型结构考虑桩—土—结构动力相互作用效应的计算模型，进行了桩—土—结构相互作用线性体系的模态分析，研究了考虑桩—土—结构相互作用体系的自振特性；进行了小展下桩—土—结构相互作用体系弹性地震反应时程分析，研究了土—结构动力相互作用效应对结构地震反应的影响；得出如下结论；考虑桩—土—结构相互作用效应后，结构体系的自振特性及结构的地震反应将有所改变。     

桩-土相互作用弹簧对倾斜液化场地-群桩-上部结构体系的影响

桩-土-上部结构体系的动力相互作用是一个复杂的过程,尤其是在倾斜液化侧向扩展流动（侧扩流）场地中,由于地震过程中场地产生地面永久大变形,桩土间有可能产生错动滑移与开裂等非线性反应,因此桩-土相互作用模拟至关重要。为了探究桩-土非线性接触对倾斜液化场地-群桩基础-上部结构体系动力响应的影响,本文基于OpenSees分别建立了考虑桩-土相互作用弹簧和桩土结点之间直接绑定的有限元数值模型。结果表明：考虑桩-土相互作用Pyliq弹簧时,土体加速度幅值略微降低,桩基对土体的约束明显变弱,土体残余位移增大。同时,具有Pyliq弹簧的模型能较好地模拟桩的曲率响应,而采用桩土结点直接绑定的模型高估了桩顶曲率,进而无法准确估计桩基抗弯最不利位置。桩-土相互作用弹簧对上部结构动力响应的影响较小。     

群桩-土-墩柱相互作用体系抗震性能试验及数值模拟研究

为进一步研究土-结构相互作用(SSI)体系的抗震性能,以1∶4比例尺桥梁墩柱模型为试验对象,考虑土-结构相互作用,通过拟静力试验观察了试验现象,得到了模型滞回曲线、耗能能力等数据。通过有限元软件ABAQUS建立了与试验情况相同的有限元模型,并分析计算。建模分析结果与试验结果一致,在此基础上,以本模型为对象进行了地震反应时程分析,得到了不同地基条件下的位移时程曲线及桩身应力、桩身位移响应曲线,并用m法对比计算桩的位移响应曲线。分析结果表明:不同地基条件下群桩-土-刚度较大墩柱结构体系破坏形式不同;在地震作用下,短桩基础在桩的中段处应力达到最大值,有必要在此处予以加强;在地震作用较大时,m法计算的桩顶位移偏小,且m法无法计入群桩效应,可能导致群桩基础内力的计算偏于不安全,建议予以重视。     

桩-土-结构相互作用分析的等效计算桩长简化模型

在一定程度上,桩长是影响桩-土-结构体系动力分析复杂程度的关键因素之一,在桩-土-结构相互作用的数值模拟中对桩长进行适当简化可以提高计算效率,尤其对具有大量长桩基础的结构体系。基于Boulanger模型和OpenSees软件,分析了软粘土地基-单桩结构体系地震反应中桩身的位移、弯矩、剪力的分布特点以及桩顶上部结构的加速度响应,探讨了结构体系振型及振型周期随桩长的变化特点,进一步提出了等效计算桩长的桩-土-结构模型。研究表明,当结构体系前3阶的振型周期的变化率控制到2.5%时,对应的等效计算桩长分析模型能实现较高的动力响应计算精度,其动力响应误差已降低至5%以内;等效计算桩长可以通过动力响应误差控制精度要求确定,对于软粘土地基中的单桩基础结构,建议将前3阶振型周期的变化率控制到2.5%时的计算桩长作为等效计算桩长。     

考虑土-结构动力相互作用的TMD结构减震控制

本文分析了TMD(Tuned mass damper)在刚性地基和柔性地基情况下的减震控制机理,以某6层钢筋混凝土框架结构为研究对象,分别考虑了土-结构动力相互作用对无TMD控制结构的影响,场地条件对TMD减震控制性能的影响和土-结构动力相互作用对TMD减震控制性能的影响。通过分析得出TMD控制系统的减震效果除了与输入地震动特性有关外,还与场地条件、上部结构和基础的动力特性等因素有关。如果土-结构动力相互作用体系的自振周期远离输入地震动的卓越周期,则相互作用体系的地震响应较小。地基土越软,框架建筑结构层间相对位移地震响应也就越小。如果考虑土-结构动力相互作用效应的影响设计TMD调频系统的自振周期,则TMD的控制效果会有一定程度的提高。     

瑞利波作用下桩筏基础的动力响应

对瑞利波作用下桩筏基础的动力响应进行了数值分析。采用薄层元素法和有限单元法建立了土-桩筏基础动力相互作用的分析模型,并讨论了桩筏基础在瑞利波作用下的水平与摇摆动力响应。讨论了一些重要参数(如桩长、桩距、桩的刚度以及土的泊松比等)对桩筏基础的动力响应的影响。结果表明:不考虑筏板-土相互作用,基础的动力响应在高频区域会产生较大的差别;在瑞利波作用下,桩筏基础会产生显著的摇摆响应,且桩距、桩长和土的泊松比对其影响较大,而桩土刚度比对其影响较小。     

爆破地震作用下桩-土-结构相互作用的数值模拟

土-结构动力相互作用是地震工程和结构抗震的重要研究内容,但目前对爆破地震作用下土-结构动力相互作用的研究较少。运用大型有限元软件ANSYS/LS-DYNA,建立了桩-土-结构相互作用体系的三维有限元模型,由桩尖输入实测爆破地震波,取得了良好的计算效果。计算结果表明：考虑桩-土-结构相互作用后,群桩基础中每个桩的位移、加速度和剪应力幅值均呈桩顶大、桩尖小的倒三角分布,桩与承台的接合部比较容易受到损坏;桩-土-结构相互作用体系在爆破地震波冲击后,还会发生几次振动,但是这些振动产生的影响要小于爆破地震产生的影响,这与实测结果相符合;爆破地震波冲击下,群桩基础中,角桩顶部表面的桩土接触压力较大,但在爆破地震波冲击后,中心桩顶部表面的桩土接触压力较大,且具有一定的周期性,直至衰减为零。     

复杂结构-桩-土振动台模型试验数据分析

本文从分析复杂结构-桩-土振动台模型试验数据入手,用加速度动力系数、最大位移、最大正应变、最大动土压力等指标对结构的地震响应全面分析,并对比桩、地下结构、地上结构的不同响应,研究发现:结构不同部分最大地震响应发生的频率不尽相同,且受地震动频谱特性及自身频率影响,天津波加载时结构的地震响应较大;地表以下,当震级较小时,土-结构对地震动起放大作用,随着震级的增加,对地震动放大作用减缓甚至减小;最大位移随结构高度增加逐渐增大,在桩与地下结构交界处和地表处,位移改变较大;地下结构柱、桩最大正应变呈中间大、两头小分布;最大动土压力随着深度增加呈两头大、中间小分布,且地表处最大;总的土压力受最大动土压力影响较大,随深度增加有先降低、后增大的趋势.     

层状土介质中群桩及其上部结构体系对任意入射地震波的响应

本文舍弃桩基动力分析中的土介平面应变假定，采用薄层内位移纡性变化条件下的动力Ｇｒｅｅｎ函数形成桩－土－桩相互作用所需的土介质柔度矩阵，用梁单元模拟桩的运动，用振型分解方法来考虑上部结构与桩基承台间的动力相互作用，以任意地震波入射为初始运动输入，建立了层状土介质中群桩及其上部结构体系对放射地震波响应的半解析分析模型，研究了桩基及其上部结构体系对不同角度入射、ＳＨ、ＳＶ和Ｐ波的动力响应，结果表明，本文     

接触对桩-土-结构动力相互作用体系的影响

为了考察桩-土接触效应对结构地震反应的影响,利用有限元软件ABAQUS建立了土-桩-框架二维有限元模型,分别采用损伤塑性模型和动力粘塑性记忆型嵌套面模型模拟混凝土和土体,利用rebar单元模拟混凝土内的钢筋,取得了较好的计算效果.计算分析中采用19条不同频谱的地震波记录,考虑了地震动强度、桩径、摩擦系数等因素,以层间位移角和桩顶最大位移为主要评价指标,揭示相互作用体系的动力响应特性.分析认为,计算结果对桩、土摩擦系数的取值不敏感;不考虑土-桩接触时,近场土体的动力反应与实际情况存在一定的误差,且上部结构和桩基的动力反应会被低估,应该考虑桩-土动力接触效应;地震动强度增加时,随着结构进入塑性状态,低估程度减小;桩径增加时,低估程度没有显著变化,虽然桩基和上部结构的反应都有所减小.     

Experimental study of the effect of inclined pile on the seismic behavior of pile group

Seismic behavior of inclined piles has been considered detrimental for years. However, recent researches show that battered piles can have a beneficial effect. In this framework, a series of centrifuge tests on an inclined pile group is performed. The analysis is based on the comparative response of two 2×1 simplified pile groups: one with vertical piles and the other with one vertical and one inclined pile. The response of these pile groups to repeated earthquakes or sinusoidal inputs is analyzed through the response frequencies, the envelop curves of bending moment profiles, the axial loads measured in both piles and the kinematic response of the cap. Results highlight that the effect of inclined pile is highly influenced by the frequency content of the input. In addition, the inclined pile induces non-negligible residual bending moments, higher horizontal stiffness at the pile cap and larger rotation.     

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.     

Kinematic bending moments in pile foundations

In this paper the kinematic seismic interaction of single piles embedded in soil deposits is evaluated by focusing the attention on the bending moments induced by the transient motion. The analysis is performed by modeling the pile like an Euler–Bernoulli beam embedded in a layered Winkler-type medium. The excitation motion is obtained by means of a one-D propagation analysis. A comprehensive parametric analysis is carried out by varying the main parameters governing the dynamic response of piles like the soil properties, the bedrock location, the diameter and embedment in the bedrock of piles. On the basis of the parametric analysis, a new design formula for predicting the kinematic bending moments for both the cross-sections at the deposit–bedrock interface and at the pile head is proposed.     

Seismic response of single-column bent on pile: evidence of beneficial role of pile and soil inelasticity

While seismic codes do not allow plastic deformation of piles, the Kobe earthquake has shown that limited structural yielding and cracking of piles may not be always detrimental. As a first attempt to investigate the consequences of pile yielding in the response of a pile-column supported bridge structure, this paper explores the soil–pile-bridge pier interaction to seismic loading, with emphasis on structural nonlinearity. The pile–soil interaction is modeled through distributed nonlinear Winkler-type springs and dashpots. Numerical analysis is performed with a constitutive model (Gerolymos and Gazetas 2005a, Soils Found 45(3):147–159, Gerolymos and Gazetas 2005b, Soils Found 45(4):119–132, Gerolymos and Gazetas 2006a, Soil Dyn Earthq Eng 26(5):363–376) materialized in the OpenSees finite element code (Mazzoni et al. 2005, OpenSees command language manual, p 375) which can simulate: the nonlinear behaviour of both pile and soil; the possible separation and gapping between pile and soil; radiation damping; loss of stiffness and strength in pile and soil. The model is applied to the analysis of pile-column supported bridge structures, focusing on the influence of soil compliance, intensity of seismic excitation, pile diameter, above-ground height of the pile, and above or below ground development of plastic hinge, on key performance measures of the pier as is: the displacement (global) and curvature (local) ductility demands and the maximum drift ratio. It is shown that kinematic expressions for performance measure parameters may lead to erroneous results when soil-structure interaction is considered.     

Dynamic pile response using two pile-driving techniques

Because of disadvantages caused by pile top driving, a new pile driving technique by which the hammer is inside the pile has been developed by contractors. The so called “down-the hole” piling system is used to drive a tubular pile in an experimental set-up in the laboratory. This new technique is tested and compared with pile top driving using similar hammer energy. A reduction of noise and the opportunity to save steel are confirmed during a field test program. Also, a reduction in driving time and a higher bearing capacity have been observed. Some similar conclusions are arrived at in the laboratory study, specially the low level of stresses in the pile and the shaft friction.     

Nonlinear analysis for dynamic lateral pile response

An analysis of pile lateral response to transient dynamic loading and to harmonic loading is presented allowing for nonlinear soil behavior, discontinuity conditions at the pile-soil interface and energy dissipation through different types of damping. Furthermore, the effect of neighbouring piles is taken into account for piles in a group. The validity of the approach was examined and a reasonable agreement with field tests and more rigorous solutions was found. Equivalent linear stiffness and damping parameters of single piles and interaction factors for approximate nonlinear analysis are presented.     

Transient kinematic pile bending in two-layer soil

The dynamic response of piles to seismic loading is explored by means of an extensive parametric study based on a properly calibrated Beam-on-Dynamic-Winkler-Foundation (BDWF) model. The investigated problem consists of a single vertical cylindrical pile, modelled as an Euler–Bernoulli beam, embedded in a subsoil consisting of two homogeneous viscoelastic layers of sharply different stiffness resting on a rigid stratum. The system is subjected to vertically propagating seismic S waves, in the form of a transient motion imposed on rock outcrop. Several accelerograms recorded in Italy are employed as input motions in the numerical analyses. The paper highlights the severity of kinematic pile bending in the vicinity of the interface separating the two soil layers. In addition to factors already investigated such as layer stiffness contrast, relative soil–pile stiffness, interface depth and intensity of ground excitation, the paper focuses on additional important factors, notably soil material damping, stiffness of Winkler springs and frequency content of earthquake excitation. Existing predictive equations for assessing kinematic pile bending at soil layer interfaces are revisited and new regression analyses are performed. A synthesis of findings in terms of a set of simple equations is provided. The use of these equations is discussed through examples.     

Soil reaction to lateral harmonic pile motion

The analytical representation of dynamic soil reaction to a laterally-loaded pile using 3D continuum modeling is revisited. The governing elastodynamic Navier equations are simplified by setting the dynamic vertical normal stresses in the soil equal to zero, which uncouples the equilibrium in vertical and horizontal directions and allows a closed-form solution to be obtained. This physically motivated approximation, correctly conforming to the existence of a free surface, was not exploited in earlier studies by Tajimi, Nogami and Novak and leads to a weaker dependence of soil response to Poisson's ratio which is in agreement with numerical solutions found in literature. The stress and displacement fields in the soil and the associated reaction to an arbitrary harmonic pile displacement are derived analytically using pertinent displacement potentials and eigenvalue expansions over the vertical coordinate. Both infinitely long piles and piles of finite length are considered. Results are presented in terms of dimensionless parameters and graphs that highlight salient aspects of the problem. A detailed discussion on wave propagation and cutoff frequencies based on the analytical findings is provided. A new dimensionless frequency parameter is introduced to demonstrate that the popular plane-strain model yields realistic values for soil reaction only at high frequencies and low Poisson's ratios.     

地震作用下液化场地变截面桩与等截面桩的动力响应对比分析

为研究液化场地变截面桩的动力响应,依托翔安大桥实体工程,采用有限元软件,建立变截面桩－土和等截面桩－土相互作用模型,模拟液化场地变截面桩及等截面桩在地震作用下的振动反应,分析在地震作用下变截面位置不同的变截面桩及等截面桩的动力响应特征。结果表明:地震作用下,液化土层不同深度处的孔压比变化规律基本相同,均从0逐渐增大最后趋于稳定;变截面桩的桩身加速度和桩身位移均大于等截面桩,且桩顶加速度峰值出现的时刻均滞后于桩底;在饱和砂土层处,桩身位移变化趋势均较陡;变截面桩的桩身弯矩峰值和桩身剪力峰值均大于等截面桩,且其峰值出现的位置较等截面桩深;地震作用下,变截面桩及等截面桩的弯矩与剪力均在安全范围之内;液化场地变截面梁桥桩基础抗震设计时,应着重分析液化土层与非液化土层分界面以下的抗弯能力设计及液化土层中抗剪能力设计。