桩土作用对渡槽动力特性影响分析

现有的渡槽数值研究中,都将结构底部作刚接的简化处理,人为地增大了结构的刚度,与实际情况不相符合.本文基于势流体假设建立渡槽-水流固耦合二维联体式渡槽模型,分别计算比较刚性地基约束情况和考虑桩土作用的弹性地基约束情况下的渡槽结构在不同水深工况下的计算结果,分析各工况下土-结构相互作用的影响变化.结果表明,槽内水体作用和槽底桩土作用均使结构自振频率降低,并且前者对后者的影响不容忽视.     

上部结构刚度改变对桩-土-杆系结构动力相互作用的影响

采用改进的Penzien模型 ,利用研制开发的桩 -土 -杆系结构动力相互作用分析程序DIPSFSA ,对带支撑和不带支撑的钢框架均按刚性基础和考虑桩土参与作用两种情况进行地震反应分析 ,研究上部结构刚度改变对桩 -土 -结构动力相互作用的影响。结果表明 ,在Ⅲ、Ⅳ类场地土条件下 ,无论上部结构是否加设支撑 ,桩土参与共同工作对上部结构地震反应均有不容忽视的影响 ,且上部结构相对于地基的刚度越大 ,桩 -土 -上部结构之间的共同工作效果越明显。从而说明 ,软土地基上结构刚度越大 ,结构不一定越安全。     

寒冷地区750kV联合构架有限元分析

通过分析讨论节点构造和刚度对联合构架连接方式的影响及在有限元建模中的处理方法。应用M IDAS/Gen结构设计有限元分析软件,建立了750kV变电联合构架的全刚性梁单元和杆梁混合单元两种模型。计算出在常用控制工况下结构的应力、位移、自振特性,得到了大风、低温作用及两种模型对联合构架的影响结果,可供高压变电联合构架的整体分析与设计参考。     

X形钢箱系杆拱人行桥设计

在分析现有十字路口人行过街方式的基础上,提出X形钢箱系杆拱人行桥方案.天桥主结构由系杆拱与相交梁两大部分组成,两段相交梁刚接于系杆跨中,平面为X形.通过杆系模型与实体模型对结构的静力指标、吊杆换索、自振频率、屈曲系数和拱脚局部受力进行分析.分析结果表明,刚性系杆刚性拱面内刚度较大,相交梁刚接在系杆跨中的方式可行,其他十...     

钢框架-筏基结构与土相互作用试验研究

介绍了室内土槽中1∶4钢框架模型的低频激振试验,利用激振器输出地震波激振钢框架模型,拾振器测量楼层加速度和位移响应,通过频谱分析测得模型的基频和自振周期,求得减震系数。采用SAP 2000对刚性地基上模型输入与试验相同的地震波,计算出模型的基频、自振周期、顶层加速度峰值和顶层位移峰值,并与试验值进行对比。结果表明,考虑土-结构相互作用后,结构的自振周期均有所延长,刚度相同、楼层质量大的模型周期延长比较大,顶层位移峰值增大,加速度峰值减小。绘制出位移放大系数和加速度折减系数随结构刚度的变化曲线,对位移放大系数和加速度折减系数随结构 (k/m)的变化规律进行拟合,得到可供工程抗震设计参考的拟合曲线。     

考虑不同桩土效应分析模型的双座串联斜拉桥动力特性分析

以双座串联大跨度斜拉桥——洪鹤大桥为背景,利用Midas/Civil分别建立了采用直接嵌固模型、等效嵌固模型、六弹簧模型、Winkler地基梁模型共四种不同桩土效应分析模型下的的全桥有限元模型,并对所有的有限元模型进行了动力特性分析。分析结果表明:双座串联大跨度斜拉桥具有自振周期较长、自振频率较低、模态密集、振型在东西桥交替出现等特点。同时发现考虑桩土作用会降低结构的整体刚度,延长结构的自振周期,会改变高阶振型,不同桩土效应模型对结构动力特性影响效果不同。对于软土场地上的大跨度斜拉桥推荐使用六弹簧模型和温克尔地基梁模型模拟桩土效应。     

土-结构相互作用下弦支穹顶结构动力性能分析

目前对弦支穹顶结构抗震计算一般基于刚性地基假定,但由于基础周围土体并非刚性,在地震作用下会产生变形。为研究弦支穹顶结构在地震作用下考虑土结构相互作用动力响应,建立跨度为92 m的弦支穹顶结构土-结构相互作用有限元模型。基于修正S-R(Sway-Rocking)方法的土体简化模型,分析土-结构相互作用下弦支穹顶结构的自振特性及地震响应特征。研究表明:考虑土-结构相互作用后,弦支穹顶结构的自振频率明显减小;在地震作用下,弦支穹顶上部网壳结构的杆件最大压应力和节点最大位移明显增大,而撑杆最大压应力和斜拉杆及环索的最大拉应力均有所减小。     

考虑桩-土效应对土质地基拱桥动力特性的影响研究

本文探讨了考虑桩-土效应的计算方法,并对采用"m法"计算土弹簧刚度给出了计算公式。通过一座土质地基上的板拱桥的实例分析,研究了土质地基条件下桩-土效应对桥梁结构动力特性的影响。研究结果表明土质地基条件下考虑桩-土效应后结构的自振频率普遍降低,频率阶次越高,降低的程度越大。根据计算结果,建议在土质地基结构动力特性分析时,必须考虑桩-土效应,土弹簧的刚度可根据"m法"进行计算。     

砂卵石土地基-筏板-巨型框架结构动力相互作用研究

以砂卵石土动力特性三轴试验为基础,结合结构与地基动力相互作用理论,利用通用有限元软件ANSYS,模拟分析了砂卵石土地基-筏板-巨型框架结构体系在地震作用下地震反应的主要规律：由于动力相互作用的影响,砂卵石土地基中相互作用体系的频率小于不考虑结构-地基相互作用的频率;基础存在平动和转动,致使相互体系与刚性地基上结构体系的顶层位移最大值和加速度有明显不同,地基土传递地震作用具有放大或减振的作用,这与地基土的性质、激励大小等因素有关,砂卵石土地基一般具有减振的作用,致使上部结构接受的地震能量较少,各层反应均较小;同时,基础的刚度对上部结构的地震反应也有明显影响.     

砂卵石土地基-筏板-巨型框架结构动力相互作用研究

以砂卵石土动力特性三轴试验为基础,结合结构与地基动力相互作用理论,利用通用有限元软件ANSYS,模拟分析了砂卵石土地基-筏板-巨型框架结构体系在地震作用下地震反应的主要规律:由于动力相互作用的影响,砂卵石土地基中相互作用体系的频率小于不考虑结构-地基相互作用的频率;基础存在平动和转动,致使相互体系与刚性地基上结构体系的顶层位移最大值和加速度有明显不同,地基土传递地震作用具有放大或减振的作用,这与地基土的性质、激励大小等因素有关,砂卵石土地基一般具有减振的作用,致使上部结构接受的地震能量较少,各层反应均较小;同时,基础的刚度对上部结构的地震反应也有明显影响。     

基于刚体准则的空间框架弹塑性非线性分析方法

基于刚体准则,采用二阶改进塑性铰模型,构建了空间弹塑性梁单元及其弹塑性刚度矩阵,建立了高效简洁的非线性增量迭代方法,可有效分析截面屈服产生的有限转动、二阶效应等柔性空间钢框架结构材料与几何非线性耦合效应。对于受初始力平衡的单元,随着单元的刚体转动与移动,其初始平衡力在当前状态下应保持大小不变,仅方向随单元做刚体转动,而实际结构的变形可以视为较大的刚体位移与较小的自然变形(弹性或非弹性)的组合。将此刚体准则植入增量迭代法,在预测阶段采用通过了刚体运动检验的弹塑性矩阵,从而合理确定迭代初始方向;在修正阶段使用刚体准则计算单元结点力,保证了计算精度。对三个典型柔性框架结构所做分析表明,本文方法能够准确预测结构的极限承载力值与塑性铰发展过程。对于空间框架结构,每根杆件仅需划分一个单元,极大提高了计算效率、降低了计算成本。与塑性区法、纤维元法以及考虑截面翘曲的修正切线刚度法等相比,本文提出的方法具有物理概念明确、单元划分少、刚度矩阵简单、分析过程简洁、计算精度与效率高等显著特点,适于工程应用。     

Implications of subsoil-foundation modelling on the dynamic characteristics of a monitored bridge

Model updating based on system identification (SI) results is a well-established procedure to evaluate the reliability of a developed numerical model. In this inverse assessment problem, soil-foundation compliance is often not explicitly considered rigorously during design and/or purely numerical assessment. The present work aims to investigate the correlation between subsoil-foundation stiffness and modal characteristics of bridges, as a means to identify a threshold beyond which rigorous subsoil modelling is a prerequisite for reliable model updating. The second Kavala Ravine Bridge, in Greece, serves as the case study for this purpose for which a reasonably reliable finite element (FE) model is developed and updated based on ambient vibration measurements. Alternative soil profiles and subsequently redesigned foundation systems are then used to examine the effect that the correspondingly variable soil compliance would have on the natural frequencies of the bridge. It is shown that soil stiffness alone is not an adequate proxy to decide on the necessity for subsoil modelling, as the foundation stiffness (particularly in cases of softer soil profiles) tends to balance the dynamic properties of the holistic soil-foundation system. The soil-foundation stiffness is therefore the key parameter that dictates the need for refined modelling of soil–structure interaction in the framework of SI-based model updating.     

Assessment of natural frequency of installed offshore wind turbines using nonlinear finite element model considering soil-monopile interaction

A nonlinear finite element model is developed to examine the lateral behaviors of monopiles, which support offshore wind turbines (OWTs) chosen from five different offshore wind farms in Europe. The simulation is using this model to accurately estimate the natural frequency of these slender structures, as a function of the interaction of the foundations with the subsoil. After a brief introduction to the wind power energy as a reliable alternative in comparison to fossil fuel, the paper focuses on concept of natural frequency as a primary indicator in designing the foundations of OWTs. Then the range of natural frequencies is provided for a safe design purpose. Next, an analytical expression of an OWT natural frequency is presented as a function of soil-monopile interaction through monopile head springs characterized by lateral stiffness K_L, rotational stiffness K_R and cross-coupling stiffness K_LR, of which the differences are discussed. The nonlinear pseudo three-dimensional finite element vertical slices model has been used to analyze the lateral behaviors of monopiles supporting the OWTs of different wind farm sites considered. Through the monopiles head movements (displacements and rotations), the values of K_L, K_R and K_LR were obtained and substituted in the analytical expression of natural frequency for comparison. The comparison results between computed and measured natural frequencies showed an excellent agreement for most cases. This confirms the convenience of the finite element model used for the accurate estimation of the monopile head stiffness.     

Two-dimensional soil arching evolution in geosynthetic-reinforced pile-supported embankments over voids

Soil arching and tensioned membrane effects are two main load transfer mechanisms for geosynthetic-reinforced pile-supported (GRPS) embankments over soft soils or voids. Evidences show that the tensioned membrane effect interacts with the soil arching effect. To investigate the soil arching evolution under different geosynthetic reinforcement stiffness and embankment height, a series of discrete element method (DEM) simulations of GRPS embankments were carried out based on physical model tests. The results indicate that the deformation pattern in the GRPS embankments changed from a concentric ellipse arch pattern to an equal settlement pattern with the increase of the embankment height. High stiffness geosynthetic hindered the development of soil arching and required more subsoil settlement to enable the development of maximum soil arching. However, soil arching in the GRPS embankments with low stiffness reinforcement degraded after reaching maximum soil arching. Appropriate stiffness reinforcement ensured the development and stability of maximum soil arching. According to the stress states on the pile top, a concentric ellipse soil arch model is proposed in this paper to describe the soil arching behavior in the GRPS embankments over voids. The predicted heights of soil arches and load efficacies on the piles agreed well with the DEM simulations and the test results from the literature.     

Comparison of end-of-drive and restrike signal matching analysis for a real case using continuum numerical modelling

The main objective of this paper is introducing a so-called continuum numerical model to overcome some of the limitations of the mass–spring–dashpot systems. Two continuum numerical schemes including a finite difference method (FDM) and a finite element method (FEM) are utilized. Unlike the previous continuum numerical models, the adopted models follow a signal matching procedure similar to lumped models. A real driven pile that has been carefully monitored is considered for model verification and validation, for which the dynamic and static test results, soil profile and soil characteristics are available. Among the major advantages of the model are considerations of soil inertia and/or radiation damping effects. A signal matching procedure is followed by changing the strength and deformation parameters of the soil and interface between pile–soil, both at the “End-Of-Drive” (EOD) and “Beginning of Restrike” (BOR). The results indicate a substantial increase in the soil strength parameters of the pile–shaft interface and the soil modulus below the pile tip at restrike. It is found that the effect of radiation damping significantly changes the pile–soil stiffness during pile hammering. The comparisons between FDM and FEM predictions show very good agreements. Two sets of parameters involved in signal matching are introduced for EOD and BOR signal matching analyses, to compare the variations during the soil setup process. The parameters are also compared against the in situ soil parameters acquired from soil investigation data.     

Analysis of second-order shear-deformable beams with semi-rigid connections

In this paper a new beam finite element is proposed for the solution of framed structures with semi-rigid connections. The element uses the second order Timoshenko's beam model. Power series expansion of kinematic relations is employed to improve solution stability. Effect of flexible and eccentric connections is considered by means of rotational and linear springs plus rigid end-offset. The proposed approach has the same computational cost of a standard beam element since additional degrees of freedom are condensed out. The resulting element stiffness matrix expression is given as the summation of the second order Timohenko's beam stiffness matrix and corrective matrices. The proposed element is readily implementable in existing finite element codes. Some benchmark problems are used to test the efficiency of the proposed procedure.     

油罐软粘土地基性状

首先,通过K₀固结三轴试验等室内试验探讨了金山粘土的应力应变关系以及强度和刚度各向异性。采用有限单元法研究了圆形贮罐上部结构、垫层与地基共同作用以及土体各向异性对沉降的效应。其次,在试验研究的基础上,提出一组考虑初始应力状态和土体固有各向异性的非线性弹性系数实用方程式。通过强度发挥度,可以把土体刚度和强度联系起来。最后,应用比奥（Biot）固结理论有限单元法分析了二只大型油罐（容量分别为10000m³和30000m³）地基试水期间的固结过程,分析中考虑了上部结构、环基和垫层的刚度。计算沉降过程线和孔隙压力过程线与现场实测值接近。     

双参数地基横向受荷桩的传递矩阵法解

为研究地基土体剪切刚度沿深度变化对横向受荷桩工作性状的影响,基于Newmark法和Pasternak双参数地基模型,假设土体剪切刚度为幂函数分布,由单元的挠曲微分方程求得了各结点的横向抗力。忽略了土体压缩变形和剪切变形的耦合作用后,对各结点的横向抗力做了简化,进而导出了单元的场传递矩阵及桩的总体传递矩阵;根据桩底边界条件,求得了桩的初始状态向量,确定了各结点的状态向量。算例分析表明:土体的剪切作用对减小顶位移和桩身最大弯矩有一定的贡献,且对中长桩的影响较长桩明显;土体剪切刚度沿深度变化对桩顶位移影响很小,地面处土体剪切刚度对其影响较大,双参数地基模型能够更好的模拟桩土的实际工况。     

桩承式路堤土拱数值模拟中路堤土本构模型的选择

数值模拟方法已成为研究桩承式路堤中土拱最重要的手段,其关键在于路堤填土要采用合理的本构模型。建立桩承式路堤平面土拱分析的弹塑性有限元模型,考虑摩尔-库伦模型（MC）、硬化土模型（HS）和小应变硬化土模型（HSS）3种不同的路堤土本构模型,用有限元方法模拟不同路堤土本构模型下桩承式路堤中的土拱形态和土拱效应。计算结果表明：3种不同路堤土本构模型下平面土拱的形态都是半个椭圆。路堤土采用HS和HSS模型,获得的土拱形态、效应和桩帽-土差异沉降相同。较之HS和HSS模型,路堤土采用MC模型时计算得到的桩帽-土差异沉降较小,桩帽荷载分担比略大。当路堤高度较小时,采用MC模型获得的土拱远小于HS和HSS模型下的计算结果。土拱效应的数值模拟中路堤土可采用简单的MC模型,但土拱形态的数值模拟中路堤土宜采用HS模型。     

Behavior of semi‐rigid connections and semi‐rigid frames

Semi‐rigid connections are widely used in different countries. These connections are usually used in semi‐rigid frames with bracing system. Considering the frequent use of these connections, studying their behavior as an individual connection or as a frame with a semi‐rigid connection is of great importance. In this paper, moment–rotation behavior of bolted angle connection as a usual semi‐rigid connection is studied, and the affecting factors on there are investigated. Finite element connection models are developed. These models are verified by comparing the results of finite element analyses with the results of experimental test, and the verified models are used to investigate the behavior of the connections. The behavior of semi‐rigid frame with bolted angle connection is then studied conducting time‐history analyses. The results show that the increase in shear stiffness of angle bolted connections significantly decreases the lateral drift and increase the frame stability against lateral loading. Copyright © 2012 John Wiley & Sons, Ltd.