Sensitivity analysis of jacket-type offshore platforms under extreme waves

Jacket-type offshore platforms play an important role in oil and gas industries in shallow and intermediate water depths such as Persian Gulf region. Such important structures need accurate considerations in analysis, design and assessment procedures. In this paper, nonlinear response of jacket-type platforms against extreme waves is examined utilizing sensitivity analyses. Results of this paper can reduce the number of random variables and consequently the computational effort in reliability analysis of jacket platforms, noticeably. Effects of foundation modeling have been neglected in majority of researches on the response of jacket platforms against wave loads. As nonlinear response of the pile foundation is one of the most important sources of potential nonlinearity in the response of offshore platforms, in this study, a powerful model which is able to consider Pile–Soil–Structure Interaction (PSSI) is employed. Therefore, PSSI parameters as well as other parameters such as uncertainties in the prediction of the wave force on jacket structure and uncertainties in structural model are utilized in sensitivity analyses. In this research, pushover methods as well as an advanced approach named “Incremental Wave Analysis (IWA)” are employed. Consequently, collapse prevention limit state of jacket platforms is investigated through different outcomes of pushover and IWA methods including Reserve Strength Ratio, ultimate capacity, collapse displacement and Collapse Wave Height indicators. In order to consider the effects of correlation between random variables, a robust method of sensitivity analysis named correlation coefficient approach is also employed.     

Nonlinear seismic response of reinforced‐concrete free‐standing towers with application to TV towers on flexible foundations

Reinforced‐concrete (R/C) free‐standing towers such as TV towers are often analysed using elastic analyses as fixed‐base cantilever beams, ignoring the effect of soil–structure interaction. To take the capacity of structures after yielding into account, most designers usually prefer to decrease the peak values of the elastic response spectrum for the maximum credible earthquake (MCE) anticipated at the site by a factor called the ductility capacity factor, which varies with the design earthquake level and the structural characteristics of the structure neglecting the effect of supporting soil. To investigate the effect of foundation flexibility on the response of R/C free‐standing towers deforming into their inelastic range during intense ground shaking, a linear sway‐rocking model is applied in numerical modelling of the soil–structure system. The effect of concrete cracking and reinforcement yielding on the elements used in the structure modelling is taken into account by introducing a nonlinear model for R/C frame elements using the moment–curvature (M–?) relation. A method called pseudo‐dynamic analysis is presented to quantify the inelastic seismic response spectrum of a soil–R/C free‐standing system using response spectrum analysis method and push‐over analysis technique. The earthquake responses of cracked and uncracked systems for a practical TV tower and a practical range of soil shear wave velocity are calculated and compared with the objective of understanding how soil–structure interaction influences structural responses. Copyright © 2002 John Wiley & Sons, Ltd.     

Trivariate analysis of soil ranking-correlated characteristics and its application to probabilistic stability assessments in geotechnical engineering problems

An important issue in the probabilistic prediction modelling of multivariate soil properties (usually including cohesion, friction angle, and unit weight) is the measurement of dependence structure among these properties. The use of Pearson's correlation as a dependence measure has several pitfalls; therefore, it may not be appropriate to use probabilistic prediction models in geotechnical engineering problems based on this correlation. As an alternative, a copula-based methodology for prediction modelling and an algorithm to simulate multivariate soil data are proposed.In this method, all different random variables are transformed to a rank/uniform domain in order to form a copula function by applying cumulative distribution function transformations. The technique of copulas, representing a promising alternative for solving multivariate problems to describe their dependence structure by a ranked correlation coefficient, is highlighted. Two existing observed soil data sets from river banks are used to fit a trivariate normal copula and a trivariate fully nested Frank copula. The ranking correlation coefficient Kendall's τ and the copula model parameters are estimated. The goodness-of-fit test to choose the best-fitting model is discussed.A series of triplet samples (i.e., cohesion, friction angle, and unit weight) simulated from the trivariate normal copula with flexible marginal distributions are used as input parameters to evaluate the uncertainties of soil properties and to define their correlations. The influence of the cross-correlation of these soil properties on reliability-based geotechnical design is demonstrated with two simple geotechnical problems: (a) the bearing capacity of a shallow foundation resting on a clayey soil and (b) the stability of a cohesive-frictional soil in a planar slope. The sensitivity analysis of their correlations of random variables on the influence of the reliability index provides a better insight into the role of the dependence structure in the reliability assessment of geotechnical engineering problems.     

Shaking table test of eccentric base-isolated structure on soft soil foundation under long-period ground motion

远场长周期地震动使基础隔震结构地震响应强烈，且软土地基-结构相互作用（SSI效应）使隔震结构地震响应更大，而偏心基础隔震结构在双向水平地震作用下将发生扭转效应，极易导致隔震层位移增加。为研究远场长周期地震动下软土地基SSI效应对偏心基础隔震结构地震响应规律及减震效果的影响，对高宽比为3的4层荷载偏心钢结构缩尺模型进行刚性、软土地基上的双向振动台试验。结果表明:软土地基上结构采用隔震技术后，周期延长比低于刚性地基结构体系，使减震效果下降；SSI效应在一定程度上会降低结构层间扭转响应，但在隔震层会出现较大扭转角；与普通地震动相比，远场长周期地震动下SSI效应对隔震结构响应影响程度更大，隔震层位移可能超限，总体减震效果较差。建议为保证软土地基上基础隔震结构的安全，应考虑SSI效应后进行结构设计，且远场长周期地震动的不利影响不可忽视。     

长周期地震动下软土地基的偏心基础隔震结构振动台试验研究

远场长周期地震动使基础隔震结构地震响应强烈,且软土地基-结构相互作用（SSI效应）使隔震结构地震响应更大,而偏心基础隔震结构在双向水平地震作用下将发生扭转效应,极易导致隔震层位移增加。为研究远场长周期地震动下软土地基SSI效应对偏心基础隔震结构地震响应规律及减震效果的影响,对高宽比为3的4层荷载偏心钢结构缩尺模型进行刚性、软土地基上的双向振动台试验。结果表明:软土地基上结构采用隔震技术后,周期延长比低于刚性地基结构体系,使减震效果下降;SSI效应在一定程度上会降低结构层间扭转响应,但在隔震层会出现较大扭转角;与普通地震动相比,远场长周期地震动下SSI效应对隔震结构响应影响程度更大,隔震层位移可能超限,总体减震效果较差。建议为保证软土地基上基础隔震结构的安全,应考虑SSI效应后进行结构设计,且远场长周期地震动的不利影响不可忽视。     

Large-Scale Experiments on Nonlinear Behavior of Shallow Foundations Subjected to Strong Earthquakes

We conducted a series of 1G large-scale shake table tests and cyclic eccentric loading tests of a shallow foundation model. The experimental parameters were the difference in loading methods (i.e., dynamic and static), input seismic motions (i.e., intensity and number of cycles), soil densities (i.e., dense and medium dense), and the ratio of horizontal and overturning moment loads. The experimental data set contains the accelerations and displacements of the soil and foundation as well as the distributions of normal and shear reaction forces at the foundation base. The experimental results provide crucial data to model the coupling effect among vertical, horizontal, and overturning loads, the accumulation of irreversible displacement, and the foundation uplift, and so is one of the most complete benchmark data sets for the development and validation of numerical models for the nonlinear response of shallow foundations to strong earthquakes.     

管棚受力和变形的分析预测方法及工程应用

采用管棚预先对浅埋软弱围岩进行支护时,对管棚力学响应的精确预测是保证管棚结构设计合理性的重要依据.在管棚的弹性地基梁分析模型基础上,考虑掌子面失稳段对管棚约束的弱化,建立基于Pasternak弹性地基梁理论的管棚分析模型来预测预支护段开挖时管棚的受力和变形.利用Euler-Bernoulli梁的控制方程以及Paster...     

局部承载黏土地层浅埋隧道稳定机制的连续上界极限分析

针对局部承载黏土地层浅埋隧道施工的横断面稳定机制问题,采用上界极限分析理论和基于Peck公式及不排水不可压缩条件的地层位移模式,分别计算研究了地层物理力学参数、浅基础荷载参数、桩基础荷载参数对隧道必要支护压力下确界的影响。在其他参数不变的前提下,隧道必要支护力下确界随地层抗剪强度的增大而线性地减小,随地层重度的增大而线性地增大;随浅基础荷载量值的增大而线性地增大,随浅基础荷载横向距离的增大而非线性地减小并趋于定值,随浅基础荷载横向范围的增大而非线性地增大并趋于定值;随桩基础侧面剪力荷载量值、底面压力荷载量值的增大而线性地增大,随桩基础荷载横向距离的增大而非线性地减小并趋于定值;对端承桩而言,当桩基长度小于（或大于）地层沉降竖向分布曲线极大值点对应的深度时,随桩基长度的增大而非线性地增大（或减小）。     

软土地基上高层隔震结构模型振动台试验研究

通过对高层隔震结构模型在刚性地基和软土地基条件下进行对比振动台试验,研究软土地基上高层隔震结构的地震反应特性和隔震效果。采用叠层剪切型土箱以减小边界影响,采用粉质黏土作为模型土,考虑基底压力的相似,采用高宽比为4的5层钢框架作为上部隔震结构模型,设计了软土地基上高层隔震结构、非隔震结构以及刚性地基上隔震、非隔震结构的振动台试验模型。试验结果表明：土-结构相互作用（SSI）效应对隔震结构的影响程度较非隔震结构轻;SSI效应显著降低了隔震结构模型的自振频率、增加了体系的阻尼;软土地基上隔震结构能够有效减轻结构的地震反应,但是相对于非隔震结构的加速度反应比值增大,隔震效果降低;SSI效应可能增加也可能减小隔震结构的地震反应,不仅与地震动类型有关还与输入地震动强度相关;软土地基上高层隔震结构基础输入地震动与自由场地震动在反应谱水平上基本一致,采用自由场地震动确定基础输入地震动是可行的,且偏于安全,而非隔震结构基础输入地震动则与地基更深层的地震动接近。     

Effects of soil-foundation-structure interaction on seismic structural response via robust Monte Carlo simulation

Uncertainties involved in the characterization and seismic response of soil-foundation-structure systems along with the inherent randomness of the earthquake ground motion result in very complex (and often controversial) effects of soil-foundation-structure interaction (SFSI) on the seismic response of structures. Conventionally, SFSI effects have been considered beneficial (reducing the structural response), however, recent evidence from strong earthquakes has highlighted the possibility of detrimental effects or increase in the structural response due to SFSI. This paper investigates the effects of SFSI on seismic response of structures through a robust Monte Carlo simulation using a wide range of realistic SFS systems and earthquake input motions in time-history analyses. The results from a total of 1.36 million analyses are used to rigorously quantify the SFSI effects on structural distortion and total horizontal displacement of the structure, and to identify conditions (system properties and earthquake motion characteristics) under which SFSI increases the structural response.     

各向异性软黏土地基上浅基础破坏包络面研究

 根据已有的关于土体强度各向异性的表达式,针对基于Tresca屈服准则的理想弹塑性模型进行改进,使之可以考虑不排水条件下软黏土强度的各向异性,并在大型通用有限元软件ABAQUS中予以数值实施。联合应用swipe加载模式和固定位移比加载模式,对于条形浅埋基础在竖向荷载V、水平荷载H与力矩荷载M的复合加载条件下的承载性能进行比较系统的数值模拟,主要探讨地基土不排水强度的各向异性、竖向荷载水平对于浅基础在H-M荷载空间内的破坏包络面的影响。首先进行浅基础分别在竖向、水平与力矩荷载单独作用下极限承载力的计算,并与已有结果进行对比。进而对于不同竖向荷载水平下浅基础在H-M荷载空间内的破坏包络面特性进行分析。计算结果表明：当浅基础埋深与宽度之比较大或基础承受三维自由度加载时,swipe加载模式得到的加载路径比真实的破坏包络面明显偏小;对于给定埋深的浅基础,其包络面的大小与地基土强度的各向异性与竖向荷载水平都有关系,而在归一化荷载平面内,包络面形状主要受到竖向荷载水平的影响,而与地基土强度的各向异性无关。     

Numerical analysis of a shaking table test on dynamic structure‐soil‐structure interaction under earthquake excitations

Structure‐soil‐structure interaction (SSSI) phenomena under earthquake excitations are investigated in this paper. Based on the results of the shaking table test, this work presents a 3‐dimensional finite element numerical simulation method using ANSYS software. In the simulation, an equivalent linear model is assumed for soil behavior, and contact elements are adopted to consider the nonlinearity state of the interface between foundation and surrounding soil. In addition, constrained equations are added to manage the uncoordinated degrees of freedom. By comparing the results of the finite element analysis with data obtained from the shaking table test, the dynamic response of the shaking table test can be simulated properly. Finally, the dynamic response of adjacent structures considering the SSSI effect is analyzed. The results show that with increased excitation, contact pressure, strain amplitude, and pile slip increase, whereas the peak acceleration magnification coefficient decreases. These results are significant for studying the effect of SSSI on seismic responses of structures.     

浅埋地下结构地震反应分析的惯性力–位移法

现有的地下结构地震反应简化分析方法,如:地震系数法、自由场变形法、柔度系数法、反应位移法、反应加速度法和Pushover分析方法等,均没有考虑上覆回填堆积土体或地震中因剪切破坏失效后的上覆堆积土体在竖向地震作用下产生的惯性力效应。已有研究表明,这种上覆土体竖向惯性力效应对浅埋地下结构支撑构件的抗震性能(抗剪强度和极限变形)有重要影响,是评价浅埋地下结构抗震安全性的关键因素之一,不能忽视。为此,针对浅埋地下结构地震反应分析问题,提出了一种考虑上覆土体竖向惯性力影响的反应位移法,简称惯性力–位移法。给出了惯性力–位移法分析模型的两个关键参数确定方法,包括地基弹簧刚度及上覆土体最大竖向惯性力。工程实例分析结果表明,建议的惯性力–位移法与传统的反应位移法相比,不仅克服了传统的反应位移法不能给出中柱轴力的缺陷外,其它反应量的计算精度与之基本相当。     

刚性桩复合地基与复合桩基工作性状对比试验研究

针对高层建筑筏板荷载分布特点,采用内密外疏布桩方式,通过现场缩尺(1:10)模型试验,完成了带上部结构的复合地基与复合桩基载荷试验,分析了筏板沉降、桩端平面以下地基沉降和筏板外侧地面沉降、筏板下桩土反力分布、桩身轴力和侧摩阻分布及桩土荷载分担比,研究了工作荷载下复合地基与复合桩基的工作性状。与复合桩基相比,得出复合地基总沉降大、差异沉降小、桩端平面以下沉降小、桩上段存在负摩阻、桩间土荷载分担比大、桩荷载分担比小的试验结果;复合地基可以更好地调动浅层地基土承载力,而复合桩基则能更好地调动深层地基土承载力,所得结论可为进一步理论研究及工程设计提供有益参考。     

Multi-perspective analysis on rainfall-induced spatial response of soil suction in a vegetated soil

In this study, an intelligent monitoring platform is established for continuous quantification of soil, vegetation, and atmosphere parameters (e.g. soil suction, rainfall, tree canopy, air temperature, and wind speed) to provide an efficient dataset for modeling suction response through machine learning. Two characteristic parameters representing suction response during wetting processes, i.e. response time and mean reduction rate of suction, are formulated through multi-gene genetic programming (MGGP) using eight selected influential parameters including depth, initial soil suction, vegetation- and atmosphere-related parameters. An error standard–based performance evaluation indicated that MGGP has appreciable potential for model development when working with even fewer than 100 data. Global sensitivity analysis revealed the importance of tree canopy and mean wind speed to estimation of response time and indicated that initial soil suction and rainfall amount have an important effect on the estimated suction reduction rate during a wetting process. Uncertainty assessment indicated that the two MGGP models describing suction response after rainfall are reliable and robust under uncertain conditions. In-depth analysis of spatial variations in suction response validated the robustness of two obtained MGGP models in prediction of suction variation characteristics under natural conditions.     

桩墩配筋率比对桥梁结构塑性区长度的影响

采用弹塑性纤维单元模型对某连续梁桥桥墩及桩基进行了考虑桩-土动力相互作用的非线性地震反应分析,分析了不同类型地震波作用下桥墩及桩基的动力响应,重点研究了不同桩墩配筋率比对结构塑性区开展程度及动力响应的影响。结果表明:随着桩墩配筋率比的改变,桥墩和桩基的反应塑性率呈现出不同的变化趋势,桩墩配筋率比的大小对桥梁结构的塑性区开展有显著影响;桥墩的配筋率大小不仅对桥墩塑性区开展有显著影响,而且对桩基的影响较大;不同类型的地震波对群桩基础桥墩结构的动力响应影响不同,长周期地震波对结构的影响最大,内陆直下型地震波次之,板块边界型地震波最小。     

Effectiveness of Soil–Structure Interaction and Dynamic Characteristics on Cable-Stayed Bridges Subjected to Multiple Support Excitation

The purpose of the study is to determine the effects of multiple support excitations (MSE) and soil–structure interaction (SSI) on the dynamic characteristics of cable-stayed bridges founded on pile foundation groups. In the design of these structures, it is important to consider the effects of spatial variability of earthquake ground motions. To do this, the time histories of the ground motions are generated based on the spatially varying ground motion components of incoherence, wave-passage, and site-response. The effects of SSI on the response of a bridge subjected to the MSE are numerically illustrated using a three-dimensional model of Quincy Bayview cable-stayed bridge in the USA. The soil around the pile is linearly elastic, homogeneous isotropic half space represented by dynamic impedance functions based on the Winkler model of soil reaction. Structural responses obtained from the dynamic analysis of the bridge system show the importance of the SSI and the MSE effects on the dynamic responses of cable-stayed bridges.     

Spatially variable soils affecting geotechnical strip foundation design

Natural soil variability is a well-known issue in geotechnical design, although not frequently managed in practice. When subsoil must be characterized in terms of mechanical properties for infrastructure design, random finite element method (RFEM) can be effectively adopted for shallow foundation design to gain a twofold purpose: (1) understanding how much the bearing capacity is affected by the spatial variability structure of soils, and (2) optimisation of the foundation dimension (i.e. width B). The present study focuses on calculating the bearing capacity of shallow foundations by RFEM in terms of undrained and drained conditions. The spatial variability structure of soil is characterized by the autocorrelation function and the scale of fluctuation (δ). The latter has been derived by geostatistical tools such as the ordinary Kriging (OK) approach based on 182 cone penetration tests (CPTs) performed in the alluvial plain in Bologna Province, Italy. Results show that the increase of the B/δ ratio not only reduces the bearing capacity uncertainty but also increases its mean value under drained conditions. Conversely, under the undrained condition, the autocorrelation function strongly affects the mean values of bearing capacity. Therefore, the authors advise caution when selecting the autocorrelation function model for describing the soil spatial variability structure and point out that undrained conditions are more affected by soil variability compared to the drained ones.     

Influence of geotextile arrangement on seismic performance of mid-rise buildings subjected to MCE shaking

Geotextile layers make it possible to construct mid-rise buildings sitting on shallow foundations in unfavourable soil conditions; this study investigates how the arrangement of geotextiles affects the seismic performance of mid-rise buildings under Maximum Considered Earthquake (MCE) shaking. The geotextile arrangement considered here includes the stiffness (5000?kN/m – 12000?kN/m), the length with respect to width of the foundation (B) (1B – 4B), the number of geotextile layers (1 – 7 layers), and their spacing (250?mm – 1000?mm). FLAC3D is used for the numerical simulation and to carry out nonlinear dynamic analysis in the time domain, and an inelastic constitutive model is used to simulate the behaviour of the structure and the geotextile layers under seismic loads. Variations in the shear modulus of soil and the corresponding damping ratio with cyclic shear strain are considered using a hysteretic damping algorithm to model the reasonable dissipation of energy in the soil. The interface between the foundation and ground surface, including the material and geometrical nonlinearities, are used to capture any possible slide and uplift in the foundations. The results are presented with regard to the geotextile arrangement considered, and include the tensile force mobilised in the geotextile layers, the response spectra at the bedrock and ground surface, the shear force developed in the structure, the maximum rocking angle of the foundation, permanent foundation settlement, maximum lateral displacement and the maximum and residual inter-storey drifts. The results show that the geotextile layers close to the edges of the foundation sustained most of the stress induced by foundation rocking, and the geotextile arrangement has a significant influence on the seismic response of mid-rise buildings. Thus, to satisfy the seismic performance of buildings and to optimise the design of foundations reinforced with geotextiles, the stiffness, length, number and spacing of the geotextile layers should be designed with great care.