Undrained cyclic triaxial behavior of saturated clays under variable confining pressure

A laboratory study on the undrained dynamic behavior of saturated clays in cyclic triaxial tests with a variable confining pressure (VCP tests) is presented. Tests were performed on remolded clayey samples using a dynamic triaxial device where the deviatoric stress and confining pressure can be varied simultaneously. Various cyclic stress paths have been applied on the specimens through varying the ratios or phase differences between the cyclic deviatoric stress and cyclic confining pressure. Specifically, the stress paths used in the present study were designed to simulate the coupling effects of simultaneously varying shear and normal stresses in clays due to earthquakes and other vibration sources. Test results obtained from this study show that the undrained response of saturated clays is strongly influenced by the variation of confining pressure, in terms of pore water pressure, development speed of cyclic strain and magnitude of cyclic strength. It is found that when strong P-waves are propagating in soil layers, VCP tests are more appropriate for the simulation of in situ stress fields than the conventional cyclic triaxial tests with a constant confining pressure (CCP tests).     

Undrained deformation behavior of saturated soft clay under long-term cyclic loading

Subgrade soils of traffic infrastructures are subjected to large numbers of load applications at a stress level below their shear strength. It is therefore of great practical relevance to study the deformation behavior of soft clay under long-term cyclic loading. In this study, a series of monotonic triaxial tests and long-term cyclic (50,000 cycles) triaxial tests have been carried out to investigate the undrained deformation behavior of undisturbed soft clay from Wenzhou, China. The stress–strain hysteretic loop, resilient modulus and permanent strain of the tested samples were found significantly dependent on CSR and confining pressure. With an increase of CSR and confining pressure, the resilient modulus decreases more significantly with increasing number of cycles and the accumulation rate of permanent strain increases. Furthermore, the shape of the stress–strain hysteretic loop almost remains unchanged and the resilient modulus tends to a steady value after a large number of cycles. Based on the experimental results, two equations are established for the prediction of long-term resilient modulus and permanent strain. Finally, a new critical value of 0.65 is suggested for CSR. When CSR>0.65, the resilient modulus for large number of cycles is reduced to a so called “asymptotic stiffness” and the accumulation rate of permanent strain significantly increases.     

复杂应力条件下饱和松砂单调与循环剪切特性的比较研究

本文利用大连理工大学新引进与开发的“土工静力-动力液压-三轴扭转多功能剪切仪”,针对福建标准砂,在不排水条件下同时进行了单调剪切试验与循环剪切试验,进而对其进行了对比分析。通过比较表明,应力-应变关系的应变软化和硬化特性与流滑变形和循环流动特性密切相关,当循环剪切应力水平高于单调剪切过程中应变软化阶段最小强度时将会发生流滑变形。无论在单调剪切中,还是在循环剪切中,稳定状态时的有效偏应力比随着大主应力方向与竖向之间夹角的增大而减小,在中主应力系数相同的条件下,循环剪切中呈现显著剪胀时的有效偏应力比和最终稳定状态时的有效偏应力比峰值分别与单调剪切中达到相变状态时的有效偏应力比和最终稳定有效偏应力比基本上一致。然而不排水条件下单调与循环剪切过程中孔隙水压力的增长特性却并不相同,循环剪切中的最大孔隙水压力随着初始主应力方向角的增大而减小,单调剪切中的最大孔隙水压力却随着主应力方向角的增大而增大。     

Overstrength of dowelled CLT connections under monotonic and cyclic loading

This paper presents an evaluation of overstrength based on an experimental study on dowelled connections in Cross Laminated Timber (CLT). Connection overstrength needs to be well understood in order to ensure that ductile system behaviour and energy dissipation can be achieved under seismic loading. Overstrength is defined as the difference between the code-based strength, using characteristic material strengths, and the 95th percentile of the true strength distribution. Many aspects contribute to total connection overstrength, which makes its definition challenging. In this study, half-hole embedment tests were performed on CLT to establish embedment strength properties and three point bending tests were performed to determine the fastener yield moment. Different connection layouts, making use of mild steel dowels and an internal steel plate, were tested under monotonic and cyclic loading to evaluate theoretically determined overstrength values and study the influence of cyclic loading on overstrength. Experimental results were compared with strength predictions from code provisions and analytical models for ductile response under monotonic loading. It was found that cyclic loading does not significantly influence overstrength for connections that respond in a mixed-mode ductile way indicating that in future more expedient monotonic test campaigns could be used. This work also provides further experimental data and theoretical considerations necessary for the estimation of a generally applicable overstrength factor for dowelled CLT connections.     

单调及低周往复荷载作用下砌体非线性分析

带构造柱和圈梁的约束砌体结构在四川灾区乡镇房屋重建中被广泛采用,其抗震性能是人们所关心的.基于绵竹市土门镇当地重建房屋常用建筑材料的实验数据以及通用有限元软件ANSYS中Solid 65单元的性质和特点,用有限元模型模拟了粘土砖砌体在不同压应力状态(σ-/fm)下沿通缝截面抗剪强度试验,给出了相关单元在模拟砖砌体开裂中闭合及开口剪力传递系数的建议值;利用这些结果,分别建立了带约束(构造柱、圈梁等)和不带约束砌体墙的有限元模型,进而分析了他们在单调荷载以及低周往复荷载作用下的抗震性能.结果表明,与不带约束的墙体相比,带约束墙体在单调水平荷载作用下的初裂性能、极值荷载和延性都有很大的提高,在低周往复荷载作用下其耗能能力得到了改善.所得结果可供相应结构抗震设计的参考.     

Cyclic and post-cyclic monotonic behavior of Adapazari soils

The August 17, 1999 Kocaeli earthquake affected the city of Adapazari, which is located in the northwest of Turkey, with severe liquefaction and bearing capacity failures causing tilting of buildings, excessive settlements and lateral displacements. To understand the stress–strain behavior and pore pressure behavior of undisturbed soils during the earthquake, the cyclic and post-cyclic shear strength tests have been conducted on soil samples obtained from Adapazari in a cyclic triaxial test system within the scope of this research. Cyclic tests have been conducted under stress controlled and undrained conditions. Post-cyclic monotonic tests have been conducted following cyclic tests. The strength curves obtained in the experiments showed that the dynamic resistance of silty sand was found to be 45% lower than those of high plasticity soils (MH). The strength of clayey soils with the plasticity index of PI=15–16% was lower compared to the strength of high plasticity soils. Also, it was observed that silty sand soils had the lowest strength. The dynamic strength of the soils increased with the increase in plasticity.     

An improved rheology model for the description of the rate-dependent cyclic behavior of high damping rubber bearings

An improved rheology model, inspired from explicit experiments is conceived to represent rate-dependent cyclic shear behavior of high damping rubber bearings at subzero and room temperatures. Total stress has been decomposed into nonlinear rate independent elasto-plastic stress, nonlinear elastic stress and nonlinear visco-elasto-plastic overstress branches. To represent nonlinear viscosity behavior, ‘overstress branch’ has been generalized by putting linear elastic spring in parallel to nonlinear elasto-plastic model, placed in series with nonlinear dashpot. Constitutive relations for model elements have been designated for respective fundamental phenomenon observed in constant strain rate experiments. An optimum calculation approach is developed to determine a unique set of overstress parameters capable not only of representing constant strain rate cyclic tests but also sinusoidal tests with variable input strain rates. Essential abilities of the proposed model and adequacy of estimated parameters have been confirmed by comparing numerical simulation results with experiments conducted at −30 °C, −10 °C and 23 °C.     

Nonlinear FE analysis of precast RC pinned beam-to-column connections under monotonic and cyclic shear loading

In precast technology, mostly (but not exclusively), frame structures with pinned beam-to-column connections are preferred, especially in low-rise buildings due to the flexibility, lower cost and more favourable behaviour they provide, especially in the case of large spans and pretensioned interconnected members. However the available literature on the behaviour of pinned connections, especially under seismic loading, can be characterized as poor, even though their use in Europe and elsewhere is rather extended. In the terms of the present research a nonlinear 3D numerical model was developed and calibrated against available experimental data to be used as an effective tool for the analytical prediction of the behaviour of pinned connections, under monotonic and cyclic shear loading. The experimental data were derived from the European FP7 project SAFECAST, Grant Agreement Number 218417. The tests were performed at the Laboratory for Earthquake Engineering of the National Technical University of Athens, Greece. From the numerical results useful information was obtained on component level about: (a) the type of the observed failure mechanism; (b) the amount of dissipated energy; (c) the location of the developed plastic hinges along the steel dowel, and (d) the evolution of stresses and strains along the dowel(s) and in the mass of the surrounding grout. However the numerical model can be further utilized to investigate and quantify the effect of several parameters (that were not experimentally investigated in depth, or were not investigated at all) on the response of pinned connections.     

Evaluation of state indices in predicting the cyclic and monotonic strength of sands with different fines contents

State parameter, ψ, has been widely used to combine the influence of void ratio, e, and stress level, p′, on the soils behavior. Stress ratio, R_s, and modified state parameter, ψ_m, have also been proposed for the same purpose. This paper aims to evaluate and compare the different state indices in combining the effect of fines content, density and stress level for five different types of sands, by processing a large number of previously published experimental data. The use of the recently established concept of equivalent interparticle void ratio, e^⁎, in definition of the state indices is also evaluated. The results indicate that the influence of fines presence, in addition to the e and p′, on the behavior is favorably reflected by the state indices. Unique correlations were derived between the cyclic or monotonic strength and each of the state indices, independent of the fines content. The correlations, for all the different types of soils, fell into limited types of common formulations. ψ and R_s worked generally better than ψ_m, whether defined in terms of e or e^⁎. The extension of straight part of critical state line was found to be an appropriate reference line for calculating R_s used in conjunction with e or e^⁎.     

Hysteretic energy dissipation capacity and the cyclic to monotonic drift ratio for rectangular RC columns in flexure

Predictions of energy dissipation capacity and of the deterioration of deformation capacity due to cumulative damage have been made by means of a non‐parametric empirical approach, called the conditional average estimator method, using empirical data on rectangular reinforced concrete columns that failed in flexure. Five input parameters were used: axial load index, index related to confinement, shear span index, concrete compressive strength, and longitudinal reinforcement index. The energy capacity was expressed in three different normalized forms and the deterioration of deformation capacity was defined as the ratio of the cyclic to the monotonic ultimate drift. The longitudinal reinforcement index, the index related to confinement, and the axial load index are the most influential input parameters in the case of energy capacity, whereas the latter two indices exhibit the most significant influence in the case of the drift ratio. Energy capacity decreases with an increasing axial load index, whereas it increases with increasing longitudinal reinforcement and with better confinement. In the case of the shear span index, the trend is more complex. Normal concrete has a higher energy dissipation capacity than high‐strength concrete. Similar trends are observed for the drift ratio, with the exception of the influence of the axial load index, where the trend is opposite. The dispersion of the results is high. Copyright © 2008 John Wiley & Sons, Ltd.     

金属橡胶支座剪切性能试验研究

针对一种金属橡胶支座,研究其剪切性能.进行3种压应力下的拟静力试验,分析竖向压力和水平剪切变形对支座剪切性能的影响;以试验数据为基准,建立支座剪切性能与压应力之间的相关性经验公式,提出能够近似模拟试验曲线的三线性恢复力模型.试验研究表明,随着支座剪切变形的增大,支座等效刚度及耗能增大,等效阻尼比减小,屈服力基本保持不变...     

A numerical investigation on the seismic isolation potential of rubber/soil mixtures

This study examines the use of granular soils mixed with tire derived aggregates(TDA) as an underlying soil layer for surface foundations. The benefit of this geotechnical seismic isolation(GSI) scheme is threefold: the seismic force and displacement are significantly reduced; the construction is similar to that of a regular compacted granular fill and the re-use of scrap tires has an assertive environmental impact. These features highlight the rubber/soil mixtures(RSM) as a promising seismic isolation technique for infrastructure and large-scale structures. The seismic isolation capabilities of a rubber-soil foundation layer are investigated, using well defined material properties, and the direct analysis method of the soil-structure system, in the form of a simple oscillator on a soil profile. The influence of the RSM layer on the fundamental variables of the seismic response, namely the base shear and the total drift displacement of the structure on deformable soil, is examined. The structure′s overall stability is studied by means of monotonic lateral load analysis and incremental dynamic analysis, varying the slenderness of the structure and the synthesis of the mixture. The effectiveness and capabilities of the RSM isolation scheme are presented and discussed.     

Mechanical behavior of lead rubber bearings under and after nonproportional plane loading

Lead rubber bearings, which have been extensively applied in many seismic isolation designs for buildings, infrastructures, and facilities worldwide, were tested under unilateral reversal loading as well as nonproportional plane loading including circular, figure-eight, and square orbits in this study. The test results indicate that unlike the unilateral hysteretic behavior, the bilateral one of lead rubber bearings is too complicated to be characterized adequately by a simplified bilinear hysteretic model. It is mainly attributed to the bilateral coupling effect, which can be clearly observed from the abnormal deformation of the mesh pattern drawn on the rubber cover during the tests. In addition, after being subjected to nonproportional plane loading, the tested bearings reveal visible permanent twisting deformation. The profiles of the cut bearings present the fracture of the inside lead plugs. Even so, the further unilateral reversal loading test results prove that the fracture might not affect the whole hysteretic behavior and mechanical properties very much. The applicability, robustness, and generalization of adopting three previously developed analytical models for describing the coupled bilateral hysteretic behavior of lead rubber bearings are further demonstrated by comparing their predictions with the nonproportional plane loading test results. Although the coefficients are identified from unilateral reversal loading tests, the three analytical models can still have an acceptable prediction capability.     

Nonlinear elastic behavior of fiber-reinforced soil under cyclic loading

Experimental investigations and modeling of nonlinear elasticity of fiber-reinforced soil under cyclic loading at small strain are conducted in this paper. The investigations include three aspects. First, cyclic shear tests are conducted using conventional triaxial apparatus. Twenty-seven specimens with three different fiber contents are employed to conduct triaxial cyclic shear tests under different confining pressure and loading repetition. Effects of geofiber, confining pressure and loading repetition on elastic shear modulus of reinforced soil are studied and analyzed. Second, a hyperbolic function is introduced to describe the nonlinear stress–strain skeletal curve under cyclic loading. Nonlinear elastic modulus is expressed as a function of shear strain and two variables A and B that are related to the initial tangential modulus and ultimate cyclic loading stress, respectively. In the present paper, variables A and B both are further assumed to be functions of geofiber content, confining pressure and loading repetition. Finally, eight constitutive coefficients of the nonlinear elastic model are calibrated using stress–strain curves from cyclic triaxial shear tests. The calibration of parameters is conducted using the technique of the linear regression for multiple variables. Impacts and effects of geofiber, confining pressure and loading repetitions on soil nonlinear elastic behavior are discussed.     

Dynamic properties of dry sand/rubber (SRM) and gravel/rubber (GRM) mixtures in a wide range of shearing strain amplitudes

Recycled tire rubber in mixtures with granular soils has been found recently applications in many civil engineering projects. The paper presents a synthesis of the dynamic strain-dependent properties of the commonly used soil/rubber mixtures, which are necessary in any seismic design. We focus herein on high-amplitude resonant column tests on granular soil/rubber mixtures with varying percentage of rubber. The most important characteristics of the dynamic properties of the mixtures like the confining pressure, the content of rubber, the grain-size characteristics of the physical portion of the mixtures as well as the relative size of soil versus rubber solids, are thoroughly discussed. We propose generic normalized shear modulus and damping ratio versus shearing strain amplitude curves for dry mixtures of sand/rubber (SRM) and gravel/rubber (GRM) appropriate for the engineering practice. Finally, we summarize analytical expressions for small-strain shear modulus and damping ratio for SRM and GRM proposed in previous studies.     

Estimation of monotonic behavior of reinforced concrete columns considering shear‐flexure‐axial load interaction

Reinforced concrete columns with insufficient transverse reinforcement and non‐seismic reinforcement details are vulnerable to brittle shear failure and to loss of axial load carrying capacity in the event of a strong earthquake. In this paper, a procedure is presented after examining the application of two macro models for displacement‐based analysis of reinforced concrete columns subjected to lateral loads. In the proposed model, lateral load‐deformation response of the column is simulated by estimating flexural and shear deformation components separately while considering their interaction and then combining these together according to a set of rules depending upon column's yield, flexural and shear strengths. In addition, lateral deformation caused by reinforcement slip in beam–column joint regions and buckling of compression bars are taken into account and considered in the analysis. Implementation of the proposed procedure produces satisfactory lateral load–displacement relationships, which are comparable with experimental data. Copyright © 2012 John Wiley & Sons, Ltd.     

橡胶集料风积沙混凝土柱抗震性能试验研究

为研究橡胶集料风积沙混凝土柱的抗震性能,制作了4根相同尺寸的混凝土柱试件,其中包括普通混凝土柱、风积沙混凝土柱、橡胶集料混凝土柱和橡胶集料风积沙混凝土柱。通过低周反复荷载试验,分析各试件的破坏现象、滞回曲线、骨架曲线、刚度退化和延性等抗震性能指标。在试验的基础上,根据能量耗散原理建立累积损伤评价模型,并对结构进行了累积损伤分析。研究结果表明:10%橡胶集料的掺入能够改善混凝土柱的变形能力,但混凝土柱的承载力有所降低;30%风积沙的掺入可以改善混凝土柱的抗震性能;橡胶集料风积沙混凝土柱具有较好的滞回性能,且耗能能力和延性性能明显高于其余试件,柱的抗震性能得到提高;通过累积损伤分析表明:适量掺入橡胶集料和风积沙的可以有效减缓试件的损伤,且该累积损伤模型能够较好的反映出各试件的损伤程度。     

应力主轴循环旋转条件下饱和砂土的变形特性

采用香港科技大学的空心圆柱扭剪仪对日本标准丰浦砂进行了3个系列、每个系列包含中主应力系数=0.1、0.5和1.0三种不同情况的纯应力主轴循环旋转排水试验。试验过程中控制作用在砂样上的有效主应力的幅值不变,仅应力主轴在0°至180°之间循环旋转。着重分析了纯应力主轴循环旋转条件下饱和砂土的剪切变形特性、体应变的发展规律及中主应力系数的影响。