The stress–strain behaviour of multiple cell geocell packs

Geocell reinforcement of soil has been used successfully for many years in a wide variety of applications, some of which have tested the boundaries of current understanding of the functioning of the geocell–soil composite systems. This paper discusses the results of uniaxial compressive tests performed on geocell packs of different sizes. The packs were instrumented and the deformation within the packs studied. The boundary conditions imposed on the geocell pack in a laboratory test governs the deformation throughout the pack and has to be taken into account when interpreting the test results. This study shows that the strength of the geocell composite structure is indirectly proportional to the size of the individual cells and that the strength reduces with an increase in the number of cells in the structure.     

Design-oriented stress–strain model for FRP-confined concrete

External confinement by the wrapping of FRP sheets (or FRP jacketing) provides a very effective method for the retrofit of reinforced concrete (RC) columns subject to either static or seismic loads. For the reliable and cost-effective design of FRP jackets, an accurate stress–strain model is required for FRP-confined concrete. In this paper, a new design-oriented stress–strain model is proposed for concrete confined by FRP wraps with fibres only or predominantly in the hoop direction based on a careful interpretation of existing test data and observations. This model is simple, so it is suitable for direct use in design, but in the meantime, it captures all the main characteristics of the stress–strain behavior of concrete confined by different types of FRP. In addition, for unconfined concrete, this model reduces directly to idealized stress–strain curves in existing design codes. In the development of this model, a number of important issues including the actual hoop strains in FRP jackets at rupture, the sufficiency of FRP confinement for a significant strength enhancement, and the effect of jacket stiffness on the ultimate axial strain, were all carefully examined and appropriately resolved. The predictions of the model are shown to agree well with test data.     

Influence of anisotropic stress path and stress history on stiffness of calcareous sands from Western Australia and the Philippines

Investigation of dynamic properties of carbonate/calcareous soils is important in earthquake and offshore engineering as these soils are commonly encountered in large-scale projects related with energy geomechanics and land reclamation.In this study,the stiffness and stiffness anisotropy of two types of calcareous sands(CS)from the Western Australia and the Philippines were examined using bender elements configured in different directions in stress path setups.Stiffness measurements were taken on specimens subjected to constant p’compression/extension and biaxial stress paths and additional tests were performed on three types of silica sands with different geological origins and particle shapes,which were used as benchmark materials in the study.Compared with the three brands of silica sands,the stiffness of the CS was found to be more significantly influenced by anisotropic loading;an important observation of the experimental results was that stress anisotropy had different weighted influences on the stiffness in different directions,thus influencing stiffness anisotropy.Comparisons were made between the specimens subjected to complex loading paths,and respected model parameters as suggested from published expressions in the literature.These comparisons further highlighted that calcareous soils have different responses in terms of stiffness,stiffness anisotropy and loading history,compared with that of silica-based sands.     

Influence of initial dry density and water content on the soil–water characteristic curve and suction stress of a reconstituted loess soil

In the northwest of China, many loess landslides have occurred without clear triggering factors (i.e., rainfall, earthquake, human activities, etc.). To better understand and analyze the hydro-mechanical properties of these slopes and then provide evidence for their stability analysis subjected to matric suction, it is essential to clarify the soil–water characteristic curve (SWCC). In this study, we conducted a set of experimental trials to examine the influences of initial dry density, water content upon the SWCCs of a loess soil taken from a loess landslide area, by using a conventional volumetric pressure plate extractor. Two common SWCC models have been investigated to evaluate which one is better for loess soil. The suction stress characteristic curves (SSCCs) were also estimated and analyzed. We found that behaviors of SWCCs would be different when the matric suction was greater than a certain value. The two SWCC equations have approximately the same performance in describing the SWCC. The rates of desorption decrease and residual water content increases with increasing the initial dry density, while the initial dry density has little, if any, influence on the air-entry value (AEV). The specimen compacted under higher initial water content would exhibit a higher AEV value and residual water content but lower rate of desorption as compared with the lower initial water content. The magnitude of suction stress had an approximately linear relationship with matric suction before the AEV value, the SSCC shapes will be markedly varied with the initial dry density and water content.     

Numerical integration of a class of 3d plastic-damage concrete models and condensation of 3d stress–strain relations for use in beam finite elements

This paper presents a method for the integration of a class of plastic-damage material models. The integration of the evolution equations results in a nonlinear problem, which is linearized and solved with the Newton–Raphson method using a sub-stepping strategy. The consistent tangent matrix can be formulated either in terms of the stress components in a general reference system or in terms of the principal stress and strain components with the former then transformed to the general reference system. In order to account for plane stress conditions, the stress–strain relations of the 3d material model are then condensed out. Plane stress conditions are imposed by the linearization of the stresses that need to be set equal to zero; thus the strain fields are updated in the corresponding directions. This solution method is extended to include transverse pressure and the effect of transverse reinforcing steel for a 3d concrete material model. The equilibrium of the stresses in the reinforcing steel and concrete is linearized and the strain fields are updated until the residual satisfies a specified tolerance. The consistent tangent matrix due to the condensation process is derived. The proposed algorithms are tested at the material and element level by comparison of numerical solutions with available experimental data.     

Generalized stress field in granular soils heap with Rayleigh–Ritz method

The stress field in granular soils heap (including piled coal) will have a non-negligible impact on the settlement of the underlying soils. It is usually obtained by measurements and numerical simulations. Because the former method is not reliable as pressure cells instrumented on the interface between piled coal and the underlying soft soil do not work well, results from numerical methods alone are necessary to be doubly checked with one more method before they are extended to more complex cases. The generalized stress field in granular soils heap is analyzed with Rayleigh–Ritz method. The problem is divided into two cases: case A without horizontal constraint on the base and case B with horizontal constraint on the base. In both cases, the displacement functions u(x, y) and v(x, y) are assumed to be cubic polynomials with 12 undetermined parameters, which will satisfy the Cauchy's partial differential equations, generalized Hooke's law and boundary equations. A function is built with the Rayleigh–Ritz method according to the principle of minimum potential energy, and the problem is converted into solving two undetermined parameters through the variation of the function, while the other parameters are expressed in terms of these two parameters. By comparison of results from the Rayleigh–Ritz method and numerical simulations, it is demonstrated that the Rayleigh–Ritz method is feasible to study the generalized stress field in granular soils heap. Solutions from numerical methods are verified before being extended to more complicated cases.     

Framework of vehicle–bridge–wind dynamic analysis

This paper presents a framework of dynamic analysis of coupled three-dimensional vehicle–bridge system under strong winds. A general formulation of this system is introduced to simulate a series of vehicles consisting of different numbers and different types of vehicles running on bridges under hurricane-induced strong winds. Each vehicle is modeled as a combination of several rigid bodies, axle mass blocks, springs, and dampers, considering wind and road roughness loads. With this vehicle–bridge model, coupled dynamic analysis of vehicles running on bridges is conducted with a numerical example. Effects of driving speeds on the dynamic performance of the vehicles as well as the bridge are discussed. It is found that the driving speeds mainly affect the vehicle's vertical relative response while they have insignificant effect on the rolling response of vehicles. Vehicle's absolute response is dominated by the bridge response when wind speed is high, while it is dominated by road roughness when the wind speed is low. Detailed accident analysis of vehicles on bridges under strong winds will be reported in an accompanying paper.     

Lateral–distortional buckling of monorails

This paper is concerned with the elastic lateral–distortional (LD) buckling of single span steel monorail I-beams and its influence on their design strengths. The distortion of a slender web reduces the elastic buckling resistance of an intermediate length beam below its flexural–torsional (FT) resistance. A finite element computer program was used to study the elastic LD buckling of single span beams. The LD to FT buckling moment ratios were generally higher for simply supported beams with bottom flange central concentrated loads than for uniform bending, and lower for shear centre concentrated loads. For beams with bottom flange loading and unrestrained bottom flanges, there were very significant reductions in these ratios, but they increased when rigid web stiffeners or top flange torsional restraints were provided at the supports. For beams with bottom flange loading and unrestrained bottom flanges, the reductions in the elastic buckling resistance were greater for beams with stocky flanges than for slender flanges. Approximations were found for estimating the reduced resistances which were generally of high accuracy or conservative, and for estimating the increased resistances caused by elastic and rigid top flange end torsional restraints. A method of designing steel beams against LD buckling was proposed and its use demonstrated by a worked example.     

Thermal–hydrologic–mechanical–chemical processes in the evolution of engineered geothermal reservoirs

In a companion paper [Taron J, Elsworth D, Min K-B. Numerical simulation of thermal–hydrologic–mechanical–chemical processes in deformable, fractured porous media. Int J Rock Mech Min Sci 2009; doi:10.1016/j.ijrmms.2009.01.008] we introduced a new methodology and numerical simulator for the modeling of thermal–hydrologic–mechanical–chemical processes in dual-porosity media. In this paper we utilize the model to examine some of the dominant behaviors and permeability-altering mechanisms that may operate in naturally fractured media. Permeability and porosity are modified as fracture apertures dilate or contract under the influence of pressure solution, thermo–hydro-mechanical compaction/dilation, and mineral precipitation/dissolution. We examine a prototypical enhanced geothermal system (EGS) for the relative, temporal arrival of hydro-mechanical vs. thermo-mechanical vs. chemical changes in fluid transmission as cold (70 °C) water is injected at geochemical disequilibrium within a heated reservoir (275 °C). For an injection-withdrawal doublet separated by ～670 m, the results demonstrate the strong influence of mechanical effects in the short-term (several days), the influence of thermal effects in the intermediate term (<1 month at injection), and the prolonged and long-term (>1 year) influence of chemical effects, especially close to injection. In most of the reservoir, cooling enhances permeability and increases fluid circulation under pressure-drive. We observe thermo-mechanical driven permeability enhancement in front of the advancing thermal sweep, counteracted by the re-precipitation of minerals previously dissolved into the cool injection water. Near injection, calcite dissolution is capable of increasing permeability by nearly an order of magnitude, while precipitation of amorphous silica onsets more slowly and can completely counteract this increase over the very long-term (>10 years). For the reinjection of highly-silica-saturated water, amorphous silica is capable of drastic reduction in permeability close to the injection well. With combined action from all mechanisms, permeability change varies by two orders of magnitude between injection and withdrawal.     

Predicting the Fire Resistance of Wood–Steel–Wood Timber Connections

This paper presents an investigation on the fire performance of wood–steel–wood timber connections with slotted-in steel plates. In the first part, a three-dimensional thermal model was employed that uses the finite element method to analyze heat transfer within timber connections exposed to the standard fire. The temperature-related properties were obtained from the literature and imported into the thermal model. A validation of the proposed thermal model was achieved by comparing predicted temperatures with experimental results. In the next phase, a reduction in the embedding strength method was adopted to estimate the load-carrying capacity of connections in fire. Based on the temperature profiles within the connection calculated by the thermal model, the reduction of the embedding strength was determined and used to calculate the load capacity at elevated temperatures. Furthermore, a formula was proposed to evaluate the fire resistance rating of timber connections and compared with the results of fire resistance tests. The parameters considered included the load level, fastener diameter and wood member thickness.     

Time–frequency analysis of structural dynamic characteristics of tall buildings

This study focuses on the correlations between the structural dynamic properties and the instantaneous response characteristics of a 492-m high building during a typhoon. An instantaneous analysis framework is established based on a combined usage of both linear-phase filtering and time–frequency techniques. This analysis framework can separate each modal response contribution without phase distortions. Random decrement technique is used to estimate the modal damping ratios. It is found that beating phenomenon widely exists in the separated modal response contributions, and the damping ratios correlate with the modulation of phase and amplitude within the beating closely. When the beating amplitude comes close to zero, the instantaneous frequency fluctuates evidently. The more intensive this fluctuation is, the larger the damping ratio becomes. An empirical model is presented to formulate the variations of damping ratio and natural frequency of this building with the fluctuation intensity of the instantaneous frequency.     

Effect of polyurethane on the stability of sand–clay mixtures

Three types of polyurethane were synthesized from mixtures of toluene diisocyanate, polyethylene glycol and polypropylene glycol for use in soil stabilization to improve the erosion resistance. The three polyurethanes were tested at different aqueous concentrations and sand:clay mixtures at weight ratios of 1:1, 1:3 and 1:5. The results of the rainfall simulation, unconfined compression and direct shear tests showed that the polyurethanes improve both strength and erosion resistance significantly. The observed improvement in soil erosion resistance is attributable to the physico-chemical interaction of the long-chain macro-molecules of polyurethane with the clay fraction of the soil.     

Numerical Studies of the Rotational Stiffness of Purlin–Sheeting System

In light-gauged steel purlin-to-sheeting roof systems, the attached sheeting can provide rotational restraints to the purlin. The magnitude of the additional rotational stiffness offered by the sheeting will affect the load bearing capacity of the purlin. The current design method in Eurocode3 (EC3) is less accurate in predicting the purlin–sheeting rotational stiffness as it neglects the effect of wall thickness of the purlin. An integral model is introduced based on the finite element method. Comparisons are made between numerical results and existing experiments and a good agreement is observed. Parametric studies are conducted based on the validated model to investigate the influences of geometric dimensions on the rotational stiffness. Two modified coefficients are proposed for calculating the rotational stiffness based on the codified formulae in EC3, where the effect of the wall thickness and the flange width of the purlin are both considered.     

Determinants of black interstate migration, 1965–70 and 1975–80

This paper presents and tests a regression-based model of black interstate migration. Explanatory variables include characteristics of origins and destinations, distance, and two migrant stock measures. The model is tested using black interstate migration flows published by the U.S. Bureau of the Census for 1965–70 and 1975–80. Three findings stand out. Firstly, the stock measures are strong determinants of black migration. They tap behavioral processes that channelize black migration streams, including information flows through familial and social networks and return migration. Secondly, the migrant stock measures attenuate effects of other explanatory variables indicating that other variables influence current migration both directly and indirectly through the stock measures. Thirdly, changes in coefficients of explanatory variables between the two periods reflect shifts in black migration patterns that occurred during the 1970s.     

Tests on cyclic performance of FRP–concrete–steel double-skin tubular columns

Eight FRP–concrete–steel double-skin tubular columns were tested under constant axial load and cyclically increasing flexural loading. The main parameters in the tests are axial load level and number of fibre reinforced polymer (FRP) layers. The influence of those parameters on the strength, ductility, stiffness, and energy dissipation was investigated. It was found that, in general, FRP–concrete–steel double-skin tubular columns exhibit high levels of energy dissipation prior to the rupture of the longitudinal FRP, but experience a sudden drop in the lateral load capacity after that. The ductility of the specimens can be improved to some extent due to the existence of the axial compressive load in current tests.     

Modified expression of Kozeny–Carman equation based on semilog–sigmoid function

A modified Kozeny–Carman (KC) equation is presented based on a semilog–sigmoid (SS) function of a soil’s particle size distribution (PSD). The KC equation is widely used to estimate soil permeability. However, a more effective use of the equation requires the accurate measurement of the specific surface area of the soil per unit volume. In this study, it is demonstrated that the specific surface area can be derived from the SS function, which simply but accurately represents the soil’s PSD. The modified KC equation is also extended in terms of uniformity and sorting coefficients for its potential use in geotechnical applications. The applicability of the modified KC equation is confirmed through a comparison of the theoretical results of over 50 permeability tests that yielded values ranging from 1?×?10^?5 to 1?×?10^?1?m/s.     

The local dynamics of parent–child relationships

The study of relationships between family members necessitates data on a network not confined to the boundaries of the household and a variety of indicators to capture the nature of the exchanges. Using the data from Biographies et entourage survey, it is possible to elaborate such indicators by combining frequency of contacts, geographical distance and affective closeness, allowing us to analyse the local or semi-coresident nature of the parent–child relationships and the cross-cohesion between respondents aged 50–60 and their children. By first considering all the members of the current family circle, the analysis confirms that these cohorts are strongly involved in the family network numerically dominated by their siblings and children. However, since it is with their children that the interactions appear to be strongest, the article provides a more detailed analysis of relationships between the respondents and their children.

Seismic evaluation of soil–foundation–superstructure system considering geometry and material nonlinearities of both soils and structures

In this paper, in order to increase the accuracy of numerical simulations using finite element methods related to soil–structure interaction problems, the influence of the geometric and material nonlinearities of structures was carefully investigated. By introducing proper models dealing with the geometric and material nonlinearities of structures, the authors have proposed a numerical method for the modeling of the nonlinearity of soils that have been carefully investigated in past research, and also for the modeling of structures based on finite deformation schemes. The accuracy of the numerical analysis related to the structures was firstly verified with an Abaqus (2008) calculation. The calculation was conducted with a program named DBLEAVES (Ye, 2007, Ye, 2011). Furthermore, 2D soil–water coupled dynamic analyses, in the finite deformation schemes of both soils and structures, were conducted on a soil-group pile foundation–superstructure system to investigate the seismic behavior of an elevated bridge during a major earthquake, in which strong nonlinear behavior of geometry and material are expected for both the soils and the structures. The applicability of the proposed numerical method to soil–structure interaction problems encountered in many fields of geotechnical and structural engineering was carefully checked. The main purpose of the research is to propose a numerical method by which it is possible to describe the soil–structure interaction problems with a level of accuracy that can satisfy the needs of both the geotechnical and structural engineering fields.