Numerical analyses of pillar behavior with variation in yield criterion,dilatancy, rock heterogeneity and length to width ratio

With recent advances in numerical modeling, design of underground structures increasingly relies on numerical modeling-based analysis approaches. While modeling tools like the discrete element method (DEM) and the combined finite-discrete element method (FDEM) are useful for investigating small-scale damage processes, continuum models remain the primary practical tool for most field-scale problems. The results obtained from such models are significantly dependent on the selection of an appropriate yield criterion and dilation angle. Towards improving its capabilities in handling mining-related problems, the authors have previously developed a new yield criterion (called progressive S-shaped criterion). The focus of the current study is to demonstrate its use in modeling rock pillars through a comparative analysis against four other yield criteria. In addition to the progressive S-shaped criterion, only one out of the four other criteria predicted a trend in strength consistent with an empirical pillar strength database compiled from the literature. Given the closely-knit relationship between yield criteria and dilation angle in controlling the overall damage process, a separate comparison was conducted using a mobilized dilation model, a zero degree dilation angle and a constant non-zero dilation angle. This study also investigates the impact of meso-scale heterogeneity in mechanical properties on the overall model response by assigning probability distributions to the input parameters. The comparisons revealed that an isotropic model using a combination of progressive S-shaped criterion and mobilized dilation angle model is sufficient in capturing the behaviors of rock pillars. Subsequently, the pillar model was used to assess the effect of L/W (length/width) ratio on the peak strength.     

电梯曳引绳张力分析

通过对曳引绳张力的定义和设计要求的探讨,分析了曳引绳张力差的影响,阐述了影响曳引绳张力的各类因素;指出了曳引绳张力测量不确定度的来源;针对曳引绳张力测量中的一些问题,探讨了修订曳引绳张力差控制标准要求的可能,提出了改进曳引绳张力测量、评价的方法以及减小曳引绳张力差的措施。     

砌体结构在地震下的非线性计算模型

针对目前砌体结构计算模型在弹塑性动力时程分析中的不足,本文采用一种三弹簧单元宏观模型进行整体结构的非线性计算.首先,将其计算结果与单片砌体构件试验结果对比,验证了模型的准确性;随后,采用该模型对一简单空间砌体结构进行静力和动力分析,并计算了不同地震烈度下的结构响应,得到令人较为满意的结果.以层间位移角定义了各墙体构件的弹性、破坏和极限3个阶段,对整体结构的损伤情况给予了评价,并提出以竖向承载构件完全失效作为结构倒塌的判别准则,使三弹簧单元模型在砌体结构抗震研究方面具备一定的适用性.     

夹绳器专用电磁铁的设计

通过对一种电梯上行超速保护装置广泛采用的夹绳器的简单分析．引出了安全部件所用的电磁铁的性能要求．详细分析了直流电磁铁的时间特性．给出了优化直流电磁铁时间特性的几种方法。     

Damage evolution mechanisms of rock in deep tunnels induced by cut blasting

The method of cut blasting is widely used in tunnelling excavation, which concerns the success in subsequent stop blasting and smooth blasting. Rock masses in deep tunnels are subjected to high in-situ stress, and the mechanisms of damage evolution of rock mass in deep tunnels induced by cut blasting are not well studied. In this paper, a tension and compression-shear damage model is developed, and then is implemented into the commercial software LS-DYNA as a user-defined material model. To validate the newly developed model, the comparison between numerical results and an existing field test is conducted. The effects of free-surface boundary conditions, in-situ stress and the coefficient of lateral pressure on cut blasting are considered in depth. Numerical results indicate that the superstition of stress wave and the reflected tension waves from free surfaces contribute to the damage interconnection near cut holes. The high in-situ stress has the resistance on the radially oriented pressure and the damage extension around cut holes. The coefficients of lateral pressure influence the extending direction of the tensile damage zone.     

一起电梯钢丝绳断股事故的调查分析

通过一起电梯曳引钢丝绳断股事故的鉴定,深入调查与分析了曳引钢丝绳断股事故的影响因素,认为曳引机导向轮设计不合理、电梯运行时钢丝绳张力偏差大、钢丝绳日常维护不到位等是造成事故的重要原因。     

Numerical modelling of the cyclic behaviour of RC elements built with plain reinforcing bars

The bond-slip mechanism is one of the features that significantly controls the response as well as damage evolution of reinforced concrete structures when subjected to severe cyclic loadings, such as those induced by earthquakes. Its effect is particularly important in structures built with plain reinforcing bars. For a rigorous simulation of the response of existing RC structures, built mainly with plain bars, the bond-slip mechanism should be considered. However, the majority of the available concrete-steel bond numerical models were developed and calibrated for elements with deformed reinforcing bars. Moreover, the available experimental data on the cyclic behaviour of RC elements built with plain bars is still limited. In this framework, the objective of the present paper is to calibrate a numerical model based on results of a cyclic test performed on a two-span RC beam built with plain bars, which was collected from an existing structure. The numerical modelling was carried out with the nonlinear OpenSees software platform. Particular awareness was devoted to the bond-slip mechanism. The numerical results obtained with the calibrated nonlinear model are presented and compared with the experimental results. The consideration of the bond-slip effect in the numerical model was fundamental to achieve a good agreement between the numerical simulation and the test results.     

Seepage and Inertia Effect on Rate-Dependent Reaction of a Pile in Liquefied Soil

The seepage and inertia effects on the rate-dependent subgrade reaction of a single pile in liquefied soil are clarified through numerical studies. The quasi-static (for seepage effect) and dynamic (for inertia effect) numerical analyses are performed with a soil-water coupled formulation and a simplified cyclic elasto-plastic constitutive model for liquefied sand. The constitutive model can explicitly deal with the liquefaction intensity by changing the lower bounds of the mean effective stress. The liquefied soil at a certain depth around a pile is modeled with finite elements under a plane stress condition. The results of the quasi-static analyses under monotonic loading conditions show that the apparent rate-dependency of the subgrade reaction is caused by the seepage of pore water around the pile due to soil positive dilatancy under large strain range. The results of the dynamic analyses under cyclic loading conditions show that the positive correlation in a p-v (subgrade reaction-pile relative velocity) relation can be explained with the phase difference in movement between a pile and the neighboring soil around the pile due to inertia under small strain range.     

Experimental and numerical evaluation of bearing capacity of steel plate girder affected by end panel corrosion

The present study deals with the effect of uniform and non-uniform local corrosion damages at the bearing stiffener and nearby web on the bearing capacity of plate girder. Five plate girder ends simulated with different uniform and non-uniform types of corrosion damage, at the base of the bearing stiffener are used in the experimental program. The experimental results are modeled and verified on a Finite Element (FE) software ABAQUS, considering shell-solid coupling elements formulation. The study is further extended to various damage cases i.e., stiffener damage, stiffer plus web damages etc. considering different damage heights and residual thicknesses. The study concludes that minimum thickness within any damage height may be used to simulate the corrosion damage in a FE analysis. At the end, some empirical relationships are also proposed to estimate the bearing strength of the plate girder affected by the local corrosion damage at plate girder end.     

电梯夹绳式限速器-安全钳联动试验失效分析

针对夹绳式限速器-安全钳联动试验的失效现象,进行失效机理分析,并给出了相应的解决措施。     

Thermal buckling analysis of rectangular microplates using higher continuity p-version finite element method

In this article, thermal buckling characteristics of rectangular flexural microplates (FMP) subjected to uniform temperature are investigated using higher continuity p-version finite element framework. Invariant form of the governing equation for a microplate with non-local effects based on “modified couple stress theory” is extended for thermal buckling analysis of FMP by considering the strain gradient effects. In this case, the constitutive equation for strain gradient model is based on one constant. Galerkin weak form of the governing equation is derived and subsequently solved for a variety of boundary conditions using higher continuity p-version finite elements to extract critical thermal buckling loads. The computational procedure is verified by comparing its predictions to those of the classical theory and analytic microplate studies that are based on the same strain gradient model. Investigations indicate that length scale parameter affects the computed flexural stiffness of a plate, and the effect is directly proportional to the value of gradient coefficient considered for that plate. Hence, there is a strong influence of length scale parameter on value of the thermal buckling load. Depending on boundary conditions and value of length scale parameter used in numerical experiments, the classical plate model severely underestimates (up to 90%) the thermal buckling load for microplates. Therefore, it is concluded and strongly suggested that the classical plate theory should not be used to predict structural response of microplates.     

Thermal buckling analysis of rectangular microplates using higher continuity p-version finite element method

In this article, thermal buckling characteristics of rectangular flexural microplates (FMP) subjected to uniform temperature are investigated using higher continuity p-version finite element framework. Invariant form of the governing equation for a microplate with non-local effects based on “modified couple stress theory” is extended for thermal buckling analysis of FMP by considering the strain gradient effects. In this case, the constitutive equation for strain gradient model is based on one constant. Galerkin weak form of the governing equation is derived and subsequently solved for a variety of boundary conditions using higher continuity p-version finite elements to extract critical thermal buckling loads. The computational procedure is verified by comparing its predictions to those of the classical theory and analytic microplate studies that are based on the same strain gradient model. Investigations indicate that length scale parameter affects the computed flexural stiffness of a plate, and the effect is directly proportional to the value of gradient coefficient considered for that plate. Hence, there is a strong influence of length scale parameter on value of the thermal buckling load. Depending on boundary conditions and value of length scale parameter used in numerical experiments, the classical plate model severely underestimates (up to 90%) the thermal buckling load for microplates. Therefore, it is concluded and strongly suggested that the classical plate theory should not be used to predict structural response of microplates.     

斜拉桥索力优化问题研究

主要介绍目前国内外有许多学者对斜拉桥的索力优化问题进行的研究,归结起来可分为三大类:指定受力状态的索力优化、无约束的索力优化和有约束的索力优化。通过对三种优化方法进行比较找出目前存在的问题。     

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.     

Comparative assessment of genotoxicity of mineral water packed in polyethylene terephthalate (PET) and glass bottles

The potential migration of genotoxic compounds into mineral water stored in polyethylene terephthalate (PET) bottles was evaluated by an integrated chemical/biological approach using short-term toxicity/genotoxicity tests and chemical analysis. Six commercial brands of still and carbonated mineral water bottled in PET and in glass were stored at 40 °C for 10 days in a stove according to the standard EEC total migration test (82/711/EEC), or at room temperature in the dark. After treatment, the samples were analysed using gas-chromatography/mass spectrometry (GC/MS) to detect volatile and non-volatile compounds, the Microtox^® test to evaluate potential toxicity of the samples, and three mutagenicity tests - Tradescantia and Allium cepa micronucleus tests and the Comet assay on human leukocytes - to detect their genotoxic activity.GC/MS analysis did not detect phthalates or acetaldehyde in the water samples. The Microtox^® test found no toxic effects. Mutagenicity tests detected genotoxic properties of some samples in both PET and glass bottles. Statistical analyses showed a positive association between mineral content and mutagenicity (micronuclei in A. cepa and DNA damage in human leukocytes). No clear effect of treatment and PET bottle was found. These results suggest the absence of toxic compounds migrating from PET regardless of time and conditions of storage. In conclusion, bottle material and stove treatment were not associated with the genotoxic properties of the water; the genotoxic effects detected in bottled water may be related to the characteristics of the water (minerals and CO₂ content).     

Long-term fatigue analysis of multi-planar tubular joints for jacket-type offshore wind turbine in time domain

Long-term fatigue analysis of welded multi-planar tubular joints for a fixed jacket offshore wind turbine designed for a North Sea site in a water depth of 70 m is performed. The dynamic response of the jacket support structure due to wind and wave loads is calculated by using a decoupled procedure with good accuracy (Gao et al., 2010). Hot-spot stresses at failure-critical locations of each reference brace for 4 different tubular joints (DK, DKT, X-type) are derived by summation of the single stress components from axial, in-plane and out of plane action, the effects of planar and non-planar braces are also considered. Both a 2-parameter Weibull function and generalized gamma function are used to fit the long-term statistical distribution of hot-spot stress ranges by a combination of time domain simulation for representative environmental conditions in operational conditions of the wind turbine. A joint probabilistic model of mean wind speed U_w, significant wave height H_s and spectral peak period T_p in the northern North Sea is used to obtain the occurrence frequencies of representative environmental conditions (Johannessen, 2002). In order to identify the contributions to fatigue damage from wind loads, wave loads and the interaction effect of wind and wave loads, 3 different load cases are analyzed: wind loads only; wave loads only; a combination of wind and wave loads. The representative environmental condition corresponding to the maximum contribution to fatigue damage is identified. Characteristic fatigue damage of the selected joints for different models is predicted and compared. The effect of brace thickness on the characteristic fatigue damage of the selected joints is also analyzed by a sensitivity study. The conclusions obtained in this paper can be used as the reference for the design of future fixed jacket offshore wind turbines in North Sea.     

Design and behavior of recentering beam-to-CFT column connections with super-elastic shape memory alloy fasteners

This study investigates the performance of new composite (steel-concrete) moment connections through numerical simulations. The innovative aspects of this research lay in the use of end-plate connections between steel beams and concrete-filled tube (CFT) columns that utilize a combination of low-carbon steel and shape memory alloy (SMA) components. In these new connections, the intent is to use the recentering effect provided by super-elastic SMA tension bars to reduce the level of building damage and residual drift after a major earthquake. The low-carbon steel components provide excellent energy dissipation. The analysis and design of these structures is complicated because the connections cannot be modeled as being simply pins or full fixity ones; they are partial restraint (PR). A refined finite element (FE) model with sophisticated three dimensional (3D) solid elements was developed to perform numerical experiments on the PR-CFT joints with a view to obtaining the global behavior of the connection. Based on the behavioral information obtained from these FE tests, simplified connection models were formulated using 2D joint elements with nonlinear spring components. The behavior of entire connections under cyclic loads was examined. The results were compared with the connection behavior obtained from the 3D FE simulations and corresponding connection tests. Good agreement was found between the simple and sophisticated models, confirming the robustness of the approach.     

The effects of explosive mass ratio on residual compressive capacity of contact blast damaged composite columns

The risks associated with suitcase bombs are of serious concern because they can be easily handled and placed within close proximity of key structural components of building structures. The most common failure mode of structures subjected to blast loads from satchel and suitcase bombs is progressive collapse. The high-fidelity physics-based computer program, LS-DYNA is utilized in this study to provide numerical simulations of the dynamic response and residual axial capacity of composite columns subjected to blast loads. A field test using contact explosive was conducted on one composite column specimen. The test results were compared with the analytical results to validate the finite element model. An extensive parametric study was conducted to investigate the relationship between residual axial capacity and structural and loading parameters such as material strength, column detail and blast conditions. Two empirical equations were derived through a multivariable regression analysis in terms of the various parameters to predict the residual capacity index based on the explosive mass ratio (ω_TNT).     

Probabilistic seismic demand analysis of a slender RC shear wall considering soil-structure interaction effects

Reinforced concrete shear walls are often used to resist the lateral loads imposed by earthquakes. Accurate evaluation of the seismic demands on shear walls requires adequate considerations of the nonlinear behavior of structural and foundation elements, the interaction between them, and the uncertainty and variability associated with earthquake ground motions. This paper presents a comprehensive probabilistic seismic demand analysis of a typical mid-rise slender shear wall in western US with a flexible foundation and evaluates the significance of soil-structure interaction (SSI) effects on their damage probability. Utilizing realistic numerical models for the shear wall and its foundation, the nonlinear time history analyses were conducted with a large number of recorded ground motions. Response quantities such as maximum inter-story drift ratio, base shear, foundation displacement and rotation are monitored and related to the intensity measure of ground motions (i.e. the inelastic spectral displacement S_di) for the cases with and without considering the SSI effects. Subsequently, the fragility functions of the shear wall are derived and the impact of SSI effects is investigated. It is found that the SSI generally reduces the damage probability of the shear wall, especially when soil nonlinearity is taken into account. The sensitivity of various seismic demands to soil parameters is also discussed. Under strong ground shakings, SSI effects on the maximum inter-story drift are most sensitive to the friction angle of the soil. It is suggested that the damages in foundation and surrounding soil should also be considered in order to systematically evaluate the SSI effects on damage probability of shear wall buildings.