Reliability analysis of intermediate crack-induced debonding failure in FRP-strengthened concrete members

This paper presents a study on the reliability analysis of intermediate crack-induced (IC) debonding failure, which is a dominant failure mode in concrete members strengthened with externally bonded fibre-reinforced polymer composites. First, the probabilistic characteristics of the model uncertainties for several typical IC debonding models are evaluated and presented, based on a comprehensive test database that contained 217 specimens that experienced IC debonding failure. Second, a Monte Carlo simulation analysis is performed to examine the variability in the IC debonding resistance considering the scatterness of different design variables. Subsequently, the reliability index is estimated by utilising the reliability analysis method, first-order reliability method, for each design case. The effect of each design variable on the average reliability index is examined. Finally, reduction factors for the investigated IC debonding models are calibrated based on the reliability analysis with the aim of meeting a target reliability index of 3.5.     

A strain-based index for monitoring laminates of FRP-strengthened beams

Carbon fibre reinforced polymer (FRP) laminates are increasingly being used to strengthen reinforced concrete beams. Monitoring the integrity of the adhesive bond between the laminate and the concrete is important to establishing the long-term reliability of this technique. This paper discusses the development of a strain-based index in which the loss of strain compatibility can be used to identify regions where debonding has occurred. Theoretical and experimental work was conducted and the laminate tensile strain profiles were shown to be sensitive to the presence of induced debonded zones. The method has potential for broad implementation in many field monitoring applications.     

Modeling of debonding failure for RC beams strengthened in shear with NSM FRP reinforcement

The shear capacity of reinforced concrete members can be successfully increased using near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. Tests conducted thus far have shown that failure is often controlled by diagonal tension associated to debonding between the NSM reinforcement and the concrete substrate. In absence of steel stirrups and/or when the spacing of the NSM reinforcement is large, debonding involves separately each of the bars crossed by the critical shear crack. In order for shear strengthening of beams with NSM reinforcement to be safely designed, an analytical model able to encompass the failure mode mentioned above must be developed. This paper presents two possible approaches, a simplified and a more sophisticated one, to predict the FRP contribution to the shear capacity. In the first approach, suitable for immediate design use, an ideally plastic bond–slip behavior of the NSM reinforcement is assumed, which implies a complete redistribution of the bond stresses along the reinforcement at ultimate. The second approach, implemented numerically, accounts for detailed bond–slip modeling of the NSM reinforcement, considering different types of local bond–slip laws calibrated during previous experimental investigations. It also takes advantage of an approach developed by previous researchers to evaluate the interaction between the contributions of steel stirrups and FRP reinforcement to the shear capacity. The paper illustrates the two models and compares their predictions, with the ultimate goal to evaluate whether the first simple model can be used expecting the same safety in predictions of the second model.     

A rational design model for transverse web stiffeners

Existing design methods for web stiffeners differ widely in concept and in the stiffener size which results. An investigation has been conducted to provide a rational, usable and validated design method for transverse stiffeners. The proposed design method takes account of the axial compression in stiffeners caused by tension field action and external forces, the transverse forces on the stiffeners required to enforce effective division of the web plate, and the tendency for the stiffened plate to buckle overall. Design criteria are established to ensure that the resulting stiffeners do not yield before plate failure, and that the plating is effectively divided into panels. The transverse stiffener dimensions required by the proposed design model have been validated against finite element results for plates subjected to pure shear, and to combined shear and direct in-plane loading. Finally, the stiffener sizes are compared with those required by the existing methods.     

A finite segment model for shear lag analysis

A finite segment model is developed to calculate the shear lag effects for box girders with varying depth. Multiple longitudinal displacement functions are used to derive the stiffness matrix and the nodal force vector of the segment element. The shear deformation of the web is taken into consideration. A Perspex glass model of a three-span continuous box girder with varying depth is tested to provide experimental results for verifying the accuracy of the proposed method. The influences of the flange width to span ratio and girder–height ratio upon the shear lag are presented and discussed.     

确定前期固结压力的一个简单数学模型

前期固结压力是判断土应力历史的最常用指标。为能够更加准确地确定前期固结压力,首先将超固结土压缩线上曲率最大时所对应的竖向应力引入到单对数坐标中,建立正常固结土和超固结土统一的一维压缩线方程。然后,基于该方程严格推导出前期固结压力的计算式。该计算式是一个显式函数,无需采用编程计算,且只需土的压缩指数Cc及一维压缩线的渐进线在单对数坐标中的纵轴截距N两个常见参数,便于工程师应用。最后,通过两个算例验证所提数学模型的合理性。     

A simple unified stress-strain model for geotextile-wrapped soils

The stress-strain behaviour of geotextile-wrapped soils is a matter of great concern in geotechnical engineering practices. In this study, a unified stress-strain model for geotextile-wrapped soils is proposed and is applied to understand the strength and deformation characteristics of soilbags (woven bag-wrapped soils) in 2D biaxial loading. It is demonstrated that the strength of wrapped soils is greatly improved due to the wrapping effect, and the shearing contraction of the wrapped soils is more pronounced than that of the pure soils. The validity of the results is verified through numerical simulation using the discrete element method (DEM). The effect of the geometric size and material properties of the woven bag on the strength and deformation characteristics of the soilbags is also analyzed using the proposed model. The proposed model differs from existing phenomenological models in that it is able to predict the mechanical response of soilbags when the constitutive behaviours of the wrapped soils and the woven bag are given.     

A force-based model for composite steel-concrete beams with partial interaction

This paper presents a new force-based beam-column element for the nonlinear analysis of composite steel-concrete beams with partial interaction. The element is made up of three components: (a) a fiber beam-column element that models the behavior of the steel girder, (b) a fiber beam-column element that models the behavior of the concrete deck, and (c) a bond element that models the transfer of forces between the steel and concrete elements through shear connectors. The model neglects uplift and frictional effects. The fiber beam-columns are force-based elements that depend on force interpolation functions. A linear bending moment and a constant axial force serve as the interpolation functions. An important factor that favors the use of force-based elements in modeling composite structures is their ability to treat any type of distributed element loads. Distributed element loads are applied internally in a continuous manner by force superposition at the control sections. The state determination of these elements is based on an iterative solution that determines the element resisting forces and stiffness matrix. The bond element is a spring-type element that assumes a linear bond stress variation along the length. The nonlinear behavior of the composite element derives entirely from the constitutive laws of the steel, the concrete and the shear connectors. The paper concludes with a correlation study to investigate the validity of the model. Good agreement between analysis and experimental results was observed.     

梁基本频率和振型的简便计算

梁基频和振型的简单计算是结构动力学领域中一个重要的研究课题.幂法通过迭代可求得基频和振型的精确解,但计算过程繁琐.本文选择梁各质点静变形量作为初始值进行迭代,只需迭代很少次数就可求得梁基频和振型的精确解,大大简化了计算过程,同时推导出了计算静变形量所需的多自由度梁柔度矩阵的计算公式,使这种方法简单易行.     

体外预应力混凝土简支梁极限应力分析

从结构的整体变形入手，推导出计算体外预应力混凝土简支梁极限应力实用计算方法，指出体外预应力混凝土筒支梁的体外索的极限应力取决于整个构件的变形。     

两种简易可行的砖墙托换梁

介绍了两种承重砖墙托换梁,并给出了相关的设计计算方法,这两种托换梁具有受力明确、构造简单、便于施工等优点.     

A new model for analyzing the effect of fractures on triaxial deformation

Rock is porous, with a connected network of cracks and pores. The static and dynamic behaviors of a rock sample under load depend on both the solid mineral matrix and the porous phase. In general, the configuration of the pore phase is complex; thus, most studies on the effect of the porous phase on rock deformation are conducted numerically and theoretical analyses of the constitutive relations are scarce. We have studied rock deformation under axially symmetric loading by analyzing a model where the pore phase is approximated by rough planes, randomly spaced and oriented, extending through the sample. The roughness is caused by asperities, all with the same tip radii, but having heights h with a probability density distribution given by the negative exponential e^−h/λ where λ is a length parameter. Slip at contacts under local shear stress is resisted by simple Coulomb friction, with friction coefficient f. Both static and dynamic deformation were analyzed. The effect of porosity on deformation for both modes was found to be given by the non-dimensional parameter λ_j, where _j is the total area of the fault planes per unit volume. We demonstrate that stress-induced microfracturing begins as randomly oriented microslip throughout the sample. As axial load increases, microslip occurs along preferred orientations and locations, which finally leads to deformation on a single fault. The model was found to fault under static loading conditions—the axial load at faulting and the angle of the “fracture” plane agree with values of those parameters given by Coulomb's theory of fracture. Dynamic moduli and Poisson's ratio are found to be virtually elastic and independent of the friction coefficient acting at contacts. The attenuation for uniaxial dynamic loading is a strong function of the friction coefficient and increases linearly with strain amplitude, in agreement with laboratory measurements.     

A simple model for diauxic growth of denitrifying bacteria

A simple model has been formulated to simulate diauxic growth of denitrifying bacteria. It is capable of fitting the experimental results of batch growth experiments with Pseudomonas denitrificans under various conditions. It successfully predicts the observed lags when a pure culture of this bacterium switches from oxygen to nitrate as terminal electron acceptor. The model includes the effect of carbon substrate limitation and length of aerobic phase and does not run into problems when switching from anoxic to aerobic conditions, unlike prior models of diauxic growth.     

任意旋转角坐标转换的简便模型

本文提出了一种适合于任意旋转角坐标转换的简便模型,该模型的原理是首先对两套坐标系下各点的坐标进行重心化,而后求得转换矩阵。在一个模拟算例和一个工程实例中应用本文的方法进行坐标转换并对结果进行分析和比较,说明了本文提出的简便方法的实施过程和有效性。将本文提出的方法用于地铁盾构的姿态控制,取得了令人满意的效果。     

黏土和砂土简单的三维本构模型(英文)

把修正剑桥模型推广为适合于黏土和砂土的简单、统一三维本构模型是基于以下两点提出的 :第一点 ,把由笔者已提出的基于SMP准则的一个变换应力张量用于修正剑桥模型使其简单实现了三维化 ,改进的模型对于黏土实现了从剪切屈服到剪切破坏的统一及临界状态理论同SMP准则的结合 ;第二点 ,为了建立对于黏土和砂土简单、统一的本构模型 ,推导出了一个新的硬化参数 ,新硬化参数不仅能描述砂土不同程度的剪胀性 ,对于正常固结黏土又能退化成塑性体积应变 ,所提硬化参数的合理性也被各种路径下分别在三轴压缩和三轴伸长下的试验结果所证实。所提模型的计算参数仅为 5个常规试验参数 ,易于确定     

A simple model for assessing the energy performance of windows

A simple model for the annual energy balance of the window taking solar radiation and heat losses into consideration has been further developed and analysed. Hourly meteorological data for the solar irradiation and the outside temperature are used together with the optical and thermal performance of the window to evaluate the net energy heat flow through a window. The model renders a very simple way to compare different advanced windows in different geographical locations, orientations and buildings using basically only the balance temperature as building input. The energy balance and the cost efficiency for several glazing combinations are evaluated for buildings with different balance temperatures in a typical mid-Swedish climate. This model has a potential to be used for energy rating of windows.     

A simple biofilm model of bacterial competition for attached surface

A simple biofilm model of competition in bacterial growth for an attached surface is developed. Competition for the attached surface is expressed with the crowded and detachment effects. The developed model is verified by comparing simulated results with data obtained in the experiments of batch culture of nitrifier and continuous treatment of actual sewage with biofilm reactor. This model can favorably simulate the growth competition between autotrophic and heterotrophic bacteria for the attached surface. Then some parameters for nitrification process are discussed with this model. It is clarified that the effective removal of organic matter before nitrification tank is required for effective nitrification in the biofilm reactor.