周向导波在不同连续性条件多层圆筒中的传播

通过对多层厚壁圆筒中周向导波频散曲线研究,发现周向导波在多层圆筒中传播时,会发生明显的频散现象和模态干涉现象,第1阶模态在高频时接近于无频散的Rayleigh面波。通过对不同连续性条件的多层筒与单层筒的频散曲线比较,以及筒的层数对频散曲线的影响,都充分说明了检测层间界面缺陷的周向导波应该集中在低阶模态上。最后对不同连续性条件多层筒的位移研究,最终推断出第2阶模态具有在层间界面附近集中能量的特征,可应用于层间界面缺陷的周向导波无损检测技术中。     

Optimum design of concrete cable-stayed bridges

The design of cable-stayed bridges involves a significant number of design variables and design objectives. The concrete cable-stayed bridge optimization is formulated here as a multi-objective optimization problem with objectives of minimum cost, minimum deflections and minimum stresses. A numerical method is developed to obtain the optimum design of such structures. This numerical method includes: structural analysis, sensitivity analysis and optimization. The structural analysis accounts for all the relevant effects (concrete time-dependent effects, construction stages and geometrical nonlinear effects). The structural response to changes in the design variables is achieved by a discrete direct sensitivity analysis procedure, and an entropy-based approach was used for structural optimization. The features and applicability of the proposed method are demonstrated by numerical examples concerning the optimization of a real-sized concrete cable-stayed bridge.     

Optimum design of concrete cable-stayed bridges with prestressed decks

This article presents an optimization-based numerical method for the design of concrete cable-stayed bridges with prestressed decks. This method includes a structural analysis module and a sensitivity analysis and optimization module. The structural analysis considers concrete time-dependent effects, construction stages and geometrical nonlinearities. The discrete direct method is used for sensitivity analysis and an entropy-based approach is used for structural optimization. The design is formulated as a multi-objective optimization problem with objectives of minimum cost, minimum deflections and stresses. Numerical examples concerning the optimization of a real sized cable-stayed bridge are presented to illustrate the applicability of the proposed method.     

Mix design method for self compacting metakaolin concrete with different properties of coarse aggregate

This study deals with a proposed mix design method for SCC utilizing different properties of coarse aggregate. The work was conducted in three phases, i.e. paste, mortar and concrete to facilitate the mix design process. Initial investigation on cement paste determined the basis for water cement ratio and superplasticizer dosage for the concrete. For the study on mortar, metakaolin (MK) as pozzolan was used at replacement levels of 5%, 10%, 15%, and 20% by weight of cement. Self compactability of mortars was obtained by adding suitable materials such as mineral admixtures and superplasticizer which provided a sufficient balance between flowability and viscosity of the mix. The optimum MK replacement level for cement was 10% from the viewpoint of workability and strength. Flowability of mortar decreased with the use of metakaolin. Moreover, strength of mortar increased when the optimum replacement level of pozzolan was used. Different fresh concrete tests were adopted. The results obtained for fresh concrete properties showed that flowability of concrete increased with increase flowability of mortar. The mixes which contained coarse aggregate with lower volume, small size, and continuous grading affected positively the fresh properties of SCC. Finally, the mix design method used was successful in producing SCC with different coarse aggregate properties.     

Axisymmetric crack problem of thick-walled cylinder with loadings on crack surfaces

This study is concerned with the fracture of an infinite thick-walled cylinder. The inner surface of the cylinder is stress free and the outer is rigidly fixed. The cylinder having a ring-shaped crack located at the symmetry plane is subjected to distributed compressive load on its surfaces. The Hankel and Fourier transform techniques are used for the solution of the field equations. By applying the boundary conditions, the singular integral equation in terms of crack surface displacement derivative is derived. By using an appropriate quadrature formula, the integral equation is reduced to a system of linear algebraic equations. Numerical results are obtained for the stress intensity factors at the edges of the crack, surfaces of which are subjected to uniform, linear and parabolic load distributions.     

The elasticity problem for a thick-walled cylinder containing a circumferential crack

The elasticity problem for a long hollow circular cylinder containing an axisymmetric circumferential crack subjected to general nonaxisymmetric external loads is considered. The problem is formulated in terms of a system of singular integral equations with the Fourier coefficients of the derivative of the crack surface displacement as density functions. The stress intensity factors and the crack opening displacement are calculated for a cylinder under uniform tension, bending by end couples, and self-equilibrating residual stresses.     

层间光滑接触的双层厚壁圆筒中周向导波

通过对双层厚壁圆筒周向导波的频散现象的研究,以及与单层圆筒周向导波特性的比较,发现周向导波在双层厚壁圆筒中传播时,会发生明显的频散现象和模态干涉现象,其第1阶模态在高频时接近于无频散Rayleigh面波。在其高阶模态上,层间效应被弱化,因此,应将层间界面裂纹无损检测的频率激发范围,集中于低阶模态频率上。研究发现,第2阶模态,具有在层间界面上集中能量的特点,因此,可用于层间界面裂纹的周向导波无损检测技术中。     

Optimum detailed design of reinforced concrete frames using genetic algorithms

This article presents the application of the genetic algorithm to the optimum detailed design of reinforced concrete frames based on Indian Standard specifications. The objective function is the total cost of the frame which includes the cost of concrete, formwork and reinforcing steel for individual members of the frame. In order for the optimum design to be directly constructible without any further modifications, aspects such as available standard reinforcement bar diameters, spacing requirements of reinforcing bars, modular sizes of members, architectural requirements on member sizes and other practical requirements in addition to relevant codal provisions are incorporated into the optimum design model. The produced optimum design satisfies the strength, serviceability, ductility, durability and other constraints related to good design and detailing practice. The detailing of reinforcements in the beam members is carried out as a sub-level optimization problem. This strategy helps to reduce the size of the optimization problem and saves computational time. The proposed method is demonstrated through several example problems and the optimum results obtained are compared with those in the available literature. It is concluded that the proposed optimum design model can be adopted in design offices as it yields rational, reliable, economical, time-saving and practical designs.     

A simple mix design method for structural lightweight aggregate concrete

Current design methods for structural lightweight aggregate concrete (SLWAC) are usually only valid for a limited range of concrete compositions that have previously been subjected to trial tests. The SLWAC mix design is more complex than that of normal weight concrete as more parameters need to be determined. Taking this into account, a simplified design method is proposed for SLWAC made with natural sand. The major advantages of the proposed method are that it is easy to apply and it can be generalized to any type of lightweight aggregate (LWA). For this, three additional design parameters are needed: the strength of LWA in concrete; the limit strength; the SLWAC potential strength. At most, two experimental mixtures are needed to determine these parameters. A biphasic model to estimate the strength of SLWAC is evaluated and high correlations are obtained. The good performance of the suggested method is demonstrated by examples of practical application and by the comparison with experimental results reported by the authors and other investigators.     

The strength design method for reinforced concrete around the world

In the 40 years since the American Concrete Institute (ACI) 318-56 Code introduced the ultimate strength design method, it has become the primary method for reinforced concrete design in the United States. The concept of ultimate or limit state design has also been incorporated into building codes around the world. However, the codes of different countries do not agree on the factors used to relate the service loads to ultimate loads. This article reviews the basis for the strength design method and compares the treatment of the strength design concept by codes around the world.     

Calculation of stress intensity factors for a longitudinal semi-elliptical crack in a finite-length thick-walled cylinder

The purpose of this paper is to present the effect of finite boundary on the stress intensity factor of an internal semi-elliptical crack in a pressurized finite-length thick-walled cylinder  ( R _i/ t = 4)  . The three-dimensional finite element method, in conjunction with the weight function method, is used for computing the stress intensity factor at the deepest and surface points of an axial semi-elliptical crack in a cylinder. The transition aspect ratios, the aspect ratios in which the maximum stress intensity factor translates from the deepest to the surface points of the crack, are calculated for different relative depths and cylinder lengths. The results show that the stress intensity factor increases as the cylinder length decreases, especially at the corner point of the crack compared with the deepest point. The major advantage of this paper is that a closed-form expression is extracted for the stress intensity factor at the surface point of a semi-elliptical crack, which experiences higher changes due to the effect of the finite boundary of the cylinder.     

Transient thermal stress intensity factors for an internal longitudinal semi-elliptical crack in a thick-walled cylinder

In this paper, the stress intensity factors are derived for an internal semi-elliptical crack in a thick-walled cylinder subjected to transient thermal stresses. First, the problem of transient thermal stresses in a thick-walled cylinder is solved analytically. Thermal and mechanical boundary conditions are assumed to act on the inner and outer surfaces of the cylinder. The quasi-static solution of the thermoelasticity problem is derived analytically using the finite Hankel transform and then, the stress intensity factors are extracted for the deepest point and the surface points of the semi-elliptical crack using the weight function method. The results show to be in accordance with those cited in the literature in the special case of steady-state problem. Using the closed-form relations extracted for the transient thermal stress intensity factors, some conclusive results are drawn.     

Analysis of apparent fracture toughness of a thick-walled FGM cylinder with two diametrically opposed edge cracks

This study focuses on the analysis of apparent fracture toughness of a thick-walled functionally graded material (FGM) cylinder with two diametrically opposed edge cracks emanating from the inner surface of the cylinder. The crack surfaces and the inner surface of the cylinder are subjected to an internal pressure. The incompatible eigenstrain developed in the cylinder due to non-uniform coefficient of thermal expansion after cooling from sintering temperature is taken into account. Based on a generalized method of evaluating stress-intensity factor developed in our previous study, an approach is presented to evaluate apparent fracture toughness. To demonstrate the approach, some numerical results of apparent fracture toughness are presented for a TiC/Al₂O₃ FGM cylinder. The effects of material distribution, cylinder wall thickness, application temperature and number of cracks on apparent fracture toughness are investigated in details. It is found that all these factors play an important role in controlling apparent fracture toughness of a thick-walled FGM cylinder.     

双层结构隔声性能的经验预测方法

1.引言 双墙的隔声已经被广泛的研究,尤其是隔声量的预测已经有许多模型算法,但是这些方法大多是对理论计算的简化,并没有对理论和实际的误差给与修正.本文阐述了双层石膏板隔声性能的特点与实验结果,并与传统理论计算结果进行了比较.在此基础上对理论预测方法进行了改进,提出了一种新的双层结构隔声性能预测的经验方法,并将经验方法预测结果与实验以及传统理论计算结果进行了比较,改善了实验与预测结果的一致性.     

Optimum structural design with stability constraints

This paper presents an optimization method based on optimality criterion for minimum weight of structures with stability requirements. A recurrence relation is derived and the method is explained in the context of the displacement method of finite element analysis. The incipient buckling of the structure is determined by a linear eigenvalue solution. The method is programmed for trusses and frames. Illustrative problems are given to show the applicability of the method of design of structures with a large number of design variables.     

A method for optimal design of steel fiber reinforced concrete composition

Existing design approaches for steel fiber reinforced concrete composition practically do not consider the interaction between the concrete components. It decreases the design efficiency and accuracy. The paper deals with methodology for design of optimal steel fibered fine-grained concrete composition based on stiff mixtures. Such concrete is used for production of thin walled precise elements. The current investigation enables to find the influence of the main factors (water–cement ratio, fiber content, fineness and quantity of sand) on the concrete mixture stiffness, compressive and flexural strength of concrete. The study has also enabled to obtain corresponding mathematical models of concrete properties. Based on the models a methodology for design of steel fibered concrete was developed and appropriate nomograms were prepared. The proposed methodology allows obtaining of optimal steel fibered fine-grained concrete composition, taking into account the required flexural strength of concrete, sand fineness and concrete mixture workability.     

考虑不均匀界面时混凝土弹性模量预测

提出了考虑不均匀界面时混凝土弹性模量预测的解析法。根据界面层上水泥颗粒的分布特性, 给出了界面层上任一点处的局部水灰比和孔隙率。将不均匀界面层划分成一系列同心球壳单元, 通过反演方法确定了每个球壳单元和水泥石基体的弹性模量。将三相混凝土分解成一系列两相复合子结构, 应用两相复合球模型的正确解导出混凝土弹性模量。通过与文献中的两组实验结果比较验证了本文方法的有效性。数值结果表明, 对于给定的骨料体积分数, 混凝土弹性模量随着最大水泥颗粒直径和水灰比的增大而减小, 但随着最大骨料直径的增大而增大, 骨料级配对混凝土弹性模量也有一定的影响。      

Constitutive relations for split cylinder tests on bituminous concrete

The scope of this progress report was to a determine the constitutive relations for asphaltic concrete using the split cylinder test for two percents of bitumen, and b. compare the results with uniaxial tensile data on asphaltic concrete to assess the suitability of the split cylinder test for routine material characterizations. A special digital computer routine was used to reduce the experimental data, develop the constitutive relations, and perform the statistical analysis. Using the procedures reported here, the split cylinder tests, compared with uniaxial tension tests, overestimated the strain at high rates of loading, underestimated at very low rates of loading, and the ultimate strain values for the two test methods were the same for a value of R of about 0.1%/min. The split cylinder test underestimated the ultimate tensile stress by about one-half an order of magnitude for all values of strain rate that would be used in highway work.