Dynamic instability of laminated sandwich plates using an efficient finite element model

For the first time, the dynamic instability of laminated sandwich plates subjected to in-plane edge loading is studied using an efficient finite element plate model, which is developed recently by the authors. The plate model is based on a refined higher order shear deformation plate theory. In this theory, the transverse shear stresses are continuous at the layer interfaces with stress free conditions at plate top and bottom surfaces. It is interesting to note that the plate model having all these refined features requires unknowns at the reference plane only. However, this theory requires C¹ continuity of transverse displacement at the element edges, which is difficult to satisfy arbitrarily in any existing finite element. To deal with this, a new triangular element developed by the authors is used in the present paper.     

Solution to the problem of axisymmetric and asymmetric dynamic instability of three-layered annular plates

This paper presents the solution to the dynamic stability problem of three-layered, annular plate loaded by compressive stress increasing in time. The solution enables the evaluation of the critical, dynamic loads corresponding to the various modes of plate buckling. The symmetrical cross-section structure of plate is described by the classical theory of sandwich plate with the broken line hypothesis, the nonlinear Kármán’s plate equations and linear physical relations. The solution is based on Bubnov–Galerkin method and finite difference method. The values of critical dynamic loads have been calculated by means of the stability criterion presented by (Volmir (1972) [1]). The comparison of values of critical dynamic and static loads is presented, using the dynamic ratio. The obtained results have been compared with those obtained for plate model built by means of finite element method. The calculations were carried out using the ABAQS system. The dynamic response of plate models examined by two methods is consistent.     

Two-dimensional FE model for evaluation of composite beams, I: Formulation and validation

This paper develops a two-dimensional finite element model for composite beams, based on the use of the commercial package ANSYS. Different degrees of continuity can be taken into account, permitting the investigation of systems ranging from simply-supported to fully-continuous. Beams with either full or partial shear connection can be considered, as well as different slab typologies. The model represents the behaviour of all components of the composite arrangement, based on the best available current understanding, including the reinforcing bars (considering the tension-stiffening effect), the load-slip characteristic of the shear connectors, and the key components of the beam to column connection. In addition, material nonlinearity for all components is taken into account. The model has been fully verified by extensive comparisons against experimental and numerical results and it has been demonstrated that it is suitable for comprehensive parametric studies of the behaviour of composite beams. A sensitivity study focusing on both the required and the available rotation capacities and their importance for moment redistribution in semi-continuous composite beams is presented in the companion paper.     

A finite element model based on coupled refined high-order global-local theory for static analysis of electromechanical embedded shear-mode piezoelectric sandwich composite beams with various widths

In this study, a new theory for the accurate simulation of the shear-mode behaviour of thin or thick piezoelectric sandwich composite beams is developed. The effects of transverse normal stress and transverse flexibility of layers are considered in the development of the proposed formulation. In order to increase the computational accuracy, all kinematic and stress continuity conditions are satisfied at layer interfaces. Moreover, for the first time, both the electrically induced strain components and the transverse flexibility are taken into account in the proposed formulation. Despite the fact that the number of unknown mechanical parameters in this theory is only one degree higher than the first order shear deformation theory, the accuracy is surprisingly more pronounced for the thicker beams.     

Effect of transverse shear strains on plates resting on elastic foundation using modified Vlasov model

A four-noded quadrilateral (PBQ4) and an eight-noded quadrilateral (PBQ8) plate bending element based on Mindlin plate theory are adopted for the analysis of thin and thick plates resting on elastic foundation using modified Vlasov model. The terms of vertical deflection stiffness matrix and shear deformation stiffness matrix of the subsoil are evaluated using finite element method, and presented in explicit forms. Selective reduced integration technique is used in addition to full integration technique for both the types of the elements to avoid shear-locking problem that occurs under the thin plate limit. It has been demonstrated that the performance of the eight-noded quadrilateral element is excellent for thin and thick plates on elastic foundation when selective-reduced integration technique is used. General conclusion can be drawn from the results that the effect of the shear strain on the behavior of the plate resting on subsoil is always quite small for free plates compare to the supported ones.     

基于改进Vlasov模型的弹性地基上平板横向剪应变分析

运用Vlasov模型对弹性基础上的薄板和厚板进行分析,采用了基于Mindlin板理论的4结点四边形和8结点四边形弯曲单元。运用有限元法对底层土的竖向挠度刚度矩阵和剪切变形刚度矩阵进行推算。针对2种类型的板单元,采用完全积分方法和选择性简化积分方法进行分析,以避免极限状态下薄板的剪切闭锁问题。研究发现：在选择性简化积分方法中,8结点四边形板单元非常适用于分析弹性基础上的薄板和厚板。结果表明：相比有支撑的平板,剪应变对底层土上无支撑平板的影响非常小。     

Effect of the model updating on the earthquake behavior of steel storage tanks

In this paper, effect of the finite element model updating on the earthquake behavior of steel storage tanks considering fluid-structure interaction is investigated. For this purpose, a cylindrical steel storage tank filled some liquid fuel oil located in Trabzon, Turkey is selected as an example. Initial finite element model of the storage tank is developed by ANSYS software and dynamic characteristics (natural frequencies, and mode shapes) are determined analytically. Ambient vibration tests are conducted on the storage tank under natural excitations to obtain dynamic characteristics (natural frequencies, mode shapes and damping ratios), experimentally. Peak Picking technique in the frequency domain is used to extract experimental dynamic characteristics. When the analytically and experimentally identified dynamic characteristics are compared to each other, some differences are found between both results. To minimize these differences, initial finite element model of the storage tank is updated according to experimental results using some uncertainties modeling parameters such as elasticity modulus. To investigate the effect of finite element model updating on the earthquake behavior of the storage tank, earthquake analyses are performed before and after model updating. In the earthquake analyses, YPT330 component of 1999 Kocaeli earthquake is selected and applied to the models in the horizontal directions. It is seen from the analyses that the displacements and the stresses after model updating are more effective than the displacements and the stresses before model updating.     

Development and application of an interface constitutive model for fully grouted rock-bolts and cable-bolts

This paper proposes a new interface constitutive model for fully grouted rock-bolts and cable-bolts based on pull-out test results.A database was created combining published experimental data with in-house tests.By means of a comprehensive framework,a Coulomb-type failure criterion accounting for friction mobilization was defined.During the elastic phase,in which the interface joint is not yet created,the proposed model provides zero radial displacement,and once the interface joint is created,interface dilatancy is modeled using a non-associated plastic potential inspired from the behavior of rock joints.The results predicted by the proposed model are in good agreement with experimental results.The model has been implemented in a finite element method(FEM) code and numerical simulations have been performed at the elementary and the structural scales.The results obtained provide confidence in the ability of the new model to assist in the design and optimization of bolting patterns.     

Behavior and innovative design model on soil pressure at the top of large-diameter buried steel pipes

Existing design models of soil pressure at the top of buried steel pipes are limited to small-diameter pipes. This study aims to investigate the behavior and design model on the soil pressure for large-diameter pipes considering the pipe diameter, diameter-to-thickness ratio, and cover depth. The distribution and size of soil pressure are obtained from finite element models and compared with traditional design models. Sensitivity studies of soil parameters, trench parameters, and friction coefficients are discussed. Furthermore, an innovative design model of soil pressure for large-diameter pipes is proposed. The results indicate that the basic distribution pattern of soil pressure is the “inverted basin”/parabola for large/small-diameter pipes, respectively. Peak soil pressure typically appears in the pleura of pipes with great flexibility and deep cover depth. The modulus and Poisson's ratio of backfill and soil-soil friction coefficient have an influence range of within 10% for soil pressure, while trench parameters have a more significant influence. Prism load is too large at high cover depth, and Marston load is small at low cover depth. The proposed basin model for soil pressure adopts the form of “straight line + parabola” for distribution and “Marston + pleura” load for size, with high accuracy and strong adaptability.     

Nonlinear analysis of imperfect laminated thin plates under transverse and in-plane loads and resting on an elastic foundation by a semi-analytical approach

The large deflection and postbuckling behavior of imperfect composite laminated plates exposed to a combined action of transverse loads and in-plane edge compressions and resting on an elastic foundation are investigated in this paper by a semi-analytical approach. The formulations are based on the classical laminated plate theory (CLPT), and include the plate–foundation interaction effects via a two-parameter model (Pasternak-type) from which Winkler elastic foundation can be recovered as a limiting case. The present approach employs a perturbation technique, one-dimensional differential quadrature approximation and the Galerkin procedure to model the nonlinear performance of the plate with arbitrary combination of simply supported, clamped or elastic rotational edge constraints. Studies concerning its accuracy and convergence characteristics are carried out through some numerical illustrations. Effects of foundation stiffness, plate aspect ratio, total number of plies, fiber orientation, initial geometrical imperfection, the character of boundary conditions, and load patterns on the nonlinear behavior of the plate are studied. Typical numerical results are given in dimensionless graphical forms.     

Effect of stiffening rings on buckling stability of R.C. hyperbolic cooling towers

Demand for efficient and economical reinforced concrete (R.C.) hyperbolic cooling towers has driven engineers toward designing tall and thin-shell towers with a considerable high slenderness aspect ratio. This construction type is susceptible to buckling instability. To increase the R.C. hyperbolic cooling towers' structural stability while keeping the costs down, engineers could add stiffening rings. To achieve maximum buckling stability, important design parameters such as the number, dimension, and location of stiffening rings must be considered. Currently, however, there is a lack of information on these parameters. Using a series of numerical analyses, this paper defines the above parameters for maximizing the buckling stability of cooling towers and proposes a method for obtaining these parameters for any given cooling tower. Using these parameters, this paper also investigates the effect of stiffening rings on the buckling stability of R.C. cooling towers.     

Fully coupled analysis of consolidation by prefabricated vertical drains with applications of constant strain rate tests: Case studies and an open-source program

The paper proposes a new approach to use measured data of the constant strain rate test (CRST) for analysis of consolidation by prefabricated vertical drains (PVDs). Each PVD has an influence zone that idealised as a unit cell. Consolidation behaviour of a unit cell is studied with an axisymmetric finite element (FE) model based on Biot's theory. From a CRST data, ASTM-D4186 or the back-analysis method is used to obtain stress-dependent parameters for the model. An open-source FE software named CONAXIS was developed for these purposes. Data from two projects in Mekong Delta Vietnam were used in this study. In the first project, nine CRSTs with various depths from a borehole were conducted. Two tests were chosen to be simulated using the proposed approach implemented in CONAXIS and the soft soil model in PLAXIS for validation and comparison purposes. Comparing to the laboratory data, CONAXIS gave more accurate results than PLAXIS. Then CONAXIS was used to calculate the settlement of the ground surface during the construction process with different scenarios. For the second project, six CRSTs from three boreholes were used to set up the model in CONAXIS. Modelled results of both projects showed good agreements with field monitoring data.     

Research on probing and predicting the diameter of an underground pipeline by GPR during an operation period

It is very important that the existing networks of underground pipelines be clearly surveyed when the underground space of an old urban area is rebuilt and expanded. The GPR method is always used to locate the embedded pipes; however, it is hard to determine their diameters, especially, when the underground pipe is full of a lossy medium (i.e., water, oil, or gas) during the operation period. First, this paper proposes a new method for probing and predicting the diameter of underground pipelines filled with lossy media based on GPR using the shape of a certain circle determined by the coordinates of three points on this circle. The operational procedure of this method is listed in detail. Secondly, this method is used to detect the diameters of underground pipelines in a model experiment and the project for the detection of a sewage pipe network in Yi’xing chemical industrial park. The measurement value is approximately consistent with the real value. Lastly, some factors influencing the accuracy of this method were comprehensively analysed by applying the finite difference time domain method (FDTD). These factors are the buried depth of the pipe, the detecting frequency of the GPR, the material of the pipe and the spacing of the measured points. The results showed that the proposed method has sufficient applicability and accuracy for practical engineering. These works demonstrate that the proposed method achieves good result.