Showing papers in "Computers & Structures in 2004"

Abstract: Most structural optimization methods are based on mathematical algorithms that require substantial gradient information. The selection of the starting values is also important to ensure that the algorithm converges to the global optimum. This paper describes a new structural optimization method based on the harmony search (HS) meta-heuristic algorithm, which was conceptualized using the musical process of searching for a perfect state of harmony. The HS algorithm does not require initial values and uses a random search instead of a gradient search, so derivative information is unnecessary. Various truss examples with fixed geometries are presented to demonstrate the effectiveness and robustness of the new method. The results indicate that the new technique is a powerful search and optimization method for solving structural engineering problems compared to conventional mathematical methods or genetic algorithm-based approaches.

Abstract: Deterministic optimum designs that are obtained without consideration of uncertainties could lead to unreliable designs, therefore calling for reliability-based design optimization (RBDO). However, it has been reported in literatures that when RBDO involves evaluation of probabilistic constraints it is prohibitively expensive or even diverges for many large-scale applications. Therefore, the hybrid mean value (HMV) method had been proposed by authors for highly efficient and stable RBDO by evaluating the probabilistic constraint effectively. However, even with the HMV method, the RBDO process could be still expensive for large-scale applications or applications where efficient design sensitivity analysis method is unavailable. To alleviate this difficulty, a new RBDO methodology developed integrating the HMV method with a proposed response surface method, which is specifically developed for reliability analysis and optimization. A large-scale example problem is employed to demonstrate the proposed RBDO method.

Abstract: A computational methodology for dynamic analysis of multibody mechanical systems with joint clearance is presented in this work. Clearances always exist in real joints in order to ensure the correct relative motion between the connected bodies being the gap associated to them a result of machining tolerance, wear, and local deformations. Clearance at different joints is the source for impact forces, resulting in wear and tear of the joints, and consequently the degradation of the system performance. The model for planar revolute joints is based on a thorough geometric description of contact conditions and on a continuous contact force model, which represents the impact forces. It is shown that the model proposed here lead to realistic contact forces. These forces correlate well with the joint reaction forces of an ideal revolute joint, which correspond to a null joint clearance. The application to the analysis of a simple planar multibody system illustrates the use of the different models proposed.

Abstract: This paper discusses the use of the Murakami zig-zag function (MZZF) in the two-dimensional modeling of multilayered plates and shells. A literature overview of the available works is first presented. A `simple use' of the MZZF is discussed: the MZZF is used to introduce the zig-zag effect in classical and higher order theories which are formulated with only displacement unknowns. An `advanced use' of the MZZF is then considered to introduce the zig-zag effect in those theories which are formulated on the basis of both displacement and transverse stress assumptions. A number of new plate/shell theories has been considered. Numerical results encompassing, static, dynamic and thermally loaded orthotropic, simply supported plates and shells are presented to show both the effectiveness and limitations of the MZZF in the modeling of layered structures. Linear up to forth-order expansions for the in-plane and the out-of-plane displacements, in the thickness plate/shell direction, have been compared. It has been concluded that the MZZF is a valuable tool to enhance the performances of both classical and advanced theories. The conducted numerical evaluations have shown in particular that multilayered plate and shell theories can be greatly improved by the use of MZZF.

Abstract: The use of a 3D discrete element method (DEM) is proposed to study concrete submitted to dynamic loading. The model has already been validated through quasi-static simulations. This paper aims first at extending the validation of the model and at contributing to the understanding of the physical mechanisms in stake. Once the correct quasi-static identification of the model parameters is done, compressive dynamic tests are first simulated. Unchanged, the model proves able to reproduce the concrete strain rate dependency, and confirms the inertia-based hypothesis at high strain rates. Dynamic tensile tests show that a local rate effect has to be introduced to reproduce the experimental rate dependency, which would then be a material-intrinsic effect.

Abstract: We present a simple methodology to design isotropic triangular shell finite elements based on the Mixed Interpolation of Tensorial Components (MITC) approach. Several mixed-interpolated isotropic triangular shell finite elements are proposed. We perform well-established numerical tests and show the performance of the new elements.

Abstract: Over the last decade, probability theory has been studied and embedded in engineering structural design through uncertainty quantification (UQ) analysis, instead of simply assigning safety factors. However, recently it has been found by the scientific and engineering community that there are limitations in using only one framework (probability theory) to quantify the uncertainty in a system because of the impreciseness of data or knowledge. In this paper, evidence theory is proposed as an alternative to the classical probability theory to handle the imprecise data situation. The possibility of adopting evidence theory as a general tool of UQ analysis for large-scale built up structures is investigated with an algorithm that can alleviate the computational difficulties.

Abstract: A beam element formulation and solution procedure for progressive collapse analysis of planar frame structures is presented. Unlike previous research, the current study addresses the significance of dynamic load redistribution following the failure of one or more elements. The developed beam-column element utilizes a multi-linear, lumped plasticity model, and it also accounts for the interaction of axial force and bending moment. Strength and stiffness degradation are included through use of a damage-dependent constitutive relationship. A damage index is used to determine the onset of member failure. Following the failure of an element, the analysis continues in an efficient manner through use of a modified member stiffness procedure. This approach does not require the introduction of any additional degrees-of-freedom or modification of the element connectivity definitions. Finally, a methodology for updating the state of a structure at the time of member failure is presented. Analysis results indicate that dynamic redistribution of loads is a significant feature of the progressive collapse problem and should be accounted for in order to avoid estimates of capacity that are not conservative.

Abstract: In estimating the effect of a change in a random variable parameter on the (time-invariant) probability of structural failure estimated through Monte Carlo methods the usual approach is to carry out a duplicate simulation run for each parameter being varied. The associated computational cost may become prohibitive when many random variables are involved. Herein a procedure is proposed in which the numerical results from a Monte Carlo reliability estimation procedure are converted to a form that will allow the basic ideas of the first order reliability method to be employed. Using these allows sensitivity estimates of low computational cost to be made. Illustrative examples with sensitivities computed both by conventional Monte Carlo and the proposed procedure show good agreement over a range of probability distributions for the input random variables and for various complexities of the limit state function.

Abstract: Large-amplitude (geometrically nonlinear) vibrations of rectangular plates subjected to radial harmonic excitation in the spectral neighborhood of the lowest resonances are investigated. The von Karman nonlinear strain–displacement relationships are used. The formulation is also valid for orthotropic and symmetric cross-ply laminated composite plate; geometric imperfections are taken into account. The nonlinear equations of motion are studied by using a code based on arclength continuation method that allows bifurcation analysis. Comparison of calculations to numerical results available in the literature is performed for simply supported plates with immovable and movable edges. Three different boundary conditions are considered and results are compared: (i) simply supported plates with immovable edges; (ii) simply supported plates with movable edges; and (iii) fully clamped plates. An experiment has been specifically performed in laboratory in order to very the accuracy of the present numerical model; a good agreement of theoretical and experimental results has been found for large-amplitude vibrations around the fundamental resonance of the aluminum plate tested.

Abstract: Based on a fully 3-D elasto-plastic damage theory, the material behavior of all reinforced concrete components––concrete, reinforcement and bond––is, for biaxial loading, realistically modelled, including cyclic action. Thereby emphasis is layed on concrete in tension and compression. The presented model contains a minimum number of material parameters. It further enables to map exact uniaxial stress–strain curves as proposed by modern codes of practise, like the EC 2. All material parameters of the model can be readily interpreted and determined by few standard experiments, or approximated from concrete compression strength. Finally, the concrete model is verified by numerical simulation of experiments.

Abstract: In this paper a partial mixed layerwise finite element model for adaptive plate structures is presented. Static analysis of magneto-electro-elastic laminated plate structures is considered. The mixed finite element formulation is obtained by considering a Reissner mixed variational principle. The mixed functional is formulated using transverse stresses, displacement components and electric and magnetic potentials as primary variables. The other fields are calculated by post-computation through constitutive equations. The numerical results obtained by the present model are in good agreement with available three-dimensional analytical solutions.

Abstract: An algorithm for finding constrained Pareto-optimal solutions based on the features of a biological immune system is proposed. Inter-relationships within the constrained multi-objective immune algorithm (CMOIA) resemble antibody–antigen relationships in terms of specificity, germinal center, and the memory characteristics of adaptive immune responses. Gene fragment recombination and several antibody diversification schemes were incorporated to improve the balance between exploitation and exploration. Moreover the concept of cytokines is applied for handling constraints. The effectiveness of CMOIA is evaluated through six test functions and two well-known truss sizing optimization problems. The results indicate that the CMOIA provides better performance than other methods.

Abstract: Surface ship shock trials have been conducted in many countries for shock qualification of ship integrity, systems and subsystems. The ship shock trial identifies design and construction deficiencies that have a negative impact on ship and crew survivability. It also validates shock hardening criteria and performance. However, ship shock trials are costly. As a possible alternative, numerical modeling and simulation may provide viable information to look into the details of dynamic characteristics of ship including component and sub-component level. Ship shock analyses were conducted using finite element based coupled ship and fluid model. Three-dimensional ship shock modeling and simulation has been performed and the predicted results were compared with ship shock test data. Surface ship shock analysis approach is presented and the important parameters are discussed.

Abstract: Toward the application of a two-scale analysis method for nonlinear heterogeneous solids with periodic microstructures, we make a feasibility study and introduce a parallel algorithm to achieve the computational efficiency. For the feasibility, we focus our attention to the inhomogeneous deformation of the overall structure, which may imply the loss of periodicity assumed in the initial state. For parallel computations, we present an efficient algorithm for the deconcentration of computational loads by using a PC-cluster system. A simple numerical example for a three-dimensional heterogeneous body will suggest the applicability of the two-scale analysis method to practical problems.

Abstract: A constitutive model developed for the simulation of the cyclic behaviour of interface elements is proposed. Its theoretical framework is fully based on the plasticity theory. Starting from an existing monotonic model, two new yield surfaces are introduced in order to include non-linear unloading/reloading behaviour in an accurate fashion. The motion of the unloading surfaces is controlled by a mixed hardening law and, by adopting appropriate evolution rules, it is possible to reproduce non-linear behaviour during unloading. Numerical results concerning the analysis of both uniaxial tests and masonry walls are presented and discussed as application and validation examples of the constitutive model.

Abstract: Near-surface mounted (NSM) laminate strips of carbon fiber reinforced polymer (CFRP) is a promising technique for increasing the flexural and the shear strength of deficient concrete members. Using the results of an experimental program with pullout-bending tests and developing a numerical strategy, an analytical bond stress–slip relationship was obtained. This relationship was converted into a bond stress–slip constitutive law for a line interface finite element, used to simulate the concrete–CFRP bond behavior. The numerical model developed predicted all the significant aspects registered experimentally, and can be used to assess relevant information in the design of concrete structures strengthened by NSM technique.

Abstract: A finite element formulation for active vibration control of thin plate laminated structures with integrated piezoelectric layers, acting as sensors and actuators, is presented in this paper. The finite element model is a nonconforming single-layer triangular plate/shell element with 18 degrees of freedom for the generalized displacements and one electrical potential degree of freedom for each piezoelectric element layer. The model is based on the Kirchhoff classical laminated theory, and can be applied to plate and shell adaptive structures. To achieve a mechanism of active control of the structure dynamic response, a feedback control algorithm is used, coupling the sensor and active piezoelectric layers, and the Newmark method is considered to calculate the dynamic response of the laminated structures. The model is applied in the solution of four illustrative cases, and the results are presented and discussed.

Abstract: This paper proposes a method of locating and quantifying the damage in structural members using the concept of residual forces. To describe the damage in a structure, finite element (FE) models are parameterised by structural stiffness reduction parameters. The damage parameters are determined by minimising a global error derived from dynamic residual vectors, which are obtained by introducing a simulated “experimental” data into the eigenproblem. An eigenvalue prediction algorithm along with normalised residual function is employed to formulate the objective function. Two-point crossover binary coded genetic algorithm (GA) with tournament selection approach is adopted in minimising the objective and optimum set of stiffness reduction parameters are predicted. Current structural defect-identification scheme is verified and assessed using an analytically derived plane truss, a cantilever Euler–Bernoulli beam and a portal truss. Results are presented in the form of tables and graphs.

Abstract: The aim of this study concerns with the analysis of cable-stayed bridges at different erection stages during construction using the cantilever method. A finite element computation procedure is set up for the shape finding analysis of such structures during erection procedures. Two computational processes are established, one is a forward process analysis and the other is a backward process analysis. The former is performed by following the sequence of erection stages in bridge construction and the latter is carried out in the reverse direction of erection procedures. Both processes can be successfully applied for finding the initial shape of bridge structures during erection procedures. The structural behavior of the bridge structure at different erection stages has been examined in details, such as the pretension required in cable-stays and the corresponding structural configurations of the bridge, etc. The results of shape finding analysis at each erection stage not only provide the necessary data for the purpose of structural analysis and design, but also can be used for checking and controlling the erection procedure of the cable-stayed bridge during construction. The designed shape (pretension in cables and configuration) of the bridge can then be achieved and constructed.

Abstract: Experimental and theoretical studies have been performed to determine the dynamic behavior of bridges crossed by the Korean high-speed train (KHST). Through this study, significantly amplified dynamic responses compared to static responses of the bridge crossed by trains running at speed close to the critical speed have been obtained. The numerical method technique presented in this study shows reasonable results concerning the dynamic behavior analysis of bridges under trains running at high-speed. In the case of the KHST, train–bridge interaction effects related to the dynamic response of the bridge appeared to be significant at every speed.

Abstract: Two 4-node quadrilateral membrane elements, denoted as AGQ6-I and AGQ6-II, have been developed in this paper. Instead of the traditional isoparametric coordinate, the quadrilateral area coordinates were used to establish the formulations of the new elements. And several generalized conforming conditions were then introduced to determine all unknown parameters. Numerical examples showed that the presented elements exhibit excellent performances in both regular and distorted mesh divisions. They could even yield exact solutions for pure bending problems under distorted meshes and provide lock-free solutions for the MacNeal’s test problem of trapezoidal locking. Besides, the weak patch test was conducted to guarantee the convergence of both new elements. It has also been demonstrated that the area coordinate method is an efficient tool for developing simple, effective and reliable serendipity plane membrane elements.

Abstract: Cracking is responsible for the vast majority of masonry non-linear behaviour, due to the low tensile strength of the material. Masonry features orthotropic behaviour with material axes normal and parallel to the bed joints, being the response straightforward for tension normal to the bed joints and rather complex for tension parallel to the bed joints. This paper addresses the formulation and implementation of coupling between a micro-mechanical homogenisation model and an isotropic damage model for the masonry components. The non-linear homogenisation formulation requires an improved internal deformation mode of the masonry basic cell, with respect to previous works. Finally, the model is validated with a comparison with numerical results available in the literature, using interface modelling.

Abstract: This paper presents a methodological approach of wide generality for assessing the reliability of reinforced and prestressed concrete structures. As known, the numerical values of the parameters which define the geometrical and mechanical properties of this kind of structures, are affected by several sources of uncertainties. In a realistic approach such properties cannot be considered as deterministic quantities. In the present study all these uncertainties are modeled using a fuzzy criterion in which the model is not defined through a set of fixed values, but through bands of values, bounded between suitable minimum and maximum extremes. The reliability problem is formulated at the load level, with reference to several serviceability and ultimate limit states. For the critical interval associated to each limit state, the membership function of the safety factor is derived by solving a corresponding anti-optimization problem. The strategic planning of this solution process is governed by a genetic algorithm, which generates the sampling values of the parameters involved in the material and geometrical non-linear structural analyses. The effectiveness of the proposed approach and its capability to handle complex structural systems are shown by carrying out a reliability assessment of a prestressed concrete continuous beam and of a cable-stayed bridge.

Abstract: An optimization study is presented in this paper with aim to minimize the vibrational energy (VE) of vibrating beams with passive constrained layer damping (PCLD) treatment. First, the governing equation of motion of a partially PCLD covered beam is derived on the basis of energy approach, and assumed-modes method is used to solve it. Parametric studies are then performed to identify those dominant parameters on the vibration response of the damped beam. With objective to minimize the integrated global VE of the base beam over a frequency range of interest, a genetic algorithm (GA) based penalty function method is further employed to search for the optimum of the location/length of the PCLD patch and VL’s shear modulus for a simply-supported beam with a transverse force applied at its central location. Optimal solutions are given and discussed for different cases where the VE of the beam over a frequency range covering the first four resonant modes and that at a specific resonant mode is to be minimized, without and with inclusion of the restriction of minimum damping material used.

Abstract: The shooting, Newton and p-version, hierarchical finite element methods are applied to study geometrically non-linear periodic vibrations of elastic and isotropic, beams and plates. Thin and thick or first-order shear deformation theories are followed. One of the main goals of the work presented is to demonstrate that the methods suggested are highly adequate to analyse the periodic, forced non-linear dynamics of beam and plate structures. An additional purpose is to investigate the differences in the predictions of non-linear motions when thin and thick, either beam or plate theories are followed. To this ends response curves are derived, defining both stable and unstable solutions and the characteristics of the motions are investigated using time plots, phase planes and Fourier spectra.

Abstract: A topology optimization technique for systematically designing contact-aided compliant mechanisms (CCM) is presented in this paper. A CCM is a single piece elastic body that uses intermittent contacts in addition to elastic deformation to transmit force and motion. Contact interactions give rise to interesting nonlinear and nonsmooth behaviors even under the small deflection assumption made in this work. The difficulties associated with the nondifferentiability inherent in the CCM systematic synthesis problem are circumvented by using a regularized contact model. This model uses a smooth approximation of the unilateral displacement constraints that are used to model contact interactions. The use of a regularized contact model in the underlying state problem makes it possible to use efficient smooth optimization algorithms for the systematic synthesis of CCMs. The formulation of the design problem for CCMs, sensitivity analysis, and solution methodology are presented. The paper includes CCM designs that exhibit nonsmooth motion and force transmission characteristics, which are not possible or practical with compliant mechanisms that do not use intermittent contact interactions.

Abstract: This paper studies the feasibility, potential, and limitations of using high fidelity electromechanical simulation for the reliability-based analysis and design optimization of electrostatically actuated Micro-ElectroMechanical Systems (MEMS). A reliability-based analysis and design optimization framework is presented that accounts for stochastic variations in structural parameters and operating conditions. A First-Order Reliability Method (FORM) is embedded into a design optimization procedure by a modular nested loop approach. The steady-state electromechanical problem is described by a three-field formulation and solved by a staggered procedure, coupling a structural finite element model and a finite element discretization of the electrostatic field. The motion of the electrostatic mesh is described by a fictitious elastic structure. The coupled electromechanical design sensitivities and imperfection sensitivities are efficiently evaluated by direct and adjoint approaches. The computational framework is verified by the analysis and optimization of a three-dimensional MEMS device. The appropriateness of the FORM approximation on the non-linear problem is investigated by a comparison with Monte Carlo simulation results. While computationally significantly more expensive than deterministic electromechanical optimization, the example illustrates the importance of accounting for uncertainties and the need for reliability-based optimization methods in the design of MEMS.

Abstract: Wind loads on tall buildings can be quite different from those on an isolated building due to neighboring building effects. With the increase of number of tall buildings in large cities, there is a growing attention to the interference effects among adjacent buildings under wind action. While wind tunnel tests are of importance in the understanding of the physical process, the general quantitative predictions of interference effects are difficult to reach owing to many variables involved. In the present paper, a radial basis function (RBF) neural network is proposed for its strong ability in nonlinear mapping and its higher training speed. Thus the RBF neural network is applied to evaluate the interference effects (expressed by interference factor, IF) by using experimental data obtained from many sources as training patterns. The results indicate that a very good agreement is found between the predicted IF values and the experimental counterparts.

Abstract: One of the important issues in uncertainty analysis is to find an effective way for propagating uncertainty through the system. In this paper, the polynomial chaos expansion (PCE) was selected, since this approach can reduce the computational effort in large-scale engineering design applications. An implementation of PCE for different probability distributions is the focus of this paper. Two existing techniques, a generalized PCE algorithm and transformation methods, are investigated and verified for their accuracy and efficiency for non-normal random variable cases. A highly non-linear structural model of an uninhabitated joined-wing aircraft and a three pin-connected rod structure are used for demonstrating the method.