Jiří Kala

Sensitivity Analysis of Stability Problems of Steel Structures using Shell Finite Elements and Nonlinear Computation Methods

The main focus of the paper is the analysis of the influence of residual stress on the ultimate limit state of a hot‐rolled member in compression. The member was modelled using thin‐walled elements of type SHELL 181 and meshed in the programme ANSYS. Geometrical and material non‐linear analysis was used. The influence of residual stress was studied using variance‐based sensitivity analysis. In order to obtain more general results, the non‐dimensional slenderness was selected as a study parameter. Comparison of the influence of the residual stress with the influence of other dominant imperfections is illustrated in the conclusion of the paper. All input random variables were considered according to results of experimental research.

Improved Element Erosion Function for Concrete-Like Materials with the SPH Method

The subject of the paper is a description of a simple test from the field of terminal ballistics and the handling of issues arising during its simulation using the numerical techniques of the finite element method. With regard to the possible excessive reshaping of the finite element mesh there is a danger that problems will arise such as the locking of elements or the appearance of negative volumes. It is often necessary to introduce numerical extensions so that the simulations can be carried out at all. When examining local damage to structures, such as the penetration of the outer shell or its perforation, it is almost essential to introduce the numerical erosion of elements into the simulations. However, when using numerical erosion, the dissipation of matter and energy from the computational model occurs in the mathematical background to the calculation. It is a phenomenon which can reveal itself in the final result when a discrepancy appears between the simulations and the experiments. This issue can be solved by transforming the eroded elements into smoothed particle hydrodynamics particles. These newly created particles can then assume the characteristics of the original elements and preserve the matter and energy of the numerical model.

Large‐deflection‐theory Analysis of the Effect of Web Initial Curvature on the Ultimate Strength of Steel Plate Girder

The objective of the paper is to analyze the influence of initial imperfections on the behaviour of thin‐walled girders welded of slender plate elements. In parallel with experiments, one of the ultimate load tests was computer modelled. In so doing, the girder was modelled, using the geometrically and materially non‐linear variant of the shell finite element method, by the ANSYS program. The shape changing during loading process is often accompanying with sudden “snap‐through” i. e. rapid curvature change.

Lateral-torsional buckling analysis of I-beams using shell finite elements and nonlinear computation methods

The paper deals with the influence of correlation length, of Gauss random field, and of yield strength of a hotrolled I-beam under bending on the ultimate load carrying capacity limit state. Load carrying capacity is an output random quantity depending on input random imperfections. Latin Hypercube Sampling Method is used for sampling simulation. Load carrying capacity is computed by the programme ANSYS using shell finite elements and nonlinear computation methods. The nonlinear FEM computation model takes into consideration the effect of lateral-torsional buckling on the ultimate limit state.

The interaction of local buckling and stability loss of a thin-walled column under compression

The influence of input random quantities (imperfections) on the output random quantity (load-carrying capacity) variability is studied by the sensitivity analysis. The influence of the initial curvature shape of a column is discussed. The initial curvature is often supposed to have the sine function shape. The Euler method based on proportional loading in combination with the Newton-Raphson method was used. The girder symmetry and that of loading were made use of. The realizations of input imperfections is simulated using the Latin Hypercube method (improved Monte Carlo method) that makes it possible to simulate the realization of the data from the known statistics of the input values in a similar way as they could be obtained by direct measurements. The method LHS gives more accurate estimates of mean value and standard deviation for a smaller number of runs.

Using noise to generate the material structure of concrete

The contribution deals with the description and demonstration of an algorithm for the generation of spatial geometry – the material structure of concrete. The term “material structure of concrete” is taken to mean a mix of randomly distributed aggregate and cement binder, the latter being used to bond the aggregate. In order for the generated material structure to be as realistic as possible, a photograph of the material which is to be created is used as the input (i.e. data source) for the algorithm. The generation process itself involves the use of spatial noise functions, which are able to generate simple patterns as well as very complex fractals. The algorithm is based on the principle that similarities can be found between a suitably placed section cut through a noise space and the abovementioned input for the algorithm – a real photograph of the material. The functionality and individual steps of the algorithm are presented via two examples of concrete generation with differently shaped aggregate grains.The contribution deals with the description and demonstration of an algorithm for the generation of spatial geometry – the material structure of concrete. The term “material structure of concrete” is taken to mean a mix of randomly distributed aggregate and cement binder, the latter being used to bond the aggregate. In order for the generated material structure to be as realistic as possible, a photograph of the material which is to be created is used as the input (i.e. data source) for the algorithm. The generation process itself involves the use of spatial noise functions, which are able to generate simple patterns as well as very complex fractals. The algorithm is based on the principle that similarities can be found between a suitably placed section cut through a noise space and the abovementioned input for the algorithm – a real photograph of the material. The functionality and individual steps of the algorithm are presented via two examples of concrete generation with differently shaped aggregate gr...

Steel Fibre Reinforced Concrete Simulation with the SPH Method

Steel fibre reinforced concrete (SFRC) is very popular in many branches of civil engineering. Thanks to its increased ductility, it is able to resist various types of loading. When designing a structure, the mechanical behaviour of SFRC can be described by currently available material models (with equivalent material for example) and therefore no problems arise with numerical simulations. But in many scenarios, e.g. high speed loading, it would be a mistake to use such an equivalent material. Physical modelling of the steel fibres used in concrete is usually problematic, though. It is necessary to consider the fact that mesh-based methods are very unsuitable for high-speed simulations with regard to the issues that occur due to the effect of excessive mesh deformation. So-called meshfree methods are much more suitable for this purpose. The Smoothed Particle Hydrodynamics (SPH) method is currently the best choice, thanks to its advantages. However, a numerical defect known as tensile instability may appear when the SPH method is used. It causes the development of numerical (false) cracks, making simulations of ductile types of failure significantly more difficult to perform. The contribution therefore deals with the description of a procedure for avoiding this defect and successfully simulating the behaviour of SFRC with the SPH method. The essence of the problem lies in the choice of coordinates and the description of the integration domain derived from them – spatial (Eulerian kernel) or material coordinates (Lagrangian kernel). The contribution describes the behaviour of both formulations. Conclusions are drawn from the fundamental tasks, and the contribution additionally demonstrates the functionality of SFRC simulations. The random generation of steel fibres and their inclusion in simulations are also discussed. The functionality of the method is supported by the results of pressure test simulations which compare various levels of fibre reinforcement of SFRC specimens.

Optimization-Based Inverse Identification of the Parameters of a Concrete Cap Material Model

Issues concerning the advanced numerical analysis of concrete building structures in sophisticated computing systems currently require the involvement of nonlinear mechanics tools. The efforts to design safer, more durable and mainly more economically efficient concrete structures are supported via the use of advanced nonlinear concrete material models and the geometrically nonlinear approach. The application of nonlinear mechanics tools undoubtedly presents another step towards the approximation of the real behaviour of concrete building structures within the framework of computer numerical simulations. However, the success rate of this application depends on having a perfect understanding of the behaviour of the concrete material models used and having a perfect understanding of the used material model parameters meaning. The effective application of nonlinear concrete material models within computer simulations often becomes very problematic because these material models very often contain parameters (material constants) whose values are difficult to obtain. However, getting of the correct values of material parameters is very important to ensure proper function of a concrete material model used. Today, one possibility, which permits successful solution of the mentioned problem, is the use of optimization algorithms for the purpose of the optimization-based inverse material parameter identification. Parameter identification goes hand in hand with experimental investigation while it trying to find parameter values of the used material model so that the resulting data obtained from the computer simulation will best approximate the experimental data. This paper is focused on the optimization-based inverse identification of the parameters of a concrete cap material model which is known under the name the Continuous Surface Cap Model. Within this paper, material parameters of the model are identified on the basis of interaction between nonlinear computer simulations, gradient based and nature inspired optimization algorithms and experimental data, the latter of which take the form of a load-extension curve obtained from the evaluation of uniaxial tensile test results. The aim of this research was to obtain material model parameters corresponding to the quasi-static tensile loading which may be further used for the research involving dynamic and high-speed tensile loading. Based on the obtained results it can be concluded that the set goal has been reached.

Concept and numerical simulations of a reactive anti-fragment armour layer

The contribution describes the concept and numerical simulation of a ballistic protective layer which is able to actively resist projectiles or smaller colliding fragments flying at high speed. The principle of the layer was designed on the basis of the action/reaction system of reactive armour which is used for the protection of armoured vehicles. As the designed ballistic layer consists of steel plates simultaneously combined with explosive material – primary explosive and secondary explosive – the technique of coupling the Finite Element Method with Smoothed Particle Hydrodynamics was used for the simulations. Certain standard situations which the ballistic layer should resist were simulated. The contribution describes the principles for the successful execution of numerical simulations, their results, and an evaluation of the functionality of the ballistic layer.

Experimental approach of the single pedestrian-induced excitation

Experimental approach of the single pedestrian-induced excitation Pedestrian-induced vibrations are a criterion for serviceability. This loading is significant for light-weight footbridge structures, but was established as a basic loading for the ceilings of various ordinary buildings. Wide variations of this action exist. To verify the different conclusions of various authors, vertical pressure measurements invoked during walking were performed. In the article the approaches of different design codes are also shown.