Instytut Podstawowych Problemów Techniki

Streszczenie:

The paper deals with non-linear analysis of a telecommunication tower with circular flange-bolted connections (CFBCs). They are composed of two flanges, welded to the structural tubes, and then connected together with pre-tensioned bolts. A rigorous FEM analysis is performed for finding the connection stiffness in two cases. One deals with all bolts undamaged and the second one with one or more bolts broken.

The analysis, which includes contact and friction forces, shows that when joints are under tension, the bolts are not only subjected to axial forces, but also to bending moments due the prying effect. The value of stresses caused by bending depends strongly on the bolt pre-tension and flange thickness. Removing one of the six connection bolts significantly increases stresses in the remaining bolts. Knowing the behaviour of the connection, it is possible to study the behaviour of the whole structure. This is achieved by applying the multilevel substructuring approach. The first levels is related to the flanges and bolts, whereby the connection model is simplified, and compared with the rigorous one, the second level is related to the assembly of the whole tower.

The paper is illustrated with several examples of connections of different thicknesses, and different bolt pre-tensions. The considered tower comes from a real design.

Słowa kluczowe:

Bolted flange connection, Contact forces, Friction forces, Non-linear FEM analysis, Substructuring, Telecommunication tower, Structural connection damage, Structural health monitoring

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An experimental and analytical study of the relation between local defect, in a steel structure, and its higher frequencies and higher modes is discussed. The structure is a plane steel frame, assembled of beams, joined together with bolted connections. Removing some bolts from a given connection simulates the damage. In the experiment, an impulse force induced structural vibrations. Effects of vibrations were shown by data from gages, measuring accelerations with a high accuracy. From the data, it could be observed, that mode shapes, for the healthy and damaged structures didn’t show any differences for low frequencies. Only modes around thirteen showed significant gap between picks of Frequency Response Functions, for healthy and damaged frame. Moreover, looking at mode shapes, it could be observed that structural configuration may have some influence on defects to be observable. This aspect is discussed in a separate section. The experiment performed on the whole structure allows finding the place where the defect is localized. However, it can’t give detailed information on the defect itself, here defect of a bolt. For finding it, an ultrasonic measurement of pre-tensioning forces in bolts was applied. It allowed not only to determine stresses in the bolt, but also to verify, if in the process of assembling the structure was not pre-stressed.

Słowa kluczowe:

Detectable damages, Bolted connections, Experimental modal analysis, Ultrasonic measurements, Steel frames, Analytical dynamics

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In this study, a relatively simple method of discrete structural optimization with dynamic loads is presented. It is based on a tree graph, representing discrete values of the structural weight. In practical design, the number of such values may be very large. This is because they are equal to the combination numbers, arising from numbers of structural members and prefabricated elements. The starting point of the method is the weight obtained from continuous optimization, which is assumed to be the lower bound of all possible discrete weights. Applying the graph, it is possible to find a set of weights close to the continuous solution. The smallest of these values, fulfilling constraints, is assumed to be the discrete minimum weight solution. Constraints can be imposed on stresses, displacements and accelerations. The short outline of the method is presented in Sec. 2. The idea of discrete structural optimization by means of graphs. The knowledge needed to apply the method is limited to the FEM and graph representation.

Słowa kluczowe:

discrete structural optimization, combinatorial optimization, structural dynamics, stochastic loading, problem oriented optimization, graphs

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The paper is concerned with a minimum weight design of composite floors subjected to dynamic loading, deriving from the rhythmic activity of a group of people. The floor structure consists of concrete slab cast, on thick trapezoidal deck, which is supported by a grid of steel beams. The structure is vibration-prone and exhibits a number of natural frequencies, which are within a range of loading function. Minimum weight design consists in assigning, from catalogues of prefabricated plates and beams appropriate elements assuring fulfillment of imposed constraints on displacements and accelerations. Applied, practical discrete optimization method is based on graph theory and finite element analysis. Efficiency of the proposed design is demonstrated in an example of real-world engineering structure.

Słowa kluczowe:

structural optimization, discrete optimization, structural vibrations, omposite floor structure

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Streszczenie:

Many optimum structural designs are based on searching for the best of all combinations, arising from the number of structural members, and parameters of listed rolled profiles. Even, in a relatively simple design, the number of such combinations is of an order higher than ten. All known methods of finding discrete minimum of structural weight require very large number of analyses often of an order of four. In this study, are relatively simple method of solving such problems is presented. It is based on a tree graph, representing discrete values of the structural volume. The structure can be subjected to multi static loadings with constraints imposed on displacements and stresses. The number of analyses, in the proposed algorithm, is limited to the order of two. The knowledge needed to apply the method is limited to FEM and graph representation. The paper is illustrated with two examples with numbers of combinations up to 42 to the power 38.

Słowa kluczowe:

Structural optimization, Problem oriented optimization, Discrete optimization, Combinatorial optimization, Graphs

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A very simple method for finding the minimum weight of a structure designed from a list of available parameters is presented. The structure can be subjected to multiple loading conditions with constraints imposed on displacements, stresses and eigenfrequency. The method consists of a recursive removal of redundant material, starting from the heaviest structure. The number of analyses required is a factor of 10 to the power 2 less than for most stochastic methods. The knowledge needed for application of the method is limited to the finite-element method.

Słowa kluczowe:

structural optimization, combinatorial optimization, structural dynamics

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A very simple discrete optimization algorithm for minimum structural weight is presented. The main idea consists in assigning initial cross-sections to a given structure, and then searching for its least stressed elements. In the second stage of the algorithm, the cross-section of the element with minimum stress is reduced to the next catalogue entry. Such reduction of cross-sections is continued until imposed bounds are met. The presented algorithm was positively tested on example configurations with complexity of 10 to the power 10 and 30 to the power 8 combinations.

Słowa kluczowe:

discrete optimization, truss structures

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A number of modern structures such as parabolic antennae, various aerial masts, hangers, domes, towers, etc., are composed of a very large number of similar bars. These two features, that is, large number and similarity of elements, imply a special approach to the theoretical and numerical investigation of the structures in question. The aim of the present paper is to present and illustrate by several examples the methods used for solving problems of regular elastic lattice systems, widely applied in modern structural engineering.

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The purpose of this paper is to present and illustrate a general approach to the analysis of lattice structures, irrespective of their arrangement. Such generalization has been made possible by the application of the lattice of points, the basic concept of the geometry of numbers. After establishing the geometrical part of the problem, a difference-variational problem for the structure as a whole is solved. As a result a set of six difference equations of motion for an arbitrary node of an arbitrary structure is obtained. These equations are most suitable for analytical as well as numerical solutions of the problem in question. The paper is illustrated by two examples of square and hexagonal patterns of rod arrangement.

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The paper deals with regular, polar, elastic, plane frames, loaded transversely (see Fig. 1). The equilibrium equations of structural joint and adjacent members, together with force-deformation relations, lead to a set of two difference equations of second order with variable coefficients. The equations are with respect to displacements and rotations of structural nodes. The solution of the problem, in the form of a finite series, is obtained by means of Laplace's method.

At the end of the paper two numerical examples are given. Although the final solutions seem complex, on closer examination they are relatively simple and reduce to the calculation of factorials.

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The paper deals with a nonlinear analysis of a tall tower, with Circular Flange Bolted Connections (CFBC), in which friction and contact effects are taken into account. Due to these nonlinearities, a detailed dynamic model of the whole structure would lead to very complex computational problem, unable for practical solutions. To overcome these difficulties a reduced order model of CFBC is proposed. Such a model enables the simulation of the whole tower including nonlinearities in connections. The paper is illustrated with an example of model reduction and dynamic calculations for a contemporary telecommunication tower. The tower is assembled of 4 truss segments, of triangular cross section, interconnected with CFBCs. Finally, the influence of the number of modes, included in the reduced order model, on the accuracy and computational effect, is discussed.

Słowa kluczowe:

Reduced order modeling, Circular flange-bolted connections, Dynamic substructuring, Nonlinear contact and friction effects, Telecommunication towers

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A successful structural monitoring and control systems should be able to discern critical events, scan frequencies and dynamic ranges. This, in turn, requires that models, applied in these systems, are as close as possible to real structural behavior. Many structures, among them, electricity transmission towers, windmills, radio and TV masts, bridges and parabolic dishes, concentrating solar energy, are build as trusses. All of them are subjected dynamic loadings coming, mostly from wind gusts, water waves and thermal activity of sun. The paper is revising assumptions, commonly made for trusses. The main assumption made is that systems of rods are pin joint. Such structures don’t exist, in real engineering design. All, above, mentioned structures are made with rigid or flexible joints. The “pin joint” assumption was made for static analyses. It stated that if joint displacements caused by rod bending can be neglected, comparing with displacements caused by rod elongations, the structural system can be considered as pin joint. As it is commonly known, such cases occur when some necessary conditions joining number of joints and hinges are fulfilled. The “pin joint”, static assumptions has been, without any formal justification, taken for granted in dynamics. Simple examples presented in the paper show that “pin joint” assumption can lead to considerable errors. The paper is illustrated with two numerical examples (simple 2 bar structure and 25 bar transmission tower). Presented examples allowing to compare results of different assumptions applied for structural elements connections.

Słowa kluczowe:

lattice structures, monitoring, control, structural dynamics

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Topics considered in this paper include: dynamic properties of the mast and nonlinearities present in the problem, linearized equations governing guyed mast motion, conditions of stability, contollability and observability. Control forces are exerted by an anchored mechanism in such a way, that they change tension in guy cables. A control algorithm ensuring robust optimal control is also proposed. Finally, the paper is verified by a numerical simulation of a vibration control process.

Słowa kluczowe:

guyed mast, vibration control

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The purpose of this study is the mechanism of the spatial movement of a human during walking. The human is considered to be a rigid body with six degrees of freedom, and its mobility is reduced by two constraints. One of the constraints is related to an assumption of constant length of the leg, foot of which is moving along given straight line. The second constraint corresponds to an assumption that human's centre of mass can move only in vertical plane. The dynamic model of the mechanism is built under above assumptions connected to the human gait kinematics. Then, kinetic energy function is derived. In the next step of the study the muscle force in the stance leg, together with its potential, is discussed. It is assumed that this force is piecewise-linear, which can be reasonably approximated by a cubic polynomial. For that purpose a smoothing procedure is proposed and finally with the aid of the Lagrange function dynamic equations for the 3D human gait are formulated. The last part of the paper is devoted to the numerical solution of obtained nonlinear equations, arising from the Lagrange procedure.

Słowa kluczowe:

human walking, gait analysis, lagrangian dynamics

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A substructuring method for the prediction of the dynamic response of footbridges subjected to loadings induced by a pedestrian is presented. The dynamic system is composed of two independent subsystems. The first one is the model of the suspension footbridge, and the second one is the model of a pedestrian. The former is obtained using the standard finite element method, while the latter is created by applying a two-step identification approach. The first step is an inverse kinematics based on an experimental human motion analysis. The second one relies on Proper Orthogonal Decomposition extracting a number of most important modes, describing the motion of the pedestrian. The presented methodology will be demonstrated by a numerical example of the pedestrian-footbridge interaction.

Słowa kluczowe:

dynamic substructuring, human-structure interaction, suspension footbridges

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A relatively simple method of finding discrete minimum structural weight is proposed. It is based on a tree graph, representing discrete values of the structural volume. In the proposed method, the number of analyses is limited to the order of two. The paper is illustrated with an example containing up to 42 to the power 38 combinations.

Słowa kluczowe:

structural optimization, problem oriented optimization, discrete optimization, combinatorial optimization, graphs, structural static and dynamic analyses, time domain, frequency domain

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The paper is illustrated with numerical examples of two bar and 25 bar tower structures. Results of different assumptions, applied to the structural elements connections are discussed.

Słowa kluczowe:

Transmission towers, monitoring of structures

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In this study, the idea of removing redundant material is enhanced by combining the continuous and discrete solutions. At the end of the paper, several numerical examples, with numbers of combinations up to 308 are presented and their effectiveness is validated.

Słowa kluczowe:

discrete optimization, removing redundant material

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The objective of this paper is a numerical simulation of vibration control of a mast supported by tendons. This problem requires knowledge from many different disciplines such as multibody dynamics, control theory and numerical methods.

Słowa kluczowe:

vibration control, wind disturbances

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