Michèle S. Pfeil

Dynamic behavior and stability of transmission line towers under wind forces

Abstract A new analytical-numerical modelling for the structural analysis of transmission line towers (TLT) under wind action is presented and proposed as a rational procedure for stability assessment in a design stage. The numerical results obtained from a 3D finite element model are discussed in relation to the dynamic behavior and the mechanism of collapse of a typical TLT. A simplified two degree-of-freedom analytical model is also presented and shown to be a useful tool for evaluating the system fundamental frequency in early design stages. In order to reduce the TLTs top horizontal along-wind displacements in the cross-line direction, non-linear pendulum-like dampers (NLPD) installed on the towers are envisaged and their efficiency is demonstrated with the aid of comparisons between numerical results obtained from the controlled and the uncontrolled systems.

Reduction of vortex-induced oscillations of Rio–Niterói bridge by dynamic control devices

Abstract Cross-winds of relatively low velocities have often set into vortex-induced oscillations the lightly damped and remarkably long three continuous spans of the steel twin-box-girder Rio–Niteroi bridge. Whenever this happens the bridge is closed to traffic of any vehicle, for the sake of users comfort and overall safety. However, because of inherent operational difficulties, in some of these events the traffic barrier is closed too late and users are left frightened when crossing on the oscillating bridge. This deterrent aspect of the worlds largest steel-box-girder bridge is explored herein to briefly present the conceptual design of passive and active control devices to attenuate the observed oscillation amplitudes. For this an appraisal of the actual bridge dynamic behaviour is made first by using an experimentally calibrated mathematical–numerical model, including correlated aeroelastic forces along the spans. The derived dynamic modal equations, are further combined with optimization techniques to assist in designing feasible mechanical and robust dynamic passive and active control devices, to upgrade the serviceability of this bridge and users comfort. The performance of these dynamic energy absorbers is demonstrated through comparison of numerical results obtained for time responses of the original and the controlled structure.

Strengthening a reticulated spherical dome against local instabilities

Abstract The strength capacity of a reticulated spherical dome is generally associated with inelastic buckling of its slender members and more often of the partially restrained connections between members. These instability aspects were focused in the theoretical and experimental work performed to analyze the structural behaviour and to design the strengthening details to upgrade a large steel double-layer reticulated spherical dome. The paper outlines the main steps taken on the extensive stability and safety analyses of the as-built structure. Moreover it reports on the most relevant findings from tests carried out on a full-scale model of a typical substructure module, to investigate the collapse mechanisms displayed by the buckling-prone connections. A proposal is made for the use of a simple and rational expression to estimate the connection strength in the first designing stages. Finally, it is shown how structural upgrading was done by strengthening a few of the semi-rigid connections with specially designed fit-in bolted reinforcement details.

Multiple Controllers of Wind-Induced Oscillations of a Long Span Bridge

This paper discusses a new system of multiple synchronized dynamic attenuators (MSDA) that has been installed in the Rio- Niteroi bridge to prevent cross-winds of relatively low velocities set into vortex-induced oscillations on the lightly damped and long three continuous spans of the worlds largest steel twin-box-girder bridge. The conceptual design of this passive control system along with its main geometric and physical characteristics is briefly described in the paper. The paper gives a short account on how an experimentally calibrated mathematical-numerical model for the aeroelastic problem, combined with optimization techniques, were used to assist in designing feasible mechanical control devices to upgrade the serviceability of this bridge. The performance of the MSDA system is demonstrated through experimental measurements and comparisons of numerical results obtained for time responses of the original and the controlled structure.

Investigação 3D da distribuição de fibras de aço em concreto reforçado por microtomografia de raios X

The effects of the inclusion of steel fibers in concrete have been widely studied in order to investigate possible changes in mechanical properties, such as the increase in tensile strength, ductility, stiffness, toughness (energy absorption capacity), and durability. An immediate consequence of this addition is the mitigation of concretes brittle behavior, so that the material meets new quality requirements. In this context, it is important to study the spatial distribution of the entire internal structure of these materials. Three-dimensional computed microtomography is a non-destructive inspection technique used to characterize the internal structures of various materials based on X-ray interaction with the inspected object. Topological and morphological properties can be obtained directly in three dimensions by means of mathematical reconstruction of the radiographs, which allows analyzing, for example, porosity and distribution of objects. In this context, the aim of this study is to investigate the spatial distribution of steel fibers, as well as of porosity in reinforced concrete samples. To this end, we used a microtomography system calibrated to operate at a voltage of 80 kV, electric current of 100 μA and a pixel size equal to 24 µm. The results showed low porosity and that the steel fibers were not uniformly distributed throughout the sample.

Towards actual brazilian traffic load models for short span highway bridges

New live load models for highway bridge design in Brazil are under development by assembling real traffic database, traffic simulations, analytical-numerical modeling of the dynamic interaction between vehicle and structure and statistical extrapolations. This paper presents and discusses the results obtained in the first stages of this work which includes the comparison between the static effects due to the actual traffic of heavy vehicles and those generated by the live load model given in the current national code NBR 7188. It is demonstrated that this live load model is not appropriate to represent the actual traffic effects and may be, in some cases, non-conservative. The present work deals with short span bridges for two lanes single carriageway under free flow traffic scenarios. The representative static effects in these bridges due to the actual traffic of heavy vehicles are obtained by extrapolating its probability density functions to a certain return period. To this purpose, a traffic database was constructed by gathering data from several weighing stations in Brazilian highways which was then applied to perform traffic simulations through a specially developed computational tool.

Passive damping of vortex-induced oscillations of Rio-Niteroi Bridge

The remarkably long central spans of the Rio-Niteroi bridge are set into vortex-induced oscillations by cross winds of relatively low velocities. Whenever winds reach certain threshold speed ranges the response amplitudes grow large and the bridge is closed to traffic of any vehicle for the users comfort and safety. This deterrent aspect of the worlds largest span steel box girders bridge is explored to forward an explicit proposal for installing tuned vibration absorbers (TVAs) to attenuate oscillation amplitudes and, consequently, to improve bridge service life. A simple mathematical model, which yields response amplitudes that correlate favorably with wind-tunnel test results of a sectional model, is combined with optimization techniques to investigate and compare performances of both passive and active/passive feasible control devices. The obtained numerical results are then used to demonstrate that simple mechanical and robust TVAs come to be an advantageous intent to upgrade this bridges serviceability.

Bridge dynamics and aerodynamics: design and practical requirements for high structural performance and safety

Abstract Bridge dynamics and aerodynamics have been in many cases overlooked in the design stages and consequent low behaviour performance and safety margin have been not rarely observed and detected by means of structural monitoring of the bridge soon after it is brought into full service. In this paper, a selection of important issues regarding design and practical requirements for a high structural performance of main components of conventional and cable-stayed bridges are pointed out and discussed briefly. The paper encompasses sections describing some relevant aspects regarding the numerical and physical modelling of the structural system and the mathematical modelling of the dynamic and aerodynamic loads. Special attention is given to the dynamic interaction between vehicles and structures of roadways and railway bridges and also to the aeroelastic analyses of bridges′ behaviour and their consequent implications on the structural performance of a bridge during its service life. Case examples of actual bridges subjected to dynamic forces produced by the traffic of heavy vehicles and by the wind action are explored to depict the main sources of problems which in some cases caused structural misbehaviour. Some practical measures to improve the behaviour and performance of bridge structures are outlined.

Tension stiffening behavior of self compacting high strength fiber reinforced concrete incorporating river gravels

This study proposes a comprehensive analysis on the structural performance of reinforced Self Compacting High Strength Fiber Reinforced Concrete members. Particularly, it summarizes the results of an experimental investigation aimed at analyzing the tension stiffening behavior of concrete mixtures produced with different kinds of natural aggregates: crushed granite and two types of amazon river gravels (derived from two different Brazilian regions). The mixtures were proportioned in order to achieve 60 MPa of compressive strength and, moreover, beyond three plain concrete mixtures, 35 mm length steel fibers were added to these mixtures, as spread reinforcing element, in volumetric fractions equal to 0.5% and 1.0%. The results derived from this type of test furnish a comprehensive analysis on the crack formation and propagation on concrete elements as well as distributed cracking mechanisms. The results indicate that the adherence between the steel bar and the concrete is not affected by the presence of the river gravels and that the fiber reinforcement controls crack width leading to a great number of cracks and, this effect is more pronounced for higher volume fractions.

Computational modeling of dynamic control mechanisms with viscoelastic materials

In this paper numerical modeling of composite structures and damping devices conceived with one or more layers of viscoelastic materials (VEM) is applied towards practical solutions of vibration problems in structural systems subjected to dynamic loadings. Simple examples of composite structural systems are used to explore the potential of practical application of sandwich and multilayered VEM mechanisms to damp out vibrations induced by dynamic actions. The performance of these VEM damping devices are demonstrated by comparing response amplitudes of the uncontrolled and controlled structural systems. The validation of the developed computational tool and the dynamic numerical modeling is done by means of correlation between obtained numerical results and their experimental counterpart for selected case examples.