N. Roitman

• Professor (Full) at Federal University of Rio de Janeiro

Base isolation with roller bearing systems has been widely studied in recent years due to its successful performance in the seismicprotection of buildings and bridges. This paper numerically evaluates the effectiveness of a bi-directional roller bearing (RB) seismicisolation system composed of sloped bearing plates and multiple rollers arranged in...

The human-structure interaction in slender structures has received increasing attention from civil engineers and researchers lately. For this reason, several pedestrian models that take into account the biodynamic parameters of the human body have been widely studied. In this paper, results from a series of experimental tests are used to obtain par...

A set of empirical equations to estimate parameters and initial conditions for a biomechanical bipedal walking model is proposed in this study. These equations are intended to generate vertical loads induced by humans on rigid ground. Forty-five volunteers are recruited to participate in an experimental program to walk on a rigid surface covered by...

In the last few years, increasing attention has been given to obtaining reliable results in system identification as a key step in performing model updating, damage detection and structural health monitoring. Many methods have been developed to measure the uncertainty in modal estimates. Some techniques are based on signal resampling to obtain a la...

In this study, a damped bipedal model with compliant legs is adopted from the literature to represent a walking pedestrian. Forty-five volunteers are recruited to participate in an experimental program to walk on a rigid surface covered by force plates. The walking forces and accelerations at the pedestrians' center of mass are recorded in the expe...

In the development of oil and gas fields, subsea pipes are used in various applications, like pipelines and risers. During operation, risers can be subjected to accidents, such as collisions with other risers, anchors, rocks, or any heavy equipment or objects, which may lead to mechanical damages. These mechanical damages are commonly characterized...

An advantage bidirectional sloped rolling type isolation device composed of multiple rollers in both orthogonal-in-plane directions is studied in this research. The analytical model of a single direction of roller bearing (RB) system is extended to a two-direction RB system. Also, a 3D linear-elastic frame element to build the finite element model...

Tuned-Mass-Dampers (TMD) have been widely used to suppress undesirable structural vibrations within a user-defined frequency band. Nevertheless, the performance of TMDs designs is quite sensitive to slight deviations in model parameters such as the ratio between the TMDs natural frequency and the frequency of the main structure. This paper proposes...

In the development of oil fields, submarine pipelines are used in various applications. These pipelines and risers are subject to accidents that may occur during operation, such as shocks between risers or shocks between a riser and an anchor, rock, or any equipment or heavy object, which may cause mechanical failure, such as dents. The objective o...

Several damage identification approaches are based on computational models, and their diagnostics depend on the set of modelling hypotheses adopted when building the model itself. Among these hypotheses, the choice of appropriate damping models seems to be one of the key issues. The goal of this paper is to analyze the impact of a set of damping mo...

Composite structures with elastic layers and viscoelastic core have been used as a passive damping treatment applied to reduce vibration amplitudes. In the design phase of this type of damping technique, many aspects ranging from computer modeling to laboratory tests should be considered. Due to the frequency dependency of mechanical properties of...

Resumo O número crescente de passarelas para pedestres vem motivando um número significativo de pesquisadores a estudarem seus comportamentos dinâmicos de forma mais refinada. Como consequência destes estudos, observam-se adequações recentes em normas internacionais usadas para o dimensionamento dessas estruturas, refletindo a necessidade de se aju...

The simulation of loads induced by walking people in civil engineering structures is still challenging, being the focus of considerable research worldwide in the recent decades due to the increasing number of reported vibration problems in pedestrian structures. Because of the complexity of the pedestrian mechanisms involved in these problems, ther...

The dynamic properties of viscoelastic materials show highly frequency-temperature dependency and numerical methods for structural systems containing this type of material require accurate mathematical models to describe their dynamical behaviour. The material behaviour here is modelled using a constitutive equation based on fractional derivative o...

As tendências atuais na construção civil conduzem a estruturas cada vez mais esbeltas e flexíveis, se fazendo necessário o estudo das ações dinâmicas devido à redução das suas frequências naturais. Nas passarelas o risco de ressonância com o carregamento humano é um aspecto que deve ser levado em consideração no projeto, já que as frequências devid...

In this paper, the coupled extensional-torsional behavior of a 4 in. flexible pipe is studied. The pipe is subjected to pure tension and two different boundary conditions are considered: ends free and prevented from axially rotating. The response of the pipe is predicted with a three-dimensional nonlinear finite element (FE) model. Some aspects of...

Na atualidade, devido aos avanços técnico-científicos, estruturas cada vez mais esbeltas e flexíveis têm sido concebidas, podendo provocar uma redução nas suas frequências naturais. No caso específico das passarelas, o risco de ressonância com o carregamento humano tende a aumentar já que as frequências de excitação desse tipo de carregamento podem...

This article presents a methodology — for Multiple Degree of Freedom (MDOF) systems — to identify the modal parameters of a steel catenary riser (SCR) by employing both the Hilbert and the wavelet transforms. To validate the proposed approach, a set of experiments were performed in a small scale model of a real SCR that is installed in the P18 plat...

In this work, the mechanical response of a damaged 2.5″ flexible pipe under combined tensile and bending loads is studied. A set of experimental tests was carried out either considering the pipe intact or with one up to four broken wires in its outer tensile armor. In these tests, the deflections along the pipe as well as the strains in its outer t...

This study presents the validation of a numerical model developed for dynamic analysis of buildings with roller seismic isolation bearings. Experimental methods allowed validation of the motion equations of a physical model of a building with and without roller bearings under base excitation. The results are presented in terms of modal parameters,...

This article presents a methodology for the experimental analysis of thin membranes using digital image processing techniques. The methodology is particularly suitable for structures that cannot be monitored using conventional systems, particularly those systems that involve contact with the structure. This methodology consists of a computer vision...

Structural health monitoring is a problem that can be addressed at many levels. One of the most promising approaches used in damage assessment problems is based on pattern recognition. The idea is to extract features from data that characterize only the normal condition and to use them as a template or reference. During structural monitoring, data...

The present work is aimed at presenting an experimental assessment of a constitutive model used to describe viscoelastic behavior. This strategy is built on the basic principles of the Verification and Validation (V&V) philosophy. The mechanical model used to describe the viscoelastic behavior is a constitutive one based on the concept of internal...

Due to its compound cross-section, the prediction of the structural response of flexible pipes to loads such as their self-weight, internal and external pressure, movements imposed by the floating system and environmental loads such as currents, waves and wind is quite complex. All these loads generate stresses and strains in the cross section of t...

This work focuses on the structural analysis of a damaged 9.13″ flexible pipe to pure and combined axisymmetric loads. A set of experimental tests was carried out considering one up to ten broken wires in the outer tensile armor of the pipe and the results obtained are compared to those provided by a previously presented finite element (FE) model a...

One of the most important issues in steel riser systems is the VIV phenomenon (Vortex Induced Vibration). VIV may induce high levels of structural vibrations and stresses what eventually lead to considerable fatigue damage and reduction in structural lifetime. The use of suppression devices is a feasible solution to tackle this problem. Nevertheles...

Structural health monitoring is based on the development of reliable and robust indicators able to detect, locate, quantify and predict damage. Studies related to damage detection in civil engineering structures have a noticeable interest for researchers in this area. Indeed, the detection of structural changes likely to become critical can avoid t...

This work presents an experimental methodology specially developed for the nonlinear large-amplitude free vibration analysis of a clamped-free thin-walled metal column under self-weight. The main contribution of this paper is related to the developed experimental methodology which is based on a remote sensing technique using a computer vision syste...

In this work, the response of a 2.5″ flexible pipe to combined and pure axisymmetric loads is studied. A set of experimental tests was carried out and the results obtained are compared to those provided by a previously presented finite element model. The pipe was firstly subjected to pure tension. After that, the response to torsion superimposed wi...

Inspections in flexible risers have detected a considerable number of damages in their top section caused by the installation process or generated during their operation, due to the contact with another riser or components of the floating facility. Among the most common damages detected, the rupture of the tensile armor wires is critical to the str...

One of the most important problems in steel riser systems is the VIV phenomenon (Vortex Induced Vibration), which may induce high stresses and high structural vibrations. It is a possible cause of considerable fatigue damage and reduction in lifetime. The increase of operational lifetime of any structure is a constant occurrence for different engin...

The main purpose of this work is to present a preliminary analysis concerning the validation of the constitutive model used to characterize a viscoelastic material. In order to create an environment favourable to the accomplishment of a V&V plan (Verification and Validation) several different sandwich beam-like prototypes were built. Some prelimina...

The torsional behavior of a 4″ flexible pipe is here studied. The pipe was subjected to clockwise and anticlockwise torsion and also to torsion combined with tension. For pure torsion, two different boundary conditions were considered: ends free to elongate and prevented from elongating. When tensional and torsional loads are imposed to the pipe, o...

This paper deals with a nonlinear three-dimensional finite element (FE) model capable of predicting the mechanical response of flexible pipes subjected to axisymmetric loads focusing on their axial compression response. Moreover, in order to validate this model, experimental tests carried out at COPPE/UFRJ are also described. In these tests, a typi...

Resumo – Risers Flexíveis são estruturas largamente empregadas na indústria do petróleo, nos últimos 30 anos. Diante das sucessivas marcas alcançadas, nos últimos anos, das profundidades de exploração de petróleo em alto mar, torna-se necessário um maior conhecimento do comportamento estrutural destas linhas flexíveis. Este comportamento estrutural...

This work presents a numerical and experimental methodologies specially developed for nonlinear free vibration analysis of very flexible structures. In this work, a clamped-free thin-walled metal column under the action of self-weight is investigated. This structure is characterized for presenting, in most cases, highly nonlinear responses, which c...

There are several problems in engineering where the structural response may be affected by conventional devices for displacement measurements. This work presents an alternative methodology for the experimental dynamic analysis of structures, which cannot be monitored by conventional sensors. This methodology is based on digital image acquisition an...

Even though hydraulic collars are largely used to install flexible risers, neither the loads imposed by this equipment nor the response of the risers to these loads have been previously studied. Hence, this paper presents a three-dimensional nonlinear finite element (FE) model to predict the response of flexible risers to loads imposed by hydraulic...

In this paper, the coupled extensional-torsional behavior of a 4″ flexible pipe is studied. The pipe was subjected to pure tension and two different boundary conditions were considered: ends free and prevented from axially rotating. The response of the pipe is predicted with a three-dimensional nonlinear finite element (FE) model. Some aspects of t...

The present work is aimed at estimating the constitutive parameters of viscoelastic materials. The constitutive equation addressed takes internal dissipation into account by means of internal variables. The identification method is built on an optimization problem where the minimum of an error function is sought. The error function is based on the...

Reports on development of a simplified procedure for identifying tubular joint damage in fixed offshore framed structures. Two small scale models were designed and constructed according to results from the hydroelastic similitude theory. Dynamic model characteristics of the initially perfect model were taken as references for comparing results obta...

In this paper the buckling and post-buckling behavior of slender bars under self-weight are studied. In order to study the post-buckling behavior of the bar, a geometrically exact formulation for the non-linear analysis of uni-directional structural elements is presented, considering arbitrary load distribution and boundary conditions. From this fo...

One of the most important problems in steel riser systems is the VIV phenomena (Vortex Induced Vibration), associated to high levels of structural vibration and stress concentration, and possible cause of considerable fatigue damage and reduction in lifespan. Based on constrained viscoelastic layers experience, a structural damping test was conduct...

Krylov subspace methods are widely used in scientific computing. The Implicitly Restarted Arnoldi Method is one of these methods used for solving eigenvalues problems. This paper presents an implementation of this method applied to a generalized eigenvalue problem derived from Tuned Vibration Absorbers (TVA) connected to the main structure. TVA's s...

This research aims at investigating the possibility of using the Coulomb damping, mainly to lessen the vibrations of structures submitted to human loadings. The great advantage in using this type of damping is that we can easily obtain high levels of damping with values well controlled and adjusted to the need of the project.A computational–theoret...

To analyze operating structures it is essential to have numerical models which faithfully represent the structural behavior. These models could allow identification of structural changes and the estimation of possible damages. To obtain an adjusted numerical model is not a trivial task, due the large amount of factors that structures are subjected...

The present work is aimed at modelling and characterizing viscoelastic materials by means of a time domain technique. A constitutive equation containing internal variables is proposed. The parameter estimation problem is solved in time domain by means of the Levenberg-Marquadt technique. The ef- fectiveness of the time domain estimation is assessed...

The damage identification problem in offshore platforms, specifically in jacket platforms, has been studied since the seventies. In spite of the effort dispended, there is still no spread out methodology that can be applied in this kind of structure, due its specific characteristics (structural complexity, load and operational conditions, etc.). Th...

Offshore oil exploitation at increasing water depths has been carrying out studies focused on improving riser response prediction. In order to have a safety design of flexible risers it is necessary to evaluate its dynamic characteristics, mainly the damping factor. This parameter is obtained experimentally and the usual techniques sometimes are no...

The increasing need for petroleum is leading to an exploration in deep water, leading the structures to have a complex behavior. One of the main components in this area is the riser-like structure. They are usually arranged in catenaries and carry the oil or gas from the wells to the platforms. These structures are submitted to dynamic loads such a...

This paper reports a procedure for adjusting structural theoretical models to experimental results based on optimization techniques. It also covers some of the basic concepts of correlation techniques that are normally used to correlate natural frequencies and vibration modes. The developed method is tested performing some numerical simulations wit...

A flexible pipe is a composite structure, built up of several steel and plastic layers, which has been increasingly used in floating offshore petroleum production systems. It is characterized by presenting low bending stiffness and high capacity with regard to internal and external pressure and tension. In order to determine some physical propertie...

Experimental procedures for the determination of physical properties of viscoelastic materials are discussed in this paper. Two well-known methods-a standard ASTM method and the other based on indirect measures-are compared to the method proposed here that uses direct measures of forces and response in terms of displacements or accelerations. Two k...

Este trabalho desenvolve uma série de metodologias para adaptar uma classe especial de técnicas de processamento de sinais, denominadas transformadas tempofrequência, a três problemas distintos em engenharia civil: determinação da taxa de amortecimento de sistemas harmônicos, compressão de sinais provenientes de ensaios de laboratório e compressão...

The estimation of modal parameters through Goal Programming, an optimisation technique, is presented. The estimation, performed in the frequency domain, seeks to minimise the total squared error between experimental and estimated values of the frequency response functions. Its purpose is to obtain greater accuracy than classical methods in complex...

The present work reports on the evaluation of the performance of some damage identification methods applied on two different small scale hydroelastic models of fixed offshore platforms designed and constructed according to the Similitude Theory, where Model 1 shows greater redundancy than Model 2. Experimental tests were carried on Model 1 in order...

In buildings that give shelter to a large group of people, mainly grandstands, high levels of vibrations have been observed. This can both compromise structural safety and human comfort levels. But international codes for structural design, still do not prescribe dynamic loads due to human activities such as walking, jumping, etc. Therefore, due t...

The main goal of this paper is to describe and understand the behavior of an absorption system with Coulomb damping with two degrees of freedom. In order to achieve this goal, a theoretical-computational model is developed and adjusted to the experimental results. The experimental model utilized consists of a cantilever beam with an absorption syst...

The structural elastic behaviour of steel pipelines during on-bottom pull-in procedure is investigated experimentally by using small scale models. Despite the lack of a sea-bed soil pipeline interaction effect in the model, it is possible to show that the unevenness of the sea-bed surface and the non-uniform distribution of friction forces can lead...

Physical modeling together with analytical procedures are used to investigate the dynamic responses of a conventional lifeboat system, suspended from cables, in two conditions: integral and damaged, the latter related to failures of some structural and mechanical components. Moreover, it is shown that friction breaks and shock absorbers can be used...

Experimental tests performed on a hydroelastic small-scale model of a tension leg platform for deep water are reported. Comparison between the processed experimental dynamic signals and theoretical responses obtained with a simplified six degrees of freedom model are made. Moreover, some effects of fluid-structure interaction are briefly discussed...

Small-scale models were used to study practical aspects of a deep water pull-in procedure for largely deflected pipelines. Experimental results were used to calibrate a finite element numerical model. Model tests were conducted on a wet table with refined optical measurement and data acquisition system. (J.M.M.)

This paper describes a computer-vision based methodology for non-contact measuring of mechanical system movements. This methodology consists of frame-to-frame processing of video images corresponding to pictures acquired from vibrating structures. The main hardware and software tools are described here. Some benchmark tests are also shown so as to...

This paper reports on some important preliminary developments using an experimental modal analysis technique applied to small scale models to identify tubular joint damage which very often occurs in offshore framed structures.

Wave-basin test results from a reduced scale model for an offshore compliant tower for deep water is used to assess fluid-structure interaction effects and to calibrate finite-element models. The experimental model complies with the theoretical conditions of hydroelastic structural similitude and the fluid in which it is immersed is plain water. Th...

A hydroelastic reduced scale model of a jacket-type offshore structure was designed and constructed according to the principles of physical similitude. The techniques employed for physical modeling, construction and experimental analysis are outlined briefly and some results from free and forced vibration tests are reported. Along with these test r...

A hydroelastic reduced model of a three leg jack up platform was used for investigation of the influence of water depths and related deck levels on the structural and hydrodynamic damping factors under large amplitude displacements and fluid structure interaction.

The results of experiments with a model deep water tension leg platform to elucidate mechanical behaviour under fluid structure interaction phenomena are reported. Vibration in still water and under wave action was simulated. Measured and predicted results for natural frequencies and modes are compared.

Com o avanço tecnológico da indústria na construção civil, estruturas cada vez mais esbeltas têm sido construídas. Essas estruturas, por serem mais flexíveis, apresentam freqüências naturais cada vez mais baixas, tornando-se necessário o conhecimento de seu comportamento dinâmico principalmente quando as freqüências de excitação se aproximam das fr...