1,720,983 research outputs found
Wind-load fragility analysis of monopole towers by Layered Stochastic-Approximation-Monte-Carlo method
No full textThis paper describes a novel numerical algorithm for the simulation of the along-wind dynamic response of a prototype of slender towers under turbulent winds, using a Layered Stochastic Approximation Monte Carlo algorithm (LSAMC). The proposed algorithm is applied to derive the statistics of the dynamic response in the presence of uncertainties in the structural properties and in the wind loading. Standard “brute force” Monte-Carlo methods are also used for validating the LSAMC results. The proposed methodology efficiently estimates structural fragility curves under extreme wind loads. The methodology enables a significant speedup in the computing time compared to standard Monte Carlo sampling. Furthermore, it is demonstrated that accuracy in the estimation of structural fragility curves is superior to ordinary reliability methods (e.g. “First-order reliability methods” or FORM)
A displacement-based approach for determining non-linear effects on pre-tensioned-cable cross-braced structures
No full textPre-tensioned-cable bracing systems are widely employed in structural engineering to limit lateral deflections and stabilize structures. A suitable configuration of the pre-tensioned-cable bracing systems in a structure is an important issue since the internal force distribution, emerging from the interaction with the existing structure, significantly affects the structural dynamic behavior. The design, however, is often based on the intuition and the previous experience of the engineer. In recent years, the authors have been investigating the non-linear dynamic response of cable systems, installed on cable-stayed bridges, and in particular the so-called “cable-cross-tie systems” forming a cable network. The bracing cables (cross-ties) can exhibit slackening or snapping. Therefore, a non-linear unilateral model, combined with the taut-cable theory, is required to simulate the incipient slackening conditions in the stays. Capitalizing from this work on non-linear cable dynamics, this paper proposes a new approach to analyze, in laterally- braced truss structures, the unilateral effects and dynamic response accounting for the loss in the pre-tensioning force imparted to the bracing cables. This effect leads to non-linear vibration of the structure. In this preliminary study, the free vibrations of the structure are investigated by using the “Equivalent Linearization Method”. A performance coefficient, a real positive number between 0.5 and 1.0, is defined and employed to monitor the relative reduction in the apparent stiffness of the braces during structural vibration, “mode by mode”. It is shown that the system can exhibit alternate unilateral behavior of the cross-braces. A reduction of the performance coefficient close to fifty percent is observed in the braces when the initial pre-tensioning force is small. On the other hand the performance coefficient tends to one in the case of a high level of pre-stress. It is concluded that the performance coefficient may possibly be used as an indicator for the design of the braces since a suitable selection of the initial pre-tensioning force can avoid slackening in the braces
Flutter Control by a Multiunit Gyroscopic Stabilizer: Deterministic-Stochastic Analysis of the Messina Strait Bridge Prototype Design
No full textLong-span cable-supported bridges are increasingly vital structures. Their aerodynamic properties and deck flexibility make them susceptible to wind load effects, with flutter being a major concern as it can lead to structural failure. To address this issue, this paper utilizes a novel, multiunit gyroscopic device for flutter control. The stabilizer is installed in the interior of the deck cross section of the Messina Strait Bridge (1992 prototype design), the longest suspension bridge currently scheduled to be built. First, a mathematical model of the multiunit gyroscopic device was derived and employed to evaluate its effectiveness in altering the critical flutter wind speed. Second, structural reliability against flutter failure was evaluated accounting for random aeroelastic load perturbations, which are a primary source of uncertainty. Monte Carlo simulations were employed to predict the flutter limit-state probability, the reliability index and to investigate the stabilizer's efficiency within the practical operational range. To test the effectiveness of the proposed prototype in enhancing critical wind speed, a supplementary numerical study was conducted, varying the gyricity of the gyroscopic device. The numerical results indicate that the gyroscopic stabilizer is highly effective, with the bridge's flutter threshold increasing by more than fifty percent, depending on the gyricity of the gyroscopic device
Investigating the use of Targeted-Energy-Transfer devices for stay-cable vibration mitigation
Full text linkFree vibrations of a taut cable with an attached passive Targeted-Energy-Transfer (TET) device are investigated using an analytical formulation of the complex generalized eigenvalue problem. This problem is of considerable practical interest in the context of stay-cable vibration suppression in bridges, induced by wind, rain–wind and parametric excitation. The TET device is a nonlinear apparatus, which has been investigated and successfully ap- plied to the vibration suppression in several structural or mechanical systems. This study proposes, for the first time, the use of the TET device as a simple passive apparatus for stay-cable vibration mitigation. In this applica- tion, the device was modelled as a dashpot with a viscous damper in parallel with a power-law nonlinear elastic spring element and a lumped mass restrained to one end. The ‘flexibility of the support’ (imperfect anchorage to the deck) was also simulated by placing an elastic support (linear elastic spring) in series between the dashpot and the deck. The study derives a new family of ‘universal design curves’ for the TET device, by accounting for the effects of nonlinear elastic stiffness, lumped mass and flexibility of the support. To verify the adequacy of the universal curves and to evaluate the effectiveness of the TET devices, parametric numerical simulations were per- formed on a reference stay cable. As an application example, analytical results were employed to design the dampers of one flexible stay, installed on an existing cable-stayed bridge. In all the investigations, theoretical and numerical results were obtained and compared
Estimating the standard deviation of eigenvalue distributions for the nonlinear free-vibration stochastic dynamics of cable networks
No full textCross-ties are used on cable-stayed bridges for mitigating wind-induced stay vibration. The system obtained by connecting two adjacent stays with a transverse cross-tie is the basic component of an “in-plane cable network”. Failures in the restrainers of cable networks have motivated investigations focusing on the nonlinear dynamics of cable networks. In these problems, the nonlinearity in the restoring force transferred by the cross-tie is used to simulate the behavior at incipient failure. The “equivalent linearization method” can be used to reduce the system of nonlinear dynamic equations to an equivalent eigenvalue/eigenvector problem, which is solved algebraically as a function of a reference amplitude parameter. Since the value of the initial vibration amplitude during aeroelastic vibration is affected by uncertainties, simulations based on stochastic processes must be considered. The resulting random nonlinear eigenvalue problem can be solved with an implementation of the standard stochastic approximation (SA). We propose here a novel numerical procedure, based on a “layered” SA algorithm, to estimate not only the mean but the standard deviation of the eigenvalue distribution as well. This method is computationally efficient and can accurately evaluate the average and standard deviation of the random eigenvalue (frequency), mode by mode. Therefore, we have now enabled a more complete and numerically efficient characterization of the frequency probability distribution
STAY-CABLE VIBRATION MITIGATION USING NONLINEAR TARGETED-ENERGY- TRANSFER DEVICES: A PARAMETRIC STUDY
No full textLarge-amplitude oscillation of inclined stays are often associated with rain-wind induced phenomena, vortex shedding and parametric excitation. To suppress the problematic vibrations and to minimize the potential high cost of maintenance or repair, passive damping system have been widely employed. In most investigations the damper was modelled as a pure linear viscous device or as a linear viscous damper with internal stiffness perfectly anchored to the deck. Other authors evaluated the effectiveness of the damping system introducing the effects of the flexibility in the damper support. The aim of this paper is to investigate the use of Targeted-Energy-Transfer (TET) devices for cable vibration suppression; a new family of devices is proposed in which a nonlinear elastic stiffness force mechanism and device, modelled as a power-law elastic spring element, is placed in parallel with a linear viscous damper. The imperfect anchorage to the deck is also taken into account by an elastic support. The free vibration of a taut-cable with an attached passive TET device is investigated using an analytical formulation of the complex generalized eigenvalue problem in the presence of two cable segments. The concept of “universal design curve” was examined. A parametric study was performed on a reference cable to verify the adequacy of the “universal design curve” and to evaluate the effectiveness of TET device; the study was subsequently extended to two real stays, taken from the Fred Hartman Bridge (Houston, Texas, USA) and from the Stonecutters Bridge (Hong Kong, China). In all the investigations, theoretical and numerical results were obtained and compared
Targeted-Energy-Transfer devices for stay-cable vibration mitigation
No full textFree vibrations of a taut-cable with an attached passive Targeted-Energy-Transfer (TET) device are investigated using an analytical formulation of the complex generalized eigenvalue problem. This problem is of considerable practical interest in the context of stay-cable vibration suppression in bridges, induced by wind, wind-rain and parametric excitation. The TET device was modelled as a dashpot with a viscous damper in parallel with a power-law elastic spring element; a linear elastic spring was also added between the dashpot and the deck. Two types of TET devices were analysed: a Linear TET and a Nonlinear TET. For both the devices a family of “universal design curves” was developed, by accounting the effect of the elastic stiffness and the flexibility of the support. To verify the adequacy of the universal curves and to evaluate the effectiveness of TET devices, numerical simulations were performed on a reference cable and subsequently extended to an existing stay, taken from the Fred Hartman Bridge (Houston, Texas, USA)
Pressure coefficients for evaluating wind loads on large roofs: Comparison between Database-Assisted Design and Italian standards
No full textIn the last decade, the Database-Assisted-Design methodology has emerged as a powerful approach to estimate structural wind loads and as an alternative to prescriptive design standards, in particular for Low-Rise Buildings (e.g., Simiu et al., 2003). This work examines the applicability of this approach to wind load analysis and design as an alternative to the prescriptions currently included in the Italian design standard. In this paper, mean pressures coefficients on large roofs of industrial buildings are examined and compared to the recommendations of the current Italian national design guidelines (CNR-DT 207/2008)
Wind loss estimation in tall buildings accounting for uncertainties in wind load and damage model characterization
No full textPredicting the flutter speed of a pedestrian suspension bridge through examination of laboratory experimental errors
No full textThe paper investigates experimental error propagation and its effects on critical flutter speeds of pedestrian suspension bridges using three different experimental data sets: pressure coefficients, aerodynamic static forces and flutter derivatives. The three data sets are obtained from section model measurements in three distinct laboratories. Data sets are used to study three different geometries of pedestrian suspension bridges. Critical flutter speed is estimated using finite-element nonlinear analysis, numerical 2-DOF generalized deck model and 3-DOF full-bridge model. Flutter probability, contaminated by various experimental error sources, is examined. Experimental data sets are synthetically expanded to obtain two population sets of deck wind loads with 30 and 5.10(5) realizations, respectively. The first set is obtained using Monte-Carlo simulation approach, whereas the second one is determined using Polynomial chaos expansion theory and a basis of Hermite polynomials. The numerically-determined probability density functions are compared against empirical probability histograms (pdfs) by Kolmogorov-Smirnov tests
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