Fernando Saitta

Dynamic Behavior of Stay Cables with Rotational Dampers

Vibration reduction in stay cables by means of viscous dampers is of great interest in cable damage prevention and serviceability of structural system supported by such cables. This paper presents a study on the effectiveness, as well as the limits, of rotational viscous dampers and springs inserted at the 2 ends of a bending-stiff taut cable; influence of rotational stiffness of the springs is also studied. After a nondimensional expression of the equation of motion has been obtained, as in other cases of nonproportionally damped continuous structures, complex modal analysis is pursued, obtaining complex eigenvalues and eigenfunctions. Comparison with intermediate dampers, widely used in bridge engineering, is performed showing the range of nondimensional parameters for which the proposed approach is of interest. Finally, a numerical technique based on complex mode superposition is presented in order to evaluate time domain responses for transversal distributed excitation. As an example, the procedure is applied to a wind-exposed cable.

DYNAMICS OF SHALLOW CABLES UNDER TURBULENT WIND: A NONLINEAR FINITE ELEMENT APPROACH

In classic cable theory, vibrations are usually analyzed by writing the equations of motion in the neighborhood of the initial equilibrium configuration. Furthermore, a fundamental difference exists between out-of-plane motions, which basically corresponds to the linear behavior of a taut string and in-plane motion, where self-weight determines a sagged initial profile. This work makes use of a continuous approach to establish the initial shape of the cable when it is subjected to wind or fluid flow arbitrarily directed and employed a novel nonlinear finite element technique in order to investigate the dynamics present around the initial equilibrium shape of the cable. Stochastic solutions in the frequency domain are derived for a wind-exposed cable after linearization of the problem. By applying the proper orthogonal decomposition (POD) technique with the aim of reducing computational effort, an approach to simulate modal wind forces is proposed and applied to the nonlinear equations of motion.

Experimental analysis of base isolated buildings under low magnitude vibrations

The effects of low magnitude vibrations on the base isolated building are analysed and discussed with reference to two particular structures. The first one is a complex structure, composed by two blocks of three floors, rising up from a common base deck, which is supported by the foundation beams by means of isolation devices. During a moderate intensity earthquake, higher modes were excited, pointing out their very irregular behaviour. In the second case, which regards a building having a hemispherical shape, with three floors plus an underground one, the data recorded under ambient vibrations revealed the significant contribution of the isolation system. In both cases, the experimental behaviour was quite different from the expected one and this should be accounted for when designing base isolation systems.

Analysis of No-Tension Material Arch Bridges with Finite Compression Strength

AbstractThe mechanism method, based on the particularization of limit design to masonry structures proposed by Heyman, is certainly a very suitable procedure for the analysis of no-tension material arches, especially because of its simplicity. The literature largely treats the method under the hypothesis of infinite compressive strength of the material, whereas fewer studies are devoted to finite compression strength. In this paper the latter case is revisited with reference to masonry arch bridges, assuming a rigid-perfect plastic model for the material and using a nondimensional formulation. In this way, results are derived and discussed for a wide range of parameters. The results are compared with those obtained using Heyman’s hypotheses.

Seismic Analysis of the Stylite Tower at Umm ar-Rasas

A seismic vulnerability analysis carried out on the Stylite Tower at Umm ar-Rasas, Jordan, which is a UNESCO World Heritage Site, is presented in this paper. The monument, a unique survived example of stylite towers in the Middle East, presents evidence of structural damage due to earthquakes. The study was based on preliminary investigations, which consisted in Schmidt-hammer tests executed on stone blocks and passive tromography tests on the ground. Two models were implemented for the tower. In the first one, the tower was considered as a rigid structure supported by means of an elasto-perfect plastic cushion at the base. In the second one, a finite element model was setup, with solid elements characterized by a Drucker-Prager yield criterion. In both cases a push over analysis was performed, which resulted in a very low resistance of the tower to seismic actions. The work is part of a preservation effort which will include the design of a retrofit intervention.

Base Isolation of Buildings with Curved Surface Sliders: Basic Design Criteria and Critical Issues

Curved surface sliders are being used more and more in the seismic isolation of buildings. They are preferred not only because of their lower cost with respect to elastomeric isolators but also of their technical characteristics, such as the fact that the value of the period of vibration is independent of the mass and the automatic coincidence between the gravity mass center of the superstructure and the stiffness center of the isolation system. In this paper, these features are analysed with reference to simple structures, pointing out the possibility of rotations of the superstructure and the loss of contact in some devices. Finally, the importance of the static friction is also emphasized showing the experimental seismic response of an isolation system under a low-energy earthquake. For all these reasons, the use of nonlinear analysis, revised and detailed in this paper, is advisable for the isolation system made of curved surface sliders.

Vulnerability Analysis and Seismic Retrofit of a Strategic Building

AbstractThe seismic vulnerability analysis of a strategic building is presented in this paper. A refined finite-element model was set up on the basis of available experimental results, and updated on the basis of new experimental analyses on materials and structure. As demonstrated by an experimental vibration analysis, the structural behavior is influenced very much by the irregularity of the building and the presence of an adjacent building with no seismic joint. The results indicate a high vulnerability of the building to seismic actions, and the inadequacy of the pile foundation—for which a specific nonlinear analysis was performed—to support horizontal seismic loads. A retrofit intervention, based on the insertion of a seismic isolation system and the realization of a seismic joint with the adjacent building, was proposed, which allows the reduction of horizontal forces in the piles and the torsional movements of the structure.

Seismic analysis and retrofit of a mid-rise building

5th ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, COMPDYN 2015