Andrea Dall'Asta

Three-field mixed formulation for the non-linear analysis of composite beams with deformable shear connection

A three-field mixed finite element is proposed for the non-linear analysis of composite beam with deformable shear connection. The formulation considers the non-linear behaviour of materials and shear connectors. The established mixed element is compared to the locking-free displacement element from which it derives and to a refined locking-free displacement element previously tested by the authors. In order to evaluate the way to better improve the solution in the non-linear range (three-field mixed formulation or refined displacement formulation), numerical applications are performed using, as working example, a steel-concrete cantilever, representing a difficult test for composite beam elements.

On the coupling between three-dimensional bodies and slipping cables

Abstract This paper examines the system obtained by coupling a three-dimensional body with a stretched cable in the case of frictionless contact, in order to evidence the consequences of some characteristic aspects such as the inseparable coupling arising between local and global deformation. The analysis is carried out for hyperelastic system components in the range of a finite deformation theory and furnishes a description of the kinematics, balance conditions and infinitesimal stability. The proposed formulation may be used to analyze numerous engineering techniques; the application of some results obtained to a real case is illustrated by a simple but meaningful example involving a beam.

Efficient Approach for the Reliability-Based Design of Linear Damping Devices for Seismic Protection of Buildings

AbstractThis paper presents an efficient reliability-based methodology for the seismic design of viscous/viscoelastic dissipative devices in independent and/or coupled buildings. The proposed methodology is consistent with modern performance-based earthquake engineering frameworks and explicitly considers the uncertainties affecting the seismic input and the model parameters, as well as the correlation between multiple limit states. The proposed methodology casts the problem of the dampers’ design for a target performance objective in the form of a reliability-based optimization problem with a probabilistic constraint. The general approach proposed in this study is specialized to stochastic seismic excitations and performance levels for which the structural behavior can be assumed as linear elastic. Under these conditions, the optimization problem is solved efficiently by taking advantage of existing analytical techniques for estimating the system reliability. This analytical design solution is an approxi...

Perturbed motion of viscoelastic columns : a variational approach

Abstract In the past, the stability of viscoelastic columns has been analysed by solving the integro-differential equations of equilibrium under static or dynamic type disturbances. The solution of these is usually difficult and the authors intend to provide an alternative formulation of variational type. By using a convolution bilinear form, the operator governing the problem becomes symmetric and a functional, which is stationary at the solution of classical equations, is obtained. This formulation makes it possible both to use the classical approximation methods of the variation calculus and to pose the problem on more natural functional spaces. Some applications show the potential of the Ritz classical spectral method in the numerical solution and introduce the problem of columns subjected to variable load history.

A variational formulation of the perturbed motion problem for a viscoelastic body

Abstract The analysis of perturbed motion is often very important for studying the progress of strain and stress in viscoelastic bodies. The authors intend to provide a variational formulation of the problem as an alternative to the differential formulation used to date, by solving the so-called inverse problem of the calculus of variations. This paper shows how the operator ruling the problem can be made symmetric by using a convolution bilinear form to obtain four functionals which are stationary at the solution of the differential problem. In conclusion, for example, the two-dimensional equations of the perturbed motion of a viscoelastic thin plate, are derived from the stationary condition of the three-dimensional functional.

Dynamic monitoring of an isolated steel arch bridge during static load test

This paper describes the dynamic monitoring carried out by means of vibration measurements during the standard static load test of a half-through steel arch bridge on the Potenza river. The structural design of the bridge is characterized by some notable aspects: the river crossing is obtained by two coupled steel arches, having a span length around 115 m, and a steel-concrete composite deck sustained by thirty couples of steel hangers; the approaching spans are realized with continuous girders on intermediate supports, having cross section with variable height; the arches are supported on rubber bearing seismic isolators. The vibration measurements were nearly continuously acquired during the loading tests to detect the occurrence of anomalies in the structural behavior. In particular, the global dynamic characteristics of the structure, in terms of modal parameters, were determined using the data set of measurements at specific phases of the load test: first on the unloaded configuration, then on two different loaded configurations and finally after the bridge unloading. Measurements of vibrations due both to the ambient and to the impulse produced by a fully loaded truck passing over a bump, were carried out. The experimental results, in terms of modal parameters of the bridge, are in agreement with the theoretical results obtained with the predictive finite element model developed for design purposes and opportunely modified to account for the real conditions of the bridge during the tests.

Dynamic response of composite frames with rubber-based dissipating devices: experimental tests

Publisher Summary The chapter discusses dynamic responses of composite frames with rubber-based dissipating devices. The chapter analyzes composite moment-resisting space frames where lateral stiffness and equivalent viscous damping are augmented by means of dissipating devices, and presents the results of a preliminary experimental campaign carried out on a real scale composite space frame equipped with a dissipating bracing system endowed by high damping natural rubber devices. Moment-resisting frames made of steel-concrete members provide an efficient earthquake resistant system. The efficiency is entrusted to the formation of plastic hinges located at the ends of the beams or at the beam-to-column joints designed as partial strength connections. This design criterion is satisfactory in preventing the structure from collapse against high intensity earthquakes, but it often leads to a very large lateral deformability which brings on excessive inter-storey drift under low intensity earthquakes. The energy dissipation associated with the ductile behavior of dissipating zones induces a large amount of structural damage which usually calls for very expensive and difficult rehabilitation works.

Asymptotic behaviour of imperfect viscoelastic beams

Abstract In this paper the problem of the lateral stability of imperfect viscoelastic beams is analysed. The problem is examined by means of the quasi-static approach and leads to a system of integro-differential equations which can be resolved by a series expansions and the Laplace transforms. The viscous critical load is defined according to the asymptotic behaviour of the beam and can be evaluated by introducing the assumption of weak fading memory, solid material and thermodynamic compatibility. The critical load does not depend either on the type or the entity of the imperfections. Furthermore, some characteristic aspects of the problem, like the faster progress of the torque moment with respect to the progress of the lateral bending moment, are underlined. For a three-element model it is possible to reach a closed-form solution. In the case of the Poisson ratio constant in time and three-element model, an interesting analogy with the asymptotic behaviour of the imperfect column is observed.

Seismic Response Analysis of Continuous Multispan Bridges with Partial Isolation

Partially isolated bridges are a particular class of bridges in which isolation bearings are placed only between the piers top and the deck whereas seismic stoppers restrain the transverse motion of the deck at the abutments. This paper proposes an analytical formulation for the seismic analysis of these bridges, modelled as beams with intermediate viscoelastic restraints whose properties describe the pier-isolator behaviour. Different techniques are developed for solving the seismic problem. The first technique employs the complex mode superposition method and provides an exact benchmark solution to the problem at hand. The two other simplified techniques are based on an approximation of the displacement field and are useful for preliminary assessment and design purposes. A realistic bridge is considered as case study and its seismic response under a set of ground motion records is analyzed. First, the complex mode superposition method is applied to study the characteristic features of the dynamic and seismic response of the system. A parametric analysis is carried out to evaluate the influence of support stiffness and damping on the seismic performance. Then, a comparison is made between the exact solution and the approximate solutions in order to evaluate the accuracy and suitability of the simplified analysis techniques for evaluating the seismic response of partially isolated bridges.

Reliability-based design of fluid viscous damper for seismic protection of building frames

Viscous dampers are energy dissipation devices widely employed to control the structural response of mechanical and civil systems subjected to dynamic loadings. In particular, such dampers have been extensively applied in earthquake engineering to enhance the seismic performance of new and existing building frames. Although simplified methodologies for the preliminary design of the optimal damper properties are well established, they are often based on deterministic or semiprobabilistic approaches neglecting the response dispersion due to the uncertainties inherent to the seismic input and the system model. This paper proposes a reliability-based methodology for the optimal design of viscous dampers in structural systems subjected to the uncertain seismic input. The methodology is capable of controlling the probability of exceeding limit states related to the structural failure, while limiting the damper forces. Thus, it provides a useful mean to identify the optimal damper properties based on performance-based criteria.