Vincenzo Sepe

Influence of low-frequency wind speed fluctuations on the aeroelastic stability of suspension bridges

Starting from the standard multimode approach to the aeroelastic instability of a long span suspension bridge, that gives the critical flutter speed in the ideal case of a wind constant both in time and in space (turbulence neglected), this paper describes a procedure to evaluate in a closed form the worst effects that can ever be expected as a consequence of a turbulence with a given spectrum. To this respect, only low-frequency wind speed fluctuations strongly correlated along the span are taken into account, neglecting the beneficial effects due to a loss of coherence or to higher frequency components. This allows to obtain a flutter speed certainly safe in comparison with any realistic turbulence field. The proposed formulation is then applied to the current design of the bridge on the Messina Strait (Italy).

Numerical validation of Equivalent-Frame Models for URM walls

In the last decades increasing attention has been devoted to masonry structures both from researchers and professionals. This is due to the awareness of the great importance of these structures in the historical urban context, together with the great risk that they suffer in seismic areas. Growing success has been obtained by a simplified approach that models masonry walls through “equivalent” plane frames, with concepts and procedures drawn from the study of reinforced concrete and steel frames. In this approach, known as Equivalent Frame Method (EFM), each masonry resisting wall is modelled as a system of linear (frame) elements repre-sentative of the behaviour of finite portions of the wall (pier and spandrel panels). Up to now EFM has proven to be effective in the case of new buildings characterized by regu-lar geometrical configurations and with openings’ dimensions for which the frame-like as-sumption is suitable. Critical issues emerge from existing buildings in European historical centres, where irregularities are almost always present and geometrical anomalies can be detected even in the case of regular walls. For the specific case of geometrically regular URM walls, this paper presents sample cases tested with linear and non-linear analyses, with the aim to explore the applicability of EF procedures and to identify its limits. A comparison between an EFM procedure with a fiber approach and a more detailed FEM method with plane elements, is adopted as a validation tool, to evaluate the accuracy of the results provided by the EFM for walls characterized by different geometrical configurations.

Validation of a modal identification procedure for linear frames under unmeasured seismic input

The paper deals with the frequency domain identification of linear structures subjected to unmeasured seismic excitations. Starting from the classical and widely acknowledged transfer-function based modal technique for structures under known excitation, one of the writers co-authored, in previous theoretical works, an approach to solve the identification problem in case of unmeasured seismic excitations. This paper reports the first results of an experimental campaign, still in progress, aimed at validating the procedure. It is found, at least for the experimental data so far obtained, that in the case of unmeasured excitation multiple solutions can be found, depending on the initial estimate of the parameters to identify (natural frequencies, damping coefficients and modal shapes). To this respect, an appropriate choice of their lower and upper boundaries can be crucial; fortunately, to some extent, such information is always available for real structures. A minimisation algorithm is therefore introduced in the paper that decompose the complex-valued transfer functions and the unknown modal parameters in their real and imaginary parts. Considered that this output-only identification procedure needs at least three known time-histories of the outputs (e.g. floor accelerations), the sample case of a three storeys shear-type frame is discussed.

MODAL ANALYSIS OF A FRAME MODEL UNDER UNMEASURED SEISMIC INPUT

The paper reports the first results of an experimental campaign aimed at the validation of a procedure, proposed in previous theoretical works by one of the writers, for the structural identification in frequency domain of linear structural systems subjected to base seismic excitation. Differently from the classical identification techniques based on the transfer-function processing, that require to measure the excitation (input-output problem), such procedure allows the solution of the identification problem also in case of unknown input (output-only problem) if the responses of at least three degrees-of-freedom are available for the system. An experimental campaign has been carried out on a three storeys shear-type frame, carefully designed in order to minimize the uncertainties and errors. After validating the model setup and the measuring chain by means of a classical input-output procedure, the paper discusses the first results of the output-only identification. The outcomes show a fair agreement with the results obtained by including the input knowledge.

Effects of POD-Based Components of Turbulent Wind on the Aeroelastic Stability of Long Span Bridges

In this paper the effects of low-frequency wind speed fluctuations on the aeroelastic stability of long span bridges are analyzed and discussed. The spatial-temporal field of wind turbulence is described by means of orthogonal loading components representing the eigenfunctions of the cross power spectral density function (cpsdf) of the wind process. The stability condition, derived through stochastic averaging technique and moment stability method, are finally applied to a bridge designed for crossing the Messina Strait (Italy).