Risposta ottima di ponti multicampata continui isolati soggetti a Sismi Near Fault e Far Field

Il presente studio si basa sull’analisi dell’ottimo di attrito per l'isolamento sismico di ponti, dotati di dispositivi a pendolo scorrevole a singola concavità (FPS). Il ponte è modellato attraverso un sistema a sei gradi di libertà mentre l’attrito dell'FPS è descritto attraverso un modello che tiene conto della dipendenza dalla velocità di scorrimento. Introducendo una scala temporale e una scala di lunghezza, è stata utilizzata un'analisi non dimensionale per risolvere le equazioni del moto. In particolare, la risposta è resa indipendente dal rapporto accelerazione/velocità di picco del ter- reno (PGA/PGV). Inoltre, sono stati considerati due diversi set di registrazioni sismiche: far field e near fault. Quindi, sono stati considerati diversi modelli di ponte modificando diversi parametri (ad esempio, periodo della pila, periodo dell'impalcato, massa dell'impalcato e della pila e coefficiente di attrito). Minimizzando la risposta della sottostruttura, viene calcolato un valore ottimale del coefficiente di attrito.

The present study analyses the optimal friction coefficient for the seismic isolation of bridges, equipped with single concave friction pendulum (FPS) devices. The bridge are modelled through a six-degree-of-freedom system while the FPS friction property is described through a model that accounts for the dependency on the velocity. By introducing a time scale and a length scale, a nondimensional analysis has been used to solve the equation of motion. In particular, the response is made independent from the peak ground acceleration-to-velocity ratio. Furthermore, two differ- ent sets of seismic records are considered: far field and near fault. Then, many bridge models are considered by changing different parameters (i.e., pier period, deck period, mass of the deck and of the pier and friction coefficient). By minimizing the substructure response, an optimum value of the friction coefficient is computed.

How to enhance the seismic performance of bridges

One of the most effective strategies for enhancing the seismic performance of bridges involves the application of isolation techniques. This is achieved by increasing the period of the isolation system [1]. Friction pendulum system (FPS) bearings have the advantages of decoupling the iso- lation period from the deck mass and dissipate energy through friction [2]-[3].

Numerous studies have analysed the behaviour of FPS isolators, employing both experimental and numerical analyses [4]–[6]. In the context of seismic isolation studies, Jangid [7]-[8] identified an optimal friction coefficient to design isolators in order to minimize structural responses during seismic events, considering both stochastic random processes and near-fault ground motions as seismic inputs. Subsequent research has delved into non-dimensional analyses of various structural systems based on key parameters [9].

The frequency content of ground motions, represented by the PGA/PGV ratio, has been considered to assess optimal properties of FPS isolators for base-isolated buildings and bridges in, respectively, [10] and [11].

This study specifically examines the seismic performance of multi-span continuous composite deck bridges isolated with single concave FPS devices. The objective is to assess the influence of ground motion characteristics on the optimal friction coefficient of isolators through non- dimensionalization of motion equations with respect to the PGA/PGV ratio. A six-degree-of- freedom (dof) model is employed, encompassing 5 dofs for lumped masses of the RC pier and 1 additional dof for the composite deck, considered infinitely rigid.

Two FPS devices are modelled on top of the elastic reinforced concrete (RC) pier and the rigid RC abutment, with the latter modelled as a fixed support. The velocity dependency of FPS device behaviour is considered according to [4]. Record-to-record variability is incorporated by considering different sets of ground motions, including both far-field (FF) and near-fault (NF) inputs [10].

Moreover, a parametric analysis is conducted, covering the pier period, the ratio between the deck period and the period associated with the ground motion input, the friction coefficient, and the ratio between the mass of the deck and the pier. This analysis enables the assessment of normalized responses in terms of peak horizontal displacement of the pier and the calculation of the optimal value for the normalized friction coefficient of the isolators.

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(L'intera memoria è in lingua inglese).

La presente relazione è stata presentata in occasione del XXIX Congresso CTA, svoltosi a Milano il 26 e 27 settembre 2024.