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Many bridges are built on tall and slender piers, whose seismic response may be strongly influenced by second-order effects and higher modes contributions related to the distributed pier mass. In bridge design practice, the second-order effects are often taken into account by introducing amplification factors based on an oversimplified SDOF description of the pier behavior. In this paper, the influence of both axial load and higher-order modes on the dynamic behavior and seismic response of slender bridge piers is investigated by an analytical formulation and a continuous model that overcome some limitations of the models employed in previous studies. A dimensional analysis of the eigenvalue problem is carried out to make explicit the characteristic parameters governing the system dynamic behavior, and a parametric study is performed to study their influence on the modal properties. Further, a solving method is proposed for the seismic analysis and applied to a realistic bridge. The accuracy of the proposed model and of the kinematic assumptions, leading to a linear problem, is finally tested by comparison with the results obtained by a large displacement formulation. The case study results put in evidence some aspects characteristic of the dynamic behavior of slender piers that require further investigation. The proposed analytical formulation may be efficiently used to perform extensive parametric studies for different pier and seismic input properties. © 2014 John Wiley & Sons, Ltd

This paper deals with the seismic protection of building frames by means of external dissipative systems. Dampers and external framing system can be arranged in several configurations, involving different kinematic behaviours and seismic performances. This study analyses a recently-developed solution called "dissipative tower", which exploits the rocking motion of a steel braced frame, hinged at the foundation level, for activating the dampers. This system aims at controlling both the global response and the local storey deformation of the frame, by using a reduced number of viscous dampers. A state space formulation of the dynamic problem is presented in general terms, together with the solution of the seismic problem via the modal decomposition method. A parametric study is carried out to evaluate the influence of the added damping and of the braced frame stiffness on the modal properties and seismic response of a benchmark reinforced concrete frame retrofitted with the external dissipative towers. It is shown that the addition of the towers yields a regularization and reduction of the drift demand along the building height, but it may induce significant changes, not always beneficial, in the distribution of internal actions of the frame and in the absolute storey accelerations.

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