This is the complete No. 4-2012
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Month: December 2012
SEISMIC RETROFIT OF A PRESTRESSED CONCRETE ROAD BRIDGE
The paper deals with the proposal of a procedure for the seismic retrofit of an existing prestressed concrete bridge. First, the seismic vulnerability assessment of the bridge was carried out. With this aim, a Nonlinear Static Procedure based on the Capacity Spectrum Method as well as the Inelastic Demand Response Spectra was applied. According to the Performance-Based Earthquake Engineering principles, this procedure makes it possible to explicitly correlate the different performance levels to the varying intensities of seismic action. A seismic protection strategy based on the use of isolating system located between pier top and deck was subsequently applied. A design process consisting of an appropriate application of capacity-design principles and the Direct Displacement-Based Design approach was implemented. Finally, the seismic response of the bridge, modeled with an “exact” damping matrix, was evaluated through a linear time-history analysis involving a solution of the complete set of equilibrium equations at each time increment. The results obtained highlight the effectiveness of the seismic retrofit strategy.
SEISMIC STRENGTH OF UNREINFORCED MASONRY WALLS: EFFECTS OF THE B SHAPE FACTOR OF THE SHEAR FAILURE CRITERION WITH DIAGONAL CRACKING
The paper deals with the shear failure mode with diagonal cracking of masonry panels in existing buildings. More specifically, this study is focused on the b shape factor of the failure criterion included in the current Italian technical Recommendations (NTC 2008) and completes the results obtained in a companion paper of the same authors /7/. Three plane unreinforced masonry walls with regular openings are considered with different slenderness of the masonry beams. The walls are modelled by finite elements and by equivalent frames and then subject to pushover analyses. In the framed models the b shape factor has been selected both according to the NTC 2008 and as proposed in the companion paper /7/. The seismic capacity diagrams of the walls show that the equivalent framed models significantly overestimate the ultimate shear of the walls with respect to the results provided by the finite element models. This result is amplified when the b values are assumed as recommended in the NTC 2008.
AN ENERGY-BASED APPROACH TO THE SEISMIC CONTROL OF ONE-WAY ASSYMETRICAL STRUCTURAL SYSTEMS BY USING SEMI-ACTIVE DEVICES
The study proposes a semiactive approach to control the dynamic response of a one-way plan-wise asymmetrical structural system by defining a set of closed-form analytical control algorithm to drive semi-active viscous devices in order to optimize different energy rates of the system to be protected, namely damping energy, elastic energy, kinetic energy and input energy. These algorithms are tested by means of a large numerical experimentation by modifying the main structural parameters of asymmetrical system and considering different accelerometric input signals. Finally, the seismic response both in terms of relative displacement and energy rates, has been comparatively evaluated in the case of uncontrolled and semiactivelly controlled systems, by using the proposed algorithms. Results showed the efficiency of the strategy, especially when flexible systems and high spread device configurations are taken into account.
ANALYSIS OF THE DYNAMIC BEHAVIOR OF BASE ISOLATED STRUCTURES BY STATE-SPACE FORMULATION
The seismic behaviour of base isolated buildings is herein analysed by using an innovative mathematical formulation in the state space, that allows us to describe the dynamic response of structures in the case of non-classical damping. Particularly, the seismic response of base isolation with linear–viscous behaviour is herein investigated by studying the mode shapes, the frequencies and the modal participation factors, that are obtained by the proposed formulation varying the main design parameters. In such a manner, the effect of these parameters on the isolated structure behaviour as a whole is evident.