Uncategorized
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.
Preface
Dear Readers,
This special publication combines issues 2 and 3 of the current volume and it is motivated by the recent sequence of shocks that affected the Emilia-Romagna region of Italy during the month of May 2012. We have been able to release it in a relatively short timeframe thanks to the efforts of the contributing authors that submitted their reconnaissance reports in a record time without forsaking thoroughness of observation and depth of analysis, considering causes, effects and indicating possible remedies, to the observed damage and disruptions.
The events underlined again some of the problems well known to the whole community but at the same time have revealed new lessons that require renewed attention. It is worth mentioning that the nature of the seismic excitation appeared characterized by a significant component of vertical acceleration as well as a large spectral displacement demand for long periods. The significant vertical acceleration level and its effects highlighted, once again, its critical and under-researched role in inducing damage. Two were the evidences that, even though well known to the scientific comunity, most dramaticaly captured the pubblic attention: the extent of liquefaction occurrences and the vulnerability of industrial buildings. Both topics are addressed in this issue. The classification of the region as a low seismicity zone does not justify however the incidences of poor engineering judgment and lack of common sense, that emerged in some cases. In Emilia Romagna, as in other Italian regions, historic buildings form a consistent portion of the urban centres. The survey of both heritage and historic buildings in the many small towns of the hit region, indicated a generic good performance with the exception of cases where wall connections were poor and low material quality and lack of maintenance was evident.
Of particular interest was also the after-earthquake approach of the agencies involved in the immediate emergency management and planning for reconstruction, in clear contrast with what observed after L’Aquila’s earthquake. The opening paper suggests thoughts on this subject that should guide us in our future work of prevention, conservation and reconstruction.
Have a good read.
SOIL LIQUEFACTION PHENOMENA OBSERVED IN RECENT SEISMIC EVENTS IN EMILIA-ROMAGNA REGION, ITALY
Significant and widespread liquefaction effects, which caused panic of inhabitants and damage to buildings and infrastructures, were observed in various areas of Emilia-Romagna region, Italy, during the seismic events of May 20 and 29, 2012, with magnitude respectively of Ml 5.9 and Ml. 5.8. In Italian seismic literature these phenomena represent an interesting case study for a number of reasons: for the exceptional impacts, for the size of the area in which liquefaction effects were observed, for the amount of damage produced, for the rarity of soil liquefaction Italian case histories. Immediately after the earthquake of 20 May an extensive field reconnaissance was conducted through the natural and built environment to capture as quickly as possible surface evidences of liquefaction and to document the extent and the severity of damage. The most significant and widespread liquefaction impacts were found in the two settlements of San Carlo and Mirabello. With the aim of a better understanding of the observed scenarios, a detailed investigation program, including geophysical surveys and geotechnical testing (soundings, cone penetration tests, seismic cone penetration tests, cross-hole and down-hole tests as well as many cyclic laboratory tests), was planned. These investigations are now in progress and will be object of forecoming technical notes. This paper summarizes the observation of soil liquefaction effects made during field investigations and presents a first interpretation, based on available information on ground shaking and soil conditions, of the factors that may have contributed to determine the low liquefaction resistance of soils.