Two finite element approaches for the dynamic analysis of seepage flows are discussed here. This represents the first step of a study on the effects of earthquakes on retaining or embedded structures in saturated granular soils. The equations governing the flow of a liquid within a porous skeleton under an acceleration field varying with time are recalled first. Then they are combined in two differential equations that, reduced to their weak form, lead to a finite formulation of the problem in terms of discharge velocity. Due to the relatively large number of nodal variables, and to the iterative structure of the time integration algorithm, this approach requires a non negligible computational cost. Then a second approach is presented, based on some simplifying assumptions, where the pore pressure represents the nodal variable and that adopts a direct time integration scheme. The finite element programs implementing the two formulations are finally applied to the solution of a bench mark problem presented in the literature. The numerical results permit drawing some conclusions on the accuracy of the two approaches that will guide in the choice of the most convenient of them in extending the study towards the analysis of coupled two-phase problems.
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Recent application of seismic isolation systems in Italy
At present there are over 10,000 structures in the world that are protected by means of seismic isolation (SI), energy dissipation (ED) and other seismic vibration passive control (SVPC) systems and devices. They are located in more than 30 countries. Italy is now fifth at worldwide level (after Japan, the P.R. China, the Russian Federation and the USA) and is first in Western Europe. In the last years, however, there was a large increase of the number of the applications completed and, especially, of those in progress or under design. This was due at first to the new Italian seismic code, enforced in May 2003 by Ordinance Nr. 3274 of the Prime Minister (mostly as a consequence of the San Giuliano di Puglia tragedy during the 2002 Molise and Puglia quake), which freed and simplified the adoption of the SVPC systems; a second cause was the 6.3 magnitude Abruzzo earthquake of April 6, 2009. The new applications in Abruzzo recently made Italy fourth at worldwide level for the number of isolated buildings. Moreover, Italy remains among the worldwide leading countries for the number and importance of bridges and viaducts protected by the SVPC systems (which are over 250) and also for the manufacturing of the latter (especially of the SI ones) for applications to all kinds of structures abroad. This paper summarizes the state-of-the-art of applications of anti-seismic techniques in Italy, with particular attention to the most recent ones of SI to buildings.
For this paper is available an extended abstract after the text in Italian
An elementary model of soil and rock avalanches, debris run-out and fast spreads triggered by earthquakes
The model considered in this paper for analysis of fast and large ground movements is based on the equations of motion, mass and energy balances and on conventional ground parameters. The model capability is checked for the Pandemonium Creek rock avalanche in British Columbia, the Shum Wan Road debris run-out in Hong Kong and a coal mine waste fast spread at Aberfan in Wales. The analytical model can be used for first estimates of basic ground movement parameters such as the travel distance, peak velocity and acceleration. Despite its simplicity, the model still requires the use of some ground parameters and an assessment of the initial mass conditions a priori by the user.
A weakness of the static method as implemented by the Italian seismic Code
The static method of seismic analysis proposed by the Italian code of practice can lead to non- conservative design. This may happen when the rigorous value of the fundamental period of the structure is adopted instead of the approximate one given by the code.The present paper shows that in the presence of eccentricity between the stiffness center and the mass center of the building floors, the static method can under-evaluate the maximum seismic effects by more than 25%. Appropriate changes in the code should be introduced in order to bring the static method to the safe side. Some possible improvements are discusses in the present paper.
For this paper is available an extended abstract after the text in Italian
Seismic design methods of concentrically braced steel frames
SUMMARY – The work described here is addressed to the evaluation and the comparison of different seismic design methods of multistory steel frame structures with concentric braces. In particular, a multistory frame is designed through both the simplified method currently proposed by the recent code provisions (OPCM 3431, DM 24/01/2008, Eurocode 8 ) and an alternative simplified one, based on the partial contribution due to the brace under compression. The assessment of the seismic response of the two structures is then performed by means of static analyses, where the non-linearities of the geometry and of the materials are included, in order to compare the related seismic performances. In addition, the seismic response of the frame designed with the new method is assessed through non-linear dynamic analyses and incremental dynamic analyses, to particularly highlight the properties of the alternative adopted method, which permit to achieve less conservative solutions, more economical in terms of amount of material and characterized by more effective collapse mechanisms.
For this paper is available an extended abstract after the text in Italian
The new bridge seismically isolated over the river Fella in Dogna (Italy): design validation as a result of structural dynamic tests
An example of structural validation as a result of dynamic identification of a new bridge over the river Fella in Dogna (Italy) follows. The structure, a prestressed concrete continuous beam, bypasses, with its two spans of 37.5 m each, the wide alluvial riverbed. The bridge, which rises in an high level seismic area, has been isolated by the use of elastomeric isolators able to reduce the heavy stresses of seismic origin. The dynamic tests performed on the bridge and the followed structural identification by means of model updating have led to a «zero reading» of the dynamic characteristics, to be used as reference data for subsequent analysis of degradation, especially following a seismic event. It will be shown a parallel between the stresses identified in the structural model (based on which the bridge was built) and the resulting model from dynamic identification, to extract a possible criterion for structural validation after the construction.
For this paper is available an extended abstract after the text in Italian
Rehabilitation of a small borough damaged by Umbria earthquake (26/09/1997)
The paper describes the works done to repair the so called «Borgo di Pascelupo». The «Borgo di Pascelupo» is an hamlet placed in Scheggia and Pascelupo’s town, located in Umbria (Italy), damaged by the 26th of September 1997 earthquake. Both the administration and techniques aspects are analyzed.
For this paper is available an extended abstract after the text in Italian
Arturo Danusso and the burden of proof
For this paper is available an extended abstract after the text in Italian
Shear resistance of unreinforced masonry walls
Shear failure, characterized by the occurrence of diagonally oriented cracks, is a typical failure mode of unreinforced and confined masonry walls subjected to in-plane seismic loads. Although other mechanisms are also possible, seismic resistance of a regular masonry structure depends predominantly on the shear resistance of structural walls. The results of cyclic lateral resistance tests of unreinforced walls with different height/length aspect ratios and tested at different compressive stress/compressive strength ratios, have been used to compare the experimentally obtained resistance values with the results of calculations. It has been shown that typical equations used for the calculation of the shear resistance of walls, which are based on either shear friction or diagonal tension failure mechanisms, do not have general validity, because they reflect the type of the shear failure, for which they had been developed. Whereas good correlation between experimental results and calculations has been obtained if the equations based on the diagonal tension failure mechanism have been used, the calculations based on the shear friction failure mechanism overestimated the actual resistance of the tested walls.
Fibrous reinforced concrete beams in seismic area: experiment and design
In the present paper the experimental results referred to monotonic and cyclic tests on fibrous reinforced concrete (FRC) beams are presented and discussed. The influence of longitudinal and transverse steel bars, of type and percentages of fibers, of the shear to depth span ratios and of the cover thickness are estimated. Experimental results highlight that fibers are a very effective shear reinforcement in R.C. beams both under monotonic and cyclic actions, but under reversal cyclic actions, if shear failure is attained, the contribution due to the fibers reflects mainly in the bridging actions across the cracks and brittle failure can be partially avoided. In these cases and especially for seismic design ductile failure in flexure is more suitable and over strength in shear is required, the latter being obtained with a good combination of fibers and stirrups. Form theoretical point of view shear and flexural strength prevision was made by using a recent model developed by the authors able to take into account of the main parameters governing the flexural behavior of fibrous reinforced concrete beams. Finally, some design consideration for the use of FRC in seismic design of beams are given.
For this paper is available an extended abstract after the text in Italian