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.
Month: July 2009
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
Seismic response of “simple” masonry buildings
The paper is aimed at evaluating the seismic response of simple masonry buildings complying with the rules provided by the new Italian national seismic code. Only a few of examined buildings fulfil the Eurocode 8 requirements. The buildings have two or three stories and differ in plan and wall geometry. Initially, the requirements for simple masonry buildings provided by Italian seismic code and Eurocode 8 are coMPared. Subsequently, results from nonlinear static analyses and modal response spectrum analyses are presented. Influence of accelera- tion at site and masonry strength is also assessed. Indications on the congruence between code rules and results from numerical analyses performed according to the code themselves are supplied. The results show that Eurocode 8 provisions seem to be more appropriate and safe, especially in the case of low masonry strength.
For this paper is available an extended abstract after the text in Italian
Dynamic analysis of the road underpass “Martiri della libertà” in Mestre-Venice
The paper describes the dynamic behaviour of an important road underpass: the “Martiri della Libertà” road underpass in Venice, 9 Km north from the historic centre. The road underpass is a reinforced concrete structure with deep diaphragms. In this paper the different safety level has been evaluated on the basis of National and European Codes for seismic and traffic loads for two different type of structure, one with two diaphragm, the other with three. The main parameters defining the different behaviour of the structure for the different type of stratigraphy are investigated.
For this paper is available an extended abstract after the text in Italian
Prevedere i terremoti: la lezione dell’Abruzzo
During the first three months of 2009 a sequence of small shocks M ≤ 2.7 occurred near the city of L’Aquila. Towards the end of March the frequency of the shocks was rising and on March 30 an M = 4 earthquake occurred. A Committee of experts was charged to examine the likelihood of an impending strong earthquake. On March 31 the Committee concluded that nowadays no short-term prediction is possible and whatever proposed forecasting is not founded on a scientific basis. In this paper some critical comments are presented both on the content of this conclusion and on the method used by the Committee for the analysis. We observe that foreshocks are considered by the scientific community as a very real precursory phenomenon, even though with a high probability of false alarm. Statistical researches carried out in Italy and in California found that this probability is of the order of 0.98, so that the probability of a strong earthquake after the foreshock is of the order of 0.02. In absolute terms the risk is very small, however it is more than 100 times larger than the basic risk of the zone. As far as the method is concerned the analysis should take into account, besides all other possible seismolgical symptoms, also the social scenario in which the foreshock happens. Some results of theoretical researches on alarm systems based on more than one precursor are then summarized. They show how, starting from the characteristics of the precursors, it is possible to define new quantities that are helpful when the matter is to issue a public warning. It is a typical case of decision in uncertain conditions (small probabilities, heavy possible damage) in which an important role is played by social repercussions.
For this paper is available an extended abstract after the text in Italian