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Month: October 2010
Seismic risk analysis of harp type cable stayed bridges
A seismic risk analysis of harp type cable stayed bridges is presented using the concept of damage probability matrix. The cable stayed bridge is modeled as a two dimensional system with the deck idealized as a continuous beam subjected to bending action and axial compression. The response of the bridge is obtained by the frequency domain spectral analysis. A double filtered power spectral density function with seismic intensity parameter taken as the magnitude of earthquake and a correlation function between the support excitations are considered as a seismic inputs. For a given magnitude of earthquake, the damage probability matrix is determined by defining three damage states namely, major, moderate and minor. The seismic risk index is determined by combining the dam- age probability matrix with the probability of occurrence of different magnitudes of earthquake. As an illustrative example, a three span cable stayed bridge is analyzed for an extensive parametric study. The parameters include degree of correlation, angle of incidence of earthquake, ratio of the components of ground motion and soil condition. Some of the important conclusions of the study indicate that i) Longitudinal component of ground motion has considerable effect on the probability of failure of the bridge deck; ii) Probability of failure is significantly more for the soft soil condition; iii) Fully correlated ground motion between support excitation provides less value of the probability of failure as compared to the uncorrelated ground motion; and iv) Probability of failure is not very sensitive to the variation of angle of incidence of earthquake.
Flexural behaviour of external fibrous reinforced concrete beam-to-column joints
A softened strut-and-tie macro model able to reproduce the flexural behavior of external beam-to- column joints with the presence of horizontal and vertical steel bars, including softening of compressed struts and yielding of main and secondary steel bars, is presented, to be used for the pushover analysis. The model proposed is able to calculate also the flexural response of fibrous reinforced concrete (FRC) beam-to-column sub-assemblages in term of a multilinear load-deflection curves. The model is able to take into account of the tensile behavior of main bars embedded in the surrounding concrete and of the softening of the compressed strut, the arrangement and percentage of the steel bars, the percentage and the geometry of steel fibers. First cracking, yielding of main steel and crushing of concrete were identified to determine the corresponding loads and displacement and to plot the simplified monotonic load-deflection curves of the sub-assemblages subjected in the column to constant vertical load and at the tip of the beam to monotonically increasing lateral force. Through these load-deflection curves the component (beam, joint and column) that first collapse can be recognized and the capacity design can be verified. The experimental results available in the literature are compared with the results obtained through the proposed model. Further, a validation of the proposed model is numerically made by using a non linear finite element program (ATENA-2D) able to analyze the flexural behavior of sub-assemblages.
For this paper is available an extended abstract after the text in Italian
Rocking motion of a masonry rigid block under seismic actions: a new strategy based on the progressive correction of the resonance response
A new strategy of analysis is presented here for the rocking motion dynamics of a rigid and thin block with damping represented by the coefficient of restitution. This is based on the construction of the “limit” artificial accelerogram which involves an upper bound of the block response and on its subsequent correction to consider more realistic situations. The “limit” artificial accelerogram is characterized by a particular sequence of instantaneous pulses and a simplified equation of motion is adopted for the analysis. It is firstly underlined that the amplitude resonance for the block is much more intense and frequent than that for the linear elastic oscillator. A “reduced” accelerogram is then defined by means of two criteria: 1) increasing the frequency of the pulses and 2) considering the limited duration of the stronger phase of an earthquake. The results are discussed with reference to the influencing parameters such as the coefficient of restitution and the size and slenderness of the block. A numerical example shows the comparison between the results from this procedure with those obtained by the Italian seismic codes NTC08 for the limit analysis of masonry blocks.
For this paper is available an extended abstract after the text in Italian
Structural behaviour of masonry spandrels of URM buildings subjected to horizontal loading: experimental analysis
The mechanical behaviour of spandrels has a very significant influence on the seismic capacity of URM multi-storey buildings, which are very common in the historical Italian towns. In this paper the seismic behaviour of masonry spandrels is investigated, in case of a horizontal tensile-resistant element exists at floor level. At this aim experimental tests have been performed on reduced scale (1:10) models of spandrels, made of both tuff masonry and homogeneous material. The specimens have been built using different arrangements of masonry, corresponding to different construction techniques adopted during the time (from XVII up to XX century). The experimental equipment has been purposely designed in order to appropriately reproduce the structural behaviour of spandrels when masonry wall is subjected to horizontal forces acting in their own plane. The observed failure mechanisms and the corresponding M-c curves are reported for all the adopted slenderness ratio and masonry arrangements. Furthermore, in order to evaluate the cyclical performance of this kind of masonry panels, the results of cyclic tests have been analyzed in detail in terms of maximum strength, stiffness at reloading and energy dissipation.
For this paper is available an extended abstract after the text in Italian
Hacia una nueva filosofía de las estructuras
This paper is available in Spanish and Italian only.