1,721,144 research outputs found
Dinamica non lineare di strutture intelaiate: analisi teoriche e sperimentazione con il metodo pseudodinamico. TESI DI DOTTORATO IN INGEGNERIA DELLE STRUTTURE, III Ciclo. Depositata presso le Biblioteche Nazionali di Firenze e di Roma nell’aprile 1990
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Effects of initial creep conditions and temporary unloading on the long-term response of stones
No full textA special part of a research programme concerning the tensile and bending creep behaviour of natural building stones is presented in this paper. The new experimental studies were aimed at inquiring the effects of temporary viscoplastic conditions on the time-delayed response produced by subsequent variations of the applied stress. Long-term tests were performed which consisted of unloading the specimens from different stages of the secondary creep branch and reloading them with the same, or decreased, shares of the ultimate-stress values. Further appreciable reductions in the delayed strengths were observed compared to the results of the continuous tests developed in the preceding phases of this research. At the same time, a series of instantaneous collapse reloading-tests highlighted a little influence of the initial deformation time-history on the relevant rupture-loads. Within the context of structural-diagnostic problems, these last values cannot be regarded as exhaustive references for estimating the safety limits of stone members, unless coherent interpretation criteria of the results deriving from in-situ extracted specimens are adopted. A simple methodology is herein suggested to this aim. The delayed elasticity effects following load removal were also analysed, and reproduced by means of a proper phenomenological law that allowed to complete the global creep model proposed in previous works
Long-term tensile and bending strength of natural building stones
No full textThe results of experimental long-term tension and bending tests carried out on four natural stones formely widely used in Italian traditional building (a white-veined Carrara marble, the "pietra Serena" and "Pietraforte" sandstones, and the Piobbico-Cesana limestone) are presented. The tests consisted of applying continually different fractions of the "instantaneous" fracture loads and monitoring the strain evolution of the specimens until a time-delayed fracture was reached. A typical three-stage creep response was obtained in all the tests. At the same time, a 60% reduction in tensile and bending strength was observed in relation to testing periods not much longer than one year. A phenomenological constitutive model is proposed for analytically reproducing the experimental viscoplastic behaviour. Furthermore, a creep time-to-rupture function giving direct estimates of the strength decay effects is provided, with a view to technical applications. The results of the identification analyses carried out to calibrate the characteristic parameters of the above laws are also reported
Parameter models for estimating in-situ tensile force in tie-rods
No full textTwo analytical models are presented for the identification of the actual static scheme and tensile force of metallic tie-rods included in masonry arches and vaults. The reference structural system consists of a moment-resistant tie-beam with elastic rotational springs at the extremities. Symmetrical restraint conditions are initially hypothesized, thus obtaining a two-parameter model in which the unknowns are the springs' rigidity along with the axial force. The preceding assumption is then removed by defining a more general, three-parameter model in which two different spring rigidities are considered. The mutual experimental data are the middle-section vertical displacement under a concentrated load and the fundamental vibration frequency; the second vibration frequency is adjunctively assumed for the three-parameter model. Closed-form equations are formulated in both cases. Laboratory tests show a notable agreement between analytical estimates and experimental measures. A statistical balance of the application of the proposed models to over 200 rods belonging to several Italian monumental as well as common ancient buildings is presented along with more-detailed information about a significant case study
Time-delayed rupture of stones under low tensile and bending stress states
No full textThe advancements of research dealing with the time-delayed behaviour of building stones are presented. In preceding papers, the results of long-term tensile and bending tests conducted on one type of Carrara marble, two sandstones, and one limestone were reported. A composite phenomenological model was also defined which permitted to suitably reproduce the recorded responses. in this paper, the tests carried out by applying the minimum percentile rate of the fracture loads (equal to 35%), having in the meantime arrived at completion, are described. The outcomes of these tests confirm the basic features of the creep response observed in the previous sections of the experimental programme. By updating the experimental time-to-rupture curves with reference to the new data, an asymptotic trend towards the elastic limit values of the materials is noted. A modified version of the time-to-rupture law associated with the tertiary branch of the phenomenological model is supplied, which allows to further improve the relevant prediction potentialities
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