Issue 51

G. Ramaglia et alii, Frattura ed Integrità Strutturale, 51 (2020) 288-312; DOI: 10.3221/IGF-ESIS.51.23 299 Another important information can be assessed with the confinement curves. They provide the confined compressive strength while changing the effective confining stress, normalized to the unconfined compressive strength. The theoretical confinement curves are shown for two values of the normalized tensile strength of masonry, α fixed at -0.1 (dashed line) and -0.2 (solid line) according to the experimentally calculated values. Fig. 5 a) shows the comparison between the Stassi- D’Alia model and the experimental results; while the Fig. 5 b) shows the theoretical confinement curves with the several experimental points. The effective confining stress has been evaluated according to the approaches previously discussed (Eqns. 13 and 18, howsoever it is remarked that in the case of square cross sections the results are equal). Comparing this model with the experimental sample, the good fitting of the theoretical results with the experimental results is clear. In particular, the Stassi-D’Alia model provides reliable results for the entire experimental sample without excessive overestimation of the theoretical previsions (Fig. 5 a). The theoretical results confirm that the confinement curve is weakly influenced by the tensile strength of masonry. It is interesting to note that the mechanical model reasonably predicts the axial capacity for the entire range of the lateral stress field. The same approach has been carried out for other theoretical models. Fig. 6 a) shows the comparison between the Drucker-Prager model and the experimental results; while Fig. 6 b) shows the theoretical confinement curves with the experimental points. (a) (b) Figure 6 : Comparison between experimental results and Drucker-Prager models: a) experimental and theoretical results comparison, b) confinement curve comparison for α =-0.1 (blue solid line) and α =-0.2 (blue dashed line). This model strongly overestimates the experimental results as shown in Fig. 6 a). In particular, this effect is very clear for high values of the confining stress. In fact, for low confining stresses, , 0 0.2 l eff m f f  the mechanical model provides reliable results if compared with the experimental tests. However, a great number of tests were carried out at high