PSI - Issue 44

Gabriele Guerrini et al. / Procedia Structural Integrity 44 (2023) 2214–2221 Gabriele Guerrini et al. / Structural Integrity Procedia 00 (2022) 000–000

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beam and the top masonry spandrel, or between the bottom spandrel and the footing. This was particularly relevant for specimen P1 in the negative loading direction, while steel brackets were added to P2 to minimize relative sliding. For specimen P1, the base-shear coefficient (i.e., lateral force normalized by the axial force at the pier base) reached its peak of 0.82 and remained nearly constant between drift ratios of 0.7% and 1.2% in the positive direction, while it reached a maximum of 0.83 at 0.5% drift ratio in the negative one. As the lateral displacement increased, progressive loss of strength was observed, due to pier toe crushing under flexure. Strength losses were recorded equal to 24% and 17% at drift ratios of 2.2% and 2.8% in the positive and negative verses, respectively, which corresponded to the end of the test under flexural failure. The behavior of P2 was more symmetrical, with base-shear coefficient achieving peaks of 0.79 in positive and of 0.78 in negative direction and remaining nearly constant between drift ratios of about 1.0% and 1.2% in both directions. As the lateral displacement increased, progressive loss of strength was observed due to diagonal shear damage of the masonry pier, which was not prevented by the detachment of the FRCM retrofit. Strength losses were recorded equal to 20% at drift ratios of 1.4% and 1.5% in the positive and negative verses, respectively. At the end of testing, associated with shear failure at drift ratios of 1.75%, strength losses of 44% and 48% were measured in the two verses, respectively. Upon removal of the FRCM, wide residual diagonally crossing cracks were detected on the masonry pier. 3.2. Bilinear relationships The envelopes of the lateral force-displacement cycles were idealized into equivalent bilinear relationship following the procedure of the Italian building code (MIT, 2019). In particular, the elastic branch is chosen to pass through the point at 70% of the peak base shear (coefficient), while the ultimate displacement (or drift ratio) is defined at a strength drop of 20% (i.e., 80% residual strength). By equating the areas below the envelope and the bilinear relationship, the yield point is determined. Fig. 5 shows these operations for specimens P1 and P2 in both positive and negative loading directions. With both CRM and FRCM jacketing and in both verses, an equivalent yield base shear coefficient of about 0.73 was obtained. Specimen P1 exhibited a flexural failure, thanks to the CRM strengthening remaining bonded with the masonry substrate also under large lateral displacements and thus preventing shear failure; this resulted in ultimate drift ratios of 2.8% and 2.2% in the positive and negative directions, respectively, with an average which is 2.5 times the 1.0% drift limit imposed by current codes (MIT, 2019) for flexural failure in masonry piers. On the other hand, specimen P2 failed in diagonal shear after the FRCM retrofit detached from the masonry surface. Because a lateral force higher than the bare masonry shear strength was achieved before loss of adhesion, the FRCM detachment triggered this failure mode, leading to ultimate drift ratios of 1.5% and 1.4% in the positive and negative verses. Nevertheless, thanks to FRCM jacketing the average ultimate drift ratio was 3.0 times the limit provided by building codes (MIT, 2019) for masonry piers failing in shear.

(a) (b) Fig. 4. Lateral force-displacement hysteretic responses (blue line) and envelopes (red line): (a) specimen P1; (b) specimen P2.