PSI - Issue 44

Sara S. Lucchini et al. / Procedia Structural Integrity 44 (2023) 2206–2213 Lucchini et al. / Structural Integrity Procedia 00 (2022) 000–000

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located within the coating layer at the top of the wall. By contrast, the collapse mechanism in the positive direction was more brittle because of the damage localization that involved the inner side of masonry. As shown in Fig. 2b and c, the ultimate behavior of the wall was governed by a three-hinged arch truss mechanism. Monotonic test The second specimen was loaded monotonically by the same test frame and set-up adopted for the cyclic test described above. The aim was to assess the OP behavior of the wall without considering the effect of cumulative damage due to reverse loading. The first part of the monotonic test consisted in increasing the lateral deflection of the wall in the positive direction until the post-peak resistance reduced to about 20% of the maximum capacity. Then, after complete unloading, the specimen was monotonically re-loaded in the opposite direction till failure. The global load-midspan displacement curve obtained from the test is shown in Fig. 3a. a b c +88.5 kN; +4.8 mm 100

Positive loading direction Negative loading direction

80

+70 kN; +7.4 mm

60

40

20

0

-20 Lateral load [kN]

-40

-45.5 kN; -11.0 mm

-60

-58.5 kN; -1.6 mm

-80

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Midspan displacement [mm]

Fig. 3. Monotonic test: (a) load vs. midspan deflection response; (b) crushing of masonry detected at the top of the wall; (c) deflected shape detected during positive loading right over the upper loading beam.

The initial response of the wall remained linear as the lateral deflection and the corresponding load were lower than +2.1 mm and +60 kN, respectively. By further increasing the lateral deflection, a horizontal crack appeared at the top of the wall thus causing a gradual decrease of the stiffness. At the same time, vertical cracks due to compression appeared on the inner surface of masonry. As expected, the absence of damages due to loading in the negative direction promoted the attainment of a maximum capacity 16% higher than that detected in the cyclic test. After reaching the peak load, the load started to reduce as the crack at the top of the wall extended to the SFRM layer. Therefore, the SFRM layer provided an additional tensile contribution to the flexural resistance and the wall experienced a small recovery of capacity. Then, crushing under the upper loading point (Fig. 3b) resulted in a sharp reduction of both the strength and the deformation capacity. The test was interrupted once the OP capacity reduced to 20% of the peak load. The corresponding displacement was higher than that achieved during the cyclic test (+7.4 mm, 54% higher than the displacement at peak load). All considered, the collapse mechanism was very similar to the three-hinged arch truss described in the previous section. After total unloading, the wall was re-loaded in the negative direction. The overall response obtained from this new test is represented by the grey curve depicted in Fig. 3a. After the initial recovery of stiffness, the curve presented an almost linear response up to a load of about -55 kN. Then, the formation of new cracks in the upper portion of the wall and in the central area between the loading points caused a gradual reduction of the stiffness up to the attainment of the peak load (-58.5 kN; -1.6 mm). The quite ductile response exhibited by the wall in the post peak stage resulted from the activation of a plastic hinge right above the upper loading point that allowed exploiting the tensile resistance of the SFRM layer (Fig. 3c). The maximum midspan displacement, corresponding to 20% reduction of the peak load, was about -11 mm.