PSI - Issue 78

Maria Teresa De Risi et al. / Procedia Structural Integrity 78 (2026) 1151–1158 M.T. De Risi, C. Del Gaudio, G.M. Verderame / Structural Integrity Procedia 00 (2025) 000 – 000

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5

4. Experimental response The main experimental response is shown in this section in terms of column shear force - column top displacement envelopes (Fig. 5), comparing as-built and retrofitted specimens. The damage states corresponding to the first cracking and the peak load conditions are also shown in Fig. 5 for each test. A first important result is that related to the lateral stiffness of the tested specimens. Indeed, considering the first loading direction (negative, i.e., pull), the initial stiffness is not significantly influenced by presence of strengthening, thus allowing defining this strategy as a “local” strengthening technique. The specimen AB, as expected, exhibited a pure shear (S-) failure, with a maximum force of 115 kN, namely significantly lower than the expected yielding load (V y,AB is equal to about 190 kN). Softening phase appears quite stiff due to the fast evolution and widening of diagonal shear cracks throughout the column. At the end of the test, the column cover appeared totally ejected and some stirrups completely opened. Retrofitted specimens reached a higher lateral load, thanks to the presence of the exterior strips: maximum achieved loads was 181 kN and 226 kN for specimens SL and SH, respectively.

Lateralload [kN]

Fessurazione negativa

Top displacement [mm]

Fessurazione positiva

a)

4III+

5I+

5I+

5I-

1 st cracking 2I-

Peak load 2I+ 4I+

1 st cracking 5I 1III

Peak load 2I+

1 st cracking

Peak load 6I+

7

AB

SL

SH

b)

c)

d)

Fig.5. Comparison of lateral force-displacement envelopes (a); cracking pattern at 1 st shear cracking and at peak load for test AB (b), SL (c) and SH (d). In tune with ex-ante assessment, SL exhibited a S-failure mode typology, as the AB, but at a higher lateral load level. On the contrary, the specimen SH overcame yielding before the peak load achievement, thus exhibiting a so called SF-failure mode. Additionally, in specimen SL, at the peak load, one of the prestressed steel strips at the base of the column opened due to the failure of a seal (where the effective transverse area of the strip is reduced). A similar failure occurred for specimen SH, as well, involving the first five strips closest to the foundation block, but only in the