PSI - Issue 44

Michele Angiolilli et al. / Procedia Structural Integrity 44 (2023) 870–877 M. Angiolilli et al./ Structural Integrity Procedia 00 (2022) 000 – 000

873

4

Table 1. Mechanical properties of the concrete (*assumed from CEB-FIB for the numerical simulations)

f c (MPa)

f ctm (MPa)

E cm (MPa)

γ c (N/mm3)

ν c (-)

G

I f (Nm/m

2 )

29.3

2.85

30,373

2.5 E-3

0.2

50

Table 2. Mechanical properties of the steel rebars.

f y (MPa)

f u (MPa)

E s (MPa) 210,000

γ s (N/mm3)

ν s (-) 0.29

ε sy ( ‰ )

ε su ( ‰ )

450

540

7.85 E-5

2.14

75

Specimens were tested imposing a precise sequence of increasing amplitude quasi-static cyclic displacements among the two ends of the column (to simulate the seismic inter-floor drifts) in combination with null external forces applied to the column. The condition of null axial load in the column was decided to reduce the positive effect by which compressive axial forces generally play improving the capacity of the strut mechanism, as the diagonal strut becomes wider with an increase of the depth of the compression zone (e.g. Nicoletti et al. (2022)). The test setup is shown in Fig. 2. The beam-column specimen was placed horizontally on a steel loading frame fixed to the floor. The column was positioned directly on hinge support to ensure that column bottom end was free to rotate but preventing any horizontal/vertical movement. The column top end was supported to the loading frame allowing the upper end to rotate and move horizontally. The load V was applied at the head column using an actuator with 500 kN capacity based on a force-controlled regime. Loading history details for all specimens were based on ACI 374.1 (2005). Loading procedure included several series of force sets; each one consisted of three fully reversed identical cycles. The drift ratio is defined as the ratio between head column displacement and the length of the column by also considering the encumbrance of the support frames (i.e., 3,340 mm). The drift ratio values were selected to evaluate the performance of the joints at different loading stages such as elastic response, before and beyond yielding, and at failure. ACI 374.1 (2005) recommended that the end of the test should be at a drift ratio that equals or exceeds 3.5%. Two linear variable displacement transducers (LVDTs), with base length of 560 mm, were used to measure the diagonal deformation in the joint. Other LVDTs were used to measure displacement of the head column as well as of the two beam ends.

Fig. 2. Photos of the RC specimen with the details of the pushing machine placed at the head-column, the pinned support at the base-column and the roller support at the beam end. 3. Test results and brief discussions The cyclic behavior of Fig.3 (on the left) was almost symmetrical up to the attainment of the maximum strength Vmax (equal to 218 kN) that was attained in the negative load direction. The initial hairline cracking in the joint panel started at the beginning of the sixth complete load cycle for an imposed shear of 61.2k N corresponding to about 0.4% of θ. Then, other cracks occurred in the joint panel, as highlighted in Fig.4. In particular, one can see that cracks occur primarily on the exterior part of the joint (see cracks n. 1, 2, 3, 4 and 5 of Fig. 4). Hence, large cracks developed along