Analysis and modelling of the seismic behaviour of high ... - Ingegneria

4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS
The monotonic test shows clearly that, at an imposed global plastic rotation equal
to 15 mrad (interstorey drift equal to 2%), a marked and sudden loss of moment
resistance characterizes the positive branch of the joint behaviour. This is due to
the fact that the concrete in the beam-to-column connection zone has been
crushed around the column flange, as shown in Figure 4.24. From this point, the
behaviour of the joint is similar to that of a composite joint with not-restrained slab
(Lee and Lu, 1989; Ryan and Bitar, 2002). The loss of resistance is evident in the
diagram of connection response too.
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Figure 4.24. Crushed concrete in the beam-to-column connection
zone around the column flange
From the re-assessment of data collection of the strain gauges in the slab and
imposing the equilibrium condition between beam and column actions, it was
possible to demonstrate that only the mechanism 1 was mobilized to react to the
column action. In Figure 4.25 it is possible to underline the correspondence during
the test between the maximum strength reached in the concrete slab (38 MPa) and
the loss of resistance of the specimen. Due to this unexpected phenomenon the
panel zone underwent very large shear distortions, but it did not show any sign of
distress. The web panel rotation alone represents more than the 65% of the total
joint rotation.
Under cyclic loading history, the specimen CJ-INT shows the same properties
illustrated in monotonic regime. Moreover, the hysteretic behaviour is characterized
by gradual strength degradation with increasing plastic rotation. However, the
specimen exhibited considerable reserve of strength and one can observe that