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

5. SEISMIC BEHAVIOUR OF RC COLUMNS EMBEDDING STEEL PROFILES
5.6 Results of the tests
As said before, the specimens belonging to the Static category have been
designed only with regard to the gravity loads, whereas the specimens belonging
to the Low Ductility and Medium Ductility categories have been designed both
considering the gravity loads and the equivalent horizontal seismic loads. The Low
Ductility and the Medium Ductility categories differ one from the other mainly for the
magnitude of the seismic forces applied to the original RC structure, depending on
the grade of ductility the designers are to attribute to it at the design stage.
Consequently, the specimens differ on the amount of longitudinal reinforcement, on
the spacing of stirrups placed in the critical zones of the main elements and on the
dimensions of the embedded steel profiles. For this reason, only an abridgement of
the results obtained from testing is hereafter illustrated along with some
commentaries. The comments can be extended to the other study cases.
The F-∆ curves of five specimens belonging to the same study case are
represented. In detail, the specimens RCT5, COT9-COT10, and COT11-COT12 for
the medium ductility design case, according to the test programme illustrated in
Subsection 5.5.2, have been presented. The test conducted on the specimen RC-T
(sample without steel profile) is used as test control and reference for the other
tests of the same category. In fact, the behaviour of this specimen relies only on
the resistance and ductility of a reinforced concrete section. The previsions
obtained in the design of the specimens have been confirmed, as the tests have
shown that the specimen collapse has been caused by failure in the joint. The
global behaviour for each test will be shown below with some pictures that illustrate
clearly the problems occurred in the joints during the tests.
5.6.1 RCT5 Specimen
The test conducted on specimen RCT5 is used as test control and reference for
the other tests of the same category. As illustrated in Figure 5.30, the behaviour of
this specimen relies only on the resistance and ductility of a reinforced concrete
section. Once again, the curves above show the classical aspects of a RC column:
regular stiffness and resistance up to a 100÷110mm = 6ey displacement from
which the column loses very fast its mechanical properties (from the higher value
of ≈110kN to the lower of ≈70kN) anticipating the final failure at the beginning of
the first 8ey cycle. The column collapsed in a brittle way in the closeness of the
connection with some cracks and spalling of the concrete at the joint panel. The
joint region was rather deteriorated, as showed in the following Figure 5.31.
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