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

5. SEISMIC BEHAVIOUR OF RC COLUMNS EMBEDDING STEEL PROFILES
Elastic and inelastic convergence studies have been conducted to evaluate and
arrive at the final mesh for the finite element models. The finite element model was
verified by comparing the measured cyclic response of specimen with the predicted
monotonic (see Figure 5.40). As far as concerns the cyclic test, the envelope curve
of the experimental tests was found and compared to the curve found using the
finite element model. Even if the model is not able to capture the loss of resistance
and stiffness due to the crushing of the concrete after a certain level of the
imposed displacement, the experimental data are in good agreement with the
numerical simulation. The elaboration of the numerical evidences has led to the
modification of some geometrical and mechanical parameters related to the
formulae and it has permitted to better figure out the experimental response of the
joint subjected to shear loading. Also the measurements obtained from the local
instrumentation by means of strain gauges helped in the understanding of the
activation and evolution of the resistance mechanisms, proposed in Section 5.4,
while testing.
5.7.2 Steel web panel shear mechanism verification
By means of a set of elements extracted in the web panel zone of the steel profile,
it was possible to monitor the evolution of the shear stress σ13 at the increasing of
the imposed top displacement. As depicted in the following Table 5.16, and as
expected, the stress state of web panel increases during the test until it yields
uniformly for a top displacement equal to 6ey. From this point the concentration of
the stresses moves in the four corners of the steel web panel, where the stiffeners
are welded. In this moment the resistance of the panel zone in the steel profile is
given to the contribution of four plastic hinges that have been formed.
This hypothesis results confirmed analysing the experimental results obtained from
the strain gauges positioned in the web and in the flange of the steel profile in
correspondence of the stiffeners. The graphs below, deduced from the
experimental data (strain gauges), give explanation of the shear mechanisms
activated in the steel web, steel flanges and stiffeners. To obtain the shear strain
by the measurement the hypothesis that the principal stress σ1 and σ3 are aligned
with the axes of the steel column is made. As depicted in Figure 5.41, the
measured web strain increases uniformly up to a displacement equal to 4ey÷6ey
when it exceeds the steel yield strain. It is the central zone of the panel that
deforms most. It can be imagined the web stresses increasing during the test until
the panel zone yields uniformly for a top displacement equal to 6ey. From this point
on, concentration of stresses shifts to the four corners of the steel web panel in
correspondence of the stiffeners, and the resistance of the panel zone is given by
the contribution of these four local plastic hinges.
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