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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
amplitude testing. The different loading histories considered are built up to
appraise the influence on the cyclic joint response, and in particular on the damage
evolution, of different sequences of cycle amplitudes, simulating some features of
the seismic input. Tests were carried out quasi-statically. Therefore, an increase in
the ductility capacity compared to equivalent specimens loaded dynamically would
be expected, considering that fracture toughness of steel decreases with the strain
rate growth. However, tests on welded beam-to-column connections performed by
Suita et al. (1998) pointed out the equivalence between quasi-static and dynamic
tests with regard to ductility. Moreover, the strength as well as the absorbed energy
is larger for dynamic loading than for quasi-static loading confirming that quasi-
static test procedures lead to a conservative appraisal of these key parameters.
3.5 Main results and preliminary seismic assessment
The results of the study are briefly presented in this section, focused on the
experimental outcomes and their first assessment in the perspective of seismic
design. The three sets of tests are here considered separately. The significance of
these results in view of the validation of the component method is dealt with in
Section 3.6.
3.5.1 Isolated Tee stubs
The applied load versus the upward displacement curves of the Tee stub web
represent an important overall indicator of the specimen’s behaviour to be
associated with the failure mode in order to understand the influence of the
parameters investigated, i.e., the plate thickness, the bolt diameter and their ratio.
Figure 3.10 illustrates a typical Tee stub response with reference to the case of 12
mm end plate thickness t and 20 mm diameter bolts. Both the monotonic and cyclic
responses are plotted, showing that the envelope of the latter one lies very close to
the monotonic curve for a first significant portion of the loading process.
Progressive strength deterioration, associated to the increase of end plate plastic
deformations and damage at the weld toe, is then occurring. As illustrated in Figure
3.11, at the ultimate limit state the TM-2 specimen was characterized by a collapse
mechanism with four yield lines located at the bolt-holes and at the weld toes in
accordance with Mode 1 failure (Eurocode 3, 2001). The corresponding specimen
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