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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
On the other hand, the corresponding strain distributions highlighted in Figure 3.45
indicate that the end plate yield zone is smaller for the specimen labelled ITS2-90.
Therefore, a balance between the two opposite requirements has to be found.
(L1)
(L2)
Figure 3.45. Equivalent plastic strain (PEEQ) distributions of TM-2 specimens at two levels
of applied displacements: (a) σy/σu=0.45 (ITS2-45); (b) σy/σu=0.90 (ITS2-90)
As far as the ITS connections with 18 mm thickness is concerned, two strength
ratios σy/σu for the end plate material equal to 0.60 and 0.90 have been considered
and the corresponding specimens are called ITS3-60 and ITS3-90, respectively.
Again, to take into account that the crack-initiation threshold increases with a
strain-hardening reduction, two crack lengths equal to 2.34 mm and 4.95 mm have
been chosen for the specimen ITS3-60 and ITS3-90, respectively. Similar results
are obtained, as shown in Figure 3.46.
So far we have estimated critical states both in 2D and 3D models through the use
of a critical value of the CTOD, which amounts to 0.31 mm for the joints under
examination. It is possible to define critical stress states through the use of
response indices, which do not require a crack modelling. One of these indices is
the Rupture Index related to the stress triaxiality ratio σm/σeff, where σm defines the
hydrostatic stress and σeff is the von Mises stress. Several FE analyses were
conducted to estimate this index at the weld toe. 2D simulations confirm that end
plates with σy/σu = 0.6 exhibit a favourable value of the triaxiality ratio of about
0.49, implying a ductile behaviour. The corresponding 3D analyses yield a
maximum triaxiality ratio sampled at the weld toe in the middle of the end plate of
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