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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
defect) was modelled at the weld toe in order to investigate the behaviour of
cracked connections. More specifically, cracking is studied through contour integral
evaluation in order to infer J values. Therefore, focused meshes are set to induce
the singularity at the crack tip and the crack propagation is not traced. Sharp
cracks have to embody singular strain fields at the crack tip for fracture mechanics
evaluations. Therefore, three nodes of the same side of eight-node isoparametric
elements (CPS8) have the same geometric location at the crack tip to produce a
1/r strain singularity field. The FE mesh used for the ITS connections is illustrated
in a. A detail of the mesh with a spider web configuration is reported in b where the
innermost ring of elements degenerate to triangles, as described above. As the
large-strain zone is very localised, sharp cracks adopted in the onset of cracking
method are modelled using small-strain assumptions, and therefore large
deformations are ignored. The FE analyses account for material nonlinearities
using the von Mises yield criterion. Isotropic hardening is assumed for the
analyses. In the analysis the measured stress-strain properties of the materials
obtained by tensile test were used. The elastic modulus and the Poisson’s ratio
were assumed as E=210000 and ν=0.3, respectively.
Convergence studies on elastic and inelastic models 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 experimental responses
with the predicted response.
The ITS connections endowed with end plate thickness of 12 mm and 18 mm,
subjected to monotonic and cyclic loading respectively, have been simulated. A
typical deformed configuration at failure is reported in Figure 3.30. It is possible to
observe two different meshes relevant to the bolt shank and to the plate around the
hole, respectively. Moreover, an attentive reader may observe how the crack with a
fixed length of 2.34 mm exhibits a width increase at the tip. As a matter of fact, the
nodes at the crack tip are untied in order to generate the singularity by means of
inelastic elements.
The reaction force vs. the controlled displacement relevant to the TM-2 specimen
is illustrated in Figure 3.31a, where the numerical simulations are compared to the
experimental response. One may observe that experimental data and numerical
prediction are in a good agreement. The numerical simulation relevant to the cyclic
regime of the ITS TC-2 connection and the experimental response are reported in
Figure 3.31b. Only a few significant displacement cycles are simulated in order to
reduce the computational effort. The specimen yield strength is well captured as
expected owing to the use of the isotropic strain-hardening model.
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