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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
3.7.2 FE Models of the CJ specimens
Non-linear FE analyses of the tested Complete Joints were carried out both by 2D
and 3D models in a monotonic loading regime. As a matter of fact, 3D finite
element analyses of bolted connections are very demanding from a computational
standpoint. Hence, 2D models endowed with eight-node CPS8 plane stress
elements were adopted to reduce the computational expense. For completeness
some analyses have been repeated with 3D FE models, as 2D models tend to
average stresses along the joint width.
In a way similar to the models used for the Isolated Tee Stubs, 2D models of the
CJ specimens exploited FE layers to reproduce the end plate and additional FE
layers to simulate the bolt shank. Both the specimens endowed an end plate with
flange thickness of 12 mm and 18 mm are modelled. The models include details
such as boltholes and bolts; surface-to-surface contact elements are used to model
the surface interaction, neglecting friction, which has a negligible effect on the joint
response (Bursi and Jaspart, 1998). Moreover, constraint equations are introduced
to make the bolt heads continuous with the end plate. Bolt pre-tensioning is applied
by prescribed displacements at the end of the bolt shank, in order to entail a final
average shank stretch equal to 0.065 mm, similar to that detected during testing.
These displacements are held constant throughout the loading. Welding-induced
residual stresses develop unavoidably in the welds and in the base metal owing to
thermo elasto-plastic deformation. Therefore, an idealised stress distribution as the
one highlighted in Figure 3.28 has been introduced. One more time, at the weld toe
where stress concentrations are expected, a refined FE mesh is adopted. A crack
endowed with a length of 0.26 mm (small-to-moderate root 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. The model is shown in Figure 3.33 and represents the JA1-2 CJ
specimen, endowed with a 12 mm end plate thickness coupled with the column of
HEA180 profile.
Keeping the above-mentioned characteristics of the 2D model, a very complex 3D
model of the specimen was performed in order to investigate the real stress and
strain distribution along the width of the joint. With a view to confirming the
hypotheses of 2D analyses, one of the main objectives that it was be obtained with
the 3D model was increased accuracy in the computed local stress-strain state of
the beam flange region due to weld residual stress and high gradients of stresses,
where the connection has the highest fracture potential. The realised model is
reported in Figure 3.34 and it represents the JB1-3 CJ specimen, endowed with an
18 mm end plate thickness coupled with the column of HEB180 profile. It is
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