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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
3.7 Numerical analysis
In this section, the inelastic finite element (FE) analyses carried out by means of
the ABAQUS 5.8 code (Hibbitt, Karlsson & Sorensen Inc., 2001) both on isolated
Tee Stub (ITS) connections and on Complete Joints (CJ) are discussed. Therefore,
both FE models were calibrated and the stress and strain state of the
aforementioned connections was simulated both in the monotonic and in the cyclic
displacement regime. Finally, some design parameters which influence the fracture
resistance of steel bolted extended plate connections are commented upon. In
detail, the effects of the weld-to-base metal yield strength ratio, the end plate yield-
to-ultimate strength ratio, and the residual stress influence have been determined.
3.7.1 FE Models of the ITS connections
Non-linear FE analyses of the tested Isolated Tee-Stubs were carried out both in a
monotonic and in a cyclic loading regime. As a matter of fact, 3D finite element
analyses of bolted connections are very demanding from a computational
standpoint because contact problems as well as low-cycle fatigue phenomena
need to be simulated. Hence, 2D models endowed with eight-node CPS8 plane
stress elements were adopted to reduce the computational expense. Specimen
symmetry permitted the modelling of only one half of the specimen.
2D models exploited FE layers to reproduce the end plate and additional FE layers
to simulate the bolt shank. The pre-stressing condition was introduced in the model
imposing a stretching of the bolt shank, in order to entail a final average shank
stretch equal to 0.065 mm, similar to the one detected during testing.
Welding-induced residual stresses develop unavoidably in the welds and in the
base metal owing to thermo elasto-plastic deformation. Therefore, an idealised
stress magnitude/distribution as the one highlighted in Figure 3.28 has been
introduced. Such distribution of residual stresses, which takes into account the
presence of the crack, was proposed by Monahan on the basis of experimental
tests performed by Porter Goff and Payne (1985). More specifically, such
distribution assumes that part of the residual stresses is released as the low-cycle
fatigue crack penetrates the end plate, and that the remaining residual stress is
somehow redistributed throughout the uncracked ligaments. Clearly, tensile
stresses at the top and bottom end plate thickness are balanced by compression
stresses. Such distribution has been imposed to the mesh through several trials in
order to achieve equilibrium, compatibility and the proper stress magnitude
required.
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