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

3. SEISMIC BEHAVIOUR OF BOLTED END PLATE BEAM-TO-COLUMN STEEL JOINTS
as part of different specimen are noticeable. However, it has to be noted that the
initial stiffness is very sensitive to boundary conditions and lack of fit. Table 3.6
also gathers the elastic stiffness Ke,code computed in accordance with Eurocode 3
(2001). The stiffness ratios Ke/Ke,code lie in a wide range, with the level of accuracy
of the EC3 models being generally unsatisfactory.
With regard to the experimental plastic failure strength Fp, defined in accordance
with the tri-linear approximation of the response envelope, the differences for the
same component in different specimens are more limited than the ones for the
elastic stiffness. However, they are still remarkable. Such differences are a
consequence of the interaction, which affects the location and evolution of plastic
zones. With regard to the Eurocode 3 prediction model, the plastic strength (Fp,code)
was computed using measured material properties and no resistance factors.
These strength values are also collected in Table 3.6. The Eurocode
underestimates significantly the strength of thin end plates, while it tends to
overestimate, even remarkably, most of the other components. This seems to
depend mainly on the coupling effects among different components, which is not
considered in the code. Moreover, non-seismic codes do not consider the
development of low cycle fatigue phenomena, which lead to initiation and
propagation of cracks, and affect the yield lines sequence.
A further comparison relevant to the absorbed energy was limited to the 6th
elemental component column flange in bending and to the 7th elemental
component end plate in bending, which dissipate most of the energy within the
connection. The comparison is performed with reference to the relation between
the mean energy ratio and the displacement ductility in the i-th cycle (ECCS,
1986).
The elemental component column flange in bending for component parts and joints
embodying a thin (12mm) extended end plate showed similar mean energy ratios
approaching values of about 0.7 in the coupled Tee stubs and in the complete
joints, but ultimate positive partial ductilities e +
u/e +
y vary between 13 and 23, with
the higher value related to the coupled Tee stubs. With reference to the 7th
component, the ultimate partial ductility ranges from 9 (Complete Joint) to 13
(Isolated Tee Stub), while the mean energy ratios vary between 0.6 (Isolated Tee
Stub) to 0.8 (Coupled Tee stub). A similar comparison for the 6th and 7th
elemental components in joints embodying thick extended end plate of t =18 mm
shows even greater differences of ultimate displacement ductility factors e +
72
u/e +
y
and maximum values of the mean energy ratios. An evaluation of these results
leads to consider the component method not sufficiently accurate for approximating
the cyclic response, at least for the joint configuration considered in the study. The
extension of this model in seismic analysis does not seem straightforward, when