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