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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5. SEISMIC BEHAVIOUR OF RC COLUMNS EMBEDDING STEEL PROFILES<br />
<strong>the</strong>refore, <strong>the</strong> composite connection <strong>and</strong> concrete column design strength should<br />
not be less than forces developed by plastic hinging <strong>of</strong> <strong>the</strong> steel beam adjacent to<br />
<strong>the</strong> connection.<br />
Connection design strength is obtained reducing <strong>the</strong> nominal strength by a<br />
resistance factor φ. Since a large data base does not exists for statistical analysis<br />
<strong>of</strong> joint strength, <strong>the</strong> resistance factor is determined by calibration with relative<br />
factor <strong>of</strong> safety in <strong>the</strong> allowable stress specification for steel structures (AISC<br />
Specification, 1997). It practically corresponds to <strong>the</strong> inverse <strong>of</strong> <strong>the</strong> partial safety<br />
factor γM0 used in European practice (where φ=0.9 e γM0=1.1).<br />
In <strong>the</strong> presented formula <strong>the</strong> resistance factor φ proposed by <strong>the</strong> AISC<br />
Specification (1997) is not directly take into account. The value <strong>of</strong> 0.7, that<br />
multiplies <strong>the</strong> formula, is based on <strong>the</strong> following assumption:<br />
• The connections should have a greater reliability index (or factor <strong>of</strong> safety) than<br />
structural members. Allowable stress values reflect a factor <strong>of</strong> safety for<br />
connection that is 1.3 times greater than that for structural members;<br />
• When <strong>the</strong> column axial force is not negligible, <strong>the</strong> shear resistance <strong>of</strong> <strong>the</strong><br />
column web should be computed, by adopting <strong>the</strong> Hencky-Von Mises yield<br />
criterion, by means <strong>of</strong> <strong>the</strong> following relationship:<br />
1<br />
σ<br />
2<br />
f<br />
Sd , col<br />
− ⋅ ( 5.21 )<br />
yd , cw<br />
f yd , wc<br />
where σSd,col is <strong>the</strong> average normal stress in <strong>the</strong> panel zone. In <strong>the</strong> current Draft<br />
<strong>of</strong> EC3 (2000), <strong>the</strong> influence <strong>of</strong> <strong>the</strong> normal stress σSd,col due <strong>the</strong> column axial<br />
force, is approximately accounted for by means <strong>of</strong> a reduction coefficient equal<br />
to 0.9. In particular, <strong>the</strong> codified value <strong>of</strong> <strong>the</strong> reduction factor is on <strong>the</strong> safe side<br />
up to a column axial load equal to 45% <strong>of</strong> <strong>the</strong> column squash load. The use <strong>of</strong><br />
<strong>the</strong> above equations, i.e. <strong>the</strong> use <strong>of</strong> a first yield criterion for evaluating <strong>the</strong><br />
panel zone shear resistance, should be preferred when <strong>the</strong> effect <strong>of</strong> <strong>the</strong> panel<br />
zone deformation on frame stability is not considered in <strong>the</strong> structural analysis<br />
(Liew <strong>and</strong> Chen, 1994) or when is desired to limit <strong>the</strong> panel zone <strong>behaviour</strong> to<br />
<strong>the</strong> elastic range (Mazzolani <strong>and</strong> Piluso, 1996).<br />
Thus, it is simple to obtain that 0.7 = 0.9 / 1.3.<br />
For <strong>the</strong> calculation <strong>of</strong> <strong>the</strong> shear area in <strong>the</strong> web panel zone <strong>the</strong> minimum value <strong>of</strong><br />
two different contributions is take into account. Shear area Av proposed by EC3<br />
that may be taken as:<br />
( )<br />
Av = As , tot − 2⋅ bcf ⋅ tcf + tcw + 2⋅<br />
r ⋅ tcf<br />
( 5.22 )<br />
199