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

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