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

4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS
from the linear distribution point of view. Moreover, the third shape, calculated as
SRSS combinations of the load distributions is obtained from modal analyses of
the buildings. The choice of this load shape is made to take into consideration the
anticipated effect of higher modes of vibrations for moderate long period and
irregular structures, as well as for buildings with hybrid lateral resistance systems.
Another difference between the two approaches, i.e. the EC8 procedure and the
FEMA-273 procedure, is the evaluation of the target displacement. Following the
indication contained in the EC8 (2002), the capacity curve, which represents the
relation between base shear force and control node displacement, is determined
by pushover analysis for values of the control displacement ranging between zero
and the value corresponding to 150% of the target displacement. The target
displacement is defined as the seismic demand derived from the elastic response
spectrum in terms of the displacement of an equivalent SDoF system. The
procedure presented in the Annex B of the EC8 (2002) is articulated as follows.
i. The MDoF system is firstly converted into an SDoF system. The following
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relation between normalized lateral forces Fi and normalized displacements Φi
is assumed:
Fi = mi
Φ i
( 4.28 )
where mi is the mass in the i-th storey. Displacements are normalized in such a
way that Φn=1, where n is the control node (usually, n denotes the roof level),
so that Fn = mn
. The mass of an equivalent SDoF system m * is determined as:
*
m = m Φ = F
( 4.29 )
i i i
and the transformation factor is given by:
Γ =
*
m
m Φ
i
2
i
=
F
i
F
m
2
i
i
( 4.30 )
The force F * and the displacement d * of the equivalent SDoF system are then
computed as:
=
Γ
b F
F *
,
=
Γ
n d
d *
( 4.31 )