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

5. SEISMIC BEHAVIOUR OF RC COLUMNS EMBEDDING STEEL PROFILES
and seismic loads, as depicted in Figure 5.18, it is possible to define the following
relations for the bending moments acting at the left and at the right of the joint.
Mg,left Mg,right
Ms,left
+ =
Ms,right
Figure 5.18. Gravitational and seismic action in an interior joint
left s, left g, left
Mp,left Mp,right - 2Mg,right
Ms,left + Mg,left
Ms,right - Mg,right
M = M + M
( 5.47 )
M = M − M
( 5.48 )
right s, right g, right
If it is assumed that during a seismic event the left bending moment of the beam
reaches the value of the bending plastic moment, as a consequence:
M = M + M
( 5.49 )
p, left s, left g, left
Moreover, if the same allowances for the beam bending moment acting on the right
side are made, we obtain:
M = M + M
( 5.50 )
p, right s, right g, right
Hence, combining Eqs (5.48) and (5.50), it is easy to obtain that the bending
moment acting on the right of the joint is equal to:
M = M − M = M − 2M
( 5.51 )
right s, right g , right p, right g, right
This equation underlines that the gravitational loads reduce the effects of the
seismic action in terms of bending moment acting at the joint. This beneficial effect
of the gravitational loads is not included in the Eurocode prescriptions.
Nevertheless, the AISC provisions (1997) approximately account for it by means of
a reduction coefficient equal to 0,8. Besides, it has to be noted that the column
shear reduces the total joint shear action at the beam-to-column joint position and
207