Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 488

10.5.3 Response of Structures to Vertical Components
The effect of vertical components of ground motions on structures in near-source
areas (mainly produced by P waves) focused the attention in the last decades, on
the base of field observations concerning the damage which was produced by
severe vertical vibrations (Elnashai and Papazoglou, 1997, Elnashai et al, 2004).
The velocity in near-source field is very high. During the Northridge and Kobe
earthquakes, values of 175 cm/sec were recorded at the soil level. The velocity of
the vertical components dramatically increases near to the epicenter. So, in case of
near-source fields, the velocity is the most important parameter in design, replacing
the acceleration, which is the dominant parameter for far-source field.
Due to vertical components, the compressed RC columns are broken in an
explosive manner and the steel columns undergo brittle tensile failure (see
Northridge, Kobe and Greek earthquakes) (Carydis, 2005). As the vertical
components are rarely considered in the structural analysis, it follows that there is a
potential built-in deficiency in the majority of structures and their foundations to
resist vertical earthquake induced vibrations. Elnashai and Papazoglou (1997)
developed near-source spectra, with an important peak acceleration of 3.5 and
corner periods 0.05 and 0.20 sec. The frame buildings are much stiffer in the axial
than in the transverse direction and hence they possess very short periods in the
vertical direction, framing in the range of 0.10 to 0.40 sec, corresponding to the
maximum spectra amplification.
A relation between horizontal and vertical periods is presented in Figure 10.12.
This amplification, combined with the severity of near-source vertical ground
motions, suggests that large dynamic axial forces, acting both upwards and
downwards (Fig. 10.13), should be expected in near-source field. In case of RC
structures, the studies indicated that intermediate and top stories are more likely to
undergo tensile forces. The axial forces due to vertical motions, being caused by
accelerations whose magnitude is comparable with the one of horizontal motions,
are larger than the corresponding forces due to horizontal motion. The contribution
of vertical motion to axial forces in frames is ranging from 64 to 72% at the top
stories and from 21 to 56% at the ground stories, in case of 4 to 20-story buildings
(Papazoglou and Elnashai, 1996). Therefore, the contribution of vertical motions is
always more significant in the upper floors than in the lower floors. These forces
in the interior columns are even more significant, since the effect of horizontal
motion is less important there.
The modeling of vertical component as a velocity pulse (Fig. 10.14a) with very
short periods was performed by Gioncu and Mateescu (1998a). The artificial
spectra were determined for various pulse periods T p = 0.05; 0.10; 0.15; 0.20; 0.30
sec, and for symmetrical and asymmetrical pulse type (alpha = V max /V min ). Figure
10.14b shows the spectra for T p = 0.10 sec. It can be observed that the maximum
amplification occurs when the first half pulse is maximum.
A time-history analysis was performed for a steel structure (Fig 10.15) for an
asymmetric ground motion pulse (alpha = 1.6) and T p = 0.15 sec. The evolution of
plasticization in the columns was determined in function of the multiplier of the