Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 514

composed by three fields, the rise time (10 sec), the level time (10 sec) and the
decay time (40 sec). One of the maximum durations of 240 sec was composed by
the rise portion of 42 sec, the level time of 48 sec, and the decay time of 150 sec.
It is understandable that only a part of this envelope, classified as significant
duration , is important for the structure behavior, but there is not a very clear
definition of the duration from the structural point of view (Bommer and Martinez-
Pereira, 1996).The simplest definition is the bracketed duration which is the total
time elapsed between the first and last excursion of a given level of acceleration
(0.3 to 0.5g). A more advanced definition considers the accumulation of seismic
energy.
10.7.2 Response of Structures to Intraslab Deep Earthquakes
The frame of Figure 10.33a has been analyzed, by using the incremental dynamic
analysis, based on the time-history DRAIN 2D (1975) computer program, for the
accelerogram recorded at Bucharest-INCERC in 1977 (Fig. 10.33b) (a g = 0.24g).
The corner period is very large (T p = 1.6 sec), much larger than the first vibration
mode of the frame (T= 0.74 sec). The obtained lateral displacements are shown in
Figure 10.34. One can see that the displacements are dominated by the first
vibration mode due to the difference between acceleration pulse period and the first
vibration mode of the structure. Figure 10.35 shows the formation of plastic hinges
and global mechanism, formed at 032 g, corresponding to 1.33 times the design
acceleration.
10.7.3 Problems of Structural Response Control for Long Duration
Earthquakes
The structural design demands for ground motions produced by intraslab
earthquakes are very special and different from the ones of other earthquake types.
Due to the deep fault position, the near-source effects are reduced, but the affected
area around the epicenter is very large. In addition, there are some special aspects,
which must be considered in the design practice: great influence of traveling paths,
site soil conditions and, especially, the long duration of ground motions, associated
to important numbers of large yield cycles (minimum 15-20).
In case of long duration earthquakes, the effect of cyclic actions on the ductility
of structural members becomes the main factor (Anastasiadis et al, 2000). The
limitation of local ductility of members is given by the plastic buckling of flanges
(for steel structures, see Fig. 10.36a), or by the crushing of compressed concrete
(for reinforced concrete structures). The result is the reduction of the rotation
capacity, after some cycles, due to the accumulation of plastic deformations (Fig.
10.36b).
Examining the acceleration envelopes of Figure 10.37, one can see that there
are two different situations: (i) Local plastic buckling occurs in the field of
increasing of accelerations (curve 1), when the amplitude continues to increase
after the local buckling (Figure 10.37a); (ii) Local plastic buckling occurs at the
maximum peak acceleration (curve 2), when the amplitude remains practically