Seismic Analysis of Structures: Improvements Due to Probabilistic Concepts - Dynamic Behavior of Concrete and Seismic Engineering - page 309

- on the other hand, the behavior law is not generally the perfect elasto-plastic
type (for example the strain-damping model in Figure 8.11);
- the nature of the source plays a part. As a matter of fact the model is based on
the assumption of a seismic signal consisting of rare and isolated impulses that have
the oscillator enter the plastic field. This is more or less achieved with wideband
signals, yet it is completely wrong for narrow-band signals (floor motions).
This latter point can be illustrated by taking different seismic motions and by
numerically calculating the reduction factors. Figure 8.13 shows their evolution
according to Z impulses (compared with the average impulse of the seismic signal)
for a given P value.
REMARKS:
1) Besides the factors discussed above, the inelastic spectrum technique assumes
that the highest impulse will cause collapse (notion of absolute maximum contained
in the ORS). This is not always the case. In fact, the Px e limit value can be reached
by combining several intrusions into the plastic domain. Vanmarcke has proposed a
calculation method (see [VAN 73]) that applies whenever such intrusions are rare
and isolated. It is based on the following ideas:
- the plastic threshold overstepping statistics are little modified by the non-linear
behavior. The considerations developed in the summaries about maximum statistics
can therefore apply (average overstepping number, Poisson model etc.);
- the plastic displacement during an excursion is calculated according to the
same hypotheses as for the inelastic spectra (ZW | 1 case).