PSI - Issue 11

Siro Casolo et al. / Procedia Structural Integrity 11 (2018) 20–27 Siro Casolo & Giuseppina Uva / Structural Integrity Procedia 00 (2018) 000–000

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26 September and 14 October, stations of Colfiorito and Nocera Umbra). A reliable scalar measure for sorting the destructive potential of the ground motions is an indispensable requirement to organize and discuss the results. Most of the design code standards refer to specific response spectra associated to each seismic area, and the main parameter assumed to define the intensity is often the ordinate value at the fundamental natural period of the structure under consideration. Nevertheless, damageable structures are sensitive to the spectral content over a wider range of periods and also to the time duration of the most intense phase of the shock. In fact, the accumulation of damage tends to cause a significant lengthening of the “effective period” of vibration, especially during the most intense phase of the loading history.

Figure 7. Summary of responses to the recorded sequence at Colfiorito (CLF). Left column plots refer to Rosario (ROS) façade with texture (1), for North-South (NS) component; right side refers to Transfigurazione (TRA) façade with texture (II), for West-East (WE) component.

In the present work, an estimate of this effective period of vibration is calculated on the basis of the “secant stiffness” of the heuristic springs, which accounts for the degradation of each spring. This is evaluated as the mean slope of the largest hysteretic cycle. For the definition of a scalar destructive potential for this typology we consider an extended spectrum of periods to obtain an indication with integral character. The SI defined by Housner (Housner, 1970) satisfactory responds to the previous observations, and it is also well known and widely adopted by the scientific community for these purposes. The analysis of the responses to the sequences of shocks, applied according to their actual chronology, has shown that some complex relations can be established between the main parameters that govern the problem (e.g. see Fig.7, Casolo, 2017).

Figure 8. Aftershock effect in a sequence of 2 shocks. Relation between damage increment (in terms of Δ T%) and spectral intensity (SI) of the aftershock. Left side: the fixed first shock is the WE component of the third shock of the sequence recorded at Forgaria-Cornino, with SI = 75.5 cm. Right side: the fixed first shock is the WE component of the first shock of the sequence recorded at Colfiorito, with SI = 86.3 cm. A tricky aspect regards the relationship between the shape of a ground motion spectrum and the effective periods of vibration of the examined structure, which tend to become longer for the effect of damage. An extreme situation, for example, is represented by the case of a strong aftershock having spectral content lumped in the short period range, which could induce a negligible increase of damage. Conversely, if the aftershock has a significant spectral content in the intermediate-long period range, the damage can grow significantly higher in comparison with a single shock