Generation of Asynchronous Seismic Signals Considering Different Knowledge Levels for Seismic Input and Soil

Abstract eng:
The seismic waves show a variation of the signal characteristics at different points during the motion through soils. This difference is not only due to the time lag among the signals but also to the change of the signal frequencies content for effects of physical phenomena associated to the wave propagation (reflection, filtering, etc.). Long structures as a bridge, have foundations on different distant points; the correct evaluation of the variation of the signals at each foundation point is much important to define correctly the design actions. However, the structural design codes do not consider satisfactorily this complex type of action and the time lag difference is often the only effect of the non-synchronism considered to define the design actions. In this paper, a more complete evaluation of the asynchronous action considering time lag and signal frequencies content variation is presented. In particular, the results of two generation procedures of non-synchronous signals at different surface points are discussed. The first procedure (PR1) generates directly signals at surface starting from the soil characteristics defined by Eurocode 8 and the recorded signals at two surface points for the same seismic event. The second procedure (PR2) produces surface signals by amplifying the bedrock signals obtained by a bedrock propagation. The bedrock propagation is performed starting from the signals obtained by means of a deconvolution process starting from the recorded surface signals used as inputs for the PR1 procedure. The soil characteristics are known well (shear wave velocity profiles and layer material shear moduli and damping curves) and deconvolution and amplification are performed by SHAKE91 [1]. The propagation of the signal at surface or at bedrock was performed by a procedure proposed by the authors and implemented in a computer code MATLAB (Gas) [8], [9]. The EW component of the main shock of the earthquake at L’Aquila (4-06-2009) in Italy was studied. The results of the comparisons between the different procedures are discussed in term of effect on the structure responses (acceleration response spectra) and characteristics of the generated signal (Fourier amplitude, coherences). The critical overview of these results helps to draw important considerations about reliability and safety level useful to the designer that has to define the correct actions for long structures.

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