Keywords: earthquake, spectra, real accelogram.

The earthquake ground shaking is usually presented in the form of a response spectrum of acceleration or displacement. The spectrum used as input to equivalent lateral force or spectral modal methods of analysis is usually obtained by scaling an elastic spectrum by factors that account for, amongst other phenomena, the influence of inelastic structural response. However there are problems in which the simulation of the structural response using a scaled elastic response spectrum is not considered appropriate, and full dynamic analysis is required. This is the case for either buildings designed for a high degree of ductility; structures with a configuration in plan or elevation that is highly irregular; structures for which higher modes are likely to be excited; or critical structures, the failure of which would cause unacceptable harm or disruption, such as dams, nuclear power plants etc. [1]

On the other hand there is open question if scaled elastic response spectrum are sufficient for regular buildings because as earthquake databases are being updated the spectra curves are subject to change as well.

There are three basic options available in terms of obtaining acceleration time-series: artificial earthquake generated by special software, synthetic accelograms and real records from earthquakes [1].

Common approach is to generate a power spectral density function from the smoothed response spectrum, and then to derive sinusoidal signals having random phase angles and amplitudes. The sinusoidal motions are then summed and an iterative procedure is conducted to improve the match with the target response spectrum [1].

Synthetic accelograms are generated from seismological source models and account for path and site effects. These models range from point source stochastic simulations through their extension to finite sources, to fully dynamic models of stress release. To generate synthetic accelograms the engineer will usually need a seismologist [1].

Real accelograms are free from all problems associated with artificial spectrum-compatible records. Real records are now easily accessible in large numbers. In recent years several strong motion databases are created and distributed on a CD-ROM. Several internet sites are created where users can search and download accelograms in digital form. This article will focus on the Internet Site for European Strong Motion Data (ISESD) and strong motion data used for structures in Croatia

The earthquakes to be analysed are selected from two basic criteria: magnitude of 6 or greater and peak horizontal acceleration of 2 m/s² or greater. The geographical regions chosen are Croatia, Montenegro, Yugoslavia and the Adriatic sea region (Flinn-Engdahl Region). From chosen earthquake accelograms spectra curves are computed. They are compared to spectra curve given in the Eurocode 8 regulations.

It is concluded that EC spectra is not sufficient when compared with acceleration spectra obtained from real earthquakes. When designing high ductility structures and highly irregular structures real accelograms should be used if sufficient data is available. For regular structures a new spectra curve is proposed.

1

J.J. Bommer, A.B. Acevedo, "The use of real accelograms as input for dynamic analysis", Journal of Earthquake Engineering Vol. 8, London, 2004. doi:10.1142/S1363246904001596

2

D. Cizmar, A. Nizic, D. Mestrovic, "Importance of dynamic characteristics of accelograms to structural response", SECED Conference, 2005.

3

A. Mihanovic, "Dynamics of structures", University of Split - Faculty of Civil Engineering, 1995.

4

SeismoSoft, "SeismoSignal - A computer program for signal processing of strong-motion data" [online]. Available from URL: www.seismosoft.com.

5

A. Nizic, "Seismic devices in bearing structures", M.Sc. Degree Thesis, University of Zagreb - Faculty of Civil Engineering, 2004.

6

E. Prelogovic, B. Aljinovich and S. Bahun, "New data on structural relationship in the north Dalmatian Dinaride area", Geologia Croatica, v. 48, no. 2, 1985.

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