| چکیده انگلیسی مقاله |
Near-fault earthquakes have different characteristics comparing to far-field recorded events, they often contain strong coherent dynamic long period pulses and permanent ground displacements. In the recent years, the effects of near-field and far-field earthquakes have been studied separately in various building codes. Building codes have been revised and updated depending on the improvements in the representation of ground motions, soils and structures. These revisions have been more frequently seen in recent years. One of the key changes in earthquake codes has been performed on the design spectra. Design spectra are used in seismic analysis methods such as equivalent static lateral force analysis, dynamic spectral analysis and time history dynamic analysis. Various seismological and geophysical parameters affect the shape of design spectra. Ambraseys et al. (1996) and Bommer and Acevedo (2004) presented and discussed the effects of earthquake magnitude, source-to-site distance, site classification, and style-of-faulting on the strong-motion accelerograms and consequently design spectra. The present study shows the results of 214 accelerograms, obtained from shallow crustal earthquakes with epicenral distance less than 80 km from causative fault, to determine the design spectrum in Iran. A comparison is performed with the design spectrum of Iranian building code (Standard No. 2800). So far, due to the lack of information and registered accelerograms in Iran, as well as the unclear ground conditions at the place of record registration, no effective action has been taken to determine the design spectrum in Iran. The site conditions have been classified into different categories in earthquake codes. These categories are named ground types, soil profile types, local site classes or subsoil classes. In Iranian Seismic Building Code, four types of soils I, II, III, IV have been determined, Site Class I: Rock, or hard Rock, or thin alluvium on bed rock with Vs30 more than 750 m/s; Site Class II, very dense soil or soft rock with high thickness on rock bed with Vs30 between 375 and 750 m/s; Site Class III stiff soil or soils with medium density or hardness with Vs30 of 175 to 375 meters per second; and Site Class IV, soft soils with a Vs30 of less than 175 meters per second. The accelerograms, obtained from major earthquakes in Iran, United States and Europe have been collected and then processed. After calculating the response spectra, the design spectra was plotted from the average response spectra values for all soil types, (I, II, III, IV) presented in the Standard No. 2800. To compare the results of this study with spectra presented in Iranian Seismic Building Code, records obtained from event having magnitudes greater than 5.5 and recorded in the epicentral distances less than 80 km were selected and 5% damping was used for calculations. The results show that design spectrum for soil classes I, II, and III, were consistent with Standard No. 2800 for the spectral values of Periods 0.0 to 0.39 seconds, while in the periods longer than 0.39 seconds, the design spectrum of the Standard No. 2800 is more conservative than the present design spectra. Meanwhile, the spectral ordinates for design spectra of vertical component for soil class IV were representative for higher values for the present spectra, comparing to that presented in Standard No. 2800. This might be related to epistemic uncertainty imposed by still few available records for soft soil site (class IV). |