Nowadays, considering the diversity in construction, especially in tall and irregular buildings, the use of time-history analyses has become essential for designers. For this purpose, it is necessary to refer to available accelerogram databases and select those matching the site conditions. Accelerograms from near-field, mid-field, and far-field zones have distinct frequency content. Additionally, site conditions significantly affect the amplitude and period of seismic cycles. Another critical issue is the duration of earthquakes and their varying effects on structures. All accelerograms available in this database, caused by earthquakes with moment magnitudes ranging from 5 to 7.5, were collected and categorized based on distance into three groups: near-field (less than 20 km), mid-field (20 to 50 km), and far-field (more than 50 km). Subsequently, the records were classified according to the shear wave velocity of the site for each accelerogram and the standard soil classifications of the Iranian Standard 2800 code. Given the considerable number of records in various distance-soil type categories, proper selection while observing dispersion criteria is essential.
In this research, efforts were made to select, within each distance category, a representative sample covering all seismic magnitudes from 5 and above. Additionally, an attempt was made to cover the range of shear wave velocity within each category, from its lower to higher values. This process also accounted for variability in both distance and seismic duration. The distribution pattern based on each specified criterion is well illustrated in this study. Moreover, in the classification of accelerograms, factors such as earthquake focal depth were also examined. Focal depth is a key parameter in determining the intensity and distribution of seismic energy, which is evident when comparing the response spectra of accelerograms with the standard spectrum. Although deeper earthquakes typically transmit less energy to the surface, the frequency distribution of this energy can have diverse effects on structures. In this study, various earthquake acceleration records were extracted and analyzed from the database of the Earthquake Engineering Research Center at the University of California. The records were then categorized based on various criteria, including affected zone, magnitude diversity, distance, site soil conditions, and earthquake duration.
The database of this study includes 104 accelerograms for near-field earthquakes, 128 accelerograms for mid-field earthquakes, and 149 accelerograms for far-field earthquakes, whose distributions are presented based on different criteria. The response spectra of the selected accelerograms presented in this research were compared with the standard spectrum of the Iranian Standard 2800 code (fourth edition) for each distance zone and soil type. Some accelerograms showed significantly higher acceleration than Iran’s Standard 2800, while others showed lower values. This variation depends on the seismic potential of the source near the site, evaluated through hazard analysis. Additionally, the impact of the causative fault type and its kinematic characteristics were considered in this comparison.
This study presented the uncorrected accelerograms to provide a clearer representation of actual earthquakes occurring in different zones (near-field, mid-field, and far-field). However, in practice, scaling must be performed for seismic design as per regulations. As the distance from the seismic source increases in mid- and far-field zones, the acceleration reaching the site generally decreases, and the Iranian Standard 2800 acceleration spectrum provides more conservative values.
On the other hand, it was shown that as the soil becomes softer and the average shear wave velocity decreases, there is a risk of spectral acceleration amplification at longer periods. Therefore, specific structures with long dominant periods must undergo detailed time-history analyses using accelerograms corresponding to the relevant distance zones and site conditions to prevent potential damage. Furthermore, it was suggested that supplementary analyses be conducted to evaluate the sensitivity of structures to variations in input parameters.
Finally, the accelerogram tables presented in this study significantly assist structural designers in maintaining the necessary diversity when selecting accelerograms for time-history analyses. This ensures that the designed structure has sufficient resistance against various earthquakes and substantially reduces errors in accelerogram selection. Additionally, this method guarantees enhanced seismic design accuracy and minimizes the likelihood of structural damage caused by different earthquakes.