عنوان مقاله [English]
Dynamic soil characteristics such as natural frequency, damping ratio, and shear wave velocity have important effects on seismological structures response. Earthquake codes have been used to determine frequency or damping ratio that are suggested by analytical methods and empirical amount, while due to the influence of several factors such as designing style, type of structure, construction materials and site conditions, the existing structures have different response to earthquake. Therefore, the proposed dynamic properties are not sufficiently reliable yet. In order to recognise the natural frequency and damping ratio, experimental approach can be used to evaluate the natural frequency of buildings by using the seismic records of existing buildings. The soil layer specifications are recognized by different methods, including geotechnical and geophysical as well as spectral ratio. A method that can be used to determine the structural response, and estimate the natural frequency from ambient records, is the horizontal-to-vertical spectral ratio of microtremor (H/V). This technic has been revealed by Nakamura (1989), a practical method in site characterization studies due to its low cost and its functionality.
This paper has presented a method to estimate the variation of structure’s natural frequency with stories that changes, using the spectral ratio of microtremor H/V. The study was carried out to investigate the influence of building’s age and performance on frequency changes. Accordingly, the natural frequency values obtained from the two methods of numerical analysis of structures and microtremor analysis have been compared. As a matter of fact, 12 buildings including 6 new constructed and 6 old ones in Shiraz were selected. The buildings included from 1- and 2-story masonry buildings up to 12-story framed structures. The new buildings are designed or retrofitted by new standards such as 3rd or 4th edition of Iranian Code of Practice for Resistant Design of Buildings (Standard No 2800). The old buildings have been designed and constructed by oldest earthquake codes and have more than 30 years of age. In order to evaluate the dynamic characteristics of the buildings, two methods were used. First, the analytical results obtained from ETABS software analysis, to find the dynamic properties of structures. Therefore, the architectural and structural map of these buildings was prepared and the structures were numerically analyzed again. Secondly, the empirical data resulted from H/V spectral ratio of microtremor records. Microtremor measurement were performed on the floors and roofs of the building. Microtremor H/V spectral ratios were obtained according to the SESAME H/V user guidelines. Finally, the relationship formula resulting from these two techniques were compared in terms of fundamental frequency, amplitude, exhibiting agreements and disagreements. In both methods, the correlation formula between the resonant frequency and the number of stories of buildings were compared and the effect of age, construction methods, materials and standards codes on natural frequency were evaluated. The results show that the spectral ratio method of microtremor H/V can be a useful approach for real analysis of seismic response of buildings. In addition, the numerical analysis is conservative compared to the field measurement methods. Also, the frequency variation of the building follows the power functions as the stories increased. The age of structure and changes in the earthquake standards code cause about 10% difference in the frequencies value. In addition, comparison of the formula correlation obtained from numerical analyzes with the values resulted from microtremor H/V spectral ratio show that the proposed relationships of the analytic methods are more unreliable from the values of ambient vibrations data. Finally, the results show that the properties of the soil layers, the material of the building, the regular and irregular effect of the building and the site effect have a significant influence on the frequency variations.