Evaluation of Spectrum Correction Factor of Iranian Code No.2800 to Determine the Seismic Parameters of RC Moment-Resisting Frames in Near Field Earthquakes

Iran's Standard No. 2800 provides a code for the design of structures against earthquake loads. Due to the fact that mostly, far-field records have been used to prepare the seismic design spectra, in order to consider the destructive effects of near-field earthquakes in the 4th edition of Standard 2800, the incremental spectral correction coefficient (N) was introduced. In this paper, the accuracy and estimation of the value of this coefficient for five structures of special reinforced concrete moment-resisting frame with the number of floors from 3 to 15 are evaluated. Due to the fact that the increase in seismic requirements under the pulses of near-fault earthquakes is not the same for all seismic response parameters, so different correction coefficients can be used to estimate displacement response quantities and force quantities. For this purpose, first, the structures are statically analyzed according to the criteria of Iranian Standard 2800 and are designed according to the criteria of Article 9 of the National Regulations of Iran. Then the dimensions of beams, columns and rebars required by the structures are determined. After that the response spectrum of single-degree of freedom system to a set of records (including seven far-field records, and 22 near-field records) is calculated, and then by using incremental dynamic analysis, the seismic response of structures at different seismic intensities is calculated. By calculating the response ratio of structures under near-field records to far-field records, the value of the N-coefficient is calculated. Based on the results, the value of the N-coefficient of the standard spectrum of Standard 2800 is suitable for estimating the base shear demand of structures, but this coefficient is not accurate enough to estimate the need for lateral drift of structures. In general, the coefficients obtained from elastic and inelastic analyzes for the need for displacement in reinforced concrete flexural frame structures are higher than the values provided by the Standard 2800. This difference has reached 58% in some structures. It was also observed that there is no regular relationship between the 1st period of the structures and the magnitude of the spectral correction coefficient and the magnitude of the spectrum correction decreases with increasing seismic intensity.