Analytical study to evaluate the effect of higher modes of reinforced concrete moment

The response of flexible structures with long period to near-fault earthquakes shows an imposed demand on these structures which exceeds their capacity. Also, the relationship between frequency content of earthquake and the main frequency of structure is a significant parameter to the response of structure. Therefore, the sensitivity of the response of structure to period of pulse and the lack of enough records for near-fault earthquakes with different amounts of period of pulse is the most important challenge of structural analysis. Of all methods for this analysis, proposed model by Agraval was used in this study. To achieve this goal, various ratios of period of pulse to main period of structure (Tp/T1) were considered, where the effect of higher modes on estimating displacement demands was assessed. Meanwhile, the distribution of shear forces for 6-, 12-, and 24-story reinforced concrete moment frames with steel shear wall was evaluated. The results showed that maximum displacement and force demands were obtained for different structures with Tp/T1？=？1. Meanwhile, by increasing the number of stories, the effect of higher modes decreased and structures tended to fluctuate in first mode. Furthermore, the most effect of higher modes was obtained for shear force at the roof (Vroof) and then base shear force (Vbase), where displacement of roof (Uroof) did not have any effect on the period of models