Seismic shear demands in columns of RC frames accounting for dynamic amplification effects

Abstract

This article presents a numerical study on the shear force demands suffered by columns of reinforced concrete (RC) frames subjected to seismic actions. The paper examines well established capacity-based design procedures which account for dynamic amplification effects via the dynamic magnification factor ων, which multiplies the static shear demands resulting from the development of the beam flexural strengths. It also reviews the prescriptions of the New Zealand Standard NZS3101, which requiresων = 1.3 for columns of intermediate storeys. Using the results of nonlinear dynamic analyses (NLDA) of a model representing one of the main directions of a 12-storey building structured with RC walls and special moment resisting frames designed per NCh433 and ACI318, the paper shows the calculated values of ων for different column types and storey level, as subjected to a set of 13 earthquake records. The results show that 1.0 < ων< 1.3 in the great majority of the cases, suggesting that the limit ων = 1.3 would be appropriate in this particular case. However, it cannot be considered a conservative upper bound in a conclusive way, because values as large as 1.6 were predicted in some of the cases. Acknowledging that no dynamic amplification effects are included in the calculation of the capacity-based shear demands for columns of frames according to ACI318 (current 2019 edition), it is proposed that the recommendation of NZS3101 (2006), which cover such effects, are included in the upcoming edition of the Chilean Standard NCh430.