Fast and Slow Cyclic Tests for Reinforced Concrete Columns with an Improved Axial Force Control

Cyclic tests have been widely used in earthquake engineering to experimentally evaluate the strength and postyield response of structures. It is well known that the lateral response of a structure can be significantly affected by the presence of axial load; thus, axial loads need to be correctly applied during tests to ensure the accuracy of test results. In the past, it was a challenging task to satisfy the axial force boundary condition in real time for axially stiff members. Therefore, the majority of cyclic tests were conducted slowly over an expanded time scale, which did not account for the rate-dependent behavior of structures under dynamic loads. This paper introduces a flexible loading frame (FLF) to enable large-capacity real-time axial force application to axially stiff members. Force application using the FLF was validated by conducting fast and slow cyclic tests for small-scale reinforced concrete columns. The RC columns were controlled to have the same lateral displacement path and axial load during the fast and slow cyclic tests, in which a constant target axial force was successfully applied to the RC columns using the FLF. Force-deformation relationships were obtained for each RC column, and notable differences in strength and damage patterns were observed between fast and slow cyclic tests, clearly demonstrating the importance of conducting real-time tests to better understand the true dynamic behavior of structures.