Modal Identification of Different RC Frames Using Experimental Measurements

The main purpose of this study is to determine dynamic characteristics, such as natural frequencies, mode shapes and damping ratios of reinforced concrete (RC) frames with low and normal strength concrete and compare them with each other using the operational modal analyses method under ambient vibration. For this purpose, full-scaled, one bay, and one-story RC frames with low and normal strength concrete are produced. The RC frames are vibrated by natural excitations with small impact effects, and the response signals are recorded using accelerometers during the tests. Measurement time-frequency span and effective mode number are determined by considering similar studies and literature. To obtain experimental dynamic characteristics, enhanced frequency domain decomposition and stochastic subspace identification methods are evaluated together. Results of ambient vibration tests show that dynamic characteristics change significantly depending on the material properties. It is shown that the ambient vibration measurements are enough to identify the most significant modes of RC frames as well. The first five natural frequencies are obtained experimentally for the RC frame with low and normal strength concrete between 14.99-155.20 Hz and 16.64-179.20 Hz, respectively. Dynamic characteristics should be verified using finite element analysis. Finally, inconsistency between experimental and analytical dynamic characteristics should be minimized by finite element model updating using some uncertain parameters such as material properties, boundary condition, and section properties to reflect the current behavior of the RC frames.