Effects of elastic supports and flexural cracking on low and high order modal properties of a reinforced concrete girder

A number of nondestructive evaluation (NDE) methods are available to detect cracks and damage in reinforced concrete structures. Methods such as ultrasonic, impact echo, or X-ray testing have high resolution but are time consuming to perform and therefore only used locally. Since the early 1970s, researchers have also studied the modal properties of structures for damage detection and identification. In this paper, we discuss the effects of elastic supports and flexural cracking on the dynamic response of a large-scale laboratory reinforced concrete girder. For this purpose, an instrumented hammer used for impulse response testing was employed to initiate vibrations and to subsequently estimate the natural frequencies and modes of a large-scale laboratory concrete girder. Natural frequencies up to the 13th mode were successfully extracted from the frequency response function (FRE). In addition, the mode shapes were extracted up to the 6th mode due to the limitation of the linearity of the FRF response. These modal properties were determined from the measured accelerations of two points on the girder using a roving hammer. The test procedure was performed on the uncracked beam and repeated after the beam had been cracked. The experimental measurements were verified with a finite element (FE) model consisting of one-dimensional beam elements that consider both flexural and shear deformations. Support flexibility was modeled as linear-elastic springs, and was found critical in order to explain experimentally-obtained measurements. The effects of cracking were found to be most obvious in the higher modes. Additionally, utilizing the modal flexibility method on the first mode (natural vibration frequency and mode shape) was an accurate predictor of flexural crack locations.