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Image analysis techniques have been applied to measure the displacements, strain field, and crack distribution of structures in the laboratory environment, and present strong potential for use in structural health monitoring applications. Compared with accelerometers, image analysis is good at monitoring area-based responses, such as crack patterns at critical regions of reinforced concrete (RC) structures. While the quantitative relationship between cracks and structural damage depends on many factors, cracks need to be detected and quantified in an automatic manner for further investigation into structural health monitoring. This work proposes a damage-indexing method by integrating an image-based crack measurement method and a crack quantification method. The image-based crack measurement method identifies cracks locations, opening widths, and orientations. Fractal dimension analysis gives the flexural cracks and shear cracks an overall damage index ranging between 0 and 1. According to the orientations of the cracks analyzed by image analysis, the cracks can be classified as either shear or flexural, and the overall damage index can be separated into shear and flexural damage indices. These damage indices not only quantify the damage of an RC structure, but also the contents of shear and flexural failures. While the engineering significance of the damage indices is structure dependent, when the damage indexing method is used for structural health monitoring, the damage indices safety thresholds can further be defined based on the structure type under consideration. Finally, this paper demonstrates this method by using the results of two experiments on RC tubular containment vessel structures.

A reduced-scale, planar, two-story by two-bay reinforced concrete frame with weak beam-column joints was subjected to earthquake simulations on a shaking table. The beam-column joints did not contain transverse reinforcement, as is typical in older construction designed without attention to detail for ductile response. A series of linear and nonlinear analytical models of the frame were developed in accordance with American Society of Civil Engineers standards and subjected to the input base motions. The goodness of fit between analytical and measured results depended on the details of the analytical model. Reasonably accurate reproduction of the measured response was obtained only by modeling the inelastic responses of both columns and beam-column joints. The results confirm the importance of modeling nonlinear joint behavior in older concrete buildings with deficient beam-column joints.

Four reinforced concrete frame specimens were tested on a shaking table with the objective to observe the interaction of structural elements at the onset of collapse. Each specimen consisted of a 1/2.25 scaled model of a two-bay, two-story reinforced concrete frame without seismic detailing. The specimens were tested under moderate and high column axial loads to investigate the influence of axial load on the collapse vulnerability of nonductile columns in a structure. These tests enable study of the interaction of the beams, columns, and joints as collapse is initiated. This paper presents the observations during the tests and comparison of response of the specimens. In particular, the test results suggest that collapse of a concrete frame due to failure of unconfined beam-column joints is less likely compared with collapse due to failure of nonductile columns.