The response of Lightly Reinforced Concrete Beams when subjected to different dynamic loading rates

Abstract

After the Canterbury earthquakes, engineers noticed similar cases of failure in low-rise reinforced concrete structures, where it was observed that only a few large cracks initiated in the structural members, which differs from laboratory results (fine distributed cracks across span). It was speculated that several aspects could have led to this discrepancy in results, the first could be the bond strength between steel reinforcement and concrete, where it was ventured that a high bond strength could prevent the even distribution of strains across the steel which could lead to concentrated strain and thus large cracking; secondly, the loading history that was subjected to these structural members are unknown, as not much detailed information was recorded on whether the later observed major flexural cracks were caused by an accumulation of loadings histories applied over time (first being the Darfield main shock in February, then in June and finally in December; thirdly, is whether the rate of loading (fast seismic vibrations) in most of these lightly reinforced concrete buildings could have contributed to the observed phenomenon. This thesis focuses on investigations into the behavior of reinforced concrete beams under the application of varying high strain-rate cyclic loadings. The specimens used for the experiment consisted of three nominally identical lightly reinforced concrete beams of dimensions 250 mm width and 400 mm depth with a span of 4.6 m long designed according to NZS3101:2006 Concrete Structures Standard. Results obtained indicated that the faster rates of loading does indeed have a tendency to lead to fewer but larger cracks in the lightly reinforced concrete beams near the column face, a possible justification for this could be that the strains induced by the high rate of loading does not have enough time to properly distribute along the longitudinal reinforcements within the beam, thus causing the high concentration of strain at a particular point to cause this type of large flexural cracking.