EXPERIMENTAL STUDY ON STRUCTURAL PERFORMANCE OF RC BEAM WITH OPENING AND LONGITUDINAL CUT OFF BARS

Abstract

We often construct reinforced concrete beams with openings for plumbing, and the longitudinal bars are spliced or cut off within the beams. As designing a building structure of reinforced concrete, we verify bond around cut off bars and, lap splices, and shear capacities around openings, each by each. Although these parts are weak points in the beam, we don’t pay much attention to the combined influence among them. If an opening is close to a discontinuous point generated by cutting off the bar or lap splicing, they may influence each other. On the other hand, in order to verify serviceability and repairability, allowable shear capacities for a RC beam with opening have been added to AIJ Standard for Structural Calculation of Reinforced Concrete Structure (2010). Because examination on the allowable shear capacities with the test data are limited, more data of RC beams with opening are required to make clear a relationship between cracks on the concrete surface and structural performance of the beams. In this study, we investigated the influences of longitudinal bars cut off and lap splice in a beam with opening. We prepared three RC beam specimens, and applied anti-symmetric bending in both positive and negative directions repeatedly and statically. The specimens were scaled down by about a half. Test parameters are the presence of openings and the presence of discontinuous points generated by cutting off bars or lap splicing. One specimen had two openings and the discontinuous points of bars, another had two openings but no discontinuous points of bars, the other had the discontinuous points of bars but no openings. The following conclusions were obtained from the experimental results.

(1)　The specimen without opening, after flexural yielding, failed in bond between longitudinal bars and concrete. The specimens with opening, after flexural yielding, failed in shear at the beam end region, not at the opening. That is because the inclined reinforcement transferred bond stress around the longitudinal bars.

(2)　The load-deformation curves of the beams with opening were better than that of the beam without opening because the embedded part of the inclined reinforcement contributed to the resistance to shear load.

(3)　It is considered that the margin of bond capacity on inner cut-off bar is on the safe side considerably because the beam failed in shear though we designed the beams to fail in bond around inner cut-off bars.

(4)　The beams failed in shear had almost the same ratio of calculated shear capacity to required value at the opening as at the region without opening. However, the inclined reinforcement around opening didn’t yield. It is considered that the margin of the shear capacity in the region with opening is higher than the region without opening.

(5)　When the inner bar of the beam with opening was cut off, shear forces resisted by concrete and inclined reinforcement decreased, and shear force resisted by transverse reinforcement increased.

(6)　The load when an initial shear crack occurred around opening was lower than the load when an initial shear crack occurred on the beam without opening. However, the width of shear cracks around openings was less than approximately 0.1mm after unloading from the load of allowable shear capacity, which the width was narrower than that in the region without opening.