曲げ降伏後にせん断破壊する鉄筋コンクリート部材の安全限界変形

 Structural engineers are encouraged to design reinforced concrete (RC) structures so that flexural yielding of RC members occurs first, to prevent shear failure that leads to sudden strength deterioration. In order to allow flexural yielding without exceeding the safety limit deformation of RC members, the deformation capacity has to be accurately assessed. Few methods are available to evaluate this deformation capacity since the failure mechanism of RC members after flexural yielding is still not well understood. The current AIJ Design Guidelines, Design Guidelines for Earthquake Resistant Reinforced Concrete Buildings Based on Inelastic Displacement Concept published by the Architectural Institute of Japan, provides a method to obtain deformation capacity using a compressive strut model with deteriorating strength as the plastic hinge rotates. This study considers a new failure mechanism for RC members failing in shear after flexural yielding due to concrete cracking and proposes a new method to evaluate the safety limit deformation.<br> We conducted cyclic loading tests of three RC beams designed to fail in shear after flexural yielding and one RC column to fail in shear. The proportions of the bending to shear deformation were measured by image analysis using grid points drawn on the surface of the specimens. The results revealed that the proportion of shear deformation suddenly increased after the strength deterioration, which was defined as 90% of the maximum strength of RC member in this paper, occurred. Also, compressive strut failure was not observed in the tests. Based on the experiment results, the following failure process was considered: (1) concrete flexural cracks propagate near the critical section, (2) main reinforcements start yielding and bending-shear cracks propagate, (3) main bar yielding area extends and lateral reinforcements start yielding, and (4) crack width widens until the member strength deteriorates as the crack width is wide enough that the concrete aggregates cannot maintain the stress transmission mechanism.<br> Then, we proposed a method to evaluate the safety limit deformation where the strength deterioration occurs. The proposed safety limit deformation was assumed to consist of three deformations: (a) flexural deformations in elastic area, (b) plastic deformation estimated based on the main reinforcement elongation, and (c) elastic shear deformation. The deformation (b) computed by the proposed method and the experimental deformation estimated by the photos of specimens were compared. The proposed method showed good agreement with the experimental results and the assumptions to estimate the plastic deformation was validated.<br> To investigate the accuracy of the proposed evaluation method, the method was applied to 54 specimen data collected from previous experimental studies. Then, the method in the AIJ Design Guidelines was used for the data and compared to the new method. The results showed that the proposed evaluation method can better evaluate the safety limit deformation.<br> The new method provides better estimations than previous methods, however, the estimations still have some scatterings mainly because of the variation of the strain hardening point. We investigated the appropriate assumption of the strain hardening point by parametric studies. As a result, we obtained factors of 6 or 7 for the strain hardening point to the yield strain for the safe side estimation, when the data is not provided. The estimation may improve with sufficient data of average and variation of the strain hardening points.