Yo Hibino

Evaluating the performance of reinforced concrete (RC) structures during earthquakes and the resultant damage in the structures depends on an accurate load–displacement relationship. Several experimental and analytical evaluation methods for load–displacement relationships have been proposed and specified in current design standards. However, there have been few quantitative studies on the impact of drying on the yielding behavior of RC members, including evaluations of the effective stiffness of members. In this study, to investigate changes in the mechanical properties of RC beam–slab members due to drying of the concrete, cyclic loading tests are conducted on two RC beam–slab members with and without drying. It is found that the lateral structural stiffness of the specimen with drying decreased to 77% that of the specimen without drying. This is verified in the calculation of the flexural stiffness. In this calculation, it is assumed that drying shrinkage decreases the moment of inertia of the slab in tension but not in compression. Meanwhile, no difference is observed in the flexural capacity and yield displacement between the two specimens. Thus, there is no significant impact from drying shrinkage in RC beam–slab members on the lateral structural performance, while the shrinkage instead induces greater flexural cracking, which reduces the residual stresses in the specimen with drift leading to a gradual decrease in the impact of drying.

Shake-table tests of a 3-story reinforced concrete building with cladding casted to be monolithic with frame elements, with seismic slits or cut reinforcing bars present in some cladding elements to ensure a desirable deformation mechanism, was performed at E-Defense to evaluate its post-disaster functionality. The building satisfied response objectives; behaving elastically with 0.33% drift or less during a “severe earthquake motion” defined in the Japanese Building Standard Law, and has a base shear capacity coefficient greater than 0.55. However, the damage was greater than expected due to deformations concentrating on a few cracks, resulting in continuous functionality not necessarily being achieved.

Following the observation of severe damage to structurally isolated non-structural reinforced concrete walls after major earthquakes, researchers began to reassess the effectiveness and connection detail of non-structural walls to moment-resisting frames. A method to control damages to the non-structural wall, is to cast exterior non-structural concrete wall elements to be monolithic with frame elements, without anchoring the wall longitudinal bar. The non-anchorage of the wall longitudinal bar significantly increases the drift capacity of the wall and decreases damage. Using an experimental approach, this study assesses the influence of reinforcement detailing and quantity of the transverse reinforcements on the strength and drift capacity of the non-structural hanging wall. This study further evaluates the workability mechanism of the transverse reinforcements and reinforcement detailing with concrete. The non-anchorage of hanging walls, having boundary confinements, was found to exhibit a higher drift and strength capacity than similar walls with the anchored detailing without boundary confinements. The strength capacity of the anchored detailing hanging walls with minimum amounts of reinforcements was higher than that of the non-anchored specimen. The boundary confinements were found to be more influential on the capability of the hanging wall when placed along with non-anchored detailing reinforcement.

The required base shear and drift limit for post-disaster management buildings have increased in the Japanese Building Code following major seismic events. One method to satisfy these requirements for reinforced concrete frame buildings is to cast exterior non-structural concrete wall elements to be monolithic with frame elements, but without anchoring the longitudinal wall reinforcing. This provides additional stiffness and strength while limiting significant damage in the non-structural wall. In this study, the structural performances of such elements were evaluated using static and dynamic experimental tests. The result indicates that non-structural walls that were neither isolated by seismic slits nor anchored to the adjacent walls with longitudinal reinforcements experienced less damage and higher deformability compared with walls having seismic slits. The confinement reinforcing impact was not observed on the strength and drift capacity of the beam member, owing to the large number of transverse reinforcements. However, the confinements limited the damage and nearly prevented concrete crushing. The maximum horizontal load of the specimen could be predicted using cross-sectional analysis, and the authors propose a simple equation to predict it with sufficient accuracy.

1. Introduction The latest earthquakes such as the Tohoku/Kumamoto earthquake in 2011⁴⁾/2016⁵⁾ revealed that RC columns with wing walls having high stiffness suffered from serious damage, which resulted in restoration and demolition. To discuss this issue, the authors have proposed and verified a new rein. arrangement for columns with wing walls which removes the vertical wall rein. anchorage to let the wing walls resist the only compression, and then clarified the effectiveness for damage control⁶⁾. This new structural detail is expected that the short-term allowable bending moment may be improved because the tensile exterior rein. in the wing walls is not anchored, resulting in no yielding. Therefore, static loading tests using two columns having wing walls with/without vertical wall rein. anchorage were conducted. Furthermore, the current paper discusses specific conditions the short-term allowable bending moment increases/decreases through parametric analyses. 2. Test plans Two 1:2 scale specimens were designed as classified into type FB based on AIJ Standrad⁷⁾ by modifying confining rein. ratio and vertical wall rein. ratio based on the specimen in the authors’ past study⁶⁾. Specimen CWJ2 did not have vertical wall rein. anchorage, while Specimen CWJ2A anchored vertical wall rein. to the lower stub. (Figs. 2-3 and Table 1). Static cyclic loads were applied to the specimens (Figs. 4-5 and Table 4). 3. Test results In specimen CWJ2/CWJ2A without/with vertical wall rein. anchorage, the concrete/vertical wall rein. reached the short-term allowable stress first. The curvature of specimen CWJ2 was larger than that of specimen CWJ2A because the vertical wall rein. of specimen CWJ2 was not under tension by removing the vertical wall rein. anchorage (Fig. 7). The shear force of specimen CWJ2 at the short-term allowable bending moment was equivalent to that of specimen CWJ2A because the longitudinal rein. strain of specimen CWJ2 was larger than that of specimen CWJ2A (Fig. 8-9). 4. Parametric analyses A method for bending analyses was presented to simulate the experimental behavior of the specimens (Figs. 10-11). Based on the analytical method, a series of parametric analysis was performed with five parameters of wall thickness, concrete strength, column longitudinal rein. ratio, wall length, and vertical wall rein. ratio. In the case of the column with wing walls on both sides, the short-term allowable bending moment ratio ????_r (Fig. 15) was effectively improved by the wall thickness and concrete strength because the increase of wall thickness/concrete strength increased the curvature of the specimen without anchorage, which resulted in the increase of column rein. stress (Figs. 17-18). On the other hand, the reduction of the short-term allowable bending moment was limited under the compressive axial force ratio beyond 0.1, except for specific conditions. In the case of the column with a wing wall under tension, ????_r was effectively improved by concrete strength and vertical wall rein. ratio (Fig. 22). In the case of the column with a wing wall under compression, there was little difference in ????_r due to the parameters because the tensile exterior rein. in both specimens was the column longitudinal rein., which resulted in almost no difference in the curvature at the short-term allowable stress. 5. Conclusions The present paper investigated the structural characteristics of the column having wing walls without vertical wall rein. anchorage proposed in the authors’ previous study⁶⁾ mainly focusing on the short-term allowable bending moment. The findings from the experimental and analytical studies are summarized.