Yasushi Sanada

Beam–column joints constitute critical structural components in reinforced concrete (RC) frame systems, playing a pivotal role in ensuring seismic collapse resistance. Inadequate beam anchorage strength may lead to anchorage failure during seismic events, resulting in structural integrity loss and potential building collapse. Particularly in developing countries, where numerous vulnerable structures coexist with limited financial resources and technological capabilities, there is an urgent demand for simple and cost-effective strengthening methodologies. Bangladesh, characterized by its abundant and economical clay brick resources which are extensively utilized in construction, presents a unique context for structural reinforcement. This study proposes an innovative strengthening approach through the implementation of brick wing walls for RC structures exhibiting insufficient structural integrity. Three specimens representing the deficient exterior beam–column joints were manufactured, with two strengthened using distinct wall configurations: thin-long walls and thick-short walls. Quasi-static cyclic loading tests were conducted to evaluate the strengthening efficacy. Experimental results demonstrated that the control specimen experienced structural integrity failure attributable to insufficient beam anchorage strength. In contrast, both strengthened specimens successfully prevented anchorage failure and achieved an optimal beam-yielding mechanism. The thin-long wall-strengthened specimen exhibited superior strength performance, demonstrating enhancements of 62% and 164% under positive and negative loading conditions, respectively, compared to the prototype specimen. However, rapid strength degradation was observed after reaching the peak strength. In contrast, the thick-short wall-strengthened specimen exhibited relatively lower strength but significantly improved ductility, with increases of 45% under positive loading and 151% under negative loading. Furthermore, finite element analysis was conducted to investigate the stress distribution patterns within the brick walls. The analysis revealed the formation of a compressive strut in the compressed brick wall and a tensile force in the tensile support column. Based on these findings, a comprehensive mechanical model and corresponding design methodology for structural strengthening applications were developed.

The objectives of this study were to investigate the out‐of‐plane behavior of unreinforced masonry (URM) infill walls and to identify the effectiveness of tie systems that were proposed to enhance the infill stability under both in‐ and out‐of‐plane effects. This study examined three types of tie systems: a full‐length tie system (prototype), a staggered tie system and a half‐length tie system. For this purpose, four 1/4‐scale, single‐story, one‐bay masonry infilled reinforced concrete (RC) frame specimens with and without the proposed tie systems were sequentially tested under in‐plane static cyclic loading and out‐of‐plane dynamic excitation. In the experimental study, two series of shaking table tests were conducted to investigate the out‐of‐plane behavior of the infill walls. The first series compared the fundamental out‐of‐plane performance of three proposed tie systems, and the second series verified the effectiveness of the full‐length tie system to prevent infill out‐of‐plane failure under an excitation simulating the severe floor response in medium‐rise buildings. The test results indicated the following: (1) the presence of the tie systems caused minor differences in the in‐plane capacity and ductility of each specimen, (2) the effectiveness of the full‐length and staggered tie systems was superior to that of the half‐length tie system, and (3) the full‐length tie system not only prevented the infill out‐of‐plane failure but also significantly reduced the infill damage under realistic excitation conditions. The above research outcomes provide a useful contribution of the infill to upgrade the seismic resistance of buildings.

Many reinforced concrete (RC) frame buildings in Nepal were significantly damaged by the 7.8 magnitude (M_w) earthquake in Nepal on April 25, 2015. To contribute to mitigate future earthquake disasters, the current study focuses on two specific characteristics of residential RC frame buildings in the capital city of Nepal, Kathmandu: the application of brick masonry infill to exterior and partition walls, and the conventional vertical extension of building stories different from the design. Although these factors are likely to significantly affect the seismic performance, their effects are frequently neglected in practical design and construction management in developing countries. Hence, the main objective of this research is to investigate and clarify the seismic performance of RC frame buildings considering the above factors through experimental and numerical investigations. The present paper (1) briefly introduces the characteristics of a typical residential RC frame building in Kathmandu, (2) illustrates the numerical modeling parametrically considering three different contributions of brick masonry infill walls and (3) investigates the seismic performance of the RC frame building considering the effects of the infill wall modeling and the vertical extension through numerical analyses. Consequently, it was found that the consideration of the in-plane stiffness and strength of the infill walls resulted in both positive and negative contributions to the seismic performance of low-rise (up to three stories) and medium-rise (more than three stories) buildings respectively, quantitatively clarifying significant effects of the presence of infill and the vertical extension. These findings contribute to provide realistic solutions to upgrade the seismic performance by utilizing or removing the brick masonry infill walls or by managing the building stories to mitigate future earthquake disasters on typical RC frame buildings not only in Nepal but also in other countries with similar backgrounds.

This study emphasizes a new strengthening scheme for seismically vulnerable flat plate‐column connections in existing structures. An experimental investigation on a series of three half‐scaled interior plate‐column connections was conducted to explore the effectiveness of the proposed seismic strengthening method using postinstalled RC wing walls and to analyze the punching shear capacity of the connections. This study focuses on typical existing flat plate structures constructed using low‐strength concrete in Bangladesh. Two of the three specimens had wing walls employed in two principal directions, that is, along the loading direction and orthogonal direction, while another specimen was tested without wing walls and served as a control specimen for comparisons between the cases with and without strengthening. Static cyclic vertical displacements with increasing amplitude were applied to the ends of the slabs to represent interstory drift under seismic action. Both strengthened specimens showed higher resistance to punching shear failure. Thus, the test results experimentally verified that installing RC wing walls is a feasible method to upgrade flat plate‐column connections, including those constructed using low‐strength concrete, to prevent collapse under seismic action.

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.

Summary: In recent earthquakes in developing countries, severe damage was observed on reinforced concrete buildings. This study focuses on exterior beam‐column joints with substandard beam rebar anchorage and seismic strengthening by installing wing walls. First, a series of experiments was conducted to investigate the seismic behavior of exterior joints with substandard beam rebar anchorage representing typical Bangladeshi buildings. Two 0.7‐scaled exterior joint specimens were tested, and these specimens showed beam rebar anchorage failure and/or joint shear failure. Prior to strengthening of the joint, a series of pullout tests was conducted on postinstalled bonded anchors in low‐strength concrete for strengthening design. Then, an experiment was performed to apply the strengthening method by wing walls to one of the exterior joint specimens to improve the integrity, and this method was intended to prevent the failure of beam rebar anchorage. The strengthening method is proposed to extend the development length of beam longitudinal bars by considering the embedment length along the wing walls. The test results verified the effectiveness and applicability of the proposed strengthening method to upgrade exterior RC beam‐column joints with deficient beam rebar anchorage.

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.