EXPERIMENTAL STRUCTURAL PERFORMANCE EVALUATION OF RC COLUMNS WITH WING WALLS WITHOUT WALL VERTICAL REBAR ANCHORAGE

Abstract

 1. Introduction

 In recent years, researches aim at developing a new structural system which uses nonstructural walls as structural elements of RC buildings proactively have been conducted based on past experiences in earthquake disasters^1-3). On the other hand, the latest earthquakes such as the Kumamoto earthquakes in 2016 revealed that RC buildings suffered serious damage to stiff members like columns with wing walls resulting in restoration/demolition, while they survived the earthquakes⁴⁾. Therefore, this study presents and verifies a new rebar arrangement for columns with wing walls which omits wall vertical rebar anchorage to let the wing walls resist only compression, thus reduce damage (Fig. 1). The current paper discusses a series of static loading experiments using three columns with/without wing walls with different confining reinforcement arrangements.

 2. Test plans

 A prototype building was designed according to the following concept: 1) to satisfy the base shear coefficient of 0.55 when considering wing walls in which wall vertical rebar anchorage was omitted, and 2) to maintain that of 0.3 with high ductility even though the wing walls fail under unexpected high seismic loads (Figs. 2-3 and Tables 1-3). Then, three 1:2 scale column specimens with/without wing walls on both sides representing the prototype building were designed: Specimen C without wing walls, Specimen CWJ with wing walls having confining reinforcement satisfying requirements for FA (with high ductility) based on AIJ Standard⁹⁾, and Specimen CWA with wing walls having higher confining reinforcement for earthquake-resistant design based on ACI code⁷⁾ (Figs. 4-6 and Tables 4-9). Static cyclic loads were applied to the specimens (Figs. 7-8 and Tables 10-11).

 3. Test results

 Compared with Specimen C without wing walls, Specimens CWJ and CWA with wing walls increased the initial stiffness and the maximum strength (Fig. 9). On the other hand, the experimental behavior and performance were similar between Specimens CWJ and CWA with different confining reinforcement arrangements. This resulted from no compression failure of confined core concrete observed in both specimens, which indicated that the confining reinforcement satisfying FA⁹⁾ was sufficient for the present specimens. The structural damage to all specimens were limited: the column suffered slight damage¹⁰⁾ with residual crack widths less than 0.1 mm up to the loading cycle to 0.5% rad, while the wing walls showed larger opening at the bottom because of the omission of wall vertical rebar anchorage (Figs. 10-12). Furthermore, stress of confining reinforcement in Specimens CWJ and CWA was limited which was likely to attribute to the omission of anchorage resulting in no tensile yielding of vertical rebar (Figs. 13-14). However, the observed equivalent damping factors of Specimens CWJ and CWA were smaller than that commonly used for practical design under large drifts because they showed slippage behavior in the hysteresis loops (Figs. 15-16).

 4. Conclusions

 In this paper, the structural performance of the columns with wing walls which omitted wall vertical rebar anchorage was experimentally evaluated. It was found that the confining reinforcement according to FA based on AIJ standard⁹⁾ provided high ductility with the drift capacity of more than 4% rad for the present specimens. The proposed omission of wall vertical rebar anchorage successfully limited not only damage to the specimens, but also stress of the confining reinforcement.