An economic and convenient seismic retrofitting technique based on the thick hybrid wall (THW) technique reported by Yamakawa1) is proposed. In the proposed technique, a cast-in-site partial hybrid wing-wall is built using additional concrete sandwiched by steel plates and high-strength steel bars (PC bars) prestressing. The aim of this technique is to enhance the lateral strength, stiffness, and ductility of soft-first story reinforced concrete (RC) buildings that are vulnerable to large seismic excitations. In the THW technique, the retrofitted section consisting of an additional wing-wall with short depth and the existing RC column are unified together as one unit using channel-shaped steel plates and tightened with PC bars. Since the additional wing-wall is not reinforced by longitudinal or transverse bars, the technique is convenient and cost effective. The important structural aspect of the THW technique is increasing the flexural strength as well as ductility by ensuring that all the longitudinal bars in the existing RC column yield in tension due to the increment of the internal moment lever arm, which results from the increase in the neutral axis depth into the additional wing-wall. To verify the efficiency of the proposed THW technique from the perspective of flexural strength, the equations to evaluate ultimate moment resistance in the retrofitted THW column section was proposed3) based on the ACI stress block parameters, which consider the condition that all longitudinal bars yielded under tension in the existing RC column, and the additional wing-wall was in the compression side. Furthermore, the equation to calculate the minimum additional wing-wall length ratio was also proposed to estimate the affordability of the THW technique in Ref. 3).
This study aimed to experimentally investigate the shear resistance and shear strength of the arch mechanism of the RC column retrofitted by the THW technique. From the test results of the retrofitted RC column showing a flexural failure mode, the proposed equations of the ultimate moment resistance3) of the THW technique were verified.
Experimental investigations were conducted on six specimens. In this study, two types of specimens were considered. One was a retrofitted RC column with no bonding force between the concrete and embedded longitudinal bars, thereby generating the arch mechanism. The other was a retrofitted RC column with bonded longitudinal bars to evaluate the flexural strength. In brief, the conclusions are as follows: (1) Bonded specimens for which the THW technique is applied showed flexural behavior with high ductility involving the tension yielding of all longitudinal bars in the existing RC column, and the calculated results of proposed equations are in good agreement with the test results. (2) The application of the THW technique not only creates a connection between the RC column and additional wing-wall, but also increases the shear resistance greatly. (3) In the unbonded specimens, the compression zone of the RC column for the arch mechanism was greater than 0.5D, and the zone was distributed from 0.8D to 1.0D. (4) Based on the test results and observations, an equation was proposed to evaluate the shear strength in the case of the THW technique following the proposed concept of the shear resistance (arch) mechanism with a nonuniform section of compression strut. The calculated results of the proposed equation are in good agreement with the test results showing shear failure mode.
