LATERAL STRENGTH OF EMBEDDED SHS COLUMN BASE REINFORCED WITH U-SHAPED REBARS

Abstract

 While the embedded column base connection is effective for securing stiffness and strength relative to the other type of column base connections, the bearing resistance of embedded portion of the column due to the prying action, may results in undesirable failure mode such as punching shear failure which lead to significant deterioration in the strength. Previous studies have reported that improved the strength, ductility capacity and decreased the strength deterioration were attributed to the arrangement of U-shaped rebars and hoops surrounding the column. Based on these experimental results, Japanese design provision specified the estimation of plastic flexural strength of exterior column base based on the lateral resistance mechanism including axial yielding of U-shaped rebars. This provision however had not clearly specified the reinforcement details.

 The objective of this study is to provide the recommended details of U-shaped rebars for achieving satisfactory performance and evaluate the ultimate strength of column base connection. Cyclic lateral loading tests were performed with SHS column-grade beam sub-assemblages to investigate the cyclic responses including failure modes and ultimate resistances with emphasizing on contribution of rebars to the lateral resistance. A total of 13 specimens were tested, with embedment lengths of column, the amount of U-shaped and hoop rebars and the spacing between the rebar and column in the loading direction taken as key test parameters (See Table 1, Figs. 1 and 2).

 Based on test results, the influence of the reinforcement details on the fracture behavior of the connection, the contribution of the rebars and resulting ultimate strength were investigated (See Table 3 and Figs. 3 to 5). It was observed that the upper U-shaped rebar yielded or remained elastic at the ultimate load, depending on the details of U-shaped rebars as shown in Fig. 7. The evaluation method of anchorage strength of 90° bending rebars [Ref. 5] was extended for embedded column base connection and used to predict the anchorage strength of U-shaped rebars. The predicted axial force of the U-shaped rebars given by the lesser of axial yielding and the anchorage strength of rebar fairly agreed with the experimental results (Fig. 11). In other words, this method can be applicable to determine proper detailing for ensuring the axial yielding of the U-shaped rebars. Furthermore, the ultimate strengths of the embedded column base connection were evaluated assuming the same resistance mechanism in current provision and using modified axial resistance of rebars given by Eq. (9). The proposed method was able to consistently predict the experimental ultimate strengths, regardless of the details of U-shaped rebars (Fig. 13).