Due to simplicity, many engineers have chosen a single-shear bolted joint for the end connections of circular hollow-structural-section (HSS) braces. A flat plate is welded to the brace, and this plate, in turn, is connected to a gusset plate using slip-critical, high-strength bolts in single shear (see Fig. 1). Where such connection is adopted, the braces may be susceptible to an undesirable buckling mode that involves formation of two plastic hinges at one bracing connection and one hinge in the other bracing connection. The fundamental cause of this buckling mode is the inherent eccentricity between the brace and gusset plate and lack of out-of-plane stiffness and strength of the bracing connection. The concerned buckling mode was observed in a number of buildings after the 1995 Kobe earthquake and 2011 Tohoku earthquake [Ref. 1]. A typical example where the buckling mode led to fracture of the connection is shown in Photo 1. Past studies, conducted by the authors [Ref. 2], have alarmed that deformation concentration in the connection results in substantially smaller compressive strength and energy dissipation capacity of the brace. Therefore, every care must be taken to avoid the buckling mode that causes deformation concentration in the bracing connections.
The analysis model proposed by Tada et al. [Ref. 3 and 4] is extended to account for the out-of-plane rotational stiffness and strength of the gusset plates (Figs. 2 to 4 and 8) and the possibility that their stiffness may be unequal between the two ends (Figs. 5 and 6). Using this model, generalized strength formulae are derived for braces in compression. The formulae indicate that, within the range of practical application, the difference in stiffness between the two end connections have little effect on the compressive strength of braces (Fig. 7). The validity of the strength formulae is examined by six monotonic loading tests (Figs. 9 and 10). The six specimens had different gusset plate width to examine a range of end stiffness and strength. The test results and corroborating finite element simulations (Fig. 11) agreed with the model in failure mode and strength (Fig. 15).
The finite element model was extended to conduct a parametric study over a wider range of geometrical properties such as brace length (Lb = 2,089 to 3,000 mm ), brace section (76.3×2.8, 76.3×4.2 and 114.3×4.5), length of bracing connection (kaLb=195 to 546 mm), isolated length of flexible segment of flat plate and gusset plate (kakbLb = 45 and 75 mm), thickness of flat plate and gusset plate (tfp = tgp =9 to 16 mm), and width of gusset plate and flat plate (bgp =130 to 540 mm; bfp =130 and 180 mm symbols defined in Table 2). Results for a total of 118 models demonstrated that the proposed formulae estimate the buckling mode and compressive strength with error margin of 10% , for the range of properties examined. The study suggests that the use of compact (short and stiff) bracing connections is effective to prevent the concerned buckling mode.