Mid-story pin column system has been proposed to realize beam yielding mechanism of a steel moment-resisting frame. In this system, a pin connection system is applied inbetween the upper steel column and the lower RC column extended from the base beam to control moment demands on both ends of the first story column. An exposed-type column base has been adopted as a pin connection connected by a single anchor bolt. Static cyclic horizontal loading tests of a partial frame consisting of the first story column and the second floor beams showed a frame with the proposed column base system exhibited stable hysteresis performance up to a story drift angle of 0.05 rad, remaining column in elastic. In addition, the RC column with the connection of a single anchor bolt exhibited slippage at the connection while this problem was solved when an anchor bolt was stiffened by cross-shaped four shear plates. A cover plate was also welded on the top of the shear plates to cover the top surface of the RC column from bearing pressure of the upper steel column. An anchor bolt with the welded plates are called an anchor bolt set, or ABS, hereinafter. Ensuring a reliable seismic performance of an upper steel frame, damages at the proposed connection have to be minimized. So far, compressive stress transfer mechanism was clarified, and the ultimate compressive strength of the connection was evaluated in the previous study.
This paper discusses shear stress transfer mechanism and the ultimate shear strength of the RC column with proposed column base connection with the ABS. An anchorage rebar was welded on each shear plates to anchor the ABS to the RC column until the connection reaches the ultimate strength. Series of static cycle horizontal loading tests were conducted. Test parameters are a diameter of anchorage rebars and stirrup arrangement. Test results showed that anchorage rebars increased the shear strength twice as large as the connection by ABS without anchorage rebars, mitigating bearing crack propagation spread from the ABS (Chapter 2).
Chapter 3 shows that Navier hypothesis is applicable to strain distribution of a group of anchorage rebars, and that their neutral axis can be estimated as the gravity center of the semicircle of the cover plate under uniform pressure. A formulation of the yielding moment of group of anchorage rebars is proposed based on these findings.
Chapter 4 discusses the bearing crack initiation strength and the ultimate shear strength of the RC section where the ABS is embedded. With a combination of lever action by the embedded ABS and reaction moment retained by a group of anchorage rebars, shear force sustained by the RC section with the embedded ABS, or QRC, is amplified or reduced from the horizontal force P acting on the connection. The shear crack initiation strength QRC’,c and the ultimate shear strength QRC’,umean sufficiently agrees with test results, calculated by AIJ RC design standard with the effective width of the RC section considering the width of the ABS.
Chapter 5 proposes the stress transmission mechanism of the connection based on that for the embedded-type column base system. The shear strength estimation formulas for the RC connection with the ABS is proposed by a superposed strength method considering the effect of reaction moment provided by a group of anchorage rebars.