Experimental Research Of The Beam-Column Knee Joint With HRB500 Steel In Earthquake-resistant Frames

Six specimens of knee joints in reinforced concrete frames under cyclic reversed loading with HRB500 longitudinal steel designed basing on"Code for Design of Concrete Structures"(GB50010-2002) are conducted, which proves the efficiency of seismic measures in knee joints, and discussion is made upon the application of seismic frame with HRB500 steel. The main points and conclusions are summarized as followed:(1) Construction measures of knee joints in reinforced concrete frames in which the steel bears beam and column negative moment: Six specimens are conducted in the test and the overlapping measures is taken according to"Code for Design of Concrete Structures"(GB50010-2002) which requires that the overlapping length in the knee joints is 1.5laE and the inner diameter is equal to 6d. The test result illustrates that the construction measures of knee joints in reinforced concrete frames with HRB500 bars can satisfy the special demand.(2) The anchorage of bottom beam bars of knee joints in reinforced concrete frames: according to"Code for Design of Concrete Structures"(GB50010-2002), the anchorage measures of bottom beam bars in exterior joints is the same with that of knee joints in reinforced concrete frames: 90°hooked anchorage horizontal length is not less than 0.4laE+15d. The test result of the six specimens shows that the anchorage measure is not effective enough when the HRB500 bars are used. The test process and analysis illustrate that the anchorage measures of bottom beam bars in exterior joints should treat differently with that in knee joints.(3) The maximum shear ratio of knee joints in reinforced concrete frames: The unified maximum joints shear value is 0.3 according to"Code for Design of Concrete Structures"(GB50010-2002). The test result of the six specimens shows that the shear ratio of knee joints in reinforced concrete frames with HRB500 bars is a little higher. The comprehensive performance of three specimens with test shear ratio higher than 0.25 is not so good.(4) Concrete strength: the minimum concrete strength of seismic specimens based on the"Code for Design of Concrete Structures"is 30MPa. By comparison, the comprehensive seismic performance of two specimens with HRB500 steel in which the concrete strength value is higher than 35MPa is better. (5) stirrup effects: the joints'deformation enlarges and the constraint effect of stirrup becomes more obvious. Stirrup steel almost yields when the load capacity of the specimens declined to 85%, which shows the constraint effect to the core concrete is obvious.