Research On Mechanical Behavior Of Concrete Filled Rectangular Steel Tube Column-beam Joint With Disconnection Column Tube At Joint Zone

There are different constructions types of the joint of concrete filled steel tubular (CFST)columns connecting with reinforced concrete (RC) beam in which the longitudinalreinforcement bars in RC beam are interrupted by the continuous steel column tube, resultingin the construction of the joint is more complicated. The research results about the joint arenot systematic. In most cases, the mechanical behavior is incompatible with convenientconstruction. It is urgent for researchers and designers to develop a new joint type withexcellent mechanic behavior, convenient construction and significant commercial benefits.In this paper, a new type of joints between rectangular CFST column and RC beam withdisconnection column tube in joint zone is presented. The main features of this joint are asfollow: The steel tube is disconnected at the joint zone, and the column tube is separated ineach floor, so the longitudinal reinforcement bars in RC beam can be continued in the zone.Because the column disconnection and the lower strength concrete to be using in the jointzone leading to the decrease of compression capacity, the enlargement of the section of jointand mesh reinforcement are used to strengthen the joint zone. The stirrups are set along theouter mesh reinforcement to improve the capability of ant-crack. The column stirrups settingin the zone of normal joint do not need, because the mesh reinforcement and stirrups alongthe outer mesh reinforcement can resist the force of joint shear. The advantages of this newtype of joint include the following aspect: the joint can be used to connect to any directionbeams; The crack resistance of joint is effectively improved with stirrups along the outer meshreinforcement; It is more convenient to construction because the steel connection parts arereduced.The paper focuses on the mechanical behavior of this joint, and the innovation researchworks are as follow.(1) Experimental studies were carried out with48rectangular specimens to investigatethe axial compression behavior of this type of joint. The main parameters in the experimentare the relative section of joints (the edge length of the joint section/the edge length of thesection column), the relative height of joints β (the height of the joint/the edge length of thesection column), the mesh reinforcement ratio ρ, the strength of concrete fcuand the length towidth ratio of column γ. The results show that, under the confinement of the meshreinforcement and enlargement of the section of joints, this type of joint has high compressivecapacity and ductility. By setting the stirrups along the outer mesh reinforcement, thecapability of ant-crack can be improved. (2) Based on the local compression formulas and conception of equivalent lateralcompressive stress in confined concrete, the peak strength of the core concrete in the joint iscalculated. The constitutive relationship of confined concrete is adopted to build theequivalent stress-strain relationship for the core concrete of the joint. This stress-strainrelationship takes a similar form of the constitutive model of confined concrete while itsparameters are modified to fit this type of joint according to experimental results. Finally, acalculation of complete stress-strain relationship curves is conducted for experimentalspecimens using this constitutive relationship. The calculation results and the experiment onesare in good agreement with each other.(3) Experimental studies were carried out with8rectangular joint specimens havingupper and lower columns, to investigate the eccentric compression behavior of this type ofjoint. The main parameters in the experiment are the relative section of joints, the relativeheight of joints β and the length to width ratio of column γ. The influence of the parameters onthe eccentric compression capacity of the joint was discussed. Comparison between theexperimental eccentric compression capacity of the joint and the calculated compressioncapacity of the CFST column with the methods of standard formula shows that thecompression capacity of the joint zone is higher than the column with the reasonable design.The results indicate that this type of joint can meet the anti-seismic design principalrequirement of strong joint-weak member. By limiting the vertical expansion deformation ofconcrete with stirrups along the outer mesh reinforcement, the emergence and development ofeach side horizontal crack can be postponed, and the crack resistance of joint is effectivelyimproved.(4) Based on the theory of local compression of concrete, the confining effects of localcompression of concrete and steel reinforcement bars to the concrete were discussed. Byintroducing the joint height coefficient, the Chinese standard formula was modified toestablish the formula Ⅰ for the ultimate axial compression capacity of joint. Based on theformula Ⅰ, by introducing the eccentric bearing capacity reduction coefficient for localcompression, the formula for calculating the eccentric bearing capacity of joint wasestablished. Based on the Mander confined concrete theory, the effective restraint coefficientkeunder local compression was discussed. Then, the formula Ⅱ for the ultimate axialcompression capacity of the joint was established. On the basis of the experimentalobservations, the concept of effective compression section was submitted. The formula Ⅲwas put forward to predict the ultimate axial compression capacity of the joint. The results predicted by above formulas are in good agreement with the experimental ones.(5) Cyclic loading experimental studies were carried out with5interior joint specimensto investigate the seismic behavior of this new type of column-beam joint. The results showthat the failure region from the joint to the frame beam with the increase of relativereinforcement ratios (the ratio of joint mesh reinforcement/the ratio of beam reinforcement)and relative dimension of joints (the section area of the joint/the section area of the column).With the reasonable design, the new type joint has good seismic behaviors, and the seismicprinciple “weak beam strong column, stronger joint” can be satisfied. The axial compressionexperiment on four specimens after the cyclic loading was carried out. The results show thatthe concrete in the joints can be still confined by the mesh reinforcement after the cyclicloading. The joint has high compressive capacity and ductility. With the reasonable design,the capacity of the joint zone is higher than the concrete filled steel tubular column, meetingthe anti-seismic design principal requirement of strong joint-weak member.(6) Based on the modified compression filed theory (MCFT), the key parameters of shearcapacity calculation for the new type joint were studied. The calculation method for shearcapacity was set up. The calculation results are in accordance with the experimental ones. Itcan be thought suggested calculation method is reasonable.