Research On Static Behavior Of Reinforced Concrete Column-Steel Beam (RCS) Moment Joints

A new composite structural system, consisting of reinforced concrete columns and steel beams (RCS), has gained popularity in recent years. Columns made of reinforced concrete have the advantage of good compressive behavior and abundant local material supply. Their large rigidity, good durability, and fireproof property result in steel saving, reduced costs and increased stability of structure. Beams made of steel have the advantage of higher strength, lighter weight and higher construction speed, resulting in reduced structural dead weight, smaller member sizes and building height, increased effective spaces, and reduced foundation costs. Engineering practices show that beams and columns made of two different materials, may fully develop the merits of each of them, and thus combine rationality with economy in terms of material selection.A large number of research programs have been conducted abroad to study the interaction between steel and concrete members in RCS frames, especially in the connections between steel beams and RC columns. RCS frame systems have also been used in practices. But many problems still exist in several aspects: researches have been made mainly through experiments, and analysis of RCS connections using finite element method still remains in the beginning stage; Investigations focus mainly on interior connections, and rarely on exterior and top-interior ones, and tests of top-corner connections have not been made. Furthermore, the strength equations for interior connections are excessively conservative in ASCE 1994 guidelines.This research explores the static behavior of RCS joints by using finite element method. Emphasis is put on the behavior of RCS exterior connections under monotonic loading through in-depth investigation of the forces acting on RCS joints, the internal force transfer and its mechanism, the strength and failure modes of joints, and the deformation of joints. Moreover, influences of various factors on the joint behavior, such as different joint details, concrete strength, axial column load, and section dimensions of beams and columns, etc., are also examined. Using this new information, design models and calculation equations for interior and exterior connections are established, which provide theoretical basis for design and construction of RCS joints.The study shows that ANSYS finite element software may simulate the behavior of RCSconnections under static loading by installing parameters reasonably, and the results agreewell with experiment ones. The research also shows that joint details, concrete strength,column axial load, and section dimensions of beams and columns may affect the behavior ofRCS exterior connections. Differences in joint details have direct effects on the joint strength,stiffness, ductility, and degree of concrete mobilization in a joint. FBPS, U-shaped stirrups,steel column, E-FBPS, steel cover plates, and steel band plates are all effective joint details,and their appropriate combination may form efficient joint details with enhanced jointstrength and stiffness. The cylinder compression strength of concrete also affects the ultimateshear carrying capacity of inner and outer concrete. The higher this strength is, the higher theultimate shear capacity of inner and outer concrete can attain. But this doesn't affect the shearstrength of steel web. Axial compression and tension loads have different effects on thebehavior of RCS joints. When the ratio of axial column load to the nominal compressivestrength of the reinforced concrete column varies within the rage of 0.04 to 0.3, axialcompression load in the column has little effect on the shear strength of the joint. However,when the ratio is greater than 0.4, axial compression load reduces the joint shear strength. Thestiffness of joint decreases and the ductility of the column increases following the ratioincrease. Axial tension load reduces the strength, stiffness, and ductility of the joint. Axialcompression or tension force has little effect on the strength of the steel web. Axialcompression force enhances the strength of the inner and outer joint concrete, only by smallamount, so that its effect can be conservatively neglected. On the other hand, the effect ofaxial tension force in the column is converse. The dimensions of the column and beam sectionmay affect the joint failure mode, its strength, stiffness, and ductility. Exterior RCSconnection failure could take place in either one of the following modes: joint failure, beamfailure, and column failure. Failure can occur in the joint panel when the joint is weaker thanthe attached members. Beam failure takes place when its capacity is smaller than the columnflexural capacity and the shear capacity of the joint. Column failure takes place when thecolumn flexural strength is lower than the beam flexural strength and the joint shear capacity.There is a substantial shear force in joint panel and panel shear failure can often take place.RCS beam-column joint is subjected to the forces transferred to the joint by adjacent beam and column, including bending, shear, and axial loads. Axial force in the beam is usually small and can be neglected. It has only minor effect to neglect axial compressive column load in case the axial load in the column is not too large (smaller than 30% of nominal compressive strength of the reinforced concrete column). Under the action of external loads, failure process of RCS joint can generally be divide into three stages: the occurrence of the first crack along diagonal line in outer concrete, the yield of steel web, and the attainment of the joint ultimate capacity. Panel shear failure and bearing failure take place in RCS joint panel. The torsional shear force transfer between the inner and outer elements always occurs irrespective of whether the joint fail by panel shear or bearing failure. A panel shear failure, which generally triggers a bearing failure, always occurs in joint panel. Three-dimensional deformations occur in the joints due to the large difference between beam and column widths. Relative rotation between the inner and outer elements always occurs under the action of external loads. The portion of the joint inside the beam flanges (inner concrete) tends to undergo more severe deformation than the outer concrete. The inner concrete deformation includes angular distortions from both joint bearing and panel shear. The outer concrete deformation is only due to panel shear distortions. RCS joint deformation is substantially the total deformation of inner concrete. The results of finite element analysis show that joint bearing deformation account for large portion of the joint total deformation, even in case join panel undergoes shear failure. This is also proved to be true by experiments in many cases. Three mechanisms were found to contribute to the shear strength of RCS joints: a shear web panel mechanism, an inner concrete strut mechanism, and an outer concrete mechanism. RCS joint design model still adopts P-M model, but several modifications are made as follows: 1) Considering that RCS joint deformation is three dimensional deformation, the inner and outer concrete is separated and regarded as in a state of plane strain respectively; 2) Considering that joint bearing deformation is large, it should also be taken into account, besides joint shear distortion, in inner concrete; But only joint shear distortion is taken into account in outer concrete. Based on the previous joint model, strength (the nominal joint strength, the yield joint strength, the crack joint strength) calculation equations and design strength equations for RCS exterior joints are proposed. The strength at 2.0% total deformation of inner concrete is chosen as the nominal joint strength, and that at 1.0% as the yield joint strength, and that at the occurrence of the first crack along diagonal line in outer concrete as the crack joint strength. RCS joint strengths (including nominal, yield, and crack joint strength) following the proposed equations in this paper, agree very well with the results of finite element. The proposed design equations for exterior joints also agree very well with design equations from ASCE 1994 guidelines.