Numerical and analytical methods to investigate the load-bearing strength of joints in slim-floor composite frame

Abstract

The load-bearing strength of double-side joints between a hat-shaped WQ-beam and a circular concrete filled composite column is investigated by using experimental, numerical and analytical methods. The WQ-beams are installed on the consoles which are welded to both sides of composite column. The top of the WQ-beam is connected by a tension bar inserted through the column. 3D finite element models considering both material and geometrical nonlinearities are created for the joint and validated experimentally. The steel is assumed to be both isotropic hardening and ideal elastic-plastic. The results of parametric studies show that the load inside the joint is transferred in three stages. At the first stage, the increasing load is carried by shear of the web. Next the bending capacity equilibrates the increasing load, where after at the third stage the web shear takes role again. The plastic collapse of the joint is reached in the second stage. The limit load is determined by both strain and deformation-limit criteria. According to the strain criterion, the plastic strain of 10% reasonably predicts the limit load of the joint, which is higher than that at the plastic strain of 5% but smaller than that at the plastic strain of 20% where material failure occurs. For the design use, the deformation limit of 2% of flange width is recommended. According to numerical results, the load from the WQ-beam is transferred to the column mainly by console. To predict the limit load of the joint, four-hinge yield line mechanisms are developed. Due to the asymmetric nature of plastic collapse, two two-parameter models are established. One is based on pure bending at the hinges, and the other is considering also the axial force. Linear and nonlinear analyses for rotations are performed for each model. The comparisons between analytical and numerical results show that all analytical models predict the limit load of the joint at the plastic strain of 10% well. The linear presentation for rotations and the axial force at hinges have positive effect on the prediction of the limit load of joint. Since the console can withstand large strain, the analytical models with pure bending are further developed together with an existing method to predict the role of the strain hardening. The investigation shows that the analytical models with linear strain hardening provide satisfied prediction. By combing the equations obtained from two analytical models, a general equation is proposed to predict the limit load of joint. The studies show that the model considering pure bending together with linear presentation of rotations is simple to use but can be slightly conservative, whereas the model including the normal force, better predicts the load-bearing capacity of the joint. Compared to the method provided in design code as EN 1993-1-8, the proposed equation predicts the higher load-bearing capacity, leading to a more economical design in practice particularly if larger indentation is allowed.