Experiment and analysis on X-shaped reinforced concrete joint in diagrid structures

Diagrid tube structures are expected to be used for the outer tubes of super tall tube-in-tube buildings. The mechanical behavior and design method of X-shaped reinforced concrete joints, which are formed by four crossed inclined columns and horizontal girders, are important issues in the design and analysis of vertical crossing lattice tube. Taking the volumetric ratio of the stirrup and the loading pattern as fundamental parameters, four X-shaped reinforced concrete joints were designed and subjected to static axial load testing. The test phenomenon-including deformation of joint zones, development of cracks, and crushing of concrete-is strongly emphasized in the analysis. The laws of the stiffness response and stress development in longitudinal bars and stirrups are revealed, and the ultimate load-bearing capacity of four joints is obtained. The test results show that the stirrups may confine the lateral expanding deformation of the joint concrete regions. The X-shaped reinforced concrete joints with normal volumetric ratios of stirrups may be designed to provide large stiffness and high load-bearing capacity, which are also effective in transmitting the inner force from four crossed inclined columns. Adopting two loading patterns-agreeing and differential loads-on four inclined columns has little influence on the load-bearing capacity of the joints. Based on the results of the test and analysis, design recommendations and formulas of the bearing capacity for X-shaped reinforced concrete joints in vertical crossing lattice structures are proposed. Copyright © 2013 American Concrete Institute.