Analysis on the Mechanical Behavior of RC Column to Truss Joint in Staggered Truss System

The staggered truss structural system originated from America in the 1960s, which has been widely used. Typical staggered truss structural system consists of columns, floors, plane trusses, etc. The H-shaped steel, steel pipe, and concrete-filled steel tube (CFT) can be used as structural columns. The slabs can be designed to be hollow slab, composite slab, cast-in-place reinforced concrete slab, etc. In addition, there are three typical types of trusses, i. e. open-web truss, parker truss, and hybrid truss. This structural system has advantages of large space, light structural weight, convenient construction process, economic benefits, high efficiency, and environmentally friendly. In addition, the structure has light weight and good integrity, resulting in a small internal force of the column. When steel columns are used, the slenderness ratio is usually considered as the design control condition, resulting in a waste of materials. The precast reinforced concrete (RC) column is characterized by solid section and convenient construction process. Application of precast RC column into the staggered truss structural system can make better use of the mechanical properties of the column and improve economic benefits of this type of structural system. However, a robust column to truss joint is necessary for the application of RC columns. China Construction Science & Technology Co., Ltd., designed and developed a prefabricated staggered truss structural system consisting of RC columns, steel trusses and pre-stressed concrete floor. The inelastic static analysis of this structural system was conducted by Chongqing University to evaluate its seismic performance. A host of experimental and theoretical studies have been conducted to investigate the seismic performance of the truss and staggered truss structural system. For the column to truss joint of staggered truss structural system, preliminary investigations are still limited, and previous studies mainly focused on the mechanical performance of steel or CFT column to truss joint.
In this study, a finite element model (FEM) is established to investigate the mechanical behavior of the proposed RC column to truss joint used in a practical project. The stress distribution, internal force and loaddisplacement curves of the joint under five kinds of most unfavorable working conditions in the structural analysis are analyzed. The investigated parameters include the axial load ratio of RC column, with/without restriction of chord horizontal displacement, and load condition of web members. The results are as follows:1) The internal force of the member can be transferred to the corbel or embedded steel plate through the gusset plate and then transferred to the column through the anchor bars, demonstrating that the joint is designed reasonably with a clear force transfer path. 2) All kinds of members yield before of reinforcement within the joint region, meeting the design requirement of "strong joint and weak member". 3) The material strength of the chord and web is better used in the case where the horizontal displacement of the chord is restricted despite of a lower degree of reinforcement stress development in the RC column. 4)The load bearing capacity and displacement are greater in the cases where the horizontal displacement of the chord is not released, which is benefit from a better use of material strength of the chord and web member. In the case whose axial load ratio is 0, the load of the web member increases approximately linearly with increasing the displacement of the web member at the initial loading stage. The plastic deformation within the joint region gradually increases once the displacement of the web member exceeds 5 mm, resulting in a slower increase of the load of the web member. 5) The bending moment and shear force of the chord are generally small in all working conditions except for the shear force in the case whose axial load ratio is 0. The axial force of the chord is significantly larger in the cases where the horizontal displacement is restricted and the axial force of the chord increases by approximately 10%. 6) It is suggested to appropriately increase the longitudinal reinforcement ratio of the RC column to alleviate the eccentric effect of the column caused by the corbel, further improve its safety reserve.