Fishbone-shaped beam-column model for steel outrigger truss-concrete wall composite joints

In high-rise and super-high-rise buildings, steel outrigger trusses are commonly used as key components for improving the structural seismic performance. The joints between the trusses and core concrete walls enable reliable transfer of large axial loads from the former to the latter and therefore deserve special attention. In the present study, an equivalent fishbone-shaped beam-column model is developed for the analysis and design of the composite joint. The joint is composed of concrete-filled steel tube (CFT) boundary columns and double-skin steel plates, which work together to transfer the axial loads. In the proposed model, elastic beam-column elements are used to represent the CFT boundary columns, double-skin steel plates, horizontal rebar, and diagonal concrete struts in the wall. Two important geometric parameters of the model, namely the length of the double-skin steel plates and the height of the CFT boundary columns, are investigated. Comprehensive parametric analyses based on elaborate finite element models with solid and shell elements are conducted to reveal the main factors that determine the two above-mentioned parameters, and to propose calculating formulas. Then the model is further verified by finite element models with shell and solid elements in terms of the axial force distribution and horizontal displacement. Finally, a method for designing the truss-wall joint using the proposed beam-column model is developed and a typical design example is presented. It is demonstrated that the design of the joint is convenient and reliable once the axial load demand is determined by seismic analysis of the entire building.