Effects of differential axial shortening on outrigger systems in high rise buildings with concrete filled steel tube columns

Abstract

Concrete Filled Steel Tube (CFST) columns are popular in high rise buildings due to their superior strength, seismic and fire resistance capacities and construction simplicity. Structural framing systems in high rise buildings are commonly coupled with reinforced concrete outrigger and belt systems to facilitate lateral load resistance. When axial shortenings of vertical elements occur due to time dependent phenomena of creep, shrinkage and elastic deformations, the horizontal stiff elements balance the shortening differentials in the vertical elements and cause load redistributing among them dynamically. This can result in high transfer stresses induced in the stiff outrigger and belt systems which need to be considered in design or mitigated during construction. To plan mitigation strategies such as the time to connect the shear core to the structural frame to effectively reduce time dependent transfer stresses, it is necessary to quantify current and future differential axial shortenings. This paper first quantifies the differential axial shortening (DAS) between the shear core and columns, considering effects of construction sequence, time dependent material properties and reinforcement and then quantifies the transfer stresses built up in outrigger and belt systems in CFST high rise buildings. This information will be useful in mitigating the adverse effects of these high transfer stresses.