Differential axial shortening and its effects in high rise buildings with composite concrete filled tube columns

Use of concrete filled steel tube columns is becoming increasingly popular in high rise buildings due to their composite action, superior strength, seismic and fire resistance capacities and construction simplicity. These composite columns and the reinforced concrete (RC) lift core in the framing systems of high rise buildings are commonly coupled with reinforced concrete outrigger and belt systems to facilitate lateral load resistance. Axial shortening (AS) of the vertical structural components due to time dependent phenomena of basic creep, shrinkage and elastic deformation, is a common problem in concrete high rise construction. The creep and shrinkage of these composite columns develop more rapidly, but are comparatively lower in magnitude than for RC columns due to the concrete core having no direct exposure to the external environment. There is a need for a comprehensive understanding of the differential axial shortening (DAS) in concrete filled tube (CFT) buildings which will be different from that in a RC building. An appraisal of the DAS and accurate quantification of all the adverse effects that can occur in a building due to DAS, are required to facilitate a safe and efficient design. This paper develops and applies a comprehensive technique to evaluate the DAS in a high rise building with composite CFTs. This technique incorporates the effects of (i) construction sequence and concrete levelling (ii) stress relaxation of concrete due to the presence of the steel tube (iii) time dependent material properties and (iv) effects of belt and outrigger systems. The technique has been validated using experimental data and is then illustrated through its application to a 60 storey building with CFT columns. The DAS between the vertical members are evaluated and its effects on the structural components are studied. Finally, the technique is applied to a similar building with RC columns and the results compared with those from the CFT building. The technique developed in this paper and the new information on DAS generated will facilitate safer designs of buildings with composite CFT columns.