Analytical buckling of slender circular concrete-filled steel tubular columns with compliant interfaces S. Schnabl a, ⁎, G. Jelenić b , I. Planinc a a University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova cesta 2, 1001 Ljubljana, Slovenia b University of Rijeka, Faculty of Civil Engineering, Radmile Matejčić 3, 51000 Rijeka, Croatia abstract article info Article history: Received 23 April 2015 Received in revised form 20 July 2015 Accepted 27 August 2015 Available online xxxx Keywords: Buckling Concrete-filled Steel Tube CFST Compliant This paper presents an efficient mathematical model for studying the global buckling behavior of concrete-filled steel tubular (CFST) columns with compliant interfaces. The present mathematical model is used to evaluate exact critical buckling loads and modes of CFST columns for the first time. The results prove that the presence of finite interface compliance may significantly reduce the critical buckling load of CFST columns. A good agree- ment between analytical and experimental buckling loads of circular CFST columns is obtained if at least one among longitudinal and radial interfacial stiffnesses is high. The design methods compared in the paper give con- servative results in comparison with the experimental results and analytical results for almost perfectly bonded layers. The parametric study reveals that critical buckling loads of CFST columns are very much affected by the diameter-to-depth ratio and concrete elastic modulus. Moreover, a material nonlinearity has a pronounced effect for short CFST columns, and a negligible effect for slender ones. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Concrete-filled steel tubular (CFST) columns have been used in- creasingly in many structural applications including columns supporting platforms of offshore structures and wind turbines, roofs of storage tanks, bridge piers, piles, and columns in seismic zones and high-rise buildings. CFST columns have superior stiffness, strength, duc- tility, seismic and fire resistance, and deformation characteristics as compared to hollow steel tubes and reinforced concrete columns. Addi- tionally, CFST columns are economical and permit rapid construction because the steel tube serves as a permanent formwork and lateral confinement to the concrete fill, located at the most efficient position. On the other hand, the concrete infill increases local and global buckling resistance of CFST columns and forces the steel tube to buckle outwards rather than inwards. Moreover, with the development of self-compacting, high-strength, ultra-high-strength, lightweight, recycled aggregate concretes, and high-strength and stainless steels, the CFST construction has become even more popular in the construc- tion industry world-wide. Accordingly, a great deal of experimental [1–12], numerical [13–24], and analytical [25–29] work has been carried out recently to investigate the behavior of CFST columns under various loading conditions. A state of the art knowledge on steel–concrete composite columns including experimental and analytical studies has been reported by Shanmugam and Lakshimi [30] to highlight the significant research in this area until 1999. Similarly, Han et al. [31], have reviewed the development and advanced applications of the family of concrete-filled steel tubular structures till today. In addition, it is well known that CFST columns can sustain large axial loads. Shorter CFST columns may fail by crushing of the concrete core accompanied by local buckling and yielding of the steel tube while slender CFST columns may fail by local or overall buckling. De- spite numerous publications on CFST columns covered in literature, most of research work is focused on short CFST columns. Much less literature is available on global buckling behavior of slender CFST col- umns, and only a few papers have dealt with this subject, see e.g. [21, 32–35]. To date, however, only Han [36] has experimentally investigat- ed circular CFST columns with very high slenderness ratios. From the above-mentioned research work done on CFST columns, most of the approaches seem to be based on a simple prediction of fully bonded interface between the concrete core and steel tube. How- ever, there is a major difficulty in the design of CFST columns, which is the imperfect interface compliance between the concrete and steel tube during the initial elastic stage with high axial loads. This happens because steel dilates more than concrete. This imperfect bonding can re- duce the confining pressure provided by the steel tube and may reduce the initial stiffness and elastic strength of CFST columns considerably. This situation can be even worse for high-strength CFST columns [37]. Nevertheless, research works on composite action in CFST columns are very limited in open literature. Over the years, only a few researchers have studied numerically and experimentally CFST columns with Journal of Constructional Steel Research 115 (2015) 252–262 ⁎ Corresponding author. E-mail address: simon.schnabl@fgg.uni-lj.si (S. Schnabl). http://dx.doi.org/10.1016/j.jcsr.2015.08.035 0143-974X/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Constructional Steel Research