Thin–Walled Structures 148 (2020) 106590 Available online 20 January 2020 0263-8231/© 2020 Elsevier Ltd. All rights reserved. Review Stiffened concrete-flled steel tubes: A systematic review Faesal Alatshan a, b, * , Siti Aminah Osman a , Roszilah Hamid a , Fidelis Mashiri c a Smart and Sustainable Township Research Centre, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia b Civil Engineering Department, College of Engineering Technology, Houn, Libya c School of Computing, Engineering and Mathematics, Western Sydney University, Sydney, Australia A R T I C L E INFO Keywords: Concrete-flled steel tube (CFST) Stiffeners Bond strength Composite Systematic review ABSTRACT The behavior of concrete flled steel tubular (CFST) elements with different types of stiffening measures has been wildly investigated during the past two decades through experimental, analytical, and numerical approaches. The previous studies highlighted the importance of the presence of internal and external stiffeners to enhance the structural performance of CFST elements under different loading conditions. In this paper, a systematic meth- odology is proposed in order to rigorously review the current knowledge in literature about stiffened CFSTs. The reviewed studies include tests of static axial compression, pure bending and combined of static loading actions as well as the cyclic behavior and the fre resistance. A coherent taxonomy regarding the research of stiffened CFSTs is proposed based on the loading conditions besides their cross-sectional shape and type of stiffeners. The study also presents a number of practical applications of stiffened CFSTs in different engineering projects. Finally, concluding remarks, current research gap and future research directions on stiffened CFST are provided and discussed. 1. Introduction In the last few decades, concrete flled steel tubular (CFST) elements have been widely utilized in civil structural applications, particularly in high-rise buildings and bridges. This demand is due to their signifcant advantages including favorable strength, good ductility, excellent seismic performance and high fre resistance [1]. For CFST elements, the steel tube has three main roles, frstly, supporting the elements against different loading conditions, secondly, provide lateral confnement resisting the crushing of the inflled concrete, and thirdly employed as permanent formwork. While the concrete has two roles of resisting the loading and postponing the steel tube inward local buckling. However, to ensure the achievement of the above advantages, the steel-concrete composite bond must be good enough to act together as one element especially during the early elastic stage. The weak concrete-steel inter- action can be due to shrinkage of concrete resulting after temperature variation or because of the difference in lateral dilation of steel tube and concrete core. Extensive research efforts have been conducted in the last two de- cades to improve the performance of CFSTs by adopting a variety of steel stiffening schemes. The steel stiffeners were utilized to enhance the bond behavior between steel tube and the concrete core and to develop the concrete confnement strength provided by the steel tube. In addition, using the right steel stiffeners can enhance the structural performance of CFST elements and thus reduce their cross-sections as well as the required construction material, simultaneously increase the indoor available space. This study aims to presents a systematic review of the performance of CFST members with steel stiffeners under various loading conditions. The study also intends to classify the previous research landscape of stiffened CFSTs and discover the emerging lines of research. 2. Search method Relevant research articles were searched using the keywords query that illustrated in Fig. 1. The keywords were divided into two parts; the frst part is to fnd articles related to CFST elements in general, while the second part was to limit the search results into the articles that study the behavior of CFST elements with steel stiffeners. Four online databases of peer-reviewed journals were employed to conduct a full-text search using the adopted query. The databases are (1) “ScienceDirect” [2], “ASCE Library” [3], “Springer Link” [4] and “Taylor * Corresponding author. Smart and Sustainable Township Research Centre, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia. E-mail address: p94378@siswa.ukm.edu.my (F. Alatshan). Contents lists available at ScienceDirect Thin-Walled Structures journal homepage: http://www.elsevier.com/locate/tws https://doi.org/10.1016/j.tws.2019.106590 Received 11 June 2019; Received in revised form 5 December 2019; Accepted 30 December 2019