Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Experimental and parametric studies of SCFs in FRP strengthened tubular T- joints under axially loaded brace Alireza Sadat Hosseini a , Mohammad Reza Bahaari a, ⁎ , Mohammad Lesani b a School of Civil Engineering, College of Engineering, University of Tehran, Iran b School of Civil Engineering, College of Engineering, Sadra University, Iran ARTICLEINFO Keywords: Tubular T-joint SCF FRP Experiment Finite element Parametric study ABSTRACT An experimental program was carried out on an axially loaded steel welded tubular T-joint made of circular hollow sections and strengthened with Glass/vinyl ester to study the efect of Glass Fiber Reinforced Polymer (GFRP) material on the Stress Concentration Factors (SCF) at both crown and saddle points on the chord member. Four tests were executed. The frst test was performed on the plain joint which served as the reference for comparison, and the remaining three tests were performed on the strengthened joints with various FRP strengthening schemes. The arrangement of tests as well as specimen details together with most pertinent results arediscussed.AsignifcantdropinSCFvalueswasobserved fortheFRP-strengthenedT-jointascomparedtothe SCFs of the corresponding un-strengthened joint. The fnite element model of the T-joint specimens were made and verifed against the experimental data. Furthermore, extensive numerical analyses comprising of1188 fnite element models were performed to achieve a better understanding of the efect of the FRP parameters as well as the joint’s geometric parameters on the chord's SCFs. Additionally, through nonlinear regression analysis, SCF design equations were proposed for the fatigue design of axially loaded FRP strengthened tubular T-joints. 1. Introduction Nowadays, the growing use of fber reinforced polymer composite materials have been successfully employed in various types of struc- tures either in land or ofshore. Generally, in civil engineering, these materials are predominantly being considered for structural re- habilitation and strengthening of damaged as well as intact structures. Since using FRP materials for the aforementioned purposes is quite diferent from that of steel as a traditional and well known reinforcing material, technical knowledge of its behavior under various loading condition is essential. The most signifcant and inherent advantage of FRP over traditional materials is its superior strength and stifness and light weight. Also, the outstanding corrosion resistance of fber re- inforced polymers contributed to its lifelong durability under harsh environmental conditions. Accordingly, in some areas like the marine structures with its complicated design nature and limitations, applica- tion of qualifed composite materials claims to be a viable and bene- fcial alternative to the conventional details and materials. Owing to the cyclic nature of the loads exerted on many types of structures,fatigueanalysisofthejointsinthesestructuresplaysamajor role in the design process. In particular, fatigue life of ofshore struc- tures is normally assessed using Stress-Life (S-N) curves, which correlates the stress range to number of cycles that the structure can resist. In this way, the stress concentration factor (SCF) can be utilized for the calculation of the stress range in hot-spot area (HSSR). Accordingly, as per API [1], the SCF values for design purposes is de- fned as the ratio of HSSR over the nominal stress at the brace. Aside from the loading conditions, in another categorization of the joints, SCFs can be considered in unstifened joints as well as stifened ones. Most researches on unstifened joints over the last few decades aimed at deriving parametric equations for SCF calculation. As ex- amples of early studies that led to introducing a set of parametric equations for SCF calculation in T, Y, X, K and KT-joints under various loading conditions are Kuang et al. [2], Wordsworth and Smedley [3], Efthymiou and Durkin [4] and Hellier et al. [5]. Parametric equations were presented through curve ftting on the existing experimental SCF data. Lloyd’s Register (LR) [6] presented a great number of SCF para- metric equations covering SCF estimation at saddle and crown points of T, Y, X, K and KT-joints. Some years later, when the need for strengthening existing tubular joints became necessary, Ramachandra et al. [7] presented design formulae after performing analytical and experimentalstudiesonSCFsinringstifenedTandY-joints.Inanother study performed by Nwosu et al. [8], the stress distribution along the ring stifened T-joints was studied and geometrical parameters as well https://doi.org/10.1016/j.engstruct.2020.110548 Received 16 October 2019; Received in revised form 16 March 2020; Accepted 18 March 2020 ⁎ Corresponding author. E-mail addresses: a.sadat@ut.ac.ir (A. Sadat Hosseini), mbahari@ut.ac.ir (M.R. Bahaari), m.lesani@sadra.ac.ir (M. Lesani). Engineering Structures 213 (2020) 110548 0141-0296/ © 2020 Elsevier Ltd. All rights reserved. T