Static strength of collar plate reinforced tubular T/Y-joints under brace compressive loading Hossein Nassiraei, Mohammad Ali Lotfollahi-Yaghin, Hamid Ahmadi ⁎ Faculty of Civil Engineering, University of Tabriz, Tabriz 5166616471, Iran abstract article info Article history: Received 30 September 2015 Received in revised form 25 November 2015 Accepted 6 December 2015 Available online 17 December 2015 In the present paper, the static strength of steel collar plate reinforced tubular T/Y-joints is numerically investi- gated. A finite element (FE) model was developed and the results were verified against the experimental data. Afterwards, a set of 168 FE models of collar plate reinforced T/Y-joints was generated and analyzed under axially compressive loads. The effect of plate size and joint geometry on the ultimate strength and failure mechanism of the joints were investigated through a parametric study. Results showed that the ultimate strength of a collar plate reinforced T/Y-joint can be up to 270% of the strength of the corresponding unreinforced joint. Despite this significant difference between the static strength of unreinforced and collar plate reinforced T/Y-joints, stud- ies on this type of reinforced joints have been limited to very few T-joint tests. Also, no design equation is avail- able to determine the ultimate strength of T/Y-joints reinforced with collar plates. Hence, after the parametric study, a new equation is proposed, through nonlinear regression analysis, for determining the ultimate strength of collar plate reinforced T/Y-joints under axially compressive loads. © 2015 Elsevier Ltd. All rights reserved. Keywords: Tubular T/Y-joint Axial compression Collar plate Strengthening Parametric design equation 1. Introduction Offshore jacket-type platforms, which are commonly used for explo- ration and exploitation of oil and gas reserves from hydrocarbon reser- voirs below the seabed, are mainly fabricated from tubular members by welding one end of the branch member (brace) to the undisturbed surface of the main member (chord), resulting in what are known as tubular joints [1]. Tubular joints are critical structural components playing a key role in transferring loads in a tubular structure [2]. Until now, several alternatives to increase the static strength of tu- bular joints have been introduced. Majority of these methods can only be used for structures during the fabrication (e.g. joint can, doubler plate, ring, and rack/rib), but there are only a few methods which can be used during both fabrication and operation (e.g. collar plate and FRP). These techniques can be valuable alternatives for repairing and strengthening joints in old and new structures. An existing tubular joint can be reinforced by introducing plates to the outside of the chord resulting in what is called collar plate reinforced joint (Fig. 1). In order to study the behavior of tubular joints and to relate this behavior simply to the geometrical characteristics of the joint, a set of dimensionless geometrical parameters has been defined. Fig. 1 shows a collar plate reinforced tubular Y-joint with the geometrical parame- ters (τ c , η, θ, γ, β, and τ) for chord and brace diameters D and d, and their corresponding wall thicknesses T and t. Results of previous studies showed that the use of the collar plate in tubular joints can significantly increase the static strength [3–6] and it is among the most effective methods of enhancing the static strength. Also, investigations on the stress concatenation factor (SCF) of tubular joints showed that the use of the collar plate can also lead to the signif- icant decrease of the SCF [7,8]. However, none of the main offshore design codes, such as API [9], ISO [10] and DNV [11] provide any recom- mendations on static strength requirements for collar plate reinforced joints. In spite of the key role of the ultimate strength in assessing the static performance of tubular joints, the ultimate strength of Y-joints re- inforced with collar plates has not been investigated. Also, there are very limited studies on the ultimate strength of T-joints reinforced with collar plates. Furthermore, no design equation is available to calcu- late the ultimate strength of T- and Y-joints reinforced with collar plates. Therefore, there is an urgent need for further research so that more de- tailed guidelines on static evaluation of collar plate reinforced joints can be formulated, which is the motivation of the present study. In this paper, the results of numerical parametric investigation on the static behavior of unreinforced and collar plate reinforced T-joints verified against experimental data provided by Choo et al. [3], Zhao et al. [12], and Bomel Engineering Consultant [13] are presented. After having observed the close match between the numerical results and ex- perimental data, the FE model was used to conduct a parametric study on 168 steel T/Y joints with a wide range of geometrical parameters. The analysis results are used to present general remarks on the effect of geometrical parameters including t c (collar plate thickness), l c (collar plate length), τ (brace-to-chord wall thickness ratio), γ (chord wall slenderness ratio), β (brace-to-chord diameter ratio), and θ (brace Journal of Constructional Steel Research 119 (2016) 39–49 ⁎ Corresponding author. E-mail addresses: h.nassiraei@tabrizu.ac.ir (H. Nassiraei), lotfollahi@tabrizu.ac.ir (M.A. Lotfollahi-Yaghin), h-ahmadi@tabrizu.ac.ir (H. Ahmadi). http://dx.doi.org/10.1016/j.jcsr.2015.12.011 0143-974X/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Constructional Steel Research