Analysis of the Behavior of Tubular T Joints Strengthened by FRP under Compressive axial Load

In recent decades, significant progress has been made on the use of Fiber Reinforced Polymers (FRP) in civil infrastructures. These materials have been used widely for the repair of concrete members, but their application to steel structures has been so far limited. Offshore structures, especially offshore platforms are very important and expensive ones, due to rising demand of energy. These structures may require repair and strengthening due to damages they may suffer in service. Searching for methods which are faster, more reliable, and less expensive led to application of FRP in repairing of offshore structures. Durability and high resistance against fatigue as well as high ratio of strength to weight enable these material superior to other conventional materials for this purpose. In addition high resistance of these materials against corrosion is an advantage for their use in marine environment. Composite materials are being increasingly used for the strengthening and repair of offshore structures. More recently, carbon fiber reinforced laminates have been used to upgrade fire walls in Mobil's Beryl Bravo platform to enable them to withstand blast loading. Also a number of corroded conductors and caissons have been repaired by composites on several Gulf of Mexico and North Sea platforms.
In current research, the behavior of T-shaped tubular joints reinforced with FRP material under compressive axial load is studied to evaluate the efficiency of these materials in strengthening the connections. For this purpose and in order to examine the effect of different variables, a numerical study was carried out using the non-linear finite element program. An elastic-perfectly plastic stress-strain curve was used for steel and glass/epoxy composite was used as the FRP. A four-node quadrilateral shell element was used to model the tubular members and composite. A perfect bond between steel and composite was considered. For considering different modes of FRP failure the criteria proposed by Hashin is used. The numerical model was verified using the data available for a T- joint which was tested earlier. The model showed acceptable accuracy especially up to the level of maximum strength of the joint. Using the verified model a number of joints with different strengthening scheme ware analysed under a monotonically increasing axial compression loads. Material and geometric nonlinearities were considered in the analyses. The analysis method was modified RIKS algorithm. The effect of number of FRP layers and the fibers direction on the ultimate capacity of reinforced tubular joints was studied. The results of numerical analysis showed improvements in the capacity of reinforced joints which were further enhanced by increasing the number of layers. Comparing FRP failure initiation load with ultimate capacity of joint, it was found FRP can bear a considerable amount of ultimate load without breaking. In addition, by comparing the Von-Misses stress and the vertical and horizontal displacement (ovalization) of chord in reinforced joints were observed that a substantial reduction in mentioned factors led to hindering the yielding of steel and increased the stiffness and ultimately the strength of connections.