Analytical solution of the response of the I-shaped beam to the tubular column connections

Off-shore platforms constructed for oil and gas production are prone to high potential fire risks. These probable fire incidents may cause local or global structural damages, which in turn can result in serious consequences such as causalities, destruction of the facilities, and damage to the environment. It is therefore necessary to design these structures so as to ensure the least amount of loss after a possible fire event. Topsides of the offshore platforms are often provided with portal or truss-type structures. The truss, usually, consists of I-beams as chords and tubulars as diagonals. For the main joints of heavy topsides, I-beam-to-tubular leg connections with external diaphragms are usually employed. I-beam-to-tubular column connections with external diaphragms are important in decks and topside structures of the oil/gas platforms. In onshore steel structures, some experimental and numerical studies have been carried out to investigate the behavior of connections in fire. However, the number of studies on the behavior of connections of offshore platforms in fire and post-fire conditions is very limited. Previous studies on fire in offshore platforms mainly deal with numerical simulation and risk assessment related to offshore structures exposed to fire. Recently, the authors have investigated the behavior of I-shaped beam to cylindrical hollow steel (CHS) column connections with external diaphragms, at elevated temperatures. In this paper, a closed-form analytical solution for the prediction of moment-rotation and the rotational stiffness-rotation curves of I-shaped beam to cylindrical column connections, commonly used in off-shore platforms, in room and elevated temperatures is presented. In order to define the behavior of the connection a bounding line moment-rotation model based on the works of Al-Bermani et al. was proposed. Observing the moment-rotation behavior of the connections using the numerical models, it was concluded that the bounding line model is suitable the model for determining M-φ and k_j-φ curves of these connections. The required yield and plastic moments in this model were provided by the authors extending Roark's relationships. Therefore, having the complete geometry of the connection and the yield stress value of the material it is easily feasible to determine the yield moment and plastic moment of the connection and its high-temperature behavior. Then, having the values of yield and plastic moments it is possible to depict M - φ and k_j - φ curves of these connections in ordinary temperatures. Comparing the analytical results to experimental and numerical results indicates that the analytical relationships present acceptable approximations. The required yield and plastic moments in this model are provided as an extension to Roark's relationships. Relating the I-shaped beam to cylindrical column connection's stiffness in high temperatures to ordinary temperatures, it is then possible to extend the M - φ and k_j - φ curves of ordinary temperatures to high temperatures using the above equations. The results of this model are compared with those of a non-linear coupled mechanical-thermal finite element model previously provided by the authors, which was in turn validated using small-scale and large-scale experimental tests. Reasonable agreement has been found between the analytical model results and the experimental/numerical modeling results.