Recently, Energy consumption due to the economic recovery of the developed countries and the economic growth of the developing countries increasing but the resource development easiest land and continental shelf resources has reached almost exhausted condition. Therefore, In order to obtain the scarce resources required the development of deep-sea resources to lowly economics, and the oder quantity of offshore plant equipment is increasing accordingly. This offshore plant says a production, storage, residental facilities, for the development of deep-sea resources, and it is consist of a structure for supporting the hullside structure and the topside structure. Because offshore plants are a various facilities installed in order to get at the most a limited space and a small accident which may occur during the operations occur a large accident that causes serious marine pollution and loss of life and property, it occur from the preliminary design stage reflects all situations and it shall ensure the stability and reliability of the structure. Surrounding environment reflects and use often the aluminium alloy in order to increase the corrosion resistance and light weight. Aluminium alloys dose not appear a definite yield point unlike steel and plastic deformation beyond the elastic limit is initiated from the initial stage has a greater work hardening. also, HAZ generated during welding wider than the steel therefore HAZ appears significantly the effects on ultimate strength. aluminium alloys material nature not applicable intactly the design criteria applicable to the steel structure. accordingly, to determine the aluminium alloy characteristics and the collapse of the structure, it is necessary to establish ultimate limit state design criteria. Due to the excellent corrosion resistance and high strength, aluminium alloys are mainly used in large structures such as marine bridges, offshore and more. In this paper, It is performed the numerical analysis for A6082-T6 alloy plate girder subjected to patch loading and we propose a reasonable prediction equation to determine the collapse behaviour characteristics. Concentrated loads or patch loading introduced in the plate girders is common load in most steel structures and it occurs to be made up a rolled beam, welded girders, cold formed girder in the structures. Up to recently, Welded I-beam girder used to consider carbon, carbon-manganese, quenched and tempered steel has been studied preponderantly. A failure mode of structures has been studied extensively through a experimental and theoretical method, a numerical analysis. it performed to the elastic-plastic large deflection series analysis using the experimental model and predicted a failure mode and ultimate strength. The collapse mode of numerical analysis model is formed a plastic hinge on loaded flange and consistent with the collapse mode of experimental model. Also, The yield line is formed in the web could observed that have occurred the crippling collapse mode and it was found that very satisfied to the experimental results and the applied rules. if it is considered to be maintain a reasonable safety level, it is possible to predict the failure modes of A6082-T6 alloy plate girders and ultimate loads. A6082-T6 alloy plate girder based on this results proposed for the structural strength evaluation predict equation and established the numerical analysis method and process. These results will be useful for evaluating the ultimate strength of A6082-T6 alloy plate girder in the preliminary design.