It is known that the expected failure mode of piping systems under excessive seismic motion would be the fatigue failure at elbows or tees. Although there are a lot of experimental and analytical investigations on the strength and the failure behavior of elbows under seismic loading, there are a few studies on tee pipe joints particularly in the inelastic range. To clarify the characteristics of the strength of tee pipes under elastic-plastic region, investigations by FEM analysis were conducted. In this analytical investigation, steel butt-welding type tee pipe joints were considered. Two kinds of parameters were considered; 1) the loading direction (in-plane bending / out-of-plane bending), 2) the branch size (equal tee / unequal tee).

From the analytical examination, the load-displacement relationship and the distribution of the accumulated plastic strain concentration position were obtained. The difference of the strength in the loading direction and the expected failure locations depending on the branch pipe size and the loading direction are discussed.

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