This paper presents the behavior of a three dimensional RC structure under bi-axial loading. The behavior is investigated in two ways: one is by experiment conducted on a test model composed of a joint with a column and two beams extending orthogonally, and the other by nonlinear finite element analysis. Good agreement was observed between the results from the analysis and those from experiments. A strut is formed from one upper side to the diagonally opposite lower side when a load is applied to both ends of one beam in opposite directions. When, by holding the tip deflections, a load is applied on the other beam in the same manner, the strut undergoes a transition and ultimately runs from one upper corner to the diagonally opposite lower corner.