This paper proposes the M-Φ model for circular steel tube with large scale diameter- to- thickness ratio (D/t). In port pier-type structures, steel pipe piles of which D/t is 100 are frequently used for construction. Full plastic moment of circular steel tube is generally calculated by multiplying plastic section modulus and yield stress of the steel. The previous research, however, pointed out that the calculated full plastic moment overestimates the actual one. The calculation method of M-Φ model proposed in the previous research cannot be applicable to circular steel tube with large D/t so that the model should be modified.
The modified M-Φ model is proposed in this paper, with special reference to “Specifications for Highway Bridges Part V Seismic Design”, which is currently used for road construction. The modified M-Φ model can calculate the experimental results (e.g. maximum horizontal load and horizontal displacement at the maximum load) within ±30% accuracy, according to 12 experimental results. The effects of axial force ratio on the maximum bending moment and ductility of circular steel tube are also investigated by using finite element (FE) analysis. Based on the FE analysis, the range in application of axial force ratio for modified M-Φ model is proposed as the function of D/t.
This modified M-Φ model will be helpful for the seismic design of the port structure with steel pipe piles with large D/t.