The pier-type structures used in ports are frequently constructed using steel-pipe piles with a large diameter-to-thickness ratio (D/t) of 100. Seismic performance evaluations with level-2 earthquake motion show that there are many cases in which a large axial force acts on the batter piles of a pile-supported wharf and on the coupled raking piles of a sheet-pile quay wall. The full plastic moment of a circular steel tube is generally calculated by multiplying the plastic section modulus by the yield stress of the steel. The results of previous research, however, revealed that the values obtained for the full plastic moment using this method exceed the actual value. The modeling of steel-pipe piles in a two-dimensional effective stress analysis, which is usually performed as part of the seismic performance evaluation of a port structure, often relies on bi-linear elastoplastic behavior, the relationship between the bending moment and curvature. The full plastic moment is applied as the maximum bending moment of this modeling and is evaluated as the limit value of the performance of the steel-pipe piles. To effectively evaluate the structural performance (e.g. the strength and maximum deformation) of a steel-pipe pile with a large diameter-to-thickness ratio, it is necessary to develop a rational modeling method which eliminates the dangerous evaluations obtained using the abovementioned full plastic moment and which is capable of taking the deformation performance (e.g. the ductility of the steel) into account.
In this research, we propose the use of a modeling method based on beam elements which is usually used in two-dimensional seismic response analysis, based on results of three-dimensional FE analysis. The proposed method can evaluate the strength of steel pipe from the diameter-to-thickness and axial force ratios. It adopts the ductility factor as a performance criterion in place of the full plastic moment. As a result, it can be used to evaluate the deformation capacity of a steel pipe with large diameter-to-thickness and axial force ratios. The structural performance of a cantilever, as determined with the proposed method, was compared with that determined with the conventional method.
Key Words: steel pipe pile, pile-supported wharf, local buckling, full plastic moment,
seismic performance evaluation, seismic response analysis