This paper discusses the design load modeling of the debris flow which acts on the steel frame check dam structure from the view point of the redundancy of the design, and proposes a revised design load model in which an uncertain offset and concentration effect of debris flow action could be taken into account. The actual damaged case of the steel frame check dam is treated with a theme of the discussion. The damaged case is analyzed step by step under the several assumption of the load model and structural model, in order to clarify the ununiform load effect existed in the most probable load condition on the damaged steel frame structure. First, based on 2D analysis, the structure is safe enough against the conventional design load condition given by design manual. Second, based on 3D analysis, the structure is also evaluated to the almost equivalent safety by using uniformly distributed load following the conventional design manual concept. Third, enlarging the action area of the debris flow load till the top of the structure following the uniform load concept, the safety of the structure is evaluated as smaller than that conventional load model. But the evaluated failure mode is not similar with the actual damage. Fourth, based on a kind of random load simulation in which the debris flow load acts randomly in the space, the most probable load condition shifting the load concentration center to the right side of river. Finally, in order to evaluate such uncertain load condition at the design stage conveniently, a modified debris flow design load model is proposed. The proposed model shows preferable prediction of actual failure of the structure.

Key words�Fsteel flame check dam, debris flow, unexpected load, redundancy, structural safety