This paper proposes a simplified estimation method for redundancy of steel check dam, and its related design method. The estimation method uses a parameter defined as �gelemental index". The mathematical significance of the elemental index is proved in this paper, from the view point of structural degree of indeterminant. Using typical model of 6 steel check dams, this paper discusses the relationship between the elemental index and structural redundancy. First, each structure is evaluated by the proposed elemental index. Secondly, the effect of elemental index on collapse load is discussed. Thirdly, residual collapse load ratio associated with the structures removed some elements is checked and its relation to respect elemental index is discussed. And, the positive relationship between the residual collapse load ratio and the elemental index is found. Based on above discussion, this paper proposes the modified allowable stress design method, i.e., the allowable stress is reduced for low elemental index structure. The modified design method tends to adapt to the lager section area or section modulus of important members of less elemental index structures, in order to complement the insufficiency of redundancy of structure. In order to evaluate the effect of proposed design method from the view point of upgrading the probability of residual collapse load, two dimensions hazard map for falling gravels and fragility curve for each elemental character are produced, respectively. The probability of an elemental failure is calculated by using both the hazard curve for position of a member and the fragility curve for resistance of a member. Finally, it is found that the proposed method properly upgrading the residual collapse load for low elemental index structure.

Key words�Fsteel check dam, uncertain rock behavior, lack of member, redundancy, elemental index