Two step design method is proposed

First Step

 For a boulder which occurs frequently, the design metod based on the allowable stress is suitable.
 Structures behave in the elastic range.
 Permanent deformation or damage must be little.

Second Step

 For a boulder  which occurs unusually, the ultimate strength must be verified.
 Maximum deformation must be restricted.
 Sudden failure must be avoided.
 Flexibility of members and deformability of material are required.

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