This paper introduces the designing thoughts and calculation steps of large span excavation platform when using large permanent sinking headframe.

On the basis of introduction to the classification of the RMG structures,the global structural computing has done by ANSYS,a kind of finite element analysis software,for the different structural types of the large span of RMG.

A simplified method is adopted in defining the maximum span distance between two supports,and calculating the maximum length of pipe free end.

Arch pipe is a long-span structure made with bending and welding steel tube which is characterized by no requirement of the arch hinges.

When the customer as well as the manufacturers selects the ADSS types, besides considering the fiber numbers of ADSS, sometimes they define ADSS type based on span.

The influences of span on the motion stability of chains were discussed, and the relationship among chain sags, chain lengthes and chain numbers was studied, and the reasons for tooth-leap were analyzed, and a new-style device preventing tooth-leap was provided.