Abstract: The traditional classification methods of member importance are often exclusively applied to linear-elas-tic structures，and they are not suitable for latticed structure with strong nonlinear effect． Meanwhile，these classi-fication methods also have limitations． To solve this problem，a classification method of member importance based on strain energy was proposed for latticed structure． Based on strain energy theory，the concept of total strain ener-gy at critical point was introduced． Taking total strain energy at critical point as the evaluation index，the influence of local member's break down on the load-carrying capability of whole structure was analyzed． Thus each member's importance was figured out． Ｒeduction coefficients of total strain energy at critical point were considered as indexes of member importance． According to present principles of member importance classification，and based on K-me-doids classification method，a classification method of member importance with non-fixed boundary was proposed．The analysis results of Kiewitt lattice shells show that the classification boundary of member importance should not be fixed to a certain value． The number and location of important members vary in different failure cases，so they should be determined based on actual information． Method proposed in the paper can provide more information
than the design stress ratio method in detecting important members in structure． Classification method of member importance based on alternative path method should not be used for the normal inspection and appraisal of struc-tures．

Key words: lattice shell structure in service, member importance, strain energy, K-medoids classification method