Exact and accurate solutions in the approximate reanalysis of structures

Combined approximations (CA) is an efficient method for reanalysis of struc tures where binomial series terms are used as basis vectors in reduced basi s approximations. In previous studies high-quality approximations have been achieved for large changes in the design, but the reasons for the high acc uracy were not fully understood. In this work some typical cases, where exa ct and accurate solutions are achieved by the method, are presented and dis cussed. Exact solutions are obtained when a basis vector is a linear combin ation of the previous vectors. Such solutions are obtained also for low-ran k modifications to structures or scaling of the initial stiffness matrix. I n general the CA method provides approximate solutions, but the results pre sented explain the high accuracy achieved with only a small number of basis vectors. Accurate solutions are achieved in many cases where the basis vec tors come close to being linearly dependent. Such solutions are achieved al so for changes in a small number of elements or when the angle between the two vectors representing the initial design and modified design is small. N umerical examples of various changes in cross sections of elements and in t he layout of the structure show that accurate results are achieved even in cases where the series of basis vectors diverges.