Assessing how changes to a structure can create gaps in the natural frequency spectrum

Structures vibrate with discrete natural frequencies, which divide the frequency spectrum into frequency-free ranges, or spectral gaps. We may need a structure to have a particular spectral gap, and if this range is found to contain frequencies then the structure must be changed. The extent of the changes depends on the number of natural frequencies that are contained in the required spectral gap – the rank of the bracing must be at least the number of frequencies to be removed. This paper first deals with bracing of this minimal rank, connecting to the least number of freedoms possible, and later generalises this to higher rank bracing, with connections to any number of freedoms. The question is thus If a required spectral gap contains n natural frequencies, can changes to the structure stiffness of rank r ⩾ n, connecting to c ⩾ r freedoms i 1, i 2,… , i c remove them? In all cases, a simple criterion is developed for answering this question, and if the answer is yes, all successful changes are identified as mappings from more fundamental sets. The cases are developed separately even though later cases imply earlier ones, for clarity of the argument.