Abstract : The present report develops electrical analogs to investigate multiple cracks on simple beams and more complex frame structures. Analog networks provide the economic tool to analyze such structures. The effect of multiple cracks on the natural frequencies of simple structures is studied in detail. It is shown that closely spaced multiple cracks are indistinguishable from an effective single crack. A severe crack on a structure can be identified if there are only minor cracks in addition to the major one. If, on the other hand, there is more than one severe crack, then the damage cannot, in general, be diagnosed with only three frequencies measurable. Nevertheless, a minimum number of cracks which are likely to be present in the structure can be established. Characteristic equations are developed in the form of linear systems for cantilever beam and general frame structures with multiple cracks. Usefulness of relative-frequency-change curves are demonstrated and rough guidelines are provided to aid the damage diagnosis process. Several numerical examples are included which illustrate the effect of multiple cracks on frequencies. (Author)