Structural health monitoring is used to control the health of regularly deteriorating special structures or those subjected to sudden loading. Damage localization and severity estimation are important goals of researchers. In this study, an existing modal strain energy-based approach is enhanced by a proposed updating procedure to detect damage and estimate severity. Numerical validation is conducted on a cantilever beam and a portal frame. The updated results obtained using the suggested technique are better compared with those of the existing method. On the basis of only the fundamental mode shape, the updating technique can successfully localize damaged elements regardless of the complexity of damage scenarios, and it can estimate severity with acceptable accuracy levels as long as the modal data are free of noise or submerged in noise at low levels.