In situ measured data concerning a large civil engineering structure, such as a building, a bridge or a dam, are often incomplete because of the cost of their acquisition. To overcome the spatial incompleteness of a measured mode shape, many modal expansion methods have been developed. However, the accuracy of the expanded mode shapes corresponding to high frequencies that are obtained using traditional modal expansion methods, such as Guyan expansion, IRS, and spline interpolation is not good enough. To overcome this difficulty, this work proposes an approach of spline interpolation using a pseudo-story for modal expansion. Based on the expanded first mode shape, the damage location index (DLI) is defined for locating damage. Two structures, a uniform structure (a cantilever beam) and a non-uniform structure (a seven-story shear building), are used to demonstrate the feasibility of using the proposed approach for locating damage to structures.