In this paper we numerically study simple two dimensional frame structure loaded by impulse-like excitations. Various configurations of time duration of a single impact load and location of this excitation is investigated. The aim of this research is to indicate the best possible load case for identification of a specific damage. There are considered two kinds of a structural modification. The fist group concerns the element stiffness changes (axial and bending) which can be simulated by alternating the parameter of Young's modulus. The second group covers the modifications of stiffness nodal connections. In a~numerical model, these kind of defects can be implemented using rotational spring with different characteristics. Obtained selected responses for original and modified structure is examined.