Due to the importance and high costs of construction, the important buildings are still in use. During their service life, these structures face various damages. Some of the damaged areas are visible and identifiable, and others are invisible. If these damages are not identified sufficiently, irreparable human and financial damages will occur. Therefore, in the past few decades, numerous experimental and theatrical studies have been carried out for structural damage deification. These damages cause some changes in the dynamic characteristics of the structures, such as stiffness and damping. This paper examines the modal strain energy method for damage identification. Due to the limitation of the number of sensors in the structures, the location of the damage cannot be determined accurately. In the study, modal strains are calculated using the cubic spline interpolation method and the number of measurement points is increased by using MATLAB software. Then, the location of the damage in the steel beam is identified by the modal strain energy method. To determine the damage location, the threshold value is defined for each damage scenario. The data used in this paper is based on the information obtained from a real bridge I-40. Based on these experimental data and utilizing modal strain energy methods, the damaged areas are identified in the steel plate girder of bridge I-40.