Operational modal analysis (OMA) is one of the most intriguing engineering fields nowadays, which ordains in the studies of modal parameters of existing small buildings under operational forces. The estimation of fundamental natural period is usually acquired from code equations which are of empirical nature, hence, the problem arises. An In-field study must be performed to acquire the fundamental period of existing structures, hence, compare the results with code equations to show the discrepancy in results. This study aims to experimentally validate the modal parameters acquired using accelerometers by comparing them with those measured using code equations and finite element method (FEM). Numerical methods such as Fourier transform, peak picking, and the frequency domain decomposition (FDD) technique were used for modal parameters extraction and validation. In addition, finite element modeling of the existing structure, including support flexibility, was investigated. The studied structures include six case studies ranged in height from five to twelve stories buildings. The structural system includes moment resisting frames (MRF) and shear wall moment resisting frames (SWMRF) resting on raft foundation. The experimental results showed that the natural frequencies obtained showed discrepancies between infield results and code equation that reached 200% in some cases. Finally, a novel method for estimating the fundamental natural period was derived using field measurements. The validity of the proposed formula paves the way for more effective identification for the fundamental natural period for moment resisting frame structures.