In conventional design and analysis, the common assumption is that connections of steel frames are fully rigid or frictionless pinned. However, today, the accepted notion is that the connections of members of a steel structure exhibit semi-rigid characteristics. Semi-rigid connections as well as damage cause changes in the dynamic characteristics of the structures. This study presents an investigation into the determination of the quality of the semi-rigid connections when considering changes in dynamic characteristics of steel structures. The investigations involve three scaled models: columns with box cross-sections, columns with rectangular cross-sections, and a 21) frame. The investigation algorithm first calculates natural frequencies and mode shapes from theoretical modal analyses by assuming the supports and joint connections are fully rigid. Secondly, experimental measurements on the models are performed to obtain natural frequencies, mode shapes and modal damping ratios. Thirdly, to reduce differences between theoretical and experimental results, linear elastic rotational springs are used on supports and joint connections of the analytical model. Finally, the connection percentages of both support and beam-to-column connections are determined using an approach improved depending on the rotational spring stiffness. (c) 2009 Elsevier Ltd. All rights reserved.