Modal properties of a steel frame used for seismic evaluation studies

A modular four storey, two by two bay steel frame of about 1:4 scale was recently acquired by the Earthquake Engineering Research Laboratory at the University of British Columbia. It will be used for research and education projects on the behaviour of steel frames under different loading conditions caused by earthquakes. The modular nature of the frame permits the investigation of different configurations, mass distributions or the inclusion of special elements like dampers or base isolators. The frame was specifically designed to fit on the UBC multi-axis shake table, a state-of-the-art facility for seismic testing and qualification. This paper reports on the calibration procedure, both experimental and analytical, that was carried out to determine the dynamic properties of the frame. Impact and ambient vibration tests were used to determine fundamental frequencies and mode shapes, which were then compared to calculated values from a linear elastic analysis. The stiffness of frame connections was shown to be one of the most significant variables, while the torsional modes proved to be the most sensitive to program input parameters.