The majority of housing tenements in developing countries constitute a class of structure termed open ground story (OGS) of medium rise and highly vulnerable to strong seismic motions. Evaluation of the displacement demand magnifier (DDM) for quantifying the vulnerability of OGS structures and variations in the DDM with reference to the stiffness ratio (between OGS and infilled story) represent the contribution of this paper. The relationship between the stiffness and frequency ratios of the first two bending frequencies is established and highlighted. A methodology for evaluating the DDM of an existing OGS structure through measurement of frequencies in situ is established and validated. Expressions for computing the demand capacity ratio (DCR) of OGS frames have also been derived. The developed procedure is applied to an OGS structure that was damaged during the 2001 Bhuj earthquake. A formulation to compute the bending frequencies of a free-free discrete frame is derived, which is also a contribution. Details of the shake table experiment conducted to validate the given hypothesis are presented. To establish proof of concept, ambient vibration measurements have been taken on an existing OGS structure and its DDM is evaluated. © 2017 American Society of Civil Engineers.