Quantification of changes in modal parameters due to the presence of passive people on a slender structure

This work proposes a method to predict changes in modal parameters of a generic structure due to the presence of passive people. The method requires to accurately know the modal model of the empty structure, i.e. natural frequencies, damping ratios and mode shapes. No restriction on the number of degrees of freedom of the structure is required. Each passive subject on the structure is modelled through his apparent mass and is introduced locally on the empty structure to obtain a model of the joint human-structure system. To include the apparent mass of the subjects in the model both measurements and models available in literature were used. The effect of using different models of the apparent mass was investigated and the effect of using apparent masses obtained with different levels of vibration was analysed. The proposed model was validated by means of experimental tests carried out on a lightly damped staircase. The empty structure was first tested to obtain its modal model in order to apply the proposed approach. After performing several tests with passive people standing on the structure, the experimental results showed small changes of natural frequencies (increase or decrease) and a high increase of damping ratios due to people's presence. Thus, the proposed approach was used to predict the above-mentioned changes. The measured and predicted frequency response functions, natural frequencies and damping ratios were compared. In all the cases taken into consideration, the predicted results fitted with the experimental evidence.