The paper presents problems of modal testing of large civil engineering objects like bridges or viaducts. Authors placed here the practical information, which need to be taken into account while planning and performing this type of measurements. Descriptions of 3 modal experiments with their results are shown. All measurements and analysis were performed for the structural health monitoring and damage assessment purposes. Particularly interesting is the third case, where authors tried to assess a damage level in the road viaduct without undamaged structure model.