Experimental modal analysis of bridges: How to employ few resources and get it right

Measuring the dynamic behavior of a structure, thus assessing its modal frequencies, shapes and damping, is a mandatory step in the tuning and validation phase of any numerical model. In this special issue, devoted to infrastructures, we focus on bridges. Many national building codes impose to experimentally assess the fundamental frequency of bridges to validate the numerical models used in the design of new structures and in the retrofit of existing ones. However, the ways in which this experimental assessment is undertaken are many and not all fruitful. In this paper, we briefly review the basic principles of the dynamic characterization of bridges and the most common acquisition practices that can lead to wrong conclusions, through a set of examples.