We address the problem of structural identification using normal operating conditions: the excitation is natural and non controlled,  not measured, and can be sometimes even nonstationary -- this is typically the case when the excitation is turbulent. We present subspace-based methods using output-only measurements. We explain how to handle non simultaneously measured data from multiple sensor setups, and how robustness against nonstationary excitation can be achieved. Experimental results for three real aplication examples are shown.

Keywords : structural identification, in operation, output-only, subspace algorithms.