A sensitivity-based finite element (FE) model updating method using experimental modal data is presented. Such a procedure aims to adjust the uncertain properties of the FE model by minimising iteratively the differences between the measured modal parameters (natural frequencies and mode shapes) and the corresponding analytical predictions. In the paper the FE model updating method is applied to damage assessment (damage localisation and quantification) of structures whose damage pattern can be represented by a reduction factor of the element bending stiffness. But in order to reduce the number of unknown variables and to obtain a physically meaningful result, a limited set of damage functions is used to determine the bending stiffness distribution over the FE model. The updating parameters are the multiplication factors of the damage functions. The procedure is illustrated by a modal test before and after damage application, on a reinforced concrete beam. (C) 2002 Elsevier Science Ltd. All rights reserved.