A tool for the identification of low r.c. structures damage levels.

J. BAÑOS; J. A. PAREDES; S. OLLER; A. BARBAT; X. MARTINEZ

Conferencia; WCCM 2012; 2012

It is well known the eigenfrequencies depend on the materials used, the shape ofthe structure and on the degrees of freedom of the numerical model and not on theapplied loads. For example: the stiffness changes when the structure is damaged and,consequently, the eigenfrequencies are affected. The study of these changes allowsestablishing a correlation between the global damage level of the structure and the valueof the eigenfrequencies. Moreover, the study of the modeshapes associated with eacheigenfrequency allows localizing the damaged zone of the structure. The aim of thiswork is to establish a correlation between the global damage evolution of reinforcedconcrete structures and the change in the eigenfrequencies and in its modeshapes, usingthe Finite Element Method. In order to obtain the evolution of the global damage andthe natural frequencies for each damaged stage, an incremental load is applied upon thestructure. The reinforced concrete has been modelled as a composite material using theserial/parallel theory[1], and the steel reinforcement has been treated as an orthotropicelasto-plastic material, while the behaviour of concrete has been represented by meansof the isotropic damage model. Technics are used considerer the global orientation ofthe reinforcement. Therefore, the stiffness of each single finite element is calculatedconsidering the adequate contribution of the stiffness of its components. For theundamaged case, the original properties of each material are used to obtain theconstitutive matrix of the composite while for the different damaged stages it has to becalculated the secant constitutive matrix of the composite using the properties of eachcomponent with the correspondent level of constitutive damage. Accordingly, thestructure will have a different stiffness matrix for each level of damage.Eigenfrequencies and modeshapes are calculated using the subspace iteration method. Aglobal damage index of the structure is then calculated relating the elastic response withthe non-lineal response of the structure for each load increment. Finally the globaldamage variation and the natural frequencies variation of the structure are correlated.Examples of a cantilever and r.c. wind tower have been analysed to illustrate theproposed methodology to determinate the damaged level of structures.