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The influence of cracking vs. temperature on dynamic characteristics of reinforced concrete structures

Vibration-based damage identification is based on the principle that dynamic characteristics of a structure are stiffness dependent. Changes in natural frequencies, damping ratios and mode shapes, which are obtained from successive modal tests, can therefore in principle be used as damage indicators. However, two major challenges are faced when this principle is applied in civil engineering: the sensitivity of dynamic characteristics to a certain type of damage may be low, and environmental factors such as temperature also influence the dynamic characteristics, especially natural frequencies. The aim of the present work is to gain insight into the influence of cracking vs. temperature on the dynamic characteristics of concrete structures. Past studies have concentrated mainly on changes in natural frequencies, yet the influence on displacement and especially strain mode shapes is less well known. To improve this understanding, nonlinear dynamic finite element simulations are performed. They mimic in detail the progressive damage tests and intermittent modal tests that have been performed on reinforced concrete beams at the Structural Mechanics Laboratory of KU Leuven. In the simulations, cracking and crushing of concrete as well as yielding of the reinforcement bars are accounted for, and direct time integration is used for generating simulated acceleration responses. From these acceleration time series, modal characteristics are identified using stochastic or combined deterministic-stochastic subspace identification, depending on whether an ambient or a force vibration test is simulated, respectively. The simulation results are first validated against the measured data, which include static deflections natural frequencies and modal displacements that have been identified from measured acceleration data. Then, the displacement and strain mode shape evolution, due to the progressed cracking, in studied in full detail. Finally, these changes are compared against the changes that are due to temperature variations, which are obtained from a combined thermal-structural linear finite element analysis.

Author(s):

Edwin Reynders    
KU Leuven
Belgium

Patrizia Moretti    
KU Leuven
Belgium

Kristof De Wilder    
KU Leuven
Belgium

Guido De Roeck    
KU Leuven
Belgium