Uniaxial material model for reinforcing bar including buckling in RC structures

Abstract

The constitutive model of steel reinforcing bar incorporating inelastic buckling is rucial to accurate seismic performance evaluation of the existing reinforced concrete structures. According to experimental observations, in presence of inelastic buckling the absolute maximum stress of the rebar in compression could reduce to half of the value in absence of buckling. In this thesis, based on fiber element model, the effect of the yield strength on the critical slenderness of the rebar is studied. Then the formulas, incorporating the effect of yield stress, are given to calculate the computational slenderness critical slenderness. Next the anisotropy of some stainless steel rebar is studied according to a series of monotonic and cyclic tests on bare stainless steel rebars. Two parameters are proposed to consider the effects of anisotropy of rebar. Considering the above studies, the modified Monti-Nuti Model is proposed to improve the applicability. In order to eliminate overestimation of stress when the model is subjected to generalized loading, the criteria to update the parameters in the model for each half branch at the reversal are discussed and new strategies to update the model parameters in different unloading or reloading cases are proposed. The Framework adopting Genetic Algorithm is designed to identify the parameters in the modified Monti-Nuti model. By minimizing the stress difference between the numerical curve and experimental curve at each strain step of the cyclic loading histories, the optimized parameters could be identified. Then the empirical formulae are proposed to calculate the values of the parameters in a simpler, more efficient and still robust way. The modified Monti-Nuti model is implemented into OpenSees as a new material model named as “steel05”. Then validation of this material model with the experimental curves of carbon steel rebars and stainless steel rebars confirms the effectiveness and significance of this new model. Furthermore, the application of the new material in the structural analysis of circular reinforced concrete piers is made and the numerical curves are compared with the experimental curves to verify the effectiveness of the new material. Finally, the effects of corrosion of the rebar, which is inevitable in aged reinforced concrete structures, are studied based on a series of experiments on the bare corroded reinforcing bars. It is found that the mean yield stresses of the corroded rebar could be different in tension and in compression. The computational length of the rebar increases resulting from the deterioration of the confinement from the transverse loop. The corrosion extended model is proposed to consider the aforementioned characteristics resulting from corrosion.