Modelling localised failure of reinforced concrete structures by impact actions
Document TypePhD thesis
Access StatusOpen Access
© 2021 Zireen Zanoofar Abdul Majeed
Impact-resistant design of reinforced concrete (RC) structures has a prominent role in mitigating hazards caused by extreme events. An example of such a structure is the RC barrier that serves as a protective measure in mountainous areas to protect lives and built infrastructure. Impact actions can be divided into localised and global actions. Local actions are controlled by the amount of impact force generated at the point of contact, and it is referred herein as contact force. The magnitude of contact force can be much higher than the quasi-static force, which can cause permanent damage to the structure. Gabion cushion layers are commonly used to protect localised damage in RC passive defence measures. Gabions are expensive and require extensive maintenance. Other structural concrete elements exposed directly to collision hazards or an abrasive environment are still required to design to withstand the contact forces developed during projected impact scenarios. Localised damage in the form of denting, spalling and punching failure are primarily resulted in RC members subjected to low-velocity impact actions. Predicting localised damage by impact action is more complex than global effects as the prediction depends on the determination of highly transient forces and stresses. These impact forces are highly influenced by the material behaviour and structural dynamic behaviour of colliding objects, which have not been incorporated in the expressions stipulated in contemporary codes of practices. Due to the complex nature of the impact, a significant amount of studies in the literature to date on localised damage are based on empirical data obtained from experimentation or numerical simulations. The major limitation with the empirical modelling is uncertainties over the scope of applicability as the derived relationships employ a specific range of impact events. Models developed based on theories also have limitations, but they are more transparent and can be amended to suit the conditions of impact, thereby has the merit of generality. Because of that, theoretical models consisting of closed-form expressions and are executable on MATLAB and Excel are proposed in this research project to determine the localised failure of RC structures. The recent development of the Hunt and Crossley theoretical model for accurately predicting the impact force has been reported for windborne debris and hail impact. The main drawback of this methodology is that the determination of model parameters requires calibration against impact experimentations using spherical impactors. In this research, an inexpensive experimental procedure based on the use of compression testing of cylindrical specimens of impactor and target objects is proposed to determine the model parameters. The innovation presented in this study waives away the need for costly and time-consuming impact experimentations and preparation of spherical samples. The accuracy of the proposed methodology to predict the impact force has been validated against impact experimentations and finite element simulations for impact by granite on concrete surfaces. This deterministic methodology can be capitalised for predicting damage to concrete. Denting and spalling of concrete on the impact surface is caused by the stress developed in the vicinity of the contact region. This study elaborates the adaptation of an analytical model derived from the fundamental theories of solid mechanics for the determination of transient effective stress contours. The accuracy of the simulated stress contours has been verified employing FE simulations which were previously validated against experimental measurements. The calculated stress contours at the instance of peak impact force and hardness of the target that is subjected to strike are to be employed to estimate the degree of damage in the proposed procedure. This theoretically based model to predict the occurrence of denting and spalling has been verified with large-scale experimentations on RC barrier specimens using solid steel and rock impactors. A deterministic analytical model has been developed to predict the occurrence of punching failure in RC based on conventional “free-body diagram” analysis. The proposed model accounts for the transient actions of impact (impact force and inertia force) generated within the shear plug in combination with material resistant forces. Given that the impact force can be predicted using the previously developed deterministic methodology, a numerical procedure that can be implemented on MATLAB or Excel to calculate the inertial resistance is introduced. The proposed punching shear failure predictive model has been verified by pendulum style impact experimentations on RC wall specimens. Further validations are incorporated employing drop test results on RC beam specimens reported in the literature. Predictive models presented in this thesis to determine the local response behaviour of RC are beneficial in the design of RC structures that are constructed in the proximity of collision hazards. Worked examples and step-by-step procedures are illustrated individually in the corresponding chapters of the thesis. In order to facilitate the uptake of the proposed methodology for industrial applications, complete design guidelines combining the deterministic approaches investigated in this research project with detailed calculation steps are presented towards the end of the thesis.
KeywordsConcrete; Reinforced concrete; Impact; Dynamic analysis; Contact force; Compression test; Rock; Rockfall; Boulder; Dynamic stiffness; Hunt and Crossley model; Barrier; Protective measure; Localised damage; Denting; Spalling; Punching shear; Failure
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