Analysis And Realization Of Structural Robustness In The Light Of Energy-Absorption Of Structures

As the course of modernization progresses rapidly, civil engineering and its surroundings become so complex that continuing safety of structures is confronted with severe challenges. At the same time, the adoption of novel materials and techniques causes substantial uncertainties in structural safety. Moreover, routine structural design and analysis are performed based on the conception that structures resist external actions in pristine states, and the influence of accidental localized damage on structural system safety is neglected. Under these situations, unconventional safety problems manifest themselves, and the risk of disproportionate failure increases excessively in case that accidental damage occurs. The tragic events of 9/11 have led to unprecedented concerns on the issues of disproportionate failure and structural robustness, and so it becomes imperative to study accidental damage and its effect on structural safety.The thesis focuses on strutcural robustness analysis and mitigation of the risk of disproportionate failure, the main work and conclusions of which are listed as follows:First, the drawbacks of current mothods, regarding structural design and analysis, in dealing with the influence of accidental localized damage on structural system safety are analysed profoundly from five aspects, i.e. the development of civil engineering, typical problems associated with structural safety, the differences between routine design methods and those related to robustness, structural durability, risk control and disaster mitigation.Secondly, the thesis summarized existing qualitative requirements of structural robustness, and argued herein that structural robustness can be interpreted as the behavior of a structure to remain stable under accidental localized damage. The thesis clarifies the core idea and extension of structural robustness further: structural robustness stresses the ability of a structure to remain system safety and avoid disproportionate failure and, hence, it doesnot necessarily reflect the load-bearing capacity, ductility, reliability, redundancy or solidness of the structure in its pristine/pre-damaged state; in addition, structural robustness relies on the behavior of structural systems and the surroundings as well. Thirdly, two typical failure mechanisms of strucures are proposed from the viewpoint of energy: first, a structure fails when it cannot absorb the input of energy; second, a structure fails since the energy transform between the structure and its environment is arrested as a result of drawbacks of structural form. Structural robustness is explained in the light of phenomenological probabilistic models, i.e. a robust structure is capable of absorbing energy input arising from external actions, while keeping the resultant failure somewhat proportional to the original cause. Following these conceptions, a new method to robustness appraisal is developed, where a dimensionless form of the amount of energy the structure absorbs serves as a robustness index, on condition that the structure satisfies its design capacity requirement.Finally, based on the viewpoint of energy absorption, the thesis proposed two basic ways to improve the robustness of a structure, i.e. by mitigating the energy input during the course of structural response and thus reducing the energy absorption demand on the structure, or by improving the energy absorbing capability of the structure. Then five strategies for realizing structural robustness are proposed which aim to provide theoretical basis for mitigating the risk of disproportionate failure.The present thesis draws several conclusions in the subjects of the core idea and extension of structural robustness, the interpretation of robustness and structural failures based on energy principles, robustness apparaisal, and risk mitigation of disproportionate failure. In the face of the increasingly complex surroundings, considerably more work is needed to investigate system effect of structural systems, and to define key factors which influence robustness and energy-absorbing capability of structures.