The Research On Non-linear Damping Properties And Seismic Response Of Reinforced Concrete Buildings

The seismic responses of buildings are remarkably affected by the damping properties of the structural system. The studies results from relative literature over the last decade revealed that damping ratios of buildings are found as non-linear parameters, and the constant damping ratios as recommended in current most codes and standards for the building design throughout the world can not reasonably reflect damping mechanisms of real buildings, and may lead to significant under-estimation of the seismic response level of buildings. Forced on non-linear damping properties of reinforced-concrete buildings and its seismic responses analytical methods, this dissertation involves the following work:1. Based on analysis of collected damping ratios test data of actual buildings, structural models and engineering materials, include reinforced-concrete, steel and cement mortar components, damping mechanisms of the real buildings are discussed in depth, main influencing factors in damping properties of reinforced-concrete building are quantitatively analyzed, and the development trend of non-linear damping ratios is analyzed in detail.2. According to the analytical results of damping mechanisms and test data of damping ratios, it is concluded that the main influencing factors of damping properties of reinforced-concrete buildings are strength of concrete, percentages of longitudinal steel, structural configurations and deformation amplitude. And then, non-linear damping ratio formulae of reinforced-concrete buildings with various structural types are firstly proposed. The proposed damping ratio formulae are able to correctly reflect damping mechanisms of buildings, and are applicable for estimating damping ratio of reinforced-concrete buildings.3. After discussing disadvantage of traditional identification methods, and expatiating on the advantages and process of Hilbert-Huang transform(HHT) method in dynamic character parameters and damage identification of buildings, how to extract model responses by empirical model decomposition technology with intermittency check criterion, how to estimate instantaneous frequency and detect damage development trend by HHT, how to identify first model shape by relative Hilbert-Huang marginal spectrum are studied. Examples are presented to demonstrate the validity of the proposed methodology.The concept of local damping ratios is proposed, identification method of local damping ratios from free dynamic response or seismic response of structures are given.Using proposed identification method, dynamic parameters of two buildings in Northridge earthquake are detected. Analytical results show that the proposed method can effectively identify instantaneous frequency and local damping ratios, and reliable estimate structural damage of analyzed buildings.4. The influences of non-linear damping ratios on dynamic response are investigated by numerical analysis, the analytical results indicate that there are remarkable differences between dynamic response based on non-linear damping ratios and constant damping ratios, and the structural seismic response level based on constant damping ratios recommended by current code for seismic design of buildings may sometimes be significant underestimate.5. After non-linear damping ratios modification coefficient of seismic design spectrum are proposed based on the non-linear damping ratio formulae proposed in chapter 2, non-linear damping ratio seismic design spectral curves are established by time history analysis. The seismic design spectral curves established in this dissertation not only take into account the effect of seismic fortification intensity, site condition, structural type and the shift of damping ratio on the seismic response of buildings, but also can be linked with current seismic design spectral curves recommended in Chinese code for seismic design of buildings(GB 50011-2001).6. Seismic design method based on the non-linear damping ratio response spectrum is discussed in detail. After the examples of two reinforced-concrete buildings are analyzed, the differences of analytical results with proposed non-linear damping ratio response spectrum and constant damping ratio spectrum recommended in current code are compared.