Research Of Cfst Arch Bridge Seismic Response And Dynamic Stability Under Near-fault Ground Motions

In recent years, global seismic activity is more and more frequent, thus it needs more strengthened research in seismic design theory. The ground motion is a strong non-stationary process, and the nonlinear response results of CFST arch bridge vary enormously because of the influence of earthquake spectrum characteristics. Meanwhile, the research of structure dynamic instability needs to proceed from seismic response, the influence of the earthquake spectrum characteristics can not be ignored in dynamic stability analysis also. To research the relationship with the nonlinear response and the earthquake spectrum characteristics is an advanced subject yet, and it will involve with digital signal processing and nonlinear dynamics interdisciplinary knowledge. At present, there is no appropriate mathematical analytical model to describe non-stationary of the earthquake yet, but time-varying spectral estimation of the earthquake can make connections roughly between time-frequency distribution characteristics of the earthquake energy and the corresponding structure nonlinear responses.This paper is mainly concerned with the influence of earthquake spectrum characteristics to seismic response and dynamic stability of the CFST arch bridge, and the other influencing factors such as material nonlinearity, geometrical nonlinearity and initial geometric imperfection are given an intensive study also. Large-span arch bridge with long period is liable to meet with a serious damage by the action of strong near-fault earthquakes. In view of this fact, the spectrum characteristics of the near-fault earthquakes and the reaction characteristics are summarized, and a novel assessment indicator is proposed consequently through wavelet analysis method to identify seismic action effect of the near-fault earthquake. An intensive study on the problem of the CFST arch bridge dynamic stability under the action of near-fault ground motion is carried out also, and the characteristics of dynamic instability forms are investigated thoroughly. The main content of the paper includes the following aspects:(1) Several modern time-frequency distribution estimate methods are studied, and they are able to describe the non-stationary characteristics of the seismic signal in a certain extent. With the advantages of fast algorithm and small calculated data, local spectral density through improved L-P wavelet analysis is proved to be of great engineering application value. A novel time-energy density analysis method through dyadic orthogonal wavelet transform is proposed, and it can be beneficial to describe seismic energy distribution characteristics along time domain.(2) Nolinear seismic response analysis is conducted through an example of CFST arch bridge with132m main span, and the bridge type is a box girder and CFST arch combination bridge. The influence of initial stress effect about modal analysis is considered also. In general, research results suggest that geometry nonlinear effect is not obvious to the seismic response, and material nonlinearity influence appears to be more importantly. With the increasing earthquake effect, the development of yielding plastic materials can change the laws of internal force distributing, and improve the structure seismic performances to some extent. The earthquake signals are taken decomposition or reconstruction process by wavelet transform to research the influence of earthquake non-stationary. It is demonstrated that response spectrum can not sufficiently show the connection between earthquake spectrum characteristics and structure response, but the wavelet method has distinctive advantages in revealing earthquake time-frequency distribution characteristics and seismic effects of certain frequency components.(3) Near-fault ground motion spectrum characteristics are studied in-depth by the records of Taiwan chi-chi earthquake (CHI CHI,09/20/99) at three sites. The results show that with the enhancing near-fault seismic effects, low-frequency energy pulses become intensively and sustainable, and the peak value of acceleration response spectrum may also appears in long cycle section. Since long-span CFST arch bridge is liable to suffer a great destruction by near-fault ground motion with low-frequency impulsive effect, the near-fault earthquake acceleration signal is decomposed by dyadic discrete wavelet analysis in frequency domain then. Combined with the characteristics of low earthquake frequency pulses, a new assessment indicator WAPA (Weighted Average Peak Acceleration) of the seismic action is presented, representing the ideology of performance-based seismic design.(4) Dynamic stability research about the CFST arch bridge is carried through, and the instability critical load is determined by the dynamic incremental analysis (IDA) method and Budiansky-Roth system response criterion. It is indicated that material nonlinearity played a dominant role in the dynamic buckling development, and there is merely theoretical meaning only if the influence of geometric nonlinear is taken into account. At the same time, the influence of initial geometric imperfection can be ignored because of low sensitivity to the structure. Near-fault pulse-like ground motions are likely to resulting extremal dynamic instability, and the corresponding critical load will be significantly dropped. If the assessment indicator WAPA is chosen as the intensity measure (IM) in IDA analysis, the discrete level of the IDA curve cluster can be alleviated observably, and the performance features of dynamic instability about CFST arch bridge caused by near-fault pulsed movements will be more apt to be represented also.