Seismic Analysis Of Reinforced Concrete Frame Structures Subjected To Near-field Earthquake Ground Motions

Recently, both foreign and domestic scholars have researched extensively about near-field earthquake. But until now the effects of focal mechanism on ground motion characteristics of near-field earthquake are not well known, the definition of near-field earthquake is still not unified, and investigations of the effects of near-field pulse-like ground motions with forward-rupture directivity and fling-step on the seismic response of buildings just started. Focused on these problems, and combined with the building damage statistics during Wenchuan earthquake, this thesis involves the following work:1. First, the knowledge on near-field pulse-like ground motions is introduced and summarized, and then, the characteristics of near-field pulse-like ground motions with forward-rupture directivity and fling-step and far field ground motions are compared. It is found that the spectrum values of near-field pulse-like ground motions with forward-rupture directivity and fling-step is larger, and the response spectrums of these records change more mildly than far field ground motions. The amplification effect of near-field pulse-like ground motions with forward-rupture directivity is significant at the whole periods, while the amplification effect of near-field pulse-like ground motions with fling-step on acceleration is significant at middle and long periods, and that on velocity is significant at middle and long periods, but that on displacement is not obvious. It is indicated that these special ground motions shall be considered in the code for Chinese seismic design of buildings.2. Three codes for seismic design of buildings in China for thirty years,《Aseismic Design Code for Industrial and Civil Buildings》TJ11-78(78 edition for short),《Aseismic Design Code for Buildings》GBJ 11-89(89 edition for short)and《Aseismic Design Code for Buildings》GB 50011-2001(01 edition for short), are compared roundly. Conclusion can be made that 89 edition and 01 edition are more advanced, which qualitatively explains the phenomena that the damage of the structures which are designed according to different seismic design codes, is obviously different during Wenchuan earthquake. But 01 edition shall be improved on structure seismic design theory, near-field earthquake resistance protection, design response spectrum, response spectra ratio in vertical and horizontal component of the acceleration records and so on.3. With the method of nonlinear time history analysis, the behaviors of two four-story reinforced concrete frames, which are designed separately according to 78 edition and 01 edition seismic codes for buildings, are studied under the action of near-field ground motions with forward directivity and fling-step and far-field ground motions. The results indicate that the seismic responses magnify much when the structures are subjected to near-field ground motions with forward directivity and fling-step. When the structure is elastic, the amplification effect on base shear (acceleration) is remarkable, after the structure enters unelastic stage, the amplification effect on roof displacement and inter-story displacement become larger than base shear. Also, the responses of the frame which is designed according to 78 edition are much larger and it collapses when encountering rare earthquake while the frame which is designed according to 01 edition satisfy the request of no collapsing with strong earthquake. This work explains the phenomena quantitatively that the damage of the structures designed according to different seismic design codes, is obviously different during Wenchuan earthquake.Capacity spectrum method is applied to estimate the seismic behavior of structures. It's found that the global deformation of the frame designed by 78 edition exceeds the limitation of 01 edition seismic code for buildings. The frame designed by 01 edition has better seismic energy dissipation performance than the frame designed by 78 edition. The method of pushover analysis seriously underestimated the seismic performance of structures subjected to near-field ground motions.4. Based on the data of 371 groups of near-field earthquake records, the effects of focal mechanism and fault distance on the characteristics of near filed earthquake ground motions are analyzed.The results show that the effect of focal mechanism on the characteristics of near filed ground motion has different features, which shall be considered in the code for Chinese seismic design of buildings.Comparing with Focal mechanism, the effects of fault distance on the characteristics of near filed earthquake ground motions is smaller, and to choose 20km as the definition of near-field zone is reasonable.