Study On The Dynamic Response Of Group-piles Foundation In Deep Water Subjected To Different Ground Motions

The group-piles foundation is an important bearing part of a deep-water bridge,the seismic performance of the group-piles foundation has attracted much attention in engineering and academic circles.The seismic design should consider the influence of dynamic water on the structure of the group-piles foundation.Under the action of ground motions,dynamic water will not only change the dynamic characteristics of the structure,but also significantly affect the structural response.At present,the study about the dynamic response of group-piles foundations is relatively scarce,and the seismic research of group-piles foundations under the special long-period ground motions is even less.Therefore,it is important to study the seismic performance of group-piles foundations under different types of ground motions in more depth.In this paper,the author selects a typical group-piles foundation as the study object.Based on the fluid-structure coupling theory,the dynamic response characteristics and failure modes of the group-piles foundation under different types of ground motions are analyzed.The main work of this paper includes:(1)The elastic analysis model of a group-piles foundation,which considering the coupling of structure and water is established.Starting from the distribution of the additional mass of dynamic water of the group-piles foundation,the differences in the calculation of the additional mass of dynamic water,including Morison equation,simplified radiation wave theory and railway seismic code are compared,and the influence to the change of water depth on the natural vibration period of the structure is studied.(2)For the near-field pulsed ground motions and the far-field harmonic ground motions,the characteristics of the long-period ground motions and the general ground motions are analyzed from the basic characteristics and energy-input characteristics of ground motions.It is pointed out that the peak values of near-field pulsed ground motions have high intensity,which are larger than other ground motions.However,the peak values of acceleration and velocity of far-field harmonic ground motions are relatively small,but the difference between the peak values of displacement is very small in comparing with near-field pulsed ground motions.The distribution of frequency bands of long-period ground motions is mainly concentrated on the low frequency part about 0.1～1.2Hz,while the distribution of frequency bands of general field motions is mainly concentrated on the high frequency part about 1.3～2.5Hz.(3)The nonlinear analysis model of pile-groups foundation is established,and the seismic response of pile-groups foundations in deep water under E1 and E2 ground motions is analyzed,which including near-field pulsed,far-field harmonic and common ground motions and comparing with the Chinese seismic code of highway bridge.In addition,by introducing the tensile and compression damage factors into the model,the damage evolution and damage mode of pile-groups foundation under ground motions are studied.(4)Based on the simplified radiation wave theory,the seismic response of pilegroups foundations under different types of ground motions with different water depths is carried out,and the safety of structures is evaluated from the perspectives of structural displacement,stress,shear force and bending moment in combination with China’s building seismic design code.When selecting the near-field coefficient of 1.5as the seismic input coefficient according to the Code for Seismic Design of Buildings(GB50011-2010),the seismic design of pile-groups foundations in deep water under near-field pulsed ground motions is not safe.Finally,I would like to express my sincere thanks to Ningbo Municipal "Science and Technology Innovation 2025" Major Special Project(2019B10076)for the financial support of this project.