地震時における杭基礎ラーメン高架橋の非線形および構造全体系を考慮した入力損失効果と耐震設計への実装に関する研究

概要

In recent years, Japan has experienced an increasing number of short-period-dominated, high-acceleration earthquake motions. These earthquake motions far exceed the standard design response spectrum for short-period regions. However, only a limited amount of damage to structures founded on piles and some other types of structures is found to be caused by such earthquake motions. One of the possible reasons for this is the earthquake motion filtering effect through kinematic interaction. It is well known that unlike soil’s free field behavior, pile foundations restrict the motion of the surrounding soil during an earthquake and modify the input earthquake motion for structures. If the filtering effect is taken into consideration in the seismic design, the structure can be rationalized. Several studies focusing on the kinematic interactions for pile foundations are available in the literature. These available studies, however, have various limitations such as:
・ a pile foundation rigidly connected to a rigid footing is often examined. However, the
filtering effect of a pile foundation, where a pile is connected to elastic beam, is largely
unknown.
・ geology and stratum are complicated in Japan and soil conditions often change greatly
within the structures. In such cases, seismic motion with a phase lag is input at each point
of the structure. Since each element of the rigid frame is rigidly connected, the rigidity and
the three-dimensional motion of the superstructure affect the motion of the pile foundation.
It is necessary to develop a phenomenon elucidation and evaluation method for filtering
effect by rigidity and motion of the whole structure system.
・ most of the studies assume that both the soil and piles behave linearly. This linearity-based
approach, however, cannot be straightforwardly applied to the filtering effect caused by
strong earthquake motions as they essentially involve nonlinearity of both the soil and piles.
・ impact of filtering effect on the short-period-dominated earthquake motions is largely
・ past studies evaluates the filtering effect by using a single layer soil model under linear
conditions. This approach, however, cannot be applied to actual soil and structural
conditions. Therefore, it is necessary to develop a simple and general-purpose evaluation
method for the filtering effect that can be applied to the static analysis method, which is the
mainstream in seismic design practice.
・ there is no research on reduction of element size and reinforcing bar that is possible by
considering filtering effect in seismic design.
In this study, theoretical and analytical studies are conducted on the abovementioned shortcomings of the available research in the literature. Considering the filtering effect of a rigid frame viaduct structure founded on piles, the phenomenon elucidation and the implementation in seismic design are examined.
This dissertation consists of a total of 7 chapters. Chapter 1 details the background of research, review of past researches, and identification of shortcomings in the past researches. Chapters 2 through 6 are roughly divided into two aspects regarding filtering effect, namely - "clarification of phenomena" and "implementation for seismic design". Chapters 2, 3, and 4 detail studies on the phenomenon elucidation, while Chapters 5 and 6 summarize the studies on implementation in seismic design. The outline of the research contents and results of Chapters 2 to 6 are as follows.
In Chapter 2, the following studies are conducted for a rigid frame viaduct with pile foundations: (1) characteristics of filtering effect due to pile foundation, (2) validity of filtering effect evaluation by mass system model, and, (3) filtering effect of pile foundation connected to an elastic beam.
Firstly, the characteristics of filtering effect due to pile foundations are examined. The characteristics are clarified by performing parametric studies with varying ground and pile foundation specifications using an analysis method combining the thin layer element method and equivalent beam method. As a result, the filtering effect due to the pile foundation has a shorter wavelength relative to the pile length when the frequency is higher, so that the effect of restraining the surrounding ground by the pile becomes greater and the characteristic of the filtering effect occurs. It is also found that greater the relative rigidity of the pile foundation to the ground, the greater the filtering effect.
For the verification of the analysis accuracy of the mass system, the validity was carried out next by comparing the analysis results of the mass system model and the detailed analysis method. Based on the results, it is clarified that the results from the mass system model and the detailed method are in a good match; the filtering effect can be evaluated accurately in the mass system model.
The filtering effect of pile foundations connected to flexible beams was examined for four foundation types: (a) single pile, (b) group pile, (c) 1 column-1 pile type, and (d) pile-vent. The filtering effect was examined by dynamic analysis using a mass system model. Results show that the filtering effect is larger in the order of: group pile, 1 column-1 pile type, pile-vent, and single pile. In the group pile, the filtering effect is the largest due to the constraint effect of the pile head behavior due to the rigid footing. In addition, in the 1 column-1 pile type and pile- vent structure, the filtering effect is caused by resisting the ground motion due to the frame structure of the pile and the beam, and it is clear that the filtering effect can be expected as in the group pile.
In Chapter 3, effects of the non-linearity of the soil and the pile foundation on the filtering effect are examined. Parametric studies were performed by changing the elasto-plasticity of each element of the soil and pile foundation, and the effect of nonlinearity on the filtering effect was evaluated. Results indicate that as the degree of nonlinearity of the soil increases, the rigidity of the pile with respect to the soil becomes relatively high and the filtering effect increases. On the other hand, it is also clarified that the nonlinearity of the pile makes the rigidity of the pile relatively small with respect to the soil and reduces the filtering effect. However, the effect is smaller than when the soil is nonlinear.
In Chapter 4, the filtering effect generated from the whole structure system is examined. Dynamic analyses were carried out using a two-dimensional and three-dimensional model for a 1 column-1 pile type rigid frame viaduct. The filtering effect mechanism of the entire structure from the response results of the 3D model was examined. As a result, it became clear that the filtering effect due to the behavior of the whole structure system is caused by the horizontal rigidity of the pile foundation and the whole structure system.
In addition, to evaluate the filtering effect of the whole structure system, results of the 2D model and the 3D model were compared. Comparison reveals that at the center cross section of the structure, a larger filtering effect is generated by considering the pile and horizontal rigidity, compared to the filtering effect (two-dimensional model) generated from the pile foundation. On the other hand, at the edge of the structure, it is found that the filtering effect considering the pile foundation and horizontal rigidity is smaller than the filtering effect of the pile foundation (two-dimensional model). This is because of the rotational movement of the structure.
In Chapter 5, methods for implementing the filtering effect of pile foundation structures in the static analysis method, which is the mainstream in seismic design, are examined. In addition, effect of filtering effect on short-period-dominated earthquake motion is also examined. Firstly, a method for evaluating the filtering effect was developed. The filtering effect due to the pile foundation was calculated by the seismic deformation method and the effect due to the superstructure was assumed to be that the ground motion was obliquely input on the foundation (superstructure), and the theoretical solution of the foundation motion was derived. The filtering effect of the whole structure was evaluated by superposing the pile foundation and superstructure. The validity of the evaluation method was then verified by comparing the results of the evaluation method and the dynamic analysis method.
Next, a method for easily reflecting the filtering effect in the nonlinear response spectrum used in the static analysis method was investigated. The method is a means of correcting the spectrum by calculating the response spectrum ratio using random vibration theory. The validity of the evaluation method was then verified by comparing the nonlinear response spectrum calculated from the evaluation method and the iterated integral method.
The effect of the filtering effect on the short-period-dominated earthquake motions was investigated. Based on the observed earthquakes in the Tohoku Earthquake, the nonlinear spectra of the structure considering the filtering effect of both the pile foundation and the whole structural system were calculated. It is clarified that the reduction of the structure response becomes remarkable in the short period region due to the filtering effect in the earthquake motion containing many short period components.
In Chapter 6, the effect of filtering effect on the response values and verification values of the structure and the rationalization of the structure are examined by conducting a trial design considering the filtering effect for a rigid frame viaduct with pile foundation. The study was carried out by the static analysis method according to the seismic design standard of railways. In the examination, response values and verification values were compared. In addition, changes in the amount of reinforcing bar and element dimensions due to the filtering effect were compared. Results show that the inertial force acting on the structure is reduced by the filtering effect. Consequentially, the cross-sectional force generated by each element decreases and the design margin increases. In addition, the element size and reinforcing bar can be reduced by considering the filtering effect. From this, it became clear that the structure could be significantly rationalized by considering the filtering effect in the seismic design.
Finally, in Chapter 7, the summary of the current study is presented followed by future direction of the current work.

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