連続して地震を受ける杭基礎建物の応答特性に関する影響評価

<p> 1. Introduction</p><p> Recently, the importance of the structural resiliency and functional continuity of buildings affected by earthquakes has become increasingly evident. Studies have reported that damage to the pile foundation often have major impacts on the continued usability of the buildings⁷⁾. However, these studies focused on the seismic performance and response characteristics affected by damages to superstructures⁵⁾. In this study, mid-rise reinforced concrete (RC) buildings having a pile foundation were simulated using the three-dimensional finite element method (3D FEM). Subsequently, the response characteristics of the superstructure and pile were evaluated after the pile foundation was damaged by a large earthquake.</p><p> 2. Overview of analysis</p><p> A six-story mid-rise RC building with a foundation made of 5×5 pre-stressed high-strength concrete (PHC) pile was used as the study subject. The superstructure was modeled as a multi-degree-of-freedom system using beam element. Soil and pile were modeled using solid element and beam element, respectively. Linear superstructure behavior and non-linear pile and soil behaviors were modeled in order to focus on the influence of the damage to piles.</p><p> The applied seismic motions were set to the wave notified in the Kobe and Random phase defined as from Level 1 to Level 2 earthquake in the Japanese seismic design codes. The seismic motions were continuously input once or twice by combining each seismic motion to vary the extent of damage to the piles from an earthquake.</p><p> 3. Response evaluation of static load analysis and single input analyses</p><p> The stress of the piles varied according to the location of the pile, which is well-known as the pile group effect^15),22). First, the validity of that effect for the analysis model was confirmed by static load analysis. Subsequently, the response characteristics of the superstructure and pile were confirmed when a seismic motion was input only once. The response of the superstructure showed signs of leveling off, while that of the piles intensified with increase in the amplitude level of the input seismic motion. Piles burdened with a large stress due to the pile group effect could reach yield and ultimate state easily.</p><p> 4. Response evaluation of series input analyses</p><p> In this chapter, response characteristics were confirmed according to the level of damage to piles when seismic motions were input twice continuously. Further, these characteristics were compared with those for no damage to piles, obtained in chapter 3. In particular, the relation between damage progression of the piles and maximum amplitude of the series seismic motion were studied. Also, the rate of increase in the response of the superstructure according to the damage state of the piles were studied.</p><p> 5. Conclusion</p><p> In this study, the response characteristics of a mid-rise pile foundation building were evaluated after the pile foundation was damaged by a large earthquake. The main results obtained were as follows:</p><p> 1) Increasing damage progression of the piles was confirmed even though the amplitude of aftershock is less than or equal to that of foreshock because of increasing soil response and decreasing energy dissipation of pile with serious damage of a part of piles according to the foreshock.</p><p> 2) The natural period of the coupled soil-building system and that of the building conspicuously increased when 40% of the piles attained the ultimate state with fracturing even though the nonlinearities of the soil are considered.</p><p> 3) The response of superstructure increased and decreased with the changes of the natural period of the coupled soil-building system and frequency response characteristics of the series earthquake because of decline in influence of energy dissipation of pile with the fracturing of piles.</p>