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Two to five piles are placed in each pile cap at a spacing 2.5 times the pile diameter (500 mm). Pile layout and pile damage locations 2.2 Foundationsįorty pre-stressed, high-strength concrete (PHC) spun piles support the building (Figure 1). After the earthquake, only the analyzed building was tilted. The building is a part of housing complex composed of 3 buildings connected with one another by expansion joints. Although the building has a nearly square plan with dimensions of 10.95 m × 12.9 m across grid points 1C to 3E, a heavy staircase and an elevator within grid points 3D to 4F make the building shape irregular. In the EW (east-west) direction, simple filled-walls are attached to the building frame. Shear walls are placed in the NS (north-south) direction except that baseline 4 is braced. The building consists of steel-reinforced concrete columns up to the fourth floor and reinforced concrete (RC) columns above. The target building 3 is an eight-storey residential building that is 23.4 m in height. 2 Outline of the analyzed building 2.1 Superstructuresįigure 1 shows the building foundation plan and the differential settlement of the footings, with the damaged piles designated by numbers.
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The inertia force and axial force at the pile heads were estimated based on the results of elastic analysis.
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The seismic response of the building and the ground deformation were evaluated based on a free field ground response analysis using ground motions recorded at an observation point near the site. In addition, the pile group effect was considered depending on the position and spacing of each pile in a group. The nonlinear characteristics of the soil springs were also adjusted in accordance with soil type and depth (overburden pressure). Based on a previous study 2 on the seismic performance of pile foundations during severe earthquakes, nonlinear flexural rigidity of the pile element and rotational stiffness at the pile head were adopted. In the analysis model, a remarkably damaged frame was removed and used as a calculation model consisting of piles with soil springs. An analytical study was performed using a foundation structure model to simulate the distinctive pile failure modes observed during the 2011 off the Pacific coast of Tohoku earthquake.
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The authors focused on a case in which pile damage resulted in building tilting although the pile foundation design was based on a simple linear elastic analysis in accordance with the existing design code in 1987. These conditions indicate that significant effects of nonlinear behavior of piles and soil materials must be considered when simulating the pile failure process.
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In addition, pile damage notably occurred in precast concrete piles known to have non-ductile deformation characteristics, and the soil in which damage was observed consisted of very soft clay, a liquefiable layer and/or non-homogeneous strata in most cases. Since approximately 26% of the damage occurred in buildings constructed since 1985, the year of construction does not appear to be a crucial factor in the damage to piles. According to the damage investigation, 1 74% of the damaged buildings were constructed before 1985. Most of the damaged buildings were constructed before the Japanese code requirements for the seismic design of foundations went into effect in 1985, while several of them were constructed after 1985. Pile foundation repair and careful jacking-up to level were required to restore the buildings for use. Such damage to pile foundations, which has been commonly observed in past major earthquakes, resulted in building structure tilting even though superstructures remained practically intact. Strong ground shaking during the 2011 off the Pacific coast of Tohoku earthquake (the 2011 Great East Japan Earthquake) damaged pile foundations over wide areas in Miyagi and Chiba prefectures in addition to causing various other forms of foundation damage through liquefaction-induced ground deformation, slope failure and complete destruction by tsunami.