Nonlinear Modeling of Mat Foundation Differential Settlements in Seismic Evaluation of High-Rise Buildings


Abstract eng:
Differential foundation settlement can impose significant demands on a building’s seismic force resisting system and reduce the structure’s seismic capability. This paper describes an inventive evaluation methodology of the effects of such settlement on a tall reinforced concrete shear wall building. Seismic evaluation of building structures experiencing large differential foundation settlement is a complex task given current limitations of commercially-available analysis software, as well as many geotechnical and structural uncertainties. Under static conditions, the effect of differential settlement on structural adequacy can be assessed by modeling the structure and its foundation, imposing a deformed shape on the foundation model or base supports to match measured or estimated settlement contours, and evaluating structural demands on the structure and foundation system. However, for seismic analysis, the foundation deformation should adjust as the mathematical representation of the building structure is subjected to simulated ground motions and as mat nonlinearity develops. The evaluation of the high-rise building and its foundation was carried out in several stages using CSI Perform- 3D. The pile cap and mat foundation supporting the building were represented by a nonlinear beam grillage model with flexural and shear hinges, which was validated through finite element analysis. Hinge properties were estimated based on multiple layers of top and bottom flexural reinforcement, as well as vertical dowels used as shear reinforcement, provided throughout the foundation. Externally applied vertical point loads in select locations of the mat and underlying vertical soil spring properties were iteratively determined to produce a settlement profile matching measured deformations, while allowing additional mat deformation under ground motion excitation. The building model included explicit representation of basement retaining walls, first floor slab, building core walls, moment resisting frames, and outriggers to capture the relative stiffness of these elements and their effect on the foundation deformation. A suite of 7 ground motion records was imposed on the combined superstructure-foundation model to assess the effect of settlements on the building’s seismic adequacy using Performance-Based acceptance criteria.

Conference Title:
Conference Title:
16th World Conference on Earthquake Engineering
Conference Venue:
Santiago (CL)
Conference Dates:
2017-01-09 / 2017-01-13
Rights:
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 Record created 2017-01-18, last modified 2017-01-18


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