Dynamic response of tall buildings with basement considering soil structure interaction

Abstract

The construction of tall buildings has gone through exponential growth in recent years, creating new earthquake engineering and design challenges. Prior studies, and existing seismic design guidelines, have suggested that the current fixed-base hypothesis, which omits the soil around the base of the structure, limits an accurate evaluation of the dynamic behavior of tall buildings. Buildings constructed in Santiago, Concepción, and Viña del Mar (Chile), for example, are getting taller and have more basement levels than before. The involved loads and dimensions make larger portions of soil volumes participate in the system’s dynamic response. Dynamic Soil-Structure Interaction (DSSI) can modify the seismic response of such tall buildings and the response of the soil around the structure. Unfortunately, DSSI effects for tall buildings have been inconclusive. Thus, from a design perspective, it is unclear under which conditions consideration of DSSI is necessary and when it can safely be avoided. The Council on Tall Buildings and Urban Habitat, in conjunction with UNESCO and CIB, recognize this necessity and urge to evaluate the modeling assumptions for seismic analysis of tall buildings, including the creation of a database of results, comparison with current design assumptions and philosophy of study, as well as determining changes of seismic performance of such structures. The central hypothesis of this research is that the internal forces and deformations in the superstructure, and the lateral earth pressures on the basement walls, are greatly affected by DSSI effects in tall buildings with large embedment depths, founded in granular soils when subjected to large-intensity ground motions. The main goal was to identify structure heights, basement depths, soil properties, and earth motion characteristics that are critical to producing significant soil-structure dynamic interaction effects in tall buildings with basement levels. The specific objectives of this project are: (1) to develop numerical models of tall buildings, including DSSI, which allow proper consideration of, e.g., radiated waves from the building to the surrounding soil; (2) to identify, at the superstructure level, how DSSI modifies the seismic response when compared against those obtained using a fixed-base condition, typically assumed in structural analysis; and (3) to evaluate the seismic earth pressures acting on basement walls considering both inertial and kinematic interaction. A combined numerical-experimental approach was used to tackle the main objective of this study. The research includes: (1) the development of 3D numerical models of existing centrifuge tests of tall buildings, (2) the calibration of the numerical models against measured values on the selected centrifuge tests, (3) parametric analyses for a suite of different structural, geotechnical, and earthquake variables, and (4) the identification of the conditions that require the incorporation of DSSI effects. For the parametric analyses, buildings of 20, and 55 stories, with 2, 4, and 7 basement levels, founded on sandy and gravelly soils, typically found in large cities in Chile, were analyzed. The research combines efforts of the Pontificia Universidad Católica de Chile and the geotechnical team of the Center for Infrastructure, Energy, and Space Testing at the University of Colorado, Boulder (USA), and the co-guidance and help from two professors from Universidad de Los Andes, Chile. Findings show how seismic response parameters, such as inter-story drifts, structural demands, lateral earth pressures acting on the basement walls, and overall structural response change when DSSI is correctly incorporated in the seismic analysis of tall buildings. Thus, this project has the potential to improve safety standards, methodologies, and seismic design codes for adequately evaluating the dynamic performance of tall buildings.