Superstructure stiffness effect on the behavior of piled raft under wind and earthquake loadings

Abstract

Earlier studies relied on simplified methods, assuming uniform or concentrated loads directly transferred from the superstructure to the raft through a fixed boundary condition neglecting the influence of the superstructure's stiffness on the soil-foundation interaction. Consequently, these simplifications result in inaccurate predictions of raft bending moments, pile loads, and overestimation of the reaction forces with insignificant settlement underestimation. In this paper, 3D finite element analyses were conducted for a 20-story building with three different structural systems to study the influence of the superstructure stiffness on the performance of piled raft under wind and earthquake loads. An iterative procedure was undertaken between PLAXIS 3D and ETABS to achieve displacements compatibility between the geotechnical and structural models. A layered soil profile was studied, consisted of soft to medium clay overlying a dense sand layer. The impact of using a 2m replacement sand layer below the raft also was evaluated on the soil-foundation response. Including the stiffness of the superstructure increased the load shared by the piles by (5.5% to 6.4%), (6.0% to 7.0%), and (1.7% to 7.6%), and reduced the raft differential settlement by (25.3% to 37.2%), (24.4% to 37.4%), and (29.7%) under gravity, wind, and earthquake loadings, respectively. Moreover, employing a 2-meter replacement layer beneath the raft had a negligible impact on the behavior of the piled raft foundation. Thus, the interaction of soil, foundation, and superstructure significantly influenced the structural response.

Keywords

• piled raft foundations
• load sharing
• settlement
• soil structure interaction
• superstructure stiffness