Global stability of high-rise buildings on foundation on piles

Sammanfattning: In Sweden as well as other countries the trend of building higher is more and more popular. The global stability of tall buildings is a very important aspect that has to be taken into account while designing. Foundation on piles, that is common in Sweden, reduces the global stability compared to foundation directly on bedrock. Using inclined piles in the foundation is inevitable for high-rise buildings, because they are essential for transferring the horizontal loads into the bedrock. The aim of this paper is to look into the influence that foundation on piles has on global stability and investigate two simple methods to asses global stability. In this thesis the influence of the stiffness of the substructure (foundation), length and inclination of the piles on the global stability were investigated. It was also looked into how does the pile center affect the rotation and thus global stability. One method that was presented was based on the equivalent stiffness. Displacement at the top of the wall is used in order to calculate the bending stiffness that is reduced due to foundation on piles and further calculate buckling load on the basis of Euler buckling. In the other method that was proposed rotation at the foundation level was taken into account so as to calculate rotational spring stiffness and later buckling load due to combined flexural and rotational buckling. The analysis was conducted on a simple two dimensional problem, namely stabilizing wall as well as a building stabilized by two towers. Three different configurations of piles were investigated for single wall as well as for the structure. The investigation showed that the position of pile center has its effect on the global stability. The closer the pile center is to the foundation on piles the better the global stability of a  structure. The length of the piles plays a role in stability as well. The longer the piles are the worse the stability is. With longer piles the overall stiffness of a structure decreases and thus the global stability. The analysis showed that the foundation of piles significantly lowers the stability of high rise building. The investigated methods showed that the one based on rotation at the base gave better results compared to the method based on the equivalent bending stiffness. But to use this first method, the position of the pile center is required to be known in order to get correct results which in a complex structure is hard to estimate. In an analysis of a building stabilized by two towers it was seen that when the inclined piles that are inclined opposite to each other in a pile group and are positioned far from rotation center of a structure it increases the global safety and rotational stiffness as well. It is recommended to use such configuration of piles that the pile center is at the foundation level in order to increase global stability.