High-rise buildings

The design of the foundation for high-rise buildings is a critical factor that determines their safety, reliability, and economic viability. Through the measured data on the foundation systems of high-rise buildings and an analysis of the causes of certain engineering accidents, it has been found that there is room for further optimization and improvement in traditional conceptual design. 　　Box foundations, raft foundations, and pile-raft foundations designed according to traditional concepts inevitably lead to butterfly-shaped settlement and saddle-shaped stress distribution, or to excessive differential deformation in the main and skirt sections. Such deformation and stress distribution patterns inevitably result in increased overall bending moments, punching forces, and shear forces in the box-raft structure, thereby causing excessive secondary stresses in the superstructure and reducing its service life. To this end, this paper proposes a concept design for variable stiffness leveling. 　　The basic idea is to adjust the stiffness distribution of the foundation and pile foundations so that the reaction forces are in harmony with the load distribution, resulting in more uniform settlement deformation, and thereby minimizing the overall bending moments, punching forces, and shear forces acting on the foundation. The specific approach is as follows: based on the structural layout, loads, and soil conditions, local reinforcement for stiffness adjustment, pile foundation stiffness adjustment, and stiffness adjustment for the connection between the main structure and the skirt structure are carried out, along with computational analysis of the interaction among the foundation (piles and soil), the base structure, and the superstructure. 　　1. Local reinforcement for stiffness equalization: When using a natural foundation, it is necessary to move beyond the traditional concepts associated with such foundations. Local reinforcement is applied to areas with high load concentrations, such as the core tube, through the use of rigid pile composite foundations or local pile foundations. This helps to match the supporting stiffness to the loads, results in more uniform settlement, and reduces the internal forces on the foundation as well as material consumption. 　　2. Variable stiffness pile arrangement for leveling In cases where pile foundations are required, a pile arrangement with variable stiffness is implemented based on the structure and load distribution as well as the geological characteristics of the site. For frame-tube or frame-shear structures with significantly uneven loads, variable pile spacing, pile diameter, and pile length can be used in the pile arrangement. For internal pile groups with high load concentration, appropriate enhancement of interactions should be considered ； The peripheral area should be appropriately weakened, and calculated as a composite pile foundation. 　　3. Adjustment of stiffness for the integrated main structure and skirt building: For buildings with an integrated main structure and skirt building, the design should follow the principle of strengthening the main structure (by using pile foundations or rigid-pile composite foundations) and weakening the skirt building (by using natural foundations, sparse pile foundations, or composite foundations). 　　4. Analysis of the interaction between the foundation (piles and soil), the foundation itself, and the superstructure To make the conceptual design based on variable stiffness more reasonable, reliable, and practical, it is advisable to conduct calculations analyzing the interaction among the foundation (piles and soil), the foundation, and the superstructure as a whole, based on this conceptual design. This allows for further adjustment of the pile layout in order to minimize differential deformations, and it also enables the determination of the stresses and reinforcement requirements for the foundation and footings. 　　The concept of variable stiffness leveling has been verified through large-scale field model tests and applied in over a dozen projects, achieving excellent technical and economic results.

Regarding the issue of the burial depth of rock foundations for high-rise structures: 1. The anti-sliding requirements must be met； 2. It must comply with Article 12.1.6 of the High-Rise Building Code: \"In high-rise buildings with a height-to-width ratio greater than 4, there should be no zero-stress zone at the base surface of the foundation.\" ； The requirement that the area of the zero-stress zone between the base surface of high-rise buildings with an aspect ratio not exceeding 4 and the foundation soil should not exceed 15% of the area of the base surface.

★The code does not specify any clear requirement regarding the burial depth for rock foundations. In practice, it is not necessary to meet the 1/15 requirement. The design must meet the requirements for overall overturning resistance and slip resistance. In the foundation beneath shear walls, zero-stress zones may occur, in which case rock anchor rods can be used.