High-Rise on Spread Footings and Piles: A Design Solution as a Flexible Mat on Piles

Depending on soil conditions, high-rise structures are commonly designed either on piles or spread footings. However, design solutions including both spread footings and piles are typically rejected given the potential for unacceptable differential settlements between the columns/shear walls supported on piles and the columns supported on spread footings. Further the potential contribution of the pile cap as a footing is commonly not considered. A design solution considering the foundations as part of a very flexible mat on piles allows the engineer to consider the pile caps as footings capable of developing soil pressure and the piles as settlement reducing elements working at 70 to 80 percent of their axial capacity. The analysis provides total deformations under footings, pile caps, and individual pile loads under all the load combinations required by code. The three-dimensional model developed for the mat-on-pile analysis allows performance of a sensitivity analysis of the expected range of soil properties. As a result, the geotechnical engineer can provide soil spring constants for footings and for the piles in tension and compression, as well as the allowable soil pressure that the analysis indicates will result in acceptable differential settlements. These parameters are the input required in commonly used structural engineering software used to determine the structural solution. The structural solution using these parameters will substantially agree with the geotechnical analysis and will provide significant savings in foundation cost. The method is illustrated on a 24-story structure. The analysis is supported by a geotechnical investigation and a pile load test that has been completed for the building.