Geotechnical and Structural Design of Piles under Highrise Buildings

Auger cast-in-place (ACIP) piles that support highrise buildings are the only structural elements of those structures that are not reinforced considering the load combinations defined in the Building Code or ASCE 7. Current design practice only considers the first mode forces that are determined by the loads transferred by the pile cap to the piles. Three-dimensional numerical models of highrise foundations indicate a second mode of forces defined by moments and shear forces’ actions caused by the soil’s lateral displacements under the building loads. These second mode forces have been confirmed by instrumentation installed along production piles that have been monitored during the building’s construction. The second mode forces act at depths that are not conventionally reinforced, leading to potential pile structural failure. The impact of considering or not the second mode forces in pile design is illustrated with the study of the performance of two highrise buildings of known settlement behavior. The effect of the second mode forces, combined with the local limestone’s sensitive behavior, is included in the foundation analysis of the two highrise buildings. The structures have similar heights and similar geotechnical conditions. The numerical modeling predicts deformations under the two buildings and the forces acting on the piles. The forces acting on the piles are compared with the pile’s structural capacity. The limestone strength and deformation modulus, based on mobilized deformation, is considered. The analysis results indicate that the pile loads, combined with the limestone-sensitive behavior, generate settlements that correlate with the length of reinforcement used to construct the piles. A three-dimensional numerical method to define the second mode forces acting on the piles is recommended to analyze highrise foundations under the required load combinations. An example of the results of such an analysis performed for a 47-story building foundation is presented here. A reduction factor for the calculated forces is recommended based on stratigraphic characteristics and numerical modeling assumptions. The resulting forces are used to define the pile’s shear and longitudinal reinforcement.