Foundation Design Considerations for Tall Buildings the Inland Northwest-PPT Presentation

PPT Presentation

Pdf Summary

The presentation reviews geotechnical challenges in tall-building design, including heavy vertical loads, wind and seismic forces, deep excavations near existing structures, and large differences in loading between towers and podiums.

Foundation design must address both <strong>ultimate limit states</strong>—preventing instability or collapse—and <strong>serviceability limit states</strong>, such as excessive settlement. While all buildings settle, differential settlement can cause architectural and structural damage. Historical data suggest damage may occur near an angular distortion of 1/300; designs often use a more conservative limit of 1/500.

Foundation selection depends on structural loads, ground conditions, construction access, impacts on neighboring structures, cost, and local practice. The presentation compares spread footings, mats, deep foundations, and piled rafts. It emphasizes that footing settlement depends on footing size as well as bearing pressure, and that designers must account for deeper compressible layers and interaction between nearby footings. For mat foundations, soil spring stiffness must be adjusted for foundation dimensions; using an unadjusted value can produce unrealistic models and incompatible structural designs.

The Falls Tower project in Spokane illustrates these principles. The 12-story mixed-use building was founded on 90 rock-socketed drilled shafts because deep existing fill made shallow support unsuitable. Shafts, 24–30 inches in diameter, extended to basalt bedrock, with a minimum socket depth of one diameter. Construction included temporary casing and quality-control checks. Installation took about 67 working days, with shafts reaching lengths of up to 70 feet.

The presentation concludes that geotechnical design must balance cost, schedule, reliability, safety, regulatory requirements, and sustainability. Because early investigations may occur before structural systems and foundation loads are known, follow-up exploration can be essential. More thorough site investigation is associated with fewer claims, change orders, and cost overruns.

Keywords

geotechnical challenges

tall-building design

foundation settlement

ultimate limit states

serviceability limit states

differential settlement

deep excavations

piled rafts

drilled shafts

site investigation