When complete, McKenzie Apartments at 2202 8th Avenue will be a 41-story building on the corner of 8th Avenue and Blanchard Street. Sited at the convergence of the Denny Triangle and South Lake Union neighborhoods, the elliptical form of the building elegantly resolves the intersection of both neighborhoods’ rotated street grids. The elliptical building maximizes view opportunities outward for the tower’s residents, while minimizing blockage of view and sun for the residents of the neighboring condominium tower. The building has 8 levels of subterranean parking, with the lowest level (top of mat foundation) approximately 80 feet below average site grade.
Hart Crowser is the geotechnical engineer-of-record for this project. Our seismic design included a seismic hazard analysis with amplification from the Seattle Basin and near fault effects to develop the design response spectrum. This was the first Pacific Northwest project to use the conditional mean spectra approach for ground motions selection and scaling. This innovative approach reduced the seismic demand by about 10%, resulting in lower construction costs. We performed this work following draft procedures proposed for Chapter 16 of ASCE 7-16 before this standard was even published. The work was peer reviewed and approved by two of the authors of ASCE 7-16.
Excavation to up to about 80 feet below existing grade, extending to below groundwater table, was required for the construction. The temporary shoring consisted of 7 rows of tiebacks on three sides of the project, and tierods on the fourth side. The tierods were necessary to retain the narrow Alley between the structure and an adjacent building which has below-grade parking as well. The north shoring wall was designed to support the surcharge of an adjacent hotel without the use of underpinning.