101 Ellsworth is a six-story office building over four levels of subterranean parking in downtown San Mateo. The building is built on a mat foundation with the lowest level 15 feet below the water table. By using a combination of braced frames in the core area and moment frames in the perimeter of the building, we were able to reduce the steel tonnage to approximately 10 psf which is extremely cost-efficient for a building located just three miles from the San Andreas fault.