The SF Millennium Tower: A Case Study in Tall Building Design and Regulation

In recent years, the design and construction of tall buildings have undergone a significant shift in San Francisco, driven by the experiences of iconic structures like the Millennium Tower. This 645-foot, 58-story skyscraper has become a benchmark for the city's building community, highlighting the importance of effective communication among development team members and illustrating how miscommunication can lead to unforeseen issues.

The foundation design of the Millennium Tower is a prime example of this trend. In order to ensure acceptable performance, the structure's piles were tipped in dense sand, as depicted in Figure 1. This approach is reflective of the prevailing design practice in San Francisco, which emphasizes foundations that extend to bedrock for tall buildings. However, this requirement may not be necessary or feasible for every tall building project.

Historically, extending foundations through the Oakland Bay Clay (OBC) to Franciscan bedrock at depths of 250± feet was not a necessity. The OBC is a compressible soil that has become lightly overconsolidated due to erosion, aging, and lower sea levels during the late-Pleistocene glacial period. Beneath this layer lies a thick deposit of Old Bay Clay (OBC) laid down before the last ice age.

The successful support of tall buildings in downtown San Francisco on pile or mat foundations, bearing in dense late-Pleistocene sands that underlie artificial fill and soft Holocene sediments, is a testament to the adaptability of these structures. The Millennium Tower, with its unique foundation design, highlights the importance of collaboration among engineers, architects, developers, and regulators to achieve successful project outcomes.

As new skyscrapers continue to rise around the world, it is essential to acknowledge that potentially significant issues may be hidden beneath the surface. The SF Millennium Tower serves as a reminder that effective communication and coordination are crucial components of tall building design and construction, and that the entire structure should be considered as a single integrated system.

Ultimately, the design team for any tall building project should strive to create an integrated system that performs optimally, taking into account the unique subsurface conditions and requirements of the site. By doing so, developers can minimize potential issues and ensure the long-term success of their projects.

Sf millennium tower