Design of an Underground Viaduct for the Expansion of the Moscone Center

The Moscone Center in San Francisco, CA, is the largest convention space in the region and a prime destination for a large number of conferences and corporate events. The convention center was originally designed and constructed during the late 1970s and early 1980s, and has since undergone several expansions and retrofits to increase its capacity and modernize its infrastructure. To modernize and maintain the competitiveness of the Moscone Center, the City of San Francisco, Department of Public Works, SF Tourism Improvement District Management Corporation and the hotel industry, have embarked on another major expansion of the popular convention center. The design team, which was led by Skidmore, Owings, and Merrill (SOM) and included SOHA Engineers and Tipping Structural Engineers (TSE), was tasked with adding approximately 305,000 sf of additional space below and above ground. A key part in providing the additional exhibition space was to connect and combine the underground portions of the existing North and South facilities below Howard Street, a main thoroughfare separating the two. To create the additional space and seamlessly integrate the two facilities, a 240’ portion of Howard Street will be excavated and replaced with a post-tensioned (PT) concrete box-girder viaduct structure. The design and construction of the Howard Street viaduct presents a variety of challenges that are addressed in unique and innovative ways. Typical considerations such as strength and deflections were complicated by long spans and heavy loads that resulted in a hybrid PT and mild-steel box-girder design. Further complications resulted from limited construction access below grade, adjacent to the viaduct. Faced with these challenges, the design team developed an elegant solution that satisfied the necessary structural requirements, is efficiently constructable, and optimizes the new usable exhibition space for the Center.