History:The story of the Westfield San Francisco Centre project begins some time ago. It starts back in the 1890's with the opening of the original Emporium, the first department store on the west coast, winds its way through the devastation of the 1906 San Francisco earthquake, through expansion and contractions of the remainder of the 20th century, and, finally, into a grand reemergence at the start of the new millennium. Today's story: an inspiring urban mixed-use retail center. The Westfield San Francisco Centre brings forward the most prominent and defining features of the Emporium's historic past and combines them with modern and forward-looking, mixed-use/retail design. The construction of this $460 million dollar mega-structure, including 1.5 million square foot of new construction, resulting in the largest urban retail center west of the Mississippi, brings this historic narrative to date. The Grand Opening in October 2006 has the Emporium reemerging as a San Francisco Landmark!
Challenges:The defining historic elements, including a 100ft diameter glazed steel lattice dome, which was destroyed in the 1906 earthquake and rebuilt shortly thereafter and an 8-story sandstone façade, which survived the 1906 earthquake intact, were to be incorporated into the new design. Inclusion of these historic elements proved to be a major design challenge.
The Dome - Engineering:The dome rested on what was the original building?s third level; while the new design had it re-situated four stories higher. The most elegant solution became clear: lift the 500,000lb dome some 60 feet higher to cap the new structure. Ultimately, compared to dismantling and reassembling the original dome, this concept saved significant time, over $1 million in construction cost, and precious historic finishes.
The job of lifting the 250 ton historic and fragile object intact required considerable analysis, experience, collaboration, and nerve. Using the original shop drawings as a guide, 3-D computer models were developed and analyzed for effects of lifting, variations in levelness, consistency while lifting, and evaluation of the structure to withstand wind and seismic forces as required by the current Code. Additionally, the original dome was supported by 24 steel columns, while the final design consisted of only eight support columns. As a result, the dome was strengthened, including the welding of the steel ring beam splice connections and the threading of a limited number of diagonal HSS braces through the main arches of the existing riveted steel dome. With the dome strengthening complete, a central structural steel shoring and lifting tower was installed, along with a new mat foundation that served as a base for stability of the lifting tower in addition to being a portion of the new building's permanent foundations. All work completed up to this point was performed within the confines of the existing building structure. Using strand jacks, the dome was lifted to the limit of the initial tower structure. Next, the new tower segments were inserted and the dome was lifted further, to just above the final proposed position. With the dome up and standing free as though some giant umbrella centered over the site, the existing building was demolished to make way for a new composite steel moment resisting frame/eccentric braced frame (dual system) structure incorporating 10,000 ton of new structural steel. The dome was then lowered back with its ring beam resting at its new home atop the steel structure. The operation was a complete success with the maximum relative vertical displacement across the 100ft diameter dome not exceeding 1/8" during the lifting operation and at the final resting position.
The Historic Façade - Engineering:The façade was backed with a new ductile reinforced "shotcrete" frame, threading it within the confines of the existing structure. Temporary steel shoring towers with micropile foundations were installed to support the façade once the remaining structure was demolished.
At this phase, the façade stood independently, extending 145 feet above the basement and foundation excavation. In the final configuration, a "temporary" shotcrete backing frame was utilized as an integral component of the modern lateral system.