Cals Memorial Stadium is ready to rock

The stadium is topped by a new cantilevered, two-story press box and University Club that has its own vertically post-tensioned core supports, allowing it to sway as much as 12 inches in an earthquake. Photo: ©Jeffrey Katz Photography

HNTB Corporation’s $474 million master plan for the renovation and seismic retrofit of the University of California’s 1923 Memorial Stadium and surrounding campus has won numerous awards for both preservation and innovation. In the end, the plan is just as notable for what it didn’t do — move the structure off the active Hayward Fault. Instead, a 1-2-3 punch kept the stadium right where it is, overlooking the Berkeley campus and the Golden Gate Bridge, exhibiting the faith of HNTB’s project leadership in a custom engineering solution. The solution was designed by structural engineer Forell/Elsesser, with input from Berkeley’s own fault rupture experts on the campus Seismic Review Committee.

Following erection of the athletic training center that wraps around the University of California’s 1923 Memorial Stadium, workers gutted the old stadium bowl while preserving the historic façade with extensive bracing. Photo: HNTB

The result is a LEED Gold structure that rests on discrete “Surface Rupture Blocks” (SRBs) positioned over the fault and mounted on flat slab foundations. The stadium is topped by a cantilevered, two-story press box and University Club that has its own vertically post-tensioned core supports, allowing it to sway as much as 12 inches in an earthquake, separated from the stadium bowl except for fluid viscous dampers that act as shock absorbers between them. The seismic system will allow the retrofitted stadium to float over a 6.7 magnitude quake, twisting and tilting to compensate for a possible 6-foot horizontal and 2-foot vertical rupture.

Completing the complex is the new 145,000-square-foot Simpson Center for high-performance athlete training that serves football and 12 other sports teams. The subterranean land form building wraps around the stadium, its roof forming an acre and a half plaza that welcomes game day crowds. The center is offset 50 feet from the fault to comply with California regulations.

HNTB’s 20-month construction schedule started with erection of the Simpson Center to facilitate the move of personnel housed in the old stadium during reconstruction. Gutting of the old stadium bowl followed, with extensive bracing to John Galen Howard’s historic Roman Coliseum façade to facilitate preservation. The project was finished in time to kick off the Golden Bears’ 2012 season, with only a year of displacement in the interim.

A new stadium inside the old

Model shows Surface Rupture Block separation due to possible displacement over the Hayward Fault. Image: Forell/Elsesser

The look and feel of the historic stadium was preserved and the renovation preserves the character of the original building. The team was able to save the integrity of the existing structure but at the same time added much-needed amenities, such as better seating, along with new key elements, such as a grand plaza in front of the stadium and a new concourse for concessions with views of the campus and the San Francisco Bay.

René Vignos, structural engineer of record, noted that although innovative, “Our solution is quite simple. We separated the building from the ground and from the press box that hovers over it. The innovation lies in the fact that no one had ever done it before. The experts showed us the types of buildings that managed to survive major quakes on top of large fault displacements without a lot of damage. This was the first time that those lessons were consciously applied on a major scale.”

The necessity of compensating for the possible surface rupture of the Hayward Fault as well as large ground motions was the spur to the structural engineering innovations. The solution was the result of interdisciplinary efforts among structural engineers, geologists, seismologists, and geotechnical engineers. It was hammered out with valuable input from the earthquake experts at the university.

“The professors are leaders in the field of understanding how structures respond to earthquakes,” Vignos said. “Working with them was a unique opportunity. We learned from them during the design phase and had the right people reviewing our designs before they were approved.”

The solution involves division of the stadium into seven separate structures. The two located over the fault are SRBs and have separating joints that will let them move independently without colliding with the others. They balance on flat slab foundations that sway and tilt. The rigid superstructures are stiff and strong enough to react like cohesive floating units in response to the gyrations.

The SRBs were defined according to their geological characteristics and determined by mathematical and physical models, plus architectural accommodation to the wall arch pattern of the façade. The Hayward fault consists of two tectonic plates sliding by each other laterally with a rightward movement. The models helped determine the most probable fault angles. Rotating them slightly clockwise causes the parts to move away from each other instead of closing in and colliding during a quake.

The foundation consists of a 4-foot-thick slab resting on 3 feet of sand wrapped by geotextile and intersected by a double layer of high-density polyethylene plastic sheeting. The sand and the plastic reduce the friction between the slab and the supporting soil and the sand moves in to fill the gaps caused by fault movements. Geofoam wedges at the ends of the SRBs will also adjust to fill in any gaps. Unconnected concrete columns extend beyond the sand layer to improve the resistance of the soil beneath.

The buildings on the two SRBs perform as rigid superstructures. They can rotate and slide as cohesive units while the surface above the fault shifts and displaces.

Flying press box

Discrete Surface Rupture Block structure and foundation features separating joints that allow independent movement without colliding with other stadium structures. Image: Forell/Elsesser

The solution of separating the press box from the stadium to allow the hovering superstructure to rock in place was not obvious from the start. It took many iterations to arrive at the combination of vertical post-tensioning in the supporting cores to allow rocking and fluid viscous dampers to buffer the motion, a system described as “damped rocking-wall support” in a paper by Forell/Elsesser (“UC Berkeley California Memorial Stadium: Protecting and Strengthening a Landmark on an Active Fault,” SEAOC 2012 Convention Proceedings).

The architecture of the minimally supported press box reinforces the engineering solution. In contemplating what the addition should look like, I immediately thought of a gleaming steel and glass structure that would “fly above” the neo-classical themed stadium. When I asked Vignos if it could be done, he responded “like a kid in a candy store” at being asked to design the supports for a stunning architectural concept.

“Fernando hit the right note in how he approached the architecture,” Vignos said. “I was happy to run with it.”

The hovering press box is framed with repeating story-deep trusses that cantilever from the main long-span truss and are supported by only four concrete cores and four mid-span steel columns. The cores contain elevators and stairs. The minimal supports and cantilevered design create the illusion of flying. The cores are connected to the stadium at their midpoints by the fluid viscous dampers that temper their acceleration.

To protect the tall slender core walls, the cores are designed to rock as rigid bodies at their bases. The vertical post-tensioning provides a restoring force and constant compression to help them remain elastic. To avoid flexural yielding of the cores, they are founded in sockets that also anchor the bottom loops of the tensioning cables. Gusseted pins at two top center points allow them to sway and tip without pulling down the box. The tops of the tensioning system are attached to the steel box beam frame that surrounds the pins.

“The cores are the only exits from the press box,” Vignos said. “They had to be able to emerge from an earthquake with minimal, controlled damage to allow people to safely exit the structure. Thanks to the steel pins, rocking towers, and dampers, they will. This type of rocking system is a true innovation in the seismic design world, enabling the press box to be built right next to the Hayward Fault.”

Seismic systems ready for quake

The most recent large earthquake on the Hayward Fault was in 1868. The last five large earthquakes on the fault have occurred about 140 years apart, so another one is due. Nevertheless, the university wanted to keep the stadium where it was with its million-dollar views, now accessible from the Club House’s cantilevered glass deck. HNTB and the project team rose to the challenge to innovate a seismic solution, which evolved during seven years of collaboration after the university began to implement its retrofit plan in 2005.

Named Engineering News-Record’s “Best of the Best” project in the Sports and Entertainment category, the stadium has been transformed into one of the best venues in the Pac-12 conference. Plans to use the facility for trade shows, career days, seminars, and graduations will only broaden its appreciative audience — making it an asset that reaps a continuous rate of return for the university.

Fernando Vazquez, RA, is design director for HNTB and project designer for renovations and seismic retrofit of Memorial Stadium at UC Berkeley. He has more than 30 years of national and international architectural design experience focused on sports, retail, and public assembly facilities. 

Posted in Uncategorized | May 20th, 2014 by

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