Structures / UC Berkeley California Memorial Stadium

• Forell|Elsesser Structural Engineers

• Mason Walters
• David Friedman
• Rene Vignos
• Chris Pettys
• Geoff Bomba

The retrofit of U.C. Berkeley’s California Memorial Stadium required innovative solutions for a unique set of present day and future challenges. Sitting directly on top of the Hayward Fault, the retrofit had to accommodate intense seismic shaking and surface fault rupture movements. To solve this, segments of the stadium overlaying the fault were decoupled from the main structure and allowed to move seperately. Another significant engineering challenge was the “floating” press box, which was designed to appear as if it were hovering above the stadium. To safely brace this structure for strong seismic movements, the team utilized rocking post-tensioned concrete core walls separated from stadium bowl, allowing the press box to move independently while linking it to the stadium with fluid viscous dampers. This retrofit allowed the preservation of the stadium as a historic monument while providing new spaces and, most importantly, protecting its occupants and allowing the stadium to be used for many years to come.

U.C. Berkeley’s California Memorial Stadium was built as a memorial to fallen heroes of World War I from California and stands as one of the most picturesque venues for college football. Originally constructed in 1923 and designed by the renowned architect John Galen Howard, Memorial Stadium sits directly over the Hayward Fault. Over the years, movement of the fault had shown up in the building through leaning columns, building cracks, and other misalignments. In 1999, the Capital Project’s Office of U.C. Berkeley issued a Request for Qualifications for the Seismic Safety Corrections of California Memorial Stadium. STUDIOS Architecture and Forell | Elsesser Structural Engineers were selected to evaluate the existing facility for seismic safety and athletic programming. HNTB architects joined the team in 2005. In 2001, a concept was provided for the seismic retrofit and modernization of the stadium. No funding was available from the state of California, so the project had to be privately funded through donations. This included developing publicity for seismic safety, which was much harder to sell than facility improvements. Other delays to the start of construction included several lawsuits around use of the land, mostly focused on the application of the Alquist Priolo Seismic Zoning Act of 1972.

As the stadium was not just close to – but on top of – the Hayward Fault, design for large ground motions and potential surface rupture dominated engineering decisions about the structure. It took interdisciplinary collaboration between the structural engineers, geologists, seismologists, and geotechnical engineers to develop a retrofit scheme to accommodate the estimated six feet of lateral movement and two feet of vertical movement of the ground during a large earthquake. Because the earthquake engineering challenges would require unique solutions, the project was provided with significant technical advisors and reviewers, including U.C. Berkeley’s seismic review committee, engineering peer reviewers, campus seismic risk advisors, and geotechnical/geoscience peer reviewers.

The final retrofit solution included rebuilding roughly two-thirds of the stadium’s bowl seating and supporting structure. In the zone of possible fault rupture, separate structures called “surface rupture blocks” (SRBs) were created as rigid blocks of structure separated from the rest of the stadium with gaps on either side. These buildings would move apart in the case of an earthquake without colliding with the adjacent structure or losing gravity support. The location and size of the SRBs was determined with the help of geotechnical engineers and geologists. The effects of surface rupture on the building were studied using small scale physical models and nonlinear finite element models, both of which informed the development of a retrofit solution. The SRBs were designed to respond to ground motions through rigid-body rotation and aimed to slide rather than deform in the case of fault rupture.

During this retrofit process, the University wanted to improve their athletic facilities. The project added a two-story subterranean crescent-shaped concrete structure wrapping around the west side of the stadium, providing a plaza area for waiting fans. Additionally, the soil berm behind the west stadium seating was eliminated, adding a triangle-shaped space separate from the rest of the stadium. A concrete structure was created in this area, with cast-in-place radial ductile concrete shear walls in addition to two concrete cores to provide lateral resistance. Additionally, the historic perimeter walls were strengthened with reinforced shotcrete. With this, the entire stadium is broken into seven discrete sections, all separated with seismic joints and the SRBs.

Another key feature of the design of Memorial Stadium was the new press box. The architectural vision for this press box was a gleaming steel and glass structure “flying above” the neo-classical themed stadium. This press box itself was designed as a three-dimensional steel space truss, allowing the structure to cantilever towards the field and span 90 feet between vertical supports. Laterally, the press box is supported by four concrete cores, 225 feet apart. These core walls house elevators and stairs, which are the only exits from the press box, and therefore had to be able to withstand an earthquake with minimal damage. To ensure this, these tall, slender core walls were designed to rock as a rigid bodies at their base, instead of allowing flexural yielding, which generally equates to damage. Vertical post-tensioning was used to allow this rocking and provide a restoring force, bringing the walls back to center after an earthquake, as well as providing constant compression which helped walls remain elastic.

An additional consideration in the design of the press box was the difference in stiffness between the relatively flexible steel structure above and the stiff concrete structure below. This had the potential to concentrate damage at their connection point – the concrete core walls. To mitigate problems this could create, the concrete core walls were separated from the stadium bowl structure, providing seismic separation and allowing them to rock. Viscous dampers were introduced connecting the two structures. These dampers buffer movement between the core walls and stadium and keep high accelerations from being transferred into the press box. This combination of rocking system and viscous dampers was a true innovation in the world of seismic design and enabled the construction of this “flying” press box right next to an active fault.

The construction of this retrofit was a massive undertaking, made possible by Webcor. The team worked on a tight 20-month schedule and created a phasing of construction that allowed continued use of critical parts of the facilities throughout the process. Because of the massive scale and intricacy of this project, the structural engineer, Forell | Elsesser, maintained a presence on the site during construction. This kept the project moving forward efficiently through these challenges and helped keep construction on schedule.

Thanks to the dedication, ingenuity, and vision of the large project team and the university, the stadium opened in 2012 with a brand-new field, cheering fans, and a gleaming structure flying above it all.

• SEAONC Excellence in Structural Engineering Award (2013)
• NCSEA Excellence in Structural Engineering Award (2013)
• ASCE Historical Renovation Project of the Year (2013)

• California Memorial Stadium Athletics
• UC Berkeley California Memorial Stadium Seismic Upgrade
• Structure Magazine Article