The 389 ft tall San Diego Central Courthouse is under construction in the city’s downtown, near the San Diego Fault. © Skidomore Owings & Merrill LLP / Alan Silverman, 2014. All rights reserved
Engineers use advanced design methods to meet enhanced seismic performance requirements for a new 389 ft tall courthouse in downtown San Diego.
April 1, 2014—The new San Diego Central Courthouse will consolidate the San Diego County, California, criminal trial, family, and civil courts under one roof. While designing a single building to accommodate all of those facilities presents many challenges, the fact that the courthouse is being constructed in a region of high seismicity, near an active fault line, makes the project even more complex. To ensure the 389 ft tall building will perform in a moderate- to high-magnitude earthquake with minimal damage, advanced seismic design methods are being used to reinforce the structure and meet enhanced seismic performance requirements.
The San Diego Central Courthouse is under construction in downtown San Diego. The project comes after the California state legislature passed legislation in 2002 transferring responsibility for the 532 court facilities within the state from local to state jurisdiction. Upon assuming ownership of the facilities, the state’s Administrative Office of the Courts (AOC) analyzed each of the existing courthouses and discovered that dozens were unsuitable for contemporary court operations—including the existing San Diego Central Courthouse, which was constructed in 1961 over the San Diego Fault within the Rose Canyon Fault Zone, a strike-slip fault that cuts through the city. As a result of its analyses, the AOC launched a program to renovate or replace nearly 40 courthouses statewide. And after issuing a request for qualifications, it selected the San Francisco office of Skidmore, Owings & Merrill LLP (SOM) as the architect and structural engineer of the new San Diego Central Courthouse.
The largest of the state’s courthouse projects, the new 704,000 sq ft San Diego Central Courthouse will have 24 above-grade stories rising over two basement levels—making it one of only a few high-rise courthouses in the nation. The building’s height is the result of the fact that it will house multiple facilities on a site the size of just one city block, says Javier Arizmendi, RA, AIA, LEED-AP, an associate director in SOM’s San Francisco office and the senior designer of the project. The courthouse is being constructed across the street from the existing courthouse on a site that is bordered to the north and south by B and C streets and to the west and east by State and Union streets. The front of the building will face east and overlook a future public park, which will be developed on the site of the existing courthouse once that building is demolished. The Hall of Justice, which houses the San Diego Superior Court, will be located immediately south of the new courthouse.
The courthouse’s exterior design addresses both the functional and aesthetic needs of the court. “From the start, the idea was that the project needs to address all of the functional requirements of the courts, which are pretty complex, but also that this building needs to have a civic presence,” Arizmendi says. To that end, the building will have three key features that are intended to enhance its gravitas. First, like many civic structures, the courthouse will extend from a plinth and will have steps leading to a lobby—in this case, a spacious four-story space. Second, the building will be clad in a combination of glass and precast concrete, materials selected for their timelessness. And finally, a prominent roof cornice will extend from the building’s eastern side, giving the structure a defining presence on the skyline. “We were interested in making the courthouse very distinct as opposed to an office building that houses a different kind of function,” Arizmendi says.
The building’s interior arrangement is designed to meet the
security and functionality requirements of a contemporary
courthouse. © Skidomore Owings & Merrill, 2014. All rights
The building’s interior arrangement will be dictated by the security and functionality requirements of a modern courthouse. Its two basement levels will accommodate secure parking, sheriff’s facilities, temporary holding cells for detainees, and mechanical rooms. Below those levels, an accommodation for a future underground tunnel will connect the courthouse with the nearby central jail to facilitate the seamless movement of detainees between the facilities. Above grade, the building’s first five levels will serve as a podium for the tower and will hold the high-traffic courtrooms and public facilities, including arraignment and jury assembly rooms. An enclosed pedestrian bridge at the third level and will traverse C Street to connect the courthouse with the Hall of Justice to the south.
Rising from the podium, the courthouse tower will be divided into public and secure areas. The public corridors will be along the tower’s east face, while such secure areas as judges’ chambers and jury deliberation rooms will be on the west side. To reflect the contrasting interior uses, the building’s exterior architecture on the east side will also be different from that of the west. Between the public and secure areas, the tower will have 71 courtrooms, typically in a pod formation of four courtrooms per level. Holding areas located between each courtroom will provide a place to securely house detainees as they wait to enter the courtrooms. Multiple elevator cores will run throughout the building to ensure judges, juries, detainees, and members of the public remain separated. The primary public circulation core will be offset, located at the northeastern end—a solution that allowed the floor plan to accommodate the multiple courtrooms on each level.
Three components of the building required particularly close integration of architectural and structural engineering design: the pedestrian bridge, the cornice, and the offset circulation core. The pedestrian bridge will extend from the courthouse at level three, across C Street, and to the existing Hall of Justice on the other side of the street. It will be constructed of thin structural steel plates with internal rib stiffeners to form an orthotropic steel deck bridge, a bridge type that allows for efficient long spans. In this case, the longitudinal plates will range from 1/2 in. to 1 in. thick with transverse diaphragm stiffener plates spaced 10 ft on center. As a result, the 165 ft long bridge will have a maximum depth of 5 ft and will taper to a minimum depth of 3 ft. The bridge’s first span will extend approximately 80 ft from the courthouse to a single reinforced concrete bent founded on drilled reinforced-concrete piers near the curb along C Street. The bridge’s second span will then cantilever approximately 85 ft from the bent to the Hall of Justice. Peter Lee, P.E., S.E., LEED-AP, M.ASCE, an associate director in SOM’s San Francisco office and the senior structural engineer for the project, notes that the bridge will not transfer any loads onto the Hall of Justice, which was constructed in the mid-1990s and is not designed to handle the additional loads.
The building’s roof cornice will be framed in hot-dip galvanized structural steel and will cantilever approximately 80 ft. The cantilever will span from the roof. Extending over the podium below, the cornice will shade the tower’s east-facing glass facade in the morning, metal panels on the underside of the cornice reflecting the sunlight. “The crown of the building is designed so that this natural light is captured and reflected in a soffit,” Arizmendi explains. “It makes the building look very luminous because of the San Diego’s special light.” The cornice will also tie into the building’s offset steel-framed circulation core, which will have two bays—one ascending to the 12th floor and another ascending all the way to the cornice. The core will also tie into the tower’s primary structure and participate in the lateral and gravity framing. “This offset core seemed to be a significant challenge when it was suggested,” Lee says. “But we were able to effectively integrate it so that the footprint of the building could have an efficient layout, with four courtrooms per floor, and fit on the one-block site.”
The new courthouse is being constructed roughly 250 ft from the San Diego Fault, so the structure is being designed to meet the “enhanced” seismic objectives of the California Trial Court Facility Standards, Section 12.4 Criteria for Rare Loads, in addition to the requirements of the 2010 California Building Code and ASCE 7-05. “An enhanced seismically designed building strives for better performance under peak earthquakes that will occur over time,” Lee explains. “That’s important for this project because this facility is in for the long haul. So the goal is to allow the building to experience less damage and less interruption in operations in a moderate to major earthquake.” To that end, a probabilistic site-specific seismic hazard analysis was undertaken to determine the best criteria to achieve the seismic requirements. On the basis of the overall project design criteria, the team developed a lateral reinforcing system that incorporates special moment frames (SMF) utilizing cruciform wide-flange columns and box columns with viscous dampening devices to achieve the building’s seismic performance objectives.
The courthouse will be founded on a reinforced-concrete mat foundation and reinforced-concrete perimeter walls extending to the 38 ft deep basements. The superstructure’s lateral frame will rise as a two-way steel moment frame with fully ductile, steel SMF connections placed on primary grid lines in both the longitudinal and transverse directions, according to an SOM paper titled “Enhanced Seismic Design of the New San Diego Central Courthouse,” which was delivered at Structures Congress 2014, a conference sponsored by ASCE and it Structural Engineering Institute. Designed to allow for biaxial bending, the typical SMF will comprise W24 and W36 beams with reduced beam section connections as well as two W30 cruciform columns and box columns ranging in section from 20 to 36 in. square. While the SMFs allow for greater flexibility in layout and the function of courthouse operations, the floor-to-floor heights of 16 ft throughout the courthouse created their own sets of structural design. “A taller-than-typical floor-to-floor height becomes a challenge in designing for peak seismic forces because the structure becomes more flexible and story drift demands are greater,” Lee says. Large box columns, measuring 24 in. wide by 36 in. deep, will be incorporated into the system to stiffen the structure.
The building’s slender form makes controlling story torsion an additional challenge. To control building torsion and story drift under peak seismic forces, 106 viscous damping devices will be introduced at levels 6 through 24—typically six devices per level. Preliminary design life-cycle and cost-benefit analyses have demonstrated that the enhanced seismic design, combining SMF with the viscous damping devices, will result in an average annual return on investment of approximately 6.7 percent for the enhanced structural system, Lee says.
The contractor broke ground on the project this month and is operating on a 31-month construction schedule, completion anticipated in early 2017. Arizmendi says he expects that the project will not only provide a safe and secure environment for court operations and an inspiring place for people to work every day but also be a welcomed architectural addition to the city’s downtown. “I hope that it becomes an asset to the city of San Diego beyond the important business that is carried out inside,” he says. “The goal is that it will enhance the downtown area and will be a civic and architectural landmark for many years to come.”