Debunking the Myth About Base Isolation

    Rendering of the new San Francisco General Hospital and Trauma Center. Image: Fong & Chan Architects

    Replacing an old, seismically vulnerable hospital facility, the new San Francisco General Hospital and Trauma Center — which is on schedule and under budget — decodes an industry-wide myth that base-isolated buildings are too costly and time intensive to build in a seismically active urban environment. The new nine-story, 284-bed general acute care hospital for the City of San Francisco also goes above and beyond the mandatory seismic safety requirements of Senate Bill 1953, as the center will continue to function directly after an earthquake event.

    This project shows that base isolation is a viable and economically feasible system that can be designed without increasing the project design schedule. Provided the project design phase is properly planned, designed, and coordinated, base isolation is not only a competitive structural system, but one that can provide significant cost savings and superior performance compared with conventional structural systems, particularly in the seismically active region of California.

    Getting it done required close collaboration among key team members — Fong & Chan Architects, Arup, the Department of Public Works of the City and County of San Francisco, and the contractor, Webcor. With the city’s adoption of integrated project delivery, the team was able to carry out the design of the base isolator and the structural design at the same time. The city also was successful in commissioning early prototype testing of the isolators, rather than following the traditional process.

    Ron Alameida, project manager of San Francisco General Hospital (SFGH) from DPW commented that the results from these tests allowed Arup to “design around fine-tuned and actual performance rather than assumed bearing properties, which led to the reduction of overall structure weight by 3,000 tons.”

    Edgar Lopez, San Francisco’s city architect and DPW deputy director for buildings, said, “We’ve got an innovative seismic system that offers superior performance at a favorable cost to the project, and the project’s structural engineer has an implementation plan that reduces the project schedule. How can we not support it?”

    Benefits of base isolation

    Conventional fixed-base structures require deep columns and beams that constrain available space for utilities and make coordination more difficult. In a major seismic event, the moment connections from beam to column in a fixed-base building would yield. Replacing them would be difficult and expensive, and would result in significant downtime.

    Compared with fixed-base hospitals, a base-isolated structure uses significantly less steel and improves the interior volume so the layout is less constrained and the coordination of utilities easier to implement. Base isolation allows the SFGH building to withstand maximum-considered-earthquake-level earthquakes and remain elastic. In fact, the bearings allow the building to re-center itself after an earthquake, whereas a fixed-base building might exhibit permanent residual drifts (i.e., be out of vertical alignment).

    The new San Francisco General Hospital and Trauma Center under construction with the Bay Bridge in the background. Image: Fong & Chan Architects

    Thus, compared with a fixed-base building at the same site and of similar dimensions, the base-isolated SFGH building provides greater protection for occupants and contents during an earthquake, is more cost-effective to build, and is expected to perform with little or no structural and non-structural damage in a major earthquake.

    Structural systems

    The building is comprised of three primary geometries: a three-story rectangular podium that houses the diagnostic and treatment functions; a six-story cylindrical tower that contains the patient beds; and a seven-story parallelogram core that accommodates the hospital’s support, utility, and vertical conveyance systems. The shape of each building component was selected to maximize programming space and to support efficient operational flow.

    The foundation and gravity-load-resisting systems are key to the building’s seismic safety. The gravity-load-resisting system includes concrete fill on metal deck floors and roof diaphragms supported on steel beams and girders. The beams and girders are designed as composite structural elements with the metal deck and concrete fill floors and roofs, and are supported by steel wide-flange and built-up cruciform columns. The lateral load-resisting system includes steel intermediate moment frames, deck and fill diaphragms, and reinforced concrete walls at the basement perimeter level.

    Computer model of the San Francisco General Hospital structural system. Image: Arup

    The foundation system is a 4-foot-thick, reinforced concrete mat foundation with 148 tie-downs. The top of the mat is located 7 feet below the basement. The mat is locally sloped and depressed below the elevators to accommodate the elevator pits. The bottom of the mat foundation is approximately 10 feet below the groundwater table.

    Key element: Triple pendulum bearing

    Arup designed the new base-isolated structure using triple friction pendulum isolators. This base isolation will allow the hospital to glide 30 inches in any direction, making it the most advanced seismic-resistant design in existence and able to withstand large earthquakes in this highly seismic region. The triple friction pendulum isolators save 3,000 tons of steel, compared with a traditional fixed-base seismic system, greatly reducing construction costs.

    The triple pendulum bearing, a recently developed class of sliding isolators, incorporates four concave surfaces and three independent pendulum mechanisms. One pendulum mechanism, which serves as the isolation system under low levels of excitation, consists of inner spherical sliding interfaces with low friction on either side of a cylindrical post. The second mechanism, which serves as the isolation system under moderate levels of excitation, consists of the lower sliding interface coming in contact with the lower concave dish. Finally, the third mechanism, which prevents sliding under an extreme level of excitation, consists of the upper sliding interface coming in contact with the upper concave dish. The three mechanisms may be selected to optimize the performance of the seismic isolated structure.

    The triple pendulum bearing is supported on a 6-foot-square concrete pedestal above the mat foundation. Two types of isolator bearings are used in the building, designated Type 1 (small) and Type 2 (large), and assigned based on column axial loads. There are 43 Type 1 and 72 Type 2 isolators with maximum rated lateral displacements of 33 inches and 32.6 inches, respectively. From the top of the mat foundation to the finished grades, there is a 36-inch-wide moat around the entire perimeter of the building. Articulated moat covers bridge across the moat, typically attached to the building on one side and resting on sliding surfaces atop the perimeter retaining wall on the other side.

    Steel framing is supported by an isolator over a concrete pedestal. Photo: © Webcor Builders

    Design testing pivotal for project speed

    Prior to the plan’s submission for review, extensive testing was integral to keeping the project on schedule. The design team performed real-time prototype testing on four prototype bearings, two each of Type 1 and Type 2. The prototype bearings were subjected to a range of vertical loads (including Pu, max from the time history analyses) and displacements for varying cycles at real-time (also termed full-speed) test rates (see Figure 1).

    In addition to the real-time prototype tests, a second set of prototype tests was performed on the same bearings at peak cyclic test velocities of 1 inch/second (also termed slow speed). Based on the prototype test results, a bounding analysis was performed to develop bearing properties for use in the analyses and design of the isolated building. Using actual bearing properties for the design not only streamlined review processes, but also avoided the use of conservative code assumptions.

    Figure 1: Sample hysteresis from full-speed prototype test program. Image: Arup

    The force-displacement behavior of a typical bearing from one of the SFGH full-speed prototype tests is shown in Figure 1. The hysteresis shown represents a cyclic test conducted at several amplitudes of displacement and at a velocity corresponding to the expected velocity under actual earthquake excitation.

    A functional trauma center, even under duress

    The new building incorporates innovative practices of sustainable design and economical construction — the project is on track to receive LEED Gold. This project promotes efficient hospital operation and establishes an environment that reflects the latest advancements in patient care, comfort, and treatment. The hospital will nearly triple in emergency department space and is only one of a few buildings that can withstand an earthquake without requiring replacement parts.

    SFGH proves that base-isolated designs are capable of coming in on schedule and under budget; and that they are, in fact, more cost-effective and more resilient to seismic events than fixed-base structures. This hospital is an exemplary model for future base-isolated projects in other hospitals and in cities around the world.

    Eric Ko, S.E., a principal at Arup, has more than 30 years of experience in resilient seismic engineering in health care, corporate headquarters, academic labs, convention centers, and hospitality projects around the world. He is the project director and the structural engineer of record for SFGH. Fong & Chan Architects and Ron Alameida, project manager at San Francisco DPW, also contributed to this article