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EiSE Award Winner Series

SEAONC Post | Published on 9/1/2020

The New Stanford Hospital is this year’s SEAONC Excellence in Structural Engineering (EiSE) Project of the Year. Here is the history and description of the project:

The project includes a $2B, 825,000 square foot replacement hospital on the Stanford University campus. The seven- story state-of-the-art Level 1 trauma center includes 368 patient rooms, 20 operating suites, imaging suites, and emergency department. Patient rooms are located within four towers and following the latest healthcare trend they are all private rooms, with large floor-to-ceiling windows overlooking the surrounding campus and foothills.

The structure system consists of a base-isolated steel moment frame, designed for Functional Recovery performance following a major seismic event. The incorporation of base isolation substantially reduces the amount of earthquake energy imparted on the building resulting in no expected structural damage in the maximum credible earthquake. NYA’s resilience-based design provided enhanced protection for critical non-structural elements, with the goal of providing a functional hospital following a major seismic event. This performance is compared to that of a new conventional fixed-base hospital which is expected to experience significant non-structural and moderate structural damage following a major event.

The engineering challenges for this project were abundant including demanding architecture, the incorporation of a new type of seismic isolation bearing, providing support for significant cantilevers, long spans, a 120 foot diameter column-free atrium space, vibration sensitive MRI suites, a 4 acre landscaped terrace and a base-isolated skyway. Over 1/3 of the overall floor area for the project is cantilevered. The dramatic 120 foot atrium dome was designed as a thin lens of small diameter welded steel tubes that rises only 12 foot at the center, creating a remarkable, light-filled space.

A long-span pedestrian skyway spans over a roadway to connect the new hospital to an existing hospital. A first-of-its-kind structural system was utilized to allow the skyway to be laterally supported by the new isolated hospital at one end and by an existing fixed-base hospital at the other end, while minimizing the seismic joint size.

Michael Gemmill of Nabih Youssef answered a series of questions about the project.

One of the most striking features of this project is the extent and size of cantilevers throughout the building. How were you able to accommodate this aspect of the design while meeting the vibration requirements for sensitive hospital equipment?
The cantilevers definitely were a challenge, and we used a number of structural systems to get them to work. At the wrapround cantilevers of the patient towers we used a Vierendeel truss which is essentially a moment frame on its side. The truss extended over several floors which generated enough stiffness to meet the vibration criteria. At the podium cantilevers we used a combination of traditional cantilever beams, Vierendeel trusses, and story-deep diagonal hangers.

Triple Friction Pendulum isolation bearings were used to provide base isolation for the building. How did you approach the use of this system in an acute care facility under the jurisdiction of OSHPD?
At the time Triple Friction Pendulum bearings were still relatively new so we relied heavily on research papers and even collaborating with researchers that were studying the bearings. We did several studies to compare the performance of Triple Friction Pendulum bearings to the performance of single pendulum bearings that most reviewers were more familiar with. Gaining approval of the Tension Capable (TC) isolators for the hospital bridge was actually a bigger challenge. These bearings were roughly 12 feet by 12 feet and were designed to resist significant seismic uplift due to the bridge overturning. They had only been used once before at a smaller scale, so the testing and modeling was completely new for everyone.
The atrium dome is a 120-foot diameter, column free dome comprised of welded steel tubes designed as a series of arches with a circular tension ring.

How did you and your team develop this approach to the support of the atrium dome? Were there other structural systems that you evaluated before deciding on this one?
We looked at a number of more traditional long span options including deep trusses and two-way space frames. The arch and tension ring concept worked so well because it was a circular opening and by far resulted in the lightest structure. It was one of the few times where the final structural result was lighter than the architect’s initial renderings.

The pedestrian skybridge between the new hospital building and the existing hospitals was designed utilizing an entirely new structural system, which allowed the seismic joints to be limited to three inches.

Can you describe the how the system works and how the different aspects of the design for the pedestrian bridge structure were developed?
The bridge looks like a hockey stick in plan and connects the new base isolated hospital (which can move about 3 feet in any direction in an earthquake) to an existing fixed-base hospital (which only moves a few inches in an earthquake). A traditional way of accommodating this difference in movement is to provide a very large and rather clunky accordion-like expanding corridor joint. Our solution was to base isolate the bridge and to add three pins to break the bridge into a linkage that can articulate. This turned the challenge of a skewed bridge geometry into an advantage. The bridge articulates to accommodate the seismic movement of the new hospital. The real benefit is that the large seismic movement is converted to a few inches of rotation at each of the pins.

What was your favorite aspect of working on this project?
Definity the people. The project had top notch consultants that were great to work with. We developed a lot of friendships over the course of the 12 year project and we still keep in touch with a lot of folks.

The San Francisco International Airport (SFO) Terminal 2 Build Back and Retrofit project is this year’s SEAONC Excellence in Structural Engineering (EiSE) award winner in the category of Retrofit/Alteration. Here is the history and description of the project:

The Central Terminal Building (CTB) at SFO was dedicated in 1954 as the airport’s main terminal. The upper floors of the CTB were transformed into an administrative office building in 2000. In 2010, the flight operations portion of the CTB was seismically retrofitted and remodeled into the current Terminal 2 (T2).

The CTB’s six-story concrete structure housed over 600,000 square feet, including the original Air Traffic Control Tower (ATCT). Following completion of the new ATCT in 2016, SFO hired Turner Construction, AE3 Partners/Woods Bagot Joint Venture, and Simpson Gumpertz & Heger (SGH) to execute a design-build project that included demolition of the original control tower and upper four stories of T2 and the “build back” of two stories above the existing structure.

The new two-story structure is programmed to accommodate airline lounges, office space, café, and exterior observation decks at each end of the building. TSA screening for T2 is located directly below the “build back” at the second level, which presented a major design challenge as TSA could not be affected during construction. Advanced analysis techniques and innovative connection detailing were necessary to prove the existing building and foundations could meet current building code requirements considering changes in design loads and minimal existing structure capacities. Higher level analysis included a rocking evaluation that avoided foundation retrofit. The new space was designed with steel special moment frames (SMF) that minimized overturning forces resulting from seismic loads. The SMF column bases were designed to transfer only shear forces and minimize moment demands on existing weak steel framing.

T2 now stands as a harmonious union between old and new at the geographic center of SFO, prepared to serve the 5.5 million passengers that will pass through its doors annually.

Kevin Moore, a senior principal at SGH, was the structural engineer of record for this project. He graciously agreed to be the inaugural interviewee for a series of articles on the EiSE award winners.

SEAONC POST: This project involved building two stories on top of an existing two-story structure. How did the existing structure inform your decisions for the gravity framing and seismic force resisting system of the upper two stories?

KEVIN MOORE: The existing structure comprised both steel and concrete, but the SFRS was concrete and recently retrofit with the intention of adding two stories sometime in the future. As we know, codes change and so do seismic forces, so the two stories of earlier code cycles didn’t match the two stories of 2018, so we had to make the structure as minimalistic as possible as we were not allowed to retrofit any foundations or run construction through the T2 TSA security space, directly below 75% of the new construction.

SEAONC POST: A rocking foundation analysis was utilized to avoid retrofit of the existing foundations. How soon in the design process did it become apparent that this type of analysis would be required and what challenges did you and your team face in developing the parameters for the rocking foundation analysis?

KEVIN MOORE: We were told that the baggage handling system could not be shut down or temporarily replaced during construction, so foundation work was off the table in the beginning of the project. If we couldn’t adapt the design, the project wouldn’t have been constructed. The foundations were retrofit as part of an earlier project, so we were able to use that strengthening, coupled with rocking to provide a code compliant building without increasing foundation capacities. The limiting factor was displacement; if we could prove that vertical displacement within the foundation element was less than the surrounding concrete, the encapsulated piers would remain viable and rocking would be acceptable. Our plan reviewer (a SEAONC member) agreed with our approach.

SEAONC POST: The two-story build back structure was constructed over the existing TSA screening area for Terminal 2, which was required to remain in continuous operation. Did any issues arise during construction that made it difficult to perform the work without affecting the existing operations of the TSA screening area? If so, how were they dealt with?

KEVIN MOORE: Fortunately, there were no issues that negatively affected TSA operations during construction. However, we had several structural elements that were designed to remain above that space, meaning we hung more from the existing structure than one might in a typical retrofit/remodel. The contractor struggled with some of the difficult conditions, but as there were no other options, they were able to complete the work and avoid disrupting airport operations.

SEAONC POST: Special steel moment frames are the seismic force resisting system for the two-story build back structure. The base connections are designed and detailed as pinned in order to minimize moment demands on the existing columns below. How did you and your team develop these connections and their load-transfer mechanisms to the existing structure below?

KEVIN MOORE: We contemplated a number of options ranging from a true pin to a base isolated type connection. In the end, we developed a connection that relies on compression blocks and tension ties such that shear is transferred directly through compression stress in the blocks welded to existing structural steel framing, while tension from overturning is resisted through a flexible steel strap that transfers very little moment between the new column and existing structural steel framing. A few on the team called it our “swiss watch” detail. The steel fabricator/erector (Olson Steel) did an excellent job with the connection and had no problem inferring our intentions.

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