By Aysegul Gogus and Atila Zekioglu
Due to its location in a high seismic region and its growing population of over 15 million, Istanbul has been taking efforts to improve its resilience. One of the key initiatives of the city’s resilience plan is providing world-class healthcare services in newly built and seismically resilient large public hospitals.
Başakşehir Pine and Sakura City Hospital is the first city hospital built in Istanbul as part of the “New City Hospitals” program initiated in 2007 by the Ministry of Health in Turkey. In April 2020, the hospital partially opened 1,700 beds to assist with the COVID-19 response in Istanbul. The official grand opening of the 2,682 bed-capacity hospital took place in May 2020, and the hospital has since been in full service to roughly 32,700 patients per day, with 90 operation theatres and 458 intensive care units.
Use of base-isolation for the new city hospitals, which are located in high-seismic zones, has been deemed mandatory by the Ministry of Health in Turkey. Spanning an area of 10 million square feet and featuring 2,068 seismic isolators, Başakşehir Pine and Sakura City Hospital has become the largest base-isolated structure in the world. At 332 feet tall, the main hospital facility consists of three specialty towers, of 14, 16, and 17 stories, three helipads, and six clinic buildings, all of which share a 5-level common podium with no seismic joints. The lower three levels of the podium accommodate underground parking to more than 8,000 cars.
The hospital was built under a Public-Private-Partnership (P3, PPP) model, by Rönesans Holding, who currently operates in the roles of main contractor and investor in 28 countries around the globe, as well as Sojitz Corporation, a Japanese investment and trade corporation. Arup was responsible for the design of the base-isolation system, the foundations, and the seismic design of the super-structure. Concept design of the building started in the second quarter of 2016, and the structural design was completed within a year.
In order to achieve enhanced performance objectives for seismic resilience, the hospital was designed to satisfy ASCE 41 “Immediate Occupancy” performance objective under a very rare earthquake (BSE-2N) with a drift limit of 1 percent, and “Operational” performance objective under the design earthquake (BSE-1N) with drifts limited to 0.5 percent. In order to further minimize damage to nonstructural components, floor accelerations were limited to 0.2g under BSE-2N seismic hazard. The final design ensures vital functionality of the hospital not only for the patients who might need urgent treatment, but also for the resilience of the community and the city.
The gravity system of the hospital consists of reinforced concrete slabs and beams supported by cast-in-place concrete columns. Given the size of the project, a core wall-only lateral system offered the best potential of story-heights and optimization of concrete and reinforcement quantities. Arup conducted an extensive wall optimization study in which a total of one hundred and eighty different wall thickness configurations were evaluated to minimize construction costs and improve floor efficiency. The study aimed at optimizing the concrete quantity, while providing sufficient lateral stiffness to the base-isolated building.
Selection of the isolation system was of utmost importance since the procurement of isolators was a critical path item for the design and the construction process, and a primary driver influencing the overall construction cost and schedule. In order to facilitate the selection of the optimum isolator type and layout for the project, Arup conducted an isolation scheme optimization study in which six different schemes were evaluated. These schemes consisted of utilizing three different types of isolators; triple friction pendulum (TFP) bearings, lead rubber bearings (LRB), and high damping rubber (HDR) bearings, with various layouts. Performance of the superstructure and the foundations were also evaluated for each of the schemes through nonlinear time-history analyses of six different nonlinear models of the entire building. The analyses were conducted using seven pairs of horizontal and seven vertical time histories, both for upper-bound and lower-bound isolator properties, resulting in a total of 168 time-history analyses. Upon presentation of results, Rönesans Holding selected the TFP bearings, the design of which ensures the hospital can displace by 27 inches at the isolation level during an earthquake.
The hospital’s large-scale structure and location in a highly active seismic region made it challenging to analyze. LS-Dyna, which is an advanced finite element analysis software, was used to expedite the non-linear analysis workflow. In conjunction with LS-Dyna, digital technology and cloud computing have been used throughout the design process, from the wall optimization and the isolator selection studies, to the performance evaluation of the final design. This improved the efficiency of the design process significantly, enabling Arup to go beyond standard computational limits and finalize the design of the hospital within a year. Isolator selection study alone was completed within two months, at the onset of design development, which enabled Rönesans Holding to choose the most optimum isolation scheme based on performance, cost, and schedule. Through traditional means of workflows, analysis, optimization and design of a structure of this scale and complexity would have taken a couple of years.
In addition to cloud computing and digital tools implemented to automate the analysis model generation and post-processing of the results, Arup also developed a web interface through which performance results of the wall optimization studies and the associated concrete quantities were shared with the client. This unique interface allowed Rönesans Holding to be engaged in the engineering design and to leverage this information as part of their procurement strategy and process.
In a profession that heavily relies on formulas and rules written in building codes, as structural engineers, instead of adopting traditional means of workflows, we should adapt to the rapid advancements in digital technology. This would allow us time to explore structural system alternatives aligned with the project needs, and deliver optimized, cost-efficient and sustainable designs of the ever-growing large-scale, complex projects in our industry without the need to extend the project schedules.
As structural engineers, we have a unique opportunity to contribute to the improvement of resiliency in our cities. Without being restricted by the minimum building code requirements, having clear communication on project specific seismic performance objectives with our clients at the onset of a project would allow us to deliver resilient buildings that would remain functional even after rare earthquake events. This, in conjunction with making digital investments in the industry, will help us achieve resiliency in our communities.