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Sabiha Gökçen Airport, Istanbul: seismic engineering

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At Sabiha Gökçen Airport in Istanbul, Arup used triple friction pendulum devices to build the world’s largest seismically isolated building,

The recent earthquake in Chile was much less damaging than January’s quake in Haiti, due in part to Chilean building codes, according to architectural aid charity Article 25. Preparation for earthquakes is a vital part of building design in certain areas.

Take Istanbul, where Tekeli-Sisa Architecture Partnership, with structural engineer Arup, has recently completed a terminal at Sabiha Gökçen Airport for client ISG. According to the project’s seismic expert, Atila Zekioglu of Arup, ‘the new terminal is located in a seismically active region with two branches of the North Anatolian Fault in close proximity. The Istanbul region experiences large earthquakes periodically.’

In 1999, a magnitude 7.4 earthquake struck north-west Turkey in the Kocaeli region, killing 17,000, injuring 50,000 and destroying 27,000 buildings, leaving 500,000 people homeless. Property damage estimates ranged from £1.9 billion to £4.2 billion.

In order to quickly boost annual passenger capacity from five million to an anticipated 22 million people, ISG wanted to complete what was to be the most technically advanced structure of its size in the world just 18 months after detailed design began. It succeeded in its aim, with contractor Limak-GMR-MAHB JV embarking on construction of the terminal foundations just two months after Arup started the design process.

Architects may remember some basic principles of earthquake-resistant design (no engineer would be so foolhardy as to use the term ‘earthquakeproof ’) - structural rigidity is one option, flexibility is another. At Sabiha Gökçen, Arup applied the principle of isolation to separate the superstructure from the substructure, using a supple triple friction pendulum device.

The whole terminal building sits on a platform that is, to a high degree, isolated from the ground below. This enabled the team to design the terminal almost as though it were situated in a non-seismic location, and to include features such as large spans, because the platform and pendulum devices mean that violent lateral ground movements will scarcely affect it.

Seismic engineering analysis and strategy

The brief was to ensure that the airport would continue to operate safely in the event of an earthquake and facilitate crucial relief operations. According to Arup’s Atila Zekioglu, the project team designed for an earthquake hazard with a 10 per cent probability of being exceeded in 50 years, equivalent to a hazard with a return period of 475 years.

‘We chose a seismic isolation strategy, which uses passive structural vibration control technology, because it is the most cost-effective way to achieve the target performance,’ says Zekioglu.

‘We developed a friction pendulum seismic-isolation system, which significantly reduces the torsional seismic response. The isolated superstructure resists lateral seismic loads through a system of steel moment frames with rigid joints.’

The terminal’s 300 isolators enable it to move in a controlled manner and to absorb seismic energy in the event of an earthquake. ‘The isolation system reduces the acceleration and drift response of the terminal by 80 per cent, relative to that of a conventional seismic system,’ says Zekioglu.

Arup used real-time earthquake simulation modelling and LS-DYNA specialist software to test the building’s integrity at 100th of a second intervals, to determine the amount of movement it
could withstand. The results of 14 potential earthquake scenario tests showed that it could withstand a force 7.5-8.0 earthquake, as measured on the Richter scale.

Triple friction pendulum isolators

The isolators at Sabiha Gökçen Airport are triple friction pendulum bearings, manufactured by Earthquake Protection Systems (EPS). The bearings use the characteristics of a pendulum to prolong a structure’s isolation. During an earthquake, the supported structure moves with small pendulum motions. Since earthquake-induced displacements occur primarily in the bearings, lateral loads and movements transmitted to the structure are greatly reduced.

Video: How friction bearings work

The properties of each of the bearing’s three pendulums are chosen to become sequentially active at different earthquake strengths. As the ground motions become stronger, the bearing displacements increase. At greater displacements, the effective pendulum length and the damping increase, resulting in lower bearing displacements.

Thus, the triple pendulum bearing’s inner isolator consists of a slider that moves across two inner concave spherical surfaces. The inner pendulum reduces peak accelerations, which act on the isolated structure and reduce shear forces that occur during what are referred to as ‘service level’ earthquakes.

The two slider concaves, which move along the two concave surfaces, comprise two more independent pendulum isolators. The second pendulum minimises the shear forces that occur during a ‘design earthquake’. The third pendulum minimises bearing displacements that occur during what is known as the ‘maximum credible earthquake’ (MCE). This reduces the size and cost of the bearings, and reduces the displacements required for the structure’s seismic gaps.

Video: Triple-friction pendulum isolator test

Triple pendulum bearings have three mechanisms, which are sequentially activated as the earthquake becomes stronger. The low-friction, short-period inner pendulum absorbs small displacement, high-frequency ground motions. For the stronger design level earthquakes, both the bearing friction and period increase, resulting in lower bearing displacements and lower structure base shears. For the strongest MCEs, both the bearing friction and lateral stiffness increase, reducing the bearing displacement.

When designed for a severe MCE, the plan dimensions of the triple pendulum bearing are approximately 60 per cent of those of an equivalent single pendulum bearing. For the triple pendulum bearings, three radii and three friction co-efficients are selected to optimise performance for different strengths and frequencies of earthquake. This provides maximum design flexibility, to accommodate moderate and extreme motion.

Project data

Scope of project New terminal with surrounding facilities, including domestic and international terminal building, parking structure, hotel and new VIP terminal
Structure The terminal comprises four storeys above and a basement floor below the isolation plane. It has a steel superstructure, with plan dimensions of 160 x 272m, without floor joints, and a total building height of approximately 32.5m
Area of terminal 200,000m²
Passenger capacity 22 million
Official opening October 2009
Developer-operator Istanbul Sabiha Gökçen International Airport Investment Development and Operation
Architect Tekeli-Sisa Architecture Partnership
Masterplanning, structural design, seismic simulation and design Arup
Main contractor Limak-GMRMAHB JV
Triple friction pendulum isolators Earthquake Protection Systems, Vallejo, California. www.earthquakeprotection.com
Simulation software LS-DYNA, developed by Livermore Software Technology Corporation

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