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Standing at more than 135m and 120ft taller than Big Ben, the Airways London Eye is a far cry from the original 250-ft Ferris wheel invented by George Ferris Jr.
The much publicized 1,800-ton structure now stands proud on the River Thames, just opposite the Houses of Parliament. It is capable of carrying up to 4,500 riders a day enabling them to see up to 25 miles across the city. Perhaps the most remarkable of all the statistics is that the main contractor had just 16 months to complete the exercise from start to finish.
Designed and developed by architects David Marks and Julie Barfield, the future of the wheel lay in the hands of the Dutch steel fabrication and construction company Hollandia b.v.
In order to complete the project on time, Hollandia employed the services of Iv-Infra as a subcontractor for the design calculations and verifications of the steel structure.
The first step was a feasibility study and preliminary calculations so that the factory could begin construction of the parts for assembly of the landmark structure.
The two-month feasibility study investigated five main issues that would determine how the final structure was designed:
Due to time restrictions, many calculations had to be worked out up front for the feasibility test and then later ratified using finite element analysis (FEA) techniques to ensure their correctness.
Once the feasibility study established that the project could be completed successfully, the team began building a complete model of the structure in ANSYS. The model was complicated because it was made from many different elements and designed to be in constant motion.
"Because of the complexity of the model we were dealing with," said Iv-Infra's Arie Lanser, we decided to use ANSYS, which made the creation of the model relatively easy, but also allowed us to verify the individual parts. The majority of the model is nonlinear, and ANSYS is recognized for its strengths in this area."
Once the model was built, parameters were added to determine the fatigue life of the structure. The movement of the rim made this very difficult, and imperfections had to be built into every position of the wheel. To work out the fatigue, consultants plotted the stress against the number of cycles. Given that the wheel turns two times an hour, that's a massive 438,000 cycles in the planned 50-year time scale.
Another major factor in the analysis of the London Eye was the dynamic behaviour of the structure due to wind loading. Hollandia also wanted to ensure the ride was as comfortable as possible for the visitors, with motion sickness being a real concern. The company decided to use tuned mass dampers in the rim near each capsule.
"Using the ANSYS/Multiphysics, we performed dynamic wind analyses on every one of the 32 capsules, according to the advice of the TNO-Bouw research institute in Holland," said Lanser." The calculation was enormous because the load on each capsule is different during the course of the cycle, so we had to investigate 6,400 loadings on each one over a period of 320 seconds. That provided us With the data required to determine the size and placement of the dampers and the behaviour of the structure."
"As well as being suitable for constructing a model of the entire structure, ANSYS also made verifying the local parts easier and offers strong contact solvers for gap and spring models," Lanser continued. "Our choice of this analysis tool was also guided by the fact that all analyses were nonlinear and that the English verification body that was approving our calculation were ANSYS users."
"All participants are delighted that the London Eye is now fully operational. The London Eye is already being hailed as a monument that future generations will be proud of, not bad considering the deadline the whole team had to work to."
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