While work continues welding the flanges to the tower cans in Holland, we are busy preparing for the next stages of the tower construction process: the secondary steel work. This includes things like the internal platforms and ladders, but also some clever little gadgets called dampers.
What is a damper?
A damper is a device used to offset any vibration of the tower caused by the wind – in particular the movements caused by vortex shedding.
What is vortex shedding?
Our tower is very tall and because of its circular nature, when the wind hits it, it is forced to flow around the structure. This creates a series of vortices – mini whirlwinds – that detach periodically from the tower on alternate sides. This phenomenon is called “vortex shedding”. If the frequency of vortex shedding matches the resonant natural frequency of the i360, the tower can start to vibrate. We talked a bit about vortex shedding in a recent blog about the science behind the i360 building.
What is resonant frequency?
The natural frequencies of vibration of a structure depend on its mass and stiffness (or how flexible it is). Tall structures tend to have more flexibility and so have lower natural frequencies. The i360 tower is particularly sensitive to vibration because it is very tall and very slender.
What frequency affects the i360?
It is not as simple as one frequency. There are several different natural frequencies that can affect the tower and make it vibrate. We have calculated the three lowest natural frequencies that could occur, which we call Mode One, Mode Two and Mode Three.
How do you prevent it vibrating?
The first and most important step is to reduce the likelihood of vortex shedding, and to assist in this we are installing aluminium cladding around the tower. The cladding has a rough surface and is perforated to allow the wind to flow through it, disrupting the regular formation of vortices and so reducing the likelihood of an occurrence of vortex shedding.
The second step is to put in place measures to help offset the vibrations, should they still occur. We have a series of measures in place and one of the most important of these is the dampers.
What do the dampers do?
We use what are called liquid sloshing dampers that are fitted inside the tower. When the wind hits the tower and it begins to move, the liquid inside the dampers sloshes to one side. The uneven height of the water produces a force acting on the tower in the opposite direction to the force of the wind, stopping, or at least limiting, the vibrations. There are Mode One, Two and Three dampers in the tower, and each type is specifically designed to operate at the frequency of the different Modes.
What do the dampers look like?
They effectively small sealed boxes – about twice the size of a tin of biscuits. They are made out of stainless steel and half-filled with liquid. The liquid has anti-freeze added to it to ensure that the dampers can still work on freezing cold days. Each of the different dampers looks similar, but the exact geometry is slightly different depending on the mode type.
The dampers are stacked on top of each other with shallower height dampers at the bottom and larger height at the top.
That small? How many do you need?
We have approximately 50 in the tower placed at three different levels, and a further eight located in the pod. The majority are at the top of the tower where they are most effective in damping Modes One and Two. Further Mode Two dampers and the Mode Three dampers are located at precisely calculated lower levels. During the build we will have an extra, temporary set at the top too. These are designed specifically to operate during the construction of the tower, when it can vibrate at different frequencies as the height is gradually increased.
Why are you building in Australia?
We did not choose Australia particularly; we chose specialist engineer Professor Max Irvine who is based in Sydney. He has designed the dampers and has commissioned them to be made in a small workshop in New South Wales, Australia so that he can closely oversee the process.
In January 2015 the tower dampers will be sent to Rotterdam to meet the tower cans. They will be fitted to the tower ready for the journey to Brighton.