Much of the recent focus has been on the works underground, but this week our Chief Engineer Dr John Roberts gave an enlightening talk to Brighton University engineering students on the design of the tower and the challenges the team have faced, taking us away from the world of diggers and trenches to the top of a sleek, futuristic build unlike anything else that exists in the world.
Most students do not have the chance to see such a huge civil engineering project of this type on their doorstep, so this was an incredible chance to quiz an eminent engineer about such an innovative tourist attraction. They were not disappointed; John was able to give a remarkable insight into how the tower will be built and the challenges faced by the engineers of the i360 at every stage.
The planning stage
The Brighton i360 is going to be a major visitor attraction that sits next to the beach beside the ruined West Pier. That presented a huge challenge to the engineers. For the first part they needed to ensure the tower and pod were able to accommodate the number of people needed to make it a successful commercial venture. They also had to take into account that they were going to build on a Grade 1 listed site close to water. Even parts of the pier hidden underground or in the sea would require permission from English Heritage if they were to be moved.
The foundations
Our foundations need to be six-and-a-half metres deep. There are six metres of beach shingle, under which is chalk rock. Analysing the rock, the engineers were faced with the first problem: the tidal water table. Two hours and 15 minutes after high tide the water in the gravel stratum is higher than the concrete base. That of course meant we have to prepare the base to sit underwater. We have also had to work around the electrical cables and Victorian sewer that sit right in the middle of our site.
We plan to remove 7,200 tonnes of gravel and chalk and replace it with a three metre thick reinforced concrete foundation weighing 4,150 tonnes. The tower itself is 1,200 tonnes. The eagle-eyed among you will have clocked that the two do not add up. We are removing more than we are putting in. The chalk rock has been compacted by the great weight of shingle so as far as the chalk is concerned, our tower is going to feel like a feather.
Proportions
Just looking at the projected picture of the tower was enough to trigger perplexed looks from the student engineers. Surely it is too slender to support the pod? When John explained that 200 people would be able to move freely within the pod as it travelled up the tower, some even looked startled…what about the balance?
This is what is always going to be the wow-factor of the i360. Its incredibly slender design is only 4.5m in diameter, 3.9m if you remove the cladding. It has a slenderness ratio of 1:41, which for all you non-engineers out there is quite remarkable. The Shard, for instance, has a ratio of 1:6. We are closer to the ratio of a palm tree at 1:74.
So what about the balance? 200 adults can weigh up to 18-20 tonnes. Imagine what would happen if someone spotted a whale on one side and everyone raced over to photograph it. Well there is no need to worry (other than for the whale). The design of the pod and the tower takes this into account, so shifting the weight to one side will not affect it. In fact it could actually take more than 200 but we have limited it for comfort and experience. There is more information about how the pod works on our previous engineering blog: an innovation in engineering.
So how do you make such a slender tower?
Building a 162metre high structure that is only 4.5metre in diameter is difficult. Mobile cranes do not go that high without restraint so you would need a tower to build the tower. This is where the engineering and construction is going to get really exciting…instead of building from the bottom to the top in the normal way, the team are going to build from the top down to the bottom.
The tower is made from 17 steel tubes ranging from 12metres to 6metres long. The wall thickness of each of these ranges from 20-85mm. It is quite incredible to think that the ratio is actually thinner than that of the walls of a can of beans.
The longer, thinner ones go at the top and the shorter, thicker ones at the bottom. This has nothing to do with flexibility but simply handling the weight. To build a tower from top down you need a jacking system to push up each of the tubes. The tower weighs 1,000 tonnes and by the time you get to the bottom sections it is going to be tough work for the jacks to hoist the tower up to insert the last piece of tube.
Each tube must be perfectly smooth. When you are dealing with such dense metal, this takes time. Each one is created from a flat piece with an angle cut off each side so that they can be welded together. It takes weeks to machine roll each piece of steel into a perfect curve. It then has to be welded by machine and placed back into the roller until the welded section is perfectly smooth. The tubes alone cost over one million pounds sterling to manufacture, which gives you an indication of how accurate they must be.
The jacking up of the tower is going to be one of the most exciting parts of the build. Watch this space as we will be charting the journey of the tower from Holland to Brighton in the New Year.
Will it deflect?
The key problem with any tower is the wind and we are regularly asked about it. We touched on the subject in an earlier blog, explaining that the ride is designed to operate when it’s windy and has state of the art dampers to ensure that in even windy conditions, the ride is smooth and comfy. John expanded on this saying that in Britain, the standard wind load you prepare for is: “the worst three second gust in the middle of the worst storm which occurs on average every 50 years.” That means the structure must cope with the 50% chance of three seconds of extreme weather within a 50 year period. Incredibly he assured the students that this would mean the structure could move more than a metre, but that it could cope. You would not even see it from ground level.
The real problem for engineers is the dynamic events of the wind. Because the i360 is a very flexible, circular structure, the wind splits around the tower and whirls around it creating a vortex. These break off at different times (it is called vortex shedding) and this makes the tower move in a different direction to the wind, causing vibration. The good news is that all of this can be precisely calculated and offset so that those in the pod do not even feel they are moving.
In typical Brighton conditions the cantilever design will manage the winds seamlessly. Should it get any windier, a series of measures are in place. Perforated cladding around the tower and the slots that the pod travels up, disrupt the flow of the wind. Random outstands on the top of the tower break it up even further. There are also damping systems in place. The Brighton i360 uses liquid sloshing dampers. If you imagine trying to slosh water up to the other end of a bath, you get a better picture of what happens. The movement of the water as it returns will be how the sloshing dampers offset the movement of the tower.
What next?
The students were buzzing with questions by the time the inspiring talk ended, but so were the staff. John was whisked off to meet a team of professors to see how they could incorporate aspects of the project into engineering fields of study. He is also going to be helping many of them in an advisory capacity for their own projects – inspiring our future engineers with his innovative designs for the i360.