A focus on flanges

December 9th, 2014 | 6 min read

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The flanges have now arrived at the factory in southern Holland, where our tower cans were originally manufactured.

This is really exciting news for all those following the progress of our build as the flanges are crucial to the design – without them there would be no way of building the i360 tower.

For the next sx weeks we will be welding the flanges to the tower cans and so we decided to investigate the process further. They may look like simple rings of steel but, as with all the elements of the i360 project that we have focussed on to date, the engineering behind them is fascinating and complex.

What is a flange?

gas pipe

A flange is an external or internal ridge / rim that is welded onto an object so that it can attach more effectively to another. Imagine two gas pipes – you need to keep them firmly together to stop gas escaping, so you would either weld the pipes to create a seal, or weld flanges to the pipes so that they could be moved into and out of position easily, forming a pressure seal once bolted together.

Why do we use flanges?

We have 17 cans for our tower and we need to keep them firmly together. The problem we have is that each one is a perfectly smooth tube with walls that are, in ratio, the thickness of a bean can. We are building from the top down, so as we lift each can we need to attach it to the one below. High strength bolts clamp them tightly together so that they are firmly fixed.

If they don’t need to be moved, why don’t we weld?

Welding steel

The best way to attach each can would be to weld them together, but for a tower of this scale we face a welder’s biggest problem: moisture. Although the south east may get less rain than the rest of the country, it does get rainfall all year. It also gets some incredible winds – great news for the Rampion Wind Farm, not so great for the welders! We also have the sea air to cope with – anyone who lives close to the coast will tell you, once the salt gets in to your metalwork, it doesn’t come out easily. The only way to build the i360 safely would be to do it indoors, which is why we are welding our flanges in a factory.

What do our flanges look like?

Flanges with the tower cans behind

The image above shows a pile of flanges ready to be welded to the cans in the background. There are 33 of them in total – 16 of the cans need one at each end but the top can only needs a flange at one end.

How are they made?

Each of the 33 flanges is being made by forging a red hot single ingot of steel into a ring shape of the correct diameter. The material used in the ingot is exactly the same strength as the steel used to make the cans.

When the forging is the right shape, the steel is allowed to cool in a very controlled manner. This improves the strength and fatigue resistance of the flange.

Won’t the flanges detract from the smooth look of the tower?

i360 can with flange attached

No. As you can see in the image above, the flanges will be hidden within the cans.

Why did we use a factory in Spain?

Because of the very precise nature of our cans, we couldn’t just forge them anywhere. The factory we used in Spain is believed to be the only factory in Europe where this operation can be carried out.

How do you ensure they are the right size if they are made in a different location?

i360 tower build

The forged flange is only approximately the correct size at the first stage in the manufacturing process, and has to be machined to the exact size required.  Now the flanges have arrived in Holland, we join them to the cans and then machine them in another large lathe. This gives them an extremely flat surface that is at exactly 90 degrees to the can. If we did not do this, the i360 could become the Leaning Tower of Brighton. We then measure the cans with a laser to make sure they are exactly the right size.

Once they have been welded on, are they finished?

i360 tower

The integrity of the weld must be checked by Quality Control. Our QA officer from Hollandia joins the team from SIF to inspect each of the welds. The inside of the can is quite dark, so they use flashlights to make sure each weld is good.

Let’s talk bolts…

Up to 72 bolts are used at each joint, spaced out evenly around the circumference, and the bolts are up to 75mm (about three inches) diameter. A single bolt of this size can carry a load of as much as 440 tonnes. If you want an indication of what this means, a single 75mm bolt could lift 30 double decker buses!

How many bolts do you need?


In total more than 1,500 flange bolts are needed at the flanges – the total weight of these bolts is about 100 tonnes. A blue whale weighs a similar amount!

What happens if a bolt malfunctions?

Each bolt is galvanised (the steel bolt is submerged into a bath of molten zinc) and then wrapped in a special impregnated tape to give long term protection against corrosion. Even so, the bolts form such an important part of the tower structure that randomly selected bolts will be inspected each year. The bolts can be safely removed and replaced (one at a time, of course) if necessary during the lifetime of the tower.

What happens if the bolt holes don’t match up during the build?

i360 tower

We’ve all drilled holes in the walls, only to find we are out by a few mm (or several cm in some cases). It would be far too late to find a mismatch while the tower is under construction, so we make sure they fit beforehand.  The holes through the flanges are drilled in pairs to ensure the bolts will all fit, then each pair of cans is “trial assembled” in the factory. This is done in sequence – can one is fitted to can two, then these two are taken apart and can two is fitted to can three, and so on.

What’s next?

Once we have welded all the flanges, we will move the cans to Hollandia’s factory where they will have all the interior steelwork added (e.g. platforms, towers, bull wheels for the cable car etc).