Learning from others

By Luke Hally

September 1, 2021

Categories: Cybersecurity foundations

This week we were tasked with investigating how an engineering problem was solved in another discipline. For my engineering problem, I’ve chosen the dome of the Florence Cathedral, or the Duomo di Firenze. It’s an amazing building that I’ve been fortunate to have seen in person. I actually rendezvoused with a friend there who happened to be in Florence at the same time as me. Looking at photos as I researched this activity I can’t believe I’ve actually been there, it’s an amazing building in an amazing city.

I’ve chosen this because I’m an ignorant architectural tragic. I love looking at buildings when I travel, although I don’t really know what I’m looking at. One feature I particularly enjoy is domes and the dome on the Florence Cathedral is one of my favourites. I recall seeing a documentary on it once and that there were some innovative solutions to the challenge of building such a huge dome.

The problem

Construction on the cathedral began in 1296, the design left room for a huge dome, the biggest in the world at the time. Florence was a bustling city at the center of the renaissance, the people wanted a statement piece for their cathedral. The design certainly delivered this. The problem was that no-one knew how to build a dome so big. After over 100 years without a dome, a competition was held to find a solution. Architects from around the known world entered. It ended up being won by a local goldsmith and sculptor, a man with no architectural or engineering experience.

It had to be built without external buttressing, which was forbidden in Florence because it was a design favoured by their northern rivals, such as Milan. The problem was that these were the only known solution to supporting such a large dome at the time.
Building such a large dome 52 metres of the ground which spanned 44 metres, would it be self supporting and not collapse?
It could not be built by the traditional technique of centering (a temporary wooden frame used to guide construction as was done with the Pantheon in Rome) – there was not enough timber in the region to build the scaffolding supports.

The solution

Until modern times it was the largest dome ever constructed and remains the largest brick dome in the world.

The solution consisted on a number of technical aspects:

A double shell, with the inner dome including containing rings, which absorbed the horizontal forces wanting to pull the dome segments outwards. Like the rings on a barrel, except they were between the shells. This also created a cavity that allowed work to take place on both shells and space for stairs to the top.
Bricks were used on the outer dome to reduce weight (compared to stone)
The bricks were laid in a manner called herringbone (the Roman “opus spicatum”) which made them self supporting as they spiralled around the dome as it went up.
The dome was built to be self supporting during the construction process, this eliminated the need for catering.

How engineers ensured openness and transparency

There were two large models made and on public display. One of the original design of the cathedral by Arnolfo di Cambio in 1296, it measured 4.6 metres high and 9.2 metres long. And one showing the solution to building the dome to guide the craftsmen. But it wasn’t total transparency, Brunelleschi made the model incomplete to maintain control of the construction.

Aside from that, this is probably a good example of what happens without openness or transparency. Brunelleschi was extremely secretretive, he was working with a previous competitor on the project and wanted to maintain control. To this day, we still only have theories of how the dome was actually constructed. It’s a shame, the techniques of Roman and Greek dome building – which inspired Brunelleschi – had at his time been lost in antiquity. So too were his own.

How engineers responded to errors

There were errors made in the cathedral base, which meant that it was an unequal octagon. The dome has an open top, to allow light in, this gives tolerance to allow for the uneven base and finish it off with the spire on top. The double shell also meant that the inner shell could be built to perfectly close and fit the imperfections, while the outer shell could maintain external integrity independently.

From what I can tell there were more challenges than errors, such as working at such a great height without scaffolding. This led Brunelleschi to invent hoisting machinery and other devices to aid in the construction.

How engineers treated testing and reviews

Because of the secrecy of the project we can’t be sure, but we know that large scale models were built and techniques were practiced before being applied. This led to the choice to use brick instead of stone on the outer dome and the herringbone brick laying.

What professional standards were used

There were a lot of innovative approaches used in building this dome, however known techniques were used for the stones of the inner dome and laying the bricks on the outer dome.

How engineers used engineering practice successfully

The use of the rings in between the dome shells was a brilliant solution to prevent it falling apart without external buttressing. It used a known technique from another field – wine barrel hoops – and applied it to a new application – a cathedral dome. Engineering was also used to create hoisting equipment to get materials up to the dome.

What security engineers can learn from this

There are some obvious lessons to be learned:

taking lessons from other fields or applications and using them in our context
That successful completion can be done with secrecy, but in a field like security, where herd immunity is important, it is not good practice.
Tolerance in design can help deal with error or mistakes at system/component interfaces.

Reflection

This really was an awe inspiring achievement. A man relying on not much else other than his intelligence, inquisitiveness and belief in himself, built something amazing that has stood for over 500 years. I can’t help but marvel at the audaciousness of it, no experience and a secret plan that he pulled off. It’s not best practice but it’s a great story. But for us mere mortals, there are so many things that could have gone wrong, that for most of us will go wrong, which is why we have standards and best practice to guide us.

But I think the real takeaway is that although we have best practice and standards, we should bever be afraid to try something different. Use standards where they exist in trying something different, but try it. We may find a more elegant solution to an existing problem or maybe create a new solution to an unknown problem. And share what we learn.