When the Millennium Bridge opened, it wobbled.

I remember it well. The media laughed.

This was before the days when we could deliver engineering constructions – and mass global sporting circuses – successfully, and on time.

I read the other day  that it wobbled because of something that wasn’t supposed to happen. (This was in a book review in the LRB – yes that edition).

Or to put it more accurately, because of something that happened but that the engineers hadn’t planned for.

It wasn’t the volume of the people.

It was what they did.

They fell into line.

They walked in synch: left, right, left.

This communal pattern did a funny thing to the way everyone’s weight was distributed through the fabric of the bridge.

It meant too much weight was being put on the bridge at one time, and was being done so in an unwitting rhythm.

So the bridge wobbled.

Turns out, this is something that bridge designers had used to guard against.

Bridges used to have signs advising people to walk out of step. It was that well known.

But apparently engineers have a habit of collectively forgetting lessons that were once very well known.

Technological advances mean that everyone looks forward, forgetting what their forbears learned through expensive failures.

The writer of the book being reviewed has calculated that this process of collective memory decay takes about 30 years, about the length of a professional career.

After this, new engineering cohorts will have to learn mistakes for themselves.

And this was one of them.

This habit – and the inevitable sense of responsibility the engineering profession feels for such mistakes – led to one fascinating corrective case study, in Canada.

Since the 1920s, Canadian engineers wear iron rings on the little finger of their writing hand.

The rings were fashioned from the remains of Quebec Bridge which collapsed in 1907. It’s supposed to remind each engineer that their hand could be the one that draws the wrong line and caused a collapse.

They’re made of steel now, but back then the iron was left to rust  and go sharp – an un-ignorable reminder.

The strange thing about all this (the mistakes, the forgetting, the sense of responsibility) is that, when a lot of these old bridges were built, no-one really understood how bridges and weight distribution really worked.

No-one knew for sure the actual paths through the constructions of steel and stone, taken by that individual loads as they transferred downwards.

The engineering maths didn’t exist then.

So engineers were working with factors they didn’t even fully understand.

So, what can we learn from all this?

1. As with the Millennium Bridge, people can’t help fitting into a social pattern. It’s just very difficult to predict when, where and why that will happen.

2. There are thousands of people in our industry who have experienced failures and mistakes. We have an awful lot to learn from people who did our job a long time ago, in supposedly simpler times – if we only bothered to listen.

3. Sometimes you need more than just an idea, or a memory, or a message, to communicate effectively. To change behaviour, in the right context, sometimes you need real-world objects that people can unconsciously carry with them.

4. People have designed things way more important than ad campaigns without understanding the factors at work. Sometimes we are allowed to accept that we simply don’t yet know what we don’t yet know. We can still create brilliant, effective, aesthetically pleasing constructions even if we don’t quite understand what’s happening.

Even if they wobble every now and again.