To build up, you need to go down. This is our first lesson in structural engineering from Paula Walsh, a director at the consultancy firm Arup. Paula leads a team of 150 engineers in London and she knows what it takes to make buildings stand up.
It seems like another glass tower appears on the London skyline every week, but it wasn't always so. The ground beneath London is mostly clay. Clay is great for tunnelling (don't we love a tunnel?), but not for tall buildings. New York City, by contrast, is built on rock — skyscrapers galore but a very shallow subway system (see our comparison of the London Underground vs the New York Subway).
It wasn't until the 1960s, when engineers figured out how to build piles deep into the London clay, that we've been able to seriously build upwards. And up we've gone.
The building that never was
Our second lesson in structural engineering is patience. "So often you work on designs and plans for buildings and they don't turn into anything," Paula tells us. "Someone loses funding or the stock market crashes and they sell the site. If you get half or a third of your designs actually built, that's quite impressive."
With zen-like detachment Paula tells us about her favourite London building that didn't make it — The Pinnacle at 22 Bishopsgate. Planned to be the highest building in the City of London's tall building cluster, the concrete core of The Pinnacle made it to seven storeys before construction stopped forever.
All is not lost for The Pinnacle. Above ground, 'The Stump' has disappeared, to make way for a new tower to be built on the site. Down below, its basement and 60m deep piles will be reused for the new building.
Tall buildings have gone up and come down since things took off in the 60s, but their piles remain. London is becoming a forest of concrete under the ground. Building piles and boring them out again is expensive, so engineers reuse them wherever they can. They keep detailed records of the piles as they are built, so future engineers know how they'll perform for the next generation of buildings.
Solving problems is fun, and solving problems in teams is more fun. This is not so much a lesson in structural engineering as Walsh's professional mantra. One of Paula's most fun projects was More London, the development surrounding City Hall.
"It is such a great site, but it had literally been left as a hole in the ground for years," Paula recalls. "I worked on the vast majority of the buildings there over a 15-year period. I get a lot of satisfaction seeing it complete and well occupied, knowing some of the gymnastics we did structurally."
The last site at More London, the PWC Building, proved most challenging. There wasn't enough room left to build foundations straight down, so Paula and her team used what they could. They transferred some of the building load onto a concrete box that was built as an underground plant room for electrical cables and services for the whole development. The PWC Building now looks nice and symmetrical, but it is being held up by an ingenious, if unorthodox, structure.
How high can we go?
Our final lesson in structural engineering is that bigger is not always better. Context is everything, says Paula. When we ask if being a structural engineer means being a tall building fanatic she tells us: "I am a fan of good architecture and good design, without a doubt. Whether it's a tall building or a short building it has to look appropriate in its context and place."
In London the biggest constraint on planning for tall buildings is the protection of views of St Paul's, the Tower of London, the Palace of Westminster and other landmarks. The rights to light for neighbours also limit tall buildings, but these can be open to negotiation. The final limit to building height in London comes from the Civil Aviation Authority. Flight path safety means that buildings in the City of London are unlikely to grow beyond 290m (950ft).