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Bascule

Started: 2019-11-16 19:15:10

Submitted: 2019-11-17 22:35:55

Visibility: World-readable

9th November 2019: In which the intrepid narrator descends into the bascule chamber under the Tower Bridge

My team at work is in the habit of having team summits every year, getting everyone together in one place to talk about strategic goals and do team building and just let us spend face-to-face time so we know that the people we're working with are actually people and not just pixelated humanoid figures on a video conference once or twice a week. Last year the summit was in Seattle (a few weeks before I joined the team); this year, even though I already had the chance to visit London once already this year, I got to go again. London is one of my favorite cities (as I have documented extensively here), so after the summit I stuck around for another day to play tourist.

Some time ago I found London blogger Ian Visits and started following his blog to keep up with the city. As I was making plans to visit London I read his post about the behind-the-scenes tour at the Tower Bridge. Most of the time when I read these blog posts I am in no position to actually visit the things he mentions; this time the tours coincided with my planned trip, so I immediately followed the link and picked up one of the tickets for my one free day in London.

South tower of the Tower Bridge
South tower of the Tower Bridge

I found the behind-the-scenes tour waiting inside the ticket counter right at the scheduled start time at 10:30. There were eight total people in the tour group, plus our guide Monika, who took us into the bridge.

The first part of the tour was almost like the regular tour. We took the lift part way up the tower to look at the exhibit discussing how divers used caissons under pressure to construct the foundations to the tower, then looked at the tower's steel frame where it was visible, including a pair of long thin cross braces running in an X across the wall. (Our guide pointed out that the steel cross brace was set back from the masonry wall so that the steel could expand and contract without impacting the wall.)

Spiral staircase inside the north tower of the Tower Bridge
Spiral staircase inside the north tower of the Tower Bridge

We took the lift to the top of the tower and studied the steel structure in greater detail, including the rivets that held the steel frame together. Most of the rivets were installed when the truss was built at the steel yards; the remainder were installed when the truss was completed on site.

Steel truss inside the Tower Bridge
Steel truss inside the Tower Bridge

Our guide took us behind a curtain and around a crowd-control rope to the end of the western walkway. This walkway was accessible on the general tour, but our took an alternate route. Our guide let us look at the view from the upper deck of the bridge and talked about the history of the bridge.

Tower Bridge pedestrian walkway
Tower Bridge pedestrian walkway

The windows included little sliding hatches to allow unobstructed views of the scenery; I took advantage of the hatches to photograph the River Thames, with the fanciful steel-and-glass towers of the City of London on the north side, towering over the Tower of London on the north bank of the river.

The Walkie-Talkie Building and the River Thames
The Walkie-Talkie Building and the River Thames

On the south bank of the river I could see The Shard looming above London City Hall, looking like an architectural rendering of the building in the morning haze rather than a real building with a real observation deck that I could visit at will.

The Shard looms above London City Hall
The Shard looms above London City Hall

Since my visit to the Tower Bridge in 2006, the bridge had installed a glass floor to provide a more thrilling tourist experience, for tourists that are not also acrophobic. I waited for the crowd to disperse then walked on the thin section of opaque floor that was left at the side of the walkway, catching only a brief glimpse of the bridge pier and the river far below.

Tourists on the glass floor on the Tower Bridge
Tourists on the glass floor on the Tower Bridge

On the far side of the bridge, we descended the south tower to the road level -- and then the tour diverged from the normal tour. A bridge operator met us and opened a locked gate and took us up a short flight of stairs into the original control room on the south side of the bridge. This control room featured a bunch of control levers to manipulate the locks and engines that would open and close the bridge as required by river traffic. This room was original when the bridge was built in the 1890s and directly controlled the water engines that moved the bridge. This control room was left in place when the bridge was converted to electric-powered oil hydraulics in 1976; the other control room on the north side of the bridge now contains the modern controls that remotely operate both sides of the bridge.

Tower Bridge control room
Tower Bridge control room

I was, by far, the youngest person in the tour (everyone else seemed to be at least 15 or 20 years older than me); but the rest of the group was clearly populated by nerds who were more than willing to pay for a special tour of the bridge and fawn over the historic engineering on display. One woman in the group made an affectionate comment to the effect of "what a group of nerds", probably in reference to her partner lingering behind the group to take another picture of the machinery.

Hydraulics under the Tower Bridge control room
Hydraulics under the Tower Bridge control room

We descended a flight of stairs at the back of the control room and I lingered to study (and photograph) the bright copper tubes running to the hydraulic valves at the bottom of the control levers in the control room above -- the tubes that operated the pieces of machinery that operated the bridge.

Original water motor inside the Tower Bridge machine room
Original water motor inside the Tower Bridge machine room

The bottom of the stairs led us into a machine room that powered the shaft that turned the cog that opened the bridge. The first thing we saw was a giant Victorian water motor, painted to a gleaming shine with its different parts highlighted in different colors. There were two stages working together on the same base that would take the pressured process water (pressurized by steam) and turn the shaft. The bridge deck was balanced with a counterweight that made opening the bridge relatively easy, though the motor still had to overcome friction in the pivot to open and close the bridge. Any one motor would have been enough to open the bridge under normal circumstances, but there were four engines operating in parallel, allowing for redundancy while one engine was serviced and also supporting the bridge's requirements that it be able to open at any time.

At the back of the motor was the massive shaft, cut off when the bridge was converted to oil hydraulics in the 1970s.

Cut-off drive shaft in the Tower Bridge machine room
Cut-off drive shaft in the Tower Bridge machine room

In the other half of the machine room, in the space that was formerly occupied by another water engine, a modern hydraulic pump and hydraulic motor sat, providing the power that actually opens the bridge today.

Active hydraulic pump in the Tower Bridge machine room
Active hydraulic pump in the Tower Bridge machine room

Two electric hydraulic pumps sat on a sled, with a common hydraulic fluid reservoir between them, drawing on the mains power to open the bridge. Immediately adjacent to the pumps sat two hydraulic motors, connected to the bridge's original drive shaft, using the hydraulic power to turn the shaft through a gearbox. The contrast between the 1890s industrial technology and the modern (albeit still older than me) 1970s industrial technology was striking.

Active hydraulic motor in the Tower Bridge machine room
Active hydraulic motor in the Tower Bridge machine room

We left the machine room behind and stepped through a door leading to a cozy spiral staircase that dropped into the floor, then opened into the top of a massive brick-lined shaft. (The door was being held open by a giant spanner, two feet long, that looked like it was in occasional use at least since the bridge was retrofitted in the 1970s.) The spiral staircase stopped on a platform suspended tens of meters above the shaft, with another staircase spiraling down at a more-gentle pitch around the edge of the shaft.

Ascending towards the spiral staircase outside the Tower Bridge pier
Ascending towards the spiral staircase outside the Tower Bridge pier

I believe the dome above our heads in the picture above is just inside the bulge visible above the water line in the main pier of the bridge. That would place the platform at the bottom of the staircase around the water level, and the shaft itself was dug deep into the mud on the river bottom.

Descending into the pier in the Tower Bridge
Descending into the pier in the Tower Bridge

We stopped on a platform two-thirds of the way down the shaft where we could get a good view of the water accumulator that acted as a giant battery, storing the power generated by the steam engines (which we hadn't seen yet, they were under the approaches to the bridge) by lifting scrap weight to keep the water under pressure. When the water pressure was needed to open the bridge, the pressure was released and the weights helped maintain the water pressure into the water engines. The steam engines could open the bridge by themselves, but the accumulators were installed as a deliberately-over-engineered safety mechanism to make sure the bridge could be opened, as mandated by parliament, whenever necessary, regardless of the state of the steam engines.

Looking at the water accumulator inside the Tower Bridge pier
Looking at the water accumulator inside the Tower Bridge pier

The white corridor in the picture above led to the centerpiece of the tour: the massive bascule chamber under the bridge deck, with the 1000-ton counterweight looming over our heads, carefully balanced with the bridge deck suspended over the water.

Inside the bascule chamber under the Tower Bridge
Inside the bascule chamber under the Tower Bridge

I had a hard time finding a good angle to adequately photograph the bascule chamber. (I brought my 360 camera in my suitcase, but neglected to actually bring it with me for the tour.) The chamber was roughly a quarter of a cylinder, lined with brick, with brick steps leading up to the wall under the white-painted counterweight, tracing the path which the counterweight dropped as the bridge opened.

Bridge counterweight under the Tower Bridge
Bridge counterweight under the Tower Bridge

Noise from the road deck above us filtered down into the chamber, echoing around the brick walls. I felt the the counterweight looming over my head, ready to drop into the space I was occupying.

Pivot structure under the Tower Bridge
Pivot structure under the Tower Bridge

I walked around the chamber, studying it from every angle, trying to grasp its sheer size and bulk, carved out of the mud at the bottom of the river, dominated by the moving weight over my head.

Tourists congregate in the bascule chamber under the Tower Bridge
Tourists congregate in the bascule chamber under the Tower Bridge

At length we departed the bascule chamber and retraced our steps past the accumulator, up the stairs through the machine room, past the control room, and back outside onto the sidewalk around the south tower.

During the tour I couldn't help but wonder what would be left of Google's industrial heritage for a tour group to see decades and centuries in the future (or, for that matter, what a tour group would actually want to see). Will Google preserve any of its data centers for history, or will everything be lost and replaced as technology marches on and the buildings are put to new uses, with new racks and servers replacing the old ones? (And then I wondered how a historical museum would try to interpret a data center and replicate the life of a hardware operations tech inside the data center, complete with brightly-painted wooden models vaguely representing hardware components that visitors would stack onto carts and wheel around the data center floor "repairing" servers.)

The last thing we saw on the tour was the original steam engines that powered the bridge, preserved in the brick arches under the bridge approaches, repainted into gleaming colors representing the colors of the engineer who built the engine. Interpretive displays showed how the coal power was turned into moving the bridge, and our guide explained everything as we marveled at the Victorian industrial machinery.

Steam engine at the Tower Bridge
Steam engine at the Tower Bridge

And with that our tour drew to a close, after nearly three hours crawling around the bridge, deep into the bridge piers, where the tours don't normally go. The whole experience was amazing and I'm thrilled I had the chance to see it.

I took a bunch of pictures of the Tower Bridge, you can see all of them at Photos on 2019-11-09.

this is a neat, if unintelligable, hack
- Jaeger's comment in BMAS::Service::Domain, 11 October 2002