Editing Tower Bridge (section)

== Design ==
=== Structure ===
[[File:Tower Bridge (aerial view).jpg|thumb|Aerial view of Tower Bridge]]
[[File:Tower Bridge, aerial view, white lights, bridge open.jpg|thumb|Aerial view with bridge open]]
The structure was originally designed by [[Horace Jones (architect)|Horace Jones]] but his design had an arch instead of the current high-level footways. Concerns about ships' masts and rigging hitting the arch meant that his design faced objections and Jones "began to think that the arched form must be given up".<ref name=":0" /> It was at this point in 1885 that [[John Wolfe Barry]] partnered with Jones and developed the concept of the "level soffit for the upper bridge" which allowed the design to proceed as it stands today.<ref name=":3" /> Unusually, Tower Bridge was designed to include three types of bridge: the two spans from the shore to the piers are a [[Suspension bridge|suspension]] bridge; the central, opening span is a [[Bascule bridge|bascule]] bridge; and the high level walkways were [[Cantilever bridge|cantilever]] bridges until converted to [[Suspension bridge|suspension]] bridges in 1960.<ref name=":4" />

The main towers "consist of a skeleton of steelwork, covered with a facing of granite and Portland stone, backed with brickwork on the inside faces."<ref name=":0" /> An octagonal steel column stands at each corner of each tower (119 feet 6 inches long). The smaller abutment towers were "generally similar...though on a smaller scale."<ref name=":0" />

"The total length of the bridge, including the abutments, is 940 feet, and the length of the approaches are 1,260 feet on the north side, and 780 feet on the south side. The width of the bridge and approaches between the parapets is 60 feet, except across the opening span where it is 49 feet. The steepest gradients are 1 in 60 on the north side, and 1 in 40 on the south side."<ref name=":0" />

The central span of {{convert|200|ft|m}} between the towers is split into two equal bascules, or leaves, which project 100 feet out each and extend backwards 62 feet 6 inches within the face of each pier. The bascules, weighing about 1,070 tons each including ballast and paving,<ref name=":0" /> are counterbalanced to minimise the force required and allow raising in five minutes and have an arc of rotation of 82° with the centre of the arc, or pivot point, being 13 feet 3 inches inside the face of each pier and 5 feet 7 inches beneath the surface of the roadway.<ref name=":0" /><ref name="phoenix">{{cite web |url=https://www.phoenixhydraulics.co.uk/Blog/londons-tower-bridge |title=London's Tower Bridge |work=Phoenix Hydraulics |access-date=28 January 2020 |archive-date=24 October 2019 |archive-url=https://web.archive.org/web/20191024184056/https://www.phoenixhydraulics.co.uk/Blog/londons-tower-bridge |url-status=live }}</ref>

The two side spans are suspension bridges, each {{convert|270|ft|m|0}} long, with the suspension rods anchored both at the abutments and through rods contained within the bridge's upper walkways. The pedestrian walkways are {{convert|143|ft|m|0}} above the river at high tide and accessed by lifts and staircases.{{sfn|Weinreb|Hibbert|Keay|Keay|2008|p=922}}{{sfn|Tower Bridge|1994|p=47}} These walkways were designed as cantilever bridges for a distance from each tower of 55 feet, with girders bridging the 120 feet between the ends of the cantilevers.<ref name=":5" />

The structure was designed to withstand wind pressures of 56 lb per square foot ({{convert|56/144|psi|disp=output only}}), a design constraint introduced following the [[Tay Bridge disaster|Tay Bridge]] disaster<ref name=":3" /> which had occurred just 15 years before the opening of Tower Bridge.

There is a chimney on the bridge that is painted to look like a lamppost. It was connected to a fireplace in a guardroom located in one of the bridge piers.<ref>{{Cite web |last=Abrahams |first=Luke |date=8 February 2018 |title=Six amazing things you never knew about Tower Bridge |url=https://www.standard.co.uk/reveller/attractions/the-history-of-tower-bridge-six-amazing-things-you-never-knew-a3750461.html |access-date=2 May 2023 |website=Evening Standard |language=en |archive-date=2 May 2023 |archive-url=https://web.archive.org/web/20230502004648/https://www.standard.co.uk/reveller/attractions/the-history-of-tower-bridge-six-amazing-things-you-never-knew-a3750461.html |url-status=live }}</ref>

=== Hydraulic system===
[[File:Tower.bridge.99.machinery.london.arp.jpg|thumb|One of the original steam engines]]
The original raising mechanism was powered by pressurised water stored in six [[hydraulic accumulator]]s.<ref>{{cite web |url=http://www.towerbridge.org.uk/TBE/EN/BridgeHistory/ |title=Bridge History |website=Tower Bridge |publisher=[[City of London Corporation]] |date=1 February 2003 |access-date=13 June 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120620160843/http://www.towerbridge.org.uk/TBE/EN/BridgeHistory |archive-date=20 June 2012 }}</ref> There were then two pairs of engines on each pier. Each pair consisted of a larger engine 8 ½ inches in diameter and a smaller one at 7 ½ inches. All eight engines had three cylinders. The reason for two pairs of engines on each pier was to build in redundancy. The machinery was "equal to twice the requirements of the Board of Trade", already very rigorous standards established following the [[Tay Bridge disaster|Tay Bridge Disaster]].<ref name=":5" />

The system was designed and installed by [[Hamilton Owen Rendel]] while working for [[Armstrong, Mitchell and Company]] of [[Newcastle upon Tyne]].<ref>{{cite news |title=Obituary – Hamilton de Marchi Owen Rendel |newspaper=The Times |date=19 September 1902 |page=8 |url=https://link.gale.com/apps/doc/CS135588659/TTDA?u=kccl&sid=bookmark-TTDA&xid=0fe40bc6 |access-date=24 June 2021 |url-access=subscription |archive-date=29 September 2024 |archive-url=https://web.archive.org/web/20240929053427/https://go.gale.com/ps/i.do?p=TTDA&u=kccl&id=GALE%7CCS135588659&v=2.1&it=r&sid=bookmark-TTDA&asid=0fe40bc6 |url-status=live }}</ref>{{sfn|Spencer-Silver|Stephens|2005|p=85}} Water at a pressure of {{cvt|750|psi|MPa}} was pumped into the accumulators by a pair of [[stationary steam engine]]s.{{sfn|Peacock|2011|p=24}} Each drove a force pump from its piston tail rod. The accumulators each comprise a {{convert|20|in|cm|adj=on}} ram which sits a very heavy weight to maintain the desired pressure.<ref name="phoenix" />

The entire hydraulic system along with the gas lighting system was installed by [[William Thomas Sugg|William Sugg & Co Ltd]]. The gas lighting was initially by open-flame burners within the lanterns, but was soon updated to the later incandescent system.<ref>{{cite web |url=http://www.williamsugghistory.co.uk/ |title=History William Sugg & Co Westminster gas lighting cooking heating appliance manufacturer. |access-date=30 June 2013 |archive-date=30 June 2013 |archive-url=https://archive.today/20130630014442/http://www.williamsugghistory.co.uk/ |url-status=live }}</ref>

In 1974, the original operating mechanism was largely replaced by a new electro-hydraulic drive system, designed by BHA Cromwell House.{{sfn|Peacock|2011|p=24}} The only remaining parts of the old system are the final pinions, which fit into the racks on the bascules and were driven by [[hydraulic motor]]s and gearing. Oil is now used in place of water as the new [[hydraulic fluid]].{{sfn|Peacock|2011|p=25}}

=== Signalling and control ===
[[File:Tower Bridge London uk.jpg|thumb|Tower Bridge in 2018]]
Originally, river traffic passing beneath the bridge was required to follow several rules and signals. Daytime control was provided by red [[Railway semaphore signal|semaphore signal]]s, mounted on small control cabins on either end of both of the bridge piers. At night, coloured lights were used, in either direction, on both of the piers: two red lights to show that the bridge was closed, and two green to show that it was open. In foggy weather, a [[gong]] was sounded as well.{{sfn|Tower Bridge|1994|p=47}}

Vessels passing through the bridge were required to display signals. By day, a black ball at least {{convert|2|ft|m|2}} in diameter was mounted high up where it could be seen. Night passage called for two red lights in the same position. Foggy weather required repeated blasts from the ship's [[steam whistle]]. If a black ball was suspended from the middle of each walkway (or a red light at night) this indicated that the bridge could not be opened. These signals were repeated about {{convert|1000|yd|m}} downstream, at [[Cherry Garden Pier]], where boats needing to pass through the bridge had to hoist their signals/lights and sound their horn, as appropriate, to alert the bridge master.{{sfn|Tower Bridge|1994|p=47}}

Some of the control mechanism for the signalling equipment has been preserved and is housed in the Tower Bridge's museum.<ref>{{cite web |url=https://www.towerbridge.org.uk/Visit-Engine-Rooms/ |title=Engine Rooms |website=Tower Bridge |publisher=[[City of London Corporation]] |access-date=30 January 2020 |archive-date=9 April 2020 |archive-url=https://web.archive.org/web/20200409193415/https://www.towerbridge.org.uk/Visit-Engine-Rooms/ |url-status=live }}</ref>