Editing Tower Bridge (section)

=== 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>