Editing Tay Bridge (section)

===Origins and concept===
Proposals to build a bridge across the Tay date to 1854 but it was not until 15 July 1870 that the North British Railway Tay Bridge Act received [[royal assent]]. On 22 July 1871, the foundation stone of the bridge was laid.{{Citation needed|date=May 2018}}

The bridge was designed by [[engineer]] [[Thomas Bouch]], who received a [[knighthood]] following the bridge's completion.<ref name="structurae">{{Structurae |id=20000445 |title=Firth of Tay Bridge (1877)}}</ref> The bridge was a lattice-grid design, combining [[Cast iron|cast]] and [[wrought iron]]. The design had been used by [[Thomas W. Kennard]] in the [[Crumlin Viaduct]] in South Wales in 1858, after the use of cast iron in [[the Crystal Palace]]. The Crystal Palace was not as heavily loaded as a railway bridge. An earlier cast-iron design, the [[Dee bridge disaster|Dee bridge]] collapsed in 1847, having failed because of poor use of cast-iron girders. [[Gustave Eiffel]] used a similar design to create several large viaducts in the [[Massif Central]] in 1867.

[[File:Dusk on a sunny summers day across Dundee, Scotland, July 17, 2007.jpg|thumb|At dusk. One of the stumps of the original bridge is silhouetted against the sunlit Firth.]]
The original design was for lattice girders supported by brick piers resting on the bedrock, shown by trial borings to lie at no great depth under the river. At either end of the bridge, the single track ran on top of the bridge girder, most of which lay below the pier tops. At the centre section of the bridge (the high girders), the railway ran inside the bridge girder, which was above the pier tops to give clearance for the passage of sailing ships. To accommodate [[thermal expansion]], there were non-rigid connections between girders and piers.{{Citation needed|date=May 2018}}

As the bridge extended out into the river, by December 1873, it became clear that the bedrock lay much deeper, too deep to act as a foundation for the bridge piers.<ref name="ReferenceA">Minutes of Evidence p. 402. – evidence of Sir T Bouch.</ref> Bouch redesigned the bridge to reduce the number of piers and increase the span of the girders. The pier foundations were no longer resting on bedrock; instead they were constructed by sinking brick-lined wrought-iron caissons onto the riverbed, removing sand until they rested on the consolidated gravel layer which had been misreported as rock, and then filling the caissons with concrete.<ref name="Grothe" />

To reduce the weight that the ground underneath the caissons would have to support, the brick piers were replaced by open lattice iron skeleton piers. Each pier had multiple cast-iron columns taking the weight of the bridging girders, with wrought iron horizontal braces and diagonal tiebars linking the columns to give rigidity and stability. The basic concept was well known, having been used by Kennard in the [[Crumlin Viaduct]] in South Wales in 1858.<ref name="Maynard">Maynard, H. M. "Handbook to the Crumlin Viaduct." J M Wilson: London, 1862. retrievable via Google Books [https://books.google.com/books?id=XoEHAAAAQAAJ] Maynard claims the loading on the Crumlin viaduct foundations to be 1/5th what they would have been had brick piers been used.</ref> Bouch had used the technique for viaducts, including the [[Belah Viaduct]] (1860) on the [[South Durham & Lancashire Union Railway]] line over [[Stainmore]], but for the Tay Bridge, even with the largest practicable caissons, the pier dimensions were constrained by their size. Bouch's pier design set six columns in a hexagon maximising the pier width but not the number of diagonal braces directly resisting sideways forces.{{Citation needed|date=May 2018}}
 {| class="wikitable"
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!Structure  !! Crumlin viaduct<ref name="Maynard" />   !! Belah viaduct<ref>most data from [http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=84]. Retrieved 22 January 2012 (the quoted height between joints/bracing is clearly inconsistent with the accompanying photo so the number of column sections is taken from another reference;)</ref>!! Tay Bridge<ref>all data from Annexe to Report of Court of Inquiry</ref>
|-
|Engineer (year of opening) || Kennard (1858) || Bouch (1860) || Bouch (1878)
|-
| Single span  || {{convert|120|ft|m|1|abbr=on}}|| {{convert|60|ft|m|1|abbr=on}} || {{convert|245|ft|m|1|abbr=on}})
|-
| Pier height || {{convert|170|ft|m|1|abbr=on}}|| {{convert|180|ft|m|1|abbr=on}} || {{convert|83|ft|m|1|abbr=on}}
|-
| Pier width at top || {{convert|30|ft|m|1|abbr=on}}|| {{convert|22|ft|m|1|abbr=on}} || {{convert|19|ft|10|in|m|abbr=on}}
|-
| Pier width at base || {{convert|60|ft|m|1|abbr=on}}|| {{convert|48|ft|m|1|abbr=on}} || {{convert|21|ft|10|in|m|abbr=on}}
|-
| Columns per pier || 14 (1-3-3-3-3-1) || 6 (2-2-2) || 6 (1-2-2-1)
|-
| Sections per column || 10 || 11<ref>From diagram 276 in [https://openlibrary.org/books/OL6531011M/History_of_bridge_engineering History of Bridge Engineering HG Tyrell Chicago 1911]</ref> || 7
|-
| Diagonal tiebars giving lateral bracing (per pier)<ref>calculable from previous 2 rows</ref> || 180 || 88 || 28
|-
| Fate||Demolished 1966-7<ref>{{cite web|url=http://www.caerphilly.gov.uk/chronicle/english/onthemove/crumlinviaduct.htm|title=Crumlin Viaduct|work=caerphilly.gov.uk}}</ref>|| Demolished 1963|| Failed in service 1879
|}