Editing Tay Bridge (section)

=== Design details ===
[[File:The Tay Rail Bridge from Wormit on the south bank.jpg|thumb|The second (current) Tay Rail Bridge from Wormit on the south bank]]
[[File:The Tay Rail Bridge as seen across the Tay Estuary from Newport-on-Tay.jpg|thumb|upright|The current Tay Rail Bridge as seen across the Tay Estuary from Newport-on-Tay]]
The engineering details on the Tay Bridge were considerably simpler, lighter, and cheaper than on the earlier viaducts. The machined base of each column section docked securely into a machined enlarged section of the top of the section below.<ref>"like the spigot and faucet of a domestic water pipe" was felt be a useful analogy at the Tay Bridge Court of Inquiry but would probably baffle modern householders</ref> The joint was then secured by bolts through matching holes on lugs (Crumlin<ref name="Maynard" />) or flanges (Belah) on the two sections. This 'spigot and faucet' configuration was used, apparently without machining, on some Tay Bridge pier columns, but on some the bolts were relied upon to ensure correct alignment. In the event, the joints were made using undersized bolts, of a smaller diameter than that which would just go through the hole. This made assembling the column easier, as the bolt holes would not need to align exactly before inserting the bolt. However, this allowed the two members, so joined, to move relative to each other under load, weakening the column.

On the Tay Bridge the diagonal bracing was by means of flat bars running from the top of one column-section diagonally down to the bottom of the adjacent column section. The top connection was to a lug that was an integral part of the column casting.<ref>"..although cast-iron lugs are peculiarly liable to fail from shock, they have been used in precisely this way successfully in tens of thousands of yards of viaduct; therefore, Sir Thomas Bouch was only following precedent in using them here. Mr Barlow will know that on the Bombay and Baroda line they were used for a great many viaducts" Minutes of Evidence – evidence of B Baker, p. 507</ref> The bottom connection was to two sling plates bolted to the base of the equivalent section on an adjacent column. The bar and sling plates all had matching longitudinal slots in them. The tie bar was placed between the sling plates with all three slots aligned and overlapping. A [[Gib and cotter|gib]] was driven through all three slots and secured. Two cotters, metal wedges, were then positioned to fill the rest of the slot overlap, and driven in hard to put the tie under tension. Horizontal bracing was provided by wrought iron channel iron.<ref>Minutes of Evidence – evidence of H Law, p. 246</ref> The various bolt heads were too close to each other, and to the column for easy tightening up with spanners; this coupled with lack of precision in the preparation of the channel iron braces led to various on site fitting expedients (one of them described by a witness to the enquiry as "about as slovenly a piece of work as ever I saw in my life".<ref>Minutes of Evidence – evidence of H Law, p. 331</ref>

On the Crumlin and [[Belah Viaduct]]s, however, horizontal bracing was provided by substantial fitted cast-iron girders securely attached to the columns, with the diagonal braces then being attached to the girders.<ref>Details of the bracing of the Crumlin viaduct are given in Maynard, see also a photograph of painters at work on a pier in 1914 to be found at [http://www.crumlinviaduct.co.uk/photo.htm Crumlin Viaduct] which will probably be found of considerable assistance</ref> The Chairman of the Court of Inquiry quoted at length from a contemporary book<ref name="Humber 1870">{{cite book|last=Humber|first=William|title=Complete Treatise on Cast and Wrought Iron Bridge Construction &c Volume 1|year=1870|publisher=Longwood|location=London|pages=264|url=https://books.google.com/books?id=1SJRAAAAYAAJ}}</ref> praising the detailed engineering of the Belah viaduct piers, and describing the viaduct as one of the lightest and cheapest of the kind that had ever been erected.
<blockquote>...&nbsp;It is a distinguishing feature in this viaduct that the cross, or distance girders<ref>made of cast iron Minutes of Evidence – evidence of Sir T Bouch, p. 430</ref> of the piers encircle the columns, which are turned up at that point, the girders being bored out to fit the turned part with great accuracy. No cement of any kind was used in the whole structure, and the piers when completed, and the vertical and horizontal wrought-iron bracings keyed up, are nearly as rigid as though they were one solid piece...</blockquote>  <blockquote>.... The fitting was all done by machines, which were specially designed for the purpose, and finished the work with mathematical accuracy The flanges of the column were all faced up and their edges turned, and every column was stepped into the one below it with a lip of about 5/8 of an inch<ref>ie c 16 mm</ref> in depth, the lip and socket for it being actually turned and bored. That portion of the column against which the cross girders rested was also turned. The whole of these operations were performed at one time, the column being centred in a hollow mandril-lathe. After being turned the columns passed on to a drilling machine, in which all the holes in each flange were drilled out of the solid simultaneously. And as this was done with them all in the same machine, the holes of course, perfectly coincided when the columns were placed one on the other in the progress of erection. Similar care was taken with the cross-girders, which were bored out at the ends by machines designed for that purpose. Thus, when the pieces of the viaduct had to be put together at the place of erection there was literally not a tool required, and neither chipping or filing to retard the progress of the work.<ref>Rothery in Court of Inquiry report pp 43–44 quoting Humber op cit p 224-5</ref></blockquote>
Either, said the chairman, the Belah viaduct had been over-engineered, or the Tay Bridge had been under engineered.<ref>Rothery in Court of Inquiry report pp 43–44</ref>