Tay Bridge disaster

A court of inquiry (a judicial enquiry under Section 7 of the Regulation of Railways Act 1871 "into the causes of, and circumstances attending" the accident) was immediately set up: Henry Cadogan Rothery, Commissioner of Wrecks, presided; supported by Inspector of Railways William Yolland and William Henry Barlow, President of the Institution of Civil Engineers. By 3 January 1880, they were taking evidence in Dundee; they then appointed Henry Law, a qualified civil engineer, to undertake detailed investigations. Whilst awaiting his report they held further hearings in Dundee (26 February – 3 March); having got it they sat at Westminster (19 April – 8 May) to consider the engineering aspects of the collapse.[23]

By then the railway, the bridge's contractor and Bouch had separate legal representation, and the NBR had sought independent advice from James Brunlees and John Cochrane,[24] both engineers with extensive experience of major cast iron structures. The terms of reference did not specify the underlying purpose of the inquiry – to prevent a repetition, to allocate blame, to apportion liability or culpability, or to establish what precisely had happened. This led to difficulties (culminating in clashes) during the Westminster sessions. When the court reported their findings at the end of June, there was both an Inquiry Report signed by Barlow and Yolland and a minority report by Rothery.

Two witnesses, viewing the high girders from the north almost end-on, had seen the lights of the train as far as the 3rd–4th high girder, when they disappeared; this was followed by three flashes from the high girders north of the train. One witness said these advanced to the north end of the high girders with about fifteen seconds between first and last;[25][note 3] the other that they were all at the north end, with less time between.[26] A third witness had seen "a mass of fire fall from the bridge" at the north end of the high girders.[27] A fourth said he had seen a girder fall into the river at the north end of the high girders, then a light had briefly appeared in the southern high girders, disappearing when another girder fell; he made no mention of fire or flashes.[28][note 4] 'Ex-Provost' Robertson[note 5] had a good view of most of the bridge from his house in Newport-on-Tay,[31] but other buildings blocked his view of the southern high girders. He had seen the train move onto the bridge; then in the northern high girders, before the train could have reached them, he saw "two columns of spray illuminated with the light, first one flash and then another" and could no longer see the lights on the bridge;[note 6] the only inference he could draw was that the lit columns of spray – slanting from north to south at about 75 degrees – were areas of spray lit up by the bridge lights as it turned over.[33]

Ex-provost Robertson had bought a season ticket between Dundee and Newport at the start of November, and became concerned about the speed of north-bound local trains through the high girders, which had been causing perceptible vibration, both vertical and lateral. After complaining on three occasions to the stationmaster at Dundee, with no effect on train speed, after mid-December he had used his season ticket to travel south only, using the ferry for north-bound crossings.

Robertson had timed the train with his pocket watch, and to give the railway the benefit of the doubt he had rounded up to the nearest five seconds. The measured time through the girders (3,149 ft or 960 m) was normally 65 or 60 seconds,[note 7] but twice it had been 50 seconds. When observing from the shore, he had measured 80 seconds for trains travelling through the girders, but not on any train he had travelled on. North-bound local trains were often held up to avoid delaying expresses, and then made up time while travelling over the bridge. The gradient onto the bridge at the northern end prevented similar high speeds on south-bound locals. Robertson said that the movement he observed was hard to quantify, although the lateral movement, which was probably one to two inches (25 to 50 mm), was definitely due to the bridge, not the train, and the effect was more marked at high speed.

Four other train passengers supported Robertson's timings but only one had noticed any movement of the bridge.[35][note 8] The Dundee stationmaster had passed Robertson's complaint about speed (he had been unaware of any concern about oscillation) on to the drivers, and then checked times from cabin to cabin (at either end of the bridge the train was travelling slowly to pick up or hand over the baton). However he had never checked speed through the high girders.[37]

Painters who had worked on the bridge in mid-1879 said that it shook when a train was on it.[38][note 9] When a train entered the southern high girders the bridge had shaken at the north end, both east–west and, more strongly, up-and-down.[41] The shaking was worse when trains were going faster, which they did: "when the Fife boat was nearly over and the train had only got to the south end of the bridge it was a hard drive".[42] A joiner who had worked on the bridge from May to October 1879 also spoke of a lateral shaking, which was more alarming than the up-and-down motion, and greatest at the southern junction between the high girders and the low girders. He was unwilling to quantify the amplitude of motion, but when pressed he offered two to three inches (50 to 75 mm). When pressed further he would only say that it was distinct, large, and visible.[43] One of the painters' foremen, however, said the only motion he had seen had been north–south, and that this had been less than one-half inch (15 mm).[44]

The North British Railway maintained the tracks, but it retained Bouch to supervise maintenance of the bridge. He appointed Henry Noble as his bridge inspector.[45] Noble, who was a bricklayer, not an engineer, had worked for Bouch on the construction of the bridge.[46]

Whilst checking the pier foundations to see if the river bed was being scoured from around them, Noble had become aware that some diagonal tie bars were 'chattering',[note 10] and in October 1878 had begun remedying this. Diagonal bracing was by flat bars running from one lug at a column section top to two sling plates bolted to a lug at the base of the equivalent section on an adjacent column. The bar and sling plates all had a matching longitudinal slot in them. The tie bar was placed between the sling plates with all three slots aligned and overlapping, and then a gib was driven through all three slots and secured. Two "cotters" (metal wedges)[note 11] were then positioned to fill the rest of the slot overlap, and driven in hard to put the tie under tension.

Noble had assumed the cotters were too small and had not been driven up hard in the first place, but on the chattering ties the cotters were loose, and even if driven fully in would not fill the slot and put the bar under tension. By fitting an additional packing piece between loose cotters and driving the cotters in, Noble had re-tightened loose ties and stopped them chattering. There were over 4,000 gib and cotter joints on the bridge, but Noble said that only about 100 had had to be re-tensioned, most in October–November 1878. On his last check in December 1879, only two ties had needed attention, both on piers north of the high girders. Noble had found cracks in four column sections – one under the high girders, three to the north of them – which had then been bound with wrought iron hoops. Noble had consulted Bouch about the cracked columns, but not the chattering ties.[48]

The workers at the Wormit foundry complained that the columns had been cast using 'Cleveland iron', which always had scum on it—it was less easy to cast than 'good Scotch metal'[49][note 12] and more likely to give defective castings. Moulds were damped with salt water,[50] cores were inadequately fastened, and moved, giving uneven column wall thickness.[51] The foundry foreman explained that where lugs had been imperfectly cast; the missing metal was added by 'burning on'.[note 13] If a casting had blowholes or other casting defects considered to be minor faults, they were filled with 'Beaumont egg'[note 14] (of which the foreman kept a stock for that purpose) and the casting was used.[55]

Gilkes' site staff were inherited from the previous contractor. Under the resident engineer there were seven subordinates including a foundry manager. The original foundry manager left before most of the high girders pier column sections were cast. His replacement was also supervising erection of the bridge, and had no previous experience of supervising foundry work.[56] He was aware of 'burning on',[57] but the use of Beaumont egg had been hidden from him by the foreman.[58] When shown defects in bridge castings, he said he would not have passed the affected columns for use, nor would he have passed columns with noticeably uneven wall thickness.[56] According to his predecessor, burning-on had only been carried out on temporary 'lifting columns', which were used to allow the girders to be lifted into place and were not part of the permanent bridge structure.[59] That was on the instructions of the resident engineer,[60] who had little foundry experience either and relied upon the foreman.[61]

Whilst the working practices were the responsibility of Gilkes, their contract with NBR provided that all work done by the contractor was subject to the approval of the workmanship by Bouch. Hence Bouch would share the blame for any resulting defective work in the finished bridge. The original foundry foreman, who had been dismissed for drunkenness, vouched for Gilkes personally testing for unevenness in the early castings: "Mr. Gilkes, sometimes once a fortnight and sometimes once a month, would tap a column with a hammer, first on one side and then on the other, and he used to go over most of them in that way sounding them."[62] Bouch had spent over £9,000 on inspection (his total fee was £10,500)[63] but did not produce any witness who had inspected castings on his behalf. Bouch himself had been up about once a week whilst the design was being changed, but "afterwards, when it was all going on, I did not go so often".[64]

Bouch kept his own 'resident engineer', William Paterson, who looked after the construction of the bridge, its approaches, the line to Leuchars, and the Newport branch. Paterson was also the engineer of the Perth General Station.[64] Bouch told the court that Paterson's age was 'very much mine' but, in fact, Paterson was 12 years older[note 15] and, by the time of the Inquiry, paralysed and unable to give evidence.[66] Another inspector appointed later[66] was by then in South Australia and also unable to give evidence. Gilkes' managers could not vouch for any inspection of castings by Bouch's inspectors.[67] The completed bridge had been inspected on Bouch's behalf for quality of assembly, but that was after the bridge had been painted (though still before the bridge opened, and before the painter witnesses were on it in the summer of 1879), which hid any cracks or signs of burning-on (though the inspector said that, in any case, he would not know those signs on sight).[68] Throughout construction, Noble had been looking after foundations and brickwork.[note 16]

The bridge after its collapse

Fallen girders, Tay Bridge

Henry Law had examined the remains of the bridge; he reported defects in workmanship and design detail. Cochrane and Brunlees, who gave evidence later, largely concurred.

Samples of the bridge materials, both cast and wrought iron, were tested by David Kirkaldy, as were a number of bolts, tiebars, and associated lugs. Both the wrought and cast iron had good strength, while the bolts "were of sufficient strength and proper iron".[84][note 19] However, both ties and sound lugs failed at loadings of about 20 tons, well below what had been expected. Both ties[80] and lugs were weakened by high local stresses where the bolt bore on them.[77] Four of the fourteen lugs tested were unsound, having failed at lower than expected loadings. Some column top lugs outlasted the wrought iron, but the bottom lugs were significantly weaker.[85]

Salvage operations underway in the Firth of Tay and dockside

Images from the Board of Trade, now in the National Library of Scotland

Bouch had designed the bridge, assisted in his calculations by Allan Stewart.[note 20] After the accident Stewart had assisted William Pole[note 21] in calculating what the bridge should have withstood.[note 22] On the authority of Stewart they had assumed that the bridge was designed against a wind loading of twenty pounds per square foot (one kilopascal) 'with the usual margin of safety'.[88][note 23] Bouch said that whilst 20 psf (0.96 kPa) had been discussed, he had been "guided by the report on the Forth Bridge" to assume 10 psf (0.5 kPa) and therefore made no special allowance for wind loading.[90] He was referring to advice given by the Astronomer Royal, Sir George Biddell Airy in 1873 when consulted about Bouch's design for a suspension bridge across the Firth of Forth; that wind pressures as high as 40 psf (2 kPa) might be encountered very locally, but averaged over a 1,600 ft (490 m) span 10 psf (0.5 kPa) would be a reasonable allowance.[91] This advice had been endorsed by a number of eminent engineers.[note 24] Bouch also mentioned advice given by Yolland in 1869 – that the Board of Trade did not require any special allowance for wind loading for spans less than 200 feet (61 m), whilst noting this was for the design of girders not piers.[90][note 25]

Evidence was taken from scientists on the current state of knowledge on wind loading and from engineers on the allowance they made for it. Airy said that the advice given was specific to suspension bridges and the Forth; 40 psf (1.9 kPa) could act over an entire span of the Tay Bridge and he would now advise designing to 120 psf (5.7 kPa)(i.e. 30 psf or 1.4 kPa with the usual margin of safety).[91] The highest pressure measured at Greenwich was 50 psf (2.4 kPa); it would probably go higher in Scotland.

Sir George Stokes agreed with Airy that 'catspaws', ripples on the water produced by gusts, could have a width of several hundred yards. Standard wind pressure measurements were of hydrostatic pressure which had to be corrected by a factor of 1.4–2 to give total wind loading – with a 60 miles per hour (97 km/h) wind this would be 12.5–18 psf (0.60–0.86 kPa).[93] Pole referred to Smeaton's work, where high winds were said to give 10 psf (0.48 kPa), with higher values being quoted for winds of 50 mph (80 km/h) or above, with the caveat that these were less certain.[94]

Brunlees had made no allowance for wind loading on the Solway viaduct because the spans were short and low – if he had had to, he would probably have designed against 30 psf (1.4 kPa) with a safety margin of 4–5 (by limiting strength of iron).[89] Both Pole and Law had used a treatment from a book by Rankine.[note 26] Law agreed with Rankine that the highest wind pressure seen in Britain was 55 psf (2.6 kPa) as the reason for designing to 200 psf (9.6 kPa) (i.e. 50 psf (2.4 kPa) with a safety factor of 4); " in important structures, I think that the greatest possible margin should be taken. It does not do to speculate upon whether it is a fair estimate or not".[95] Pole had ignored it because no reference was given; he did not believe any engineer paid any attention to it when designing bridges;[96] he thought 20 psf (0.96 kPa) a reasonable allowance; this was what Robert Stephenson had assumed for the Britannia Bridge. Benjamin Baker said he would design to 28 psf (1.3 kPa) with a safety margin, but in 15 years of looking he had yet to see wind overthrow a structure that would withstand 20 psf (0.96 kPa). He doubted Rankine's pressures because he was not an experimentalist; told that the data were observations by the Regius Professor of Astronomy at Glasgow University,[note 27] he doubted that the Professor had the equipment to take the readings.[97]

Baker argued that the wind pressure on the high girders had been no more than 15 psf (0.72 kPa), from the absence of damage to vulnerable features on buildings in Dundee and the signal cabins at the south end of the bridge. The Inquiry felt that these locations were significantly more sheltered, and therefore rejected this argument. Baker's subsequent work on wind pressures at the Forth Rail Bridge site[98] showed meteorologists were overestimating,[99] but his 15 psf (0.72 kPa) might have over-interpreted the data.[note 28]

Law had numerous criticisms of the bridge design, some echoed by other engineers:

• He thought the piers should have been wider (both to counteract toppling and to increase the horizontal component forces the tiebars could withstand) and rectangular (to increase the number of tiebars directly resisting lateral forces); at the very least there should have been lateral bracing between the outermost columns of the piers.[102]
• The lug holes should have been drilled and the tiebars secured by pins filling the holes (rather than bolts).[73] Cochrane testified that he was not surprised that boltholes had been cast conical. He noted that moulders were notorious for this, unless you stood over them. Even so, he would not rely on supervision or inspection, he would have the holes bored or reamed to ensure they were cylindrical because it had an important bearing on the stability of the structure.[103] Pole – called by Bouch's counsel – agreed.[104]
• Bouch said if he had known the holes were cast conical he would have had them bored or reamed.[63] Gilkes said casting lug holes conical would have been done "as a matter of course, and unless attention had been drawn to it, it would not be thought then so important as we think it now".[105]
• Cast-on lugs tended to make unsound castings (Cochrane said he had seen examples in the bridge ruins[103]) and had prevented facing of the outer side of flanges.[102] Cochrane added that their use meant that columns had had to be cast horizontally rather than vertically, thus giving less satisfactory castings;[106] and unless lugs were carefully packed during bolting up they could be damaged or strained.[107]
• For so tall a pier Gilkes would have preferred some other means of attaching the ties to the columns "knowing how treacherous a thing cast iron is, but if an engineer gave me such a thing to make I should make it without question, believing that he had apportioned the strength properly".[105] A letter from Bouch to Gilkes on 22 January 1875 had noted that Gilkes was "inclined to prefer making the joints of the metal columns the same as on the Beelah and Deepdale".[108] Asked by Rothery why he had departed from the bracing arrangements on the Belah Viaduct, Bouch had referred to changed views on the force of the wind; pressed for other reasons he said Belah-style ties "were so much more expensive; this was a saving of money".[109]

Both Pole and Law had calculated the wind loading needed to overturn the bridge to be over 30 psf (1.4 kPa) (taking no credit for holding-down bolts fastening the windward columns to the pier masonry)[110] and concluded that a high wind should have overturned the bridge, rather than cause it to break up (Pole calculated the tension in the ties at 20 psf (0.96 kPa) windloading to be more than the 'usual margin of safety' value of 5 tons per square inch but still only half the failure tension.[111]) Pole calculated the wind loading required to overturn the lightest carriage in the train (the second-class carriage) to be less than that needed to overturn the bridge; whereas Law – taking credit for more passengers in the carriage than Pole and for the high girders partially shielding carriages from the wind – had reached the opposite conclusion.[112]

Law concluded that the bridge as designed if perfect in execution would not have failed in the way seen[113] (Cochrane went further; it 'would be standing now').[114] The calculations assumed the bridge to be largely as designed, with all components in their intended position, and the ties reasonably evenly loaded. If the bridge had failed at lower wind loadings, this was evidence that the defects in design and workmanship he had objected to had given uneven loadings, significantly reduced the bridge strength and invalidated the calculation.[112] Hence

I consider that in such a structure the thickness of the columns should have been determined, every individual column should have been examined, and not passed until it had received upon it the mark of the person who passed it as a guarantee that it had passed under his inspection ... I consider that every bolt should have been a steady pin, and should have fitted the holes to which it was applied, that every strut should have had a firm abutment, that the joints of the columns should have been incapable of movement, and that the parts should have been accurately fitted together, storey by storey upon land and carefully marked and put together again as they had been properly fitted.[112]

Pole held that the calculation was valid; the defects were self-correcting or had little effect, and some other reason for the failure should be sought.[110] It was the cast iron lugs which had failed; cast iron was vulnerable to shock loadings, and the obvious reason for a shock loading on the lugs was one of the carriages being blown over and into a bridge girder.[110] Baker agreed, but held the wind pressure was not sufficient to blow over a carriage; derailment was either wind-assisted by a different mechanism or coincidental.[115] (Bouch's own view that collision damage to the girder was the sole cause of bridge collapse[116] found little support).

Bouch's counsel called witnesses last; hence his first attempts to suggest derailment and collision were made piecemeal in cross-examination of universally unsympathetic expert witnesses. Law had 'not seen anything to indicate that the carriages left the line' (before the bridge collapse)[117] nor had Cochrane[81] nor Brunlees.[118] The physical evidence put to them for derailment and subsequent impact of one or more carriage with the girders was limited. It was suggested that the last two vehicles (the second-class carriage and a brake van) which appeared more damaged were those derailed, but (said Law) they were of less robust construction and the other carriages were not unscathed.[119] Cochrane and Brunlees added that both sides of the carriages were damaged "very much alike".[114][120]

Bouch pointed to the rails and their chairs being smashed up in the girder holding the last two carriages, to the axle-box of the second-class carriage having become detached and ending up in the bottom boom of the eastern girder,[121] to the footboard on the east side of the carriage having been completely carried away, to the girders being broken up, and to marks on the girders showing contact with the carriage roof,[122] and to a plank with wheel marks on it having been washed up at Newport but unfortunately then washed away.[123] Bouch's assistant gave evidence of two sets of horizontal scrape marks (very slight scratches in the metal or paint on the girders) matching the heights of the roofs of the last two carriages, but did not know the heights he claimed to be matched.[124] At the start of one of these abrasions, a rivet head had lifted and splinters of wood were lodged between a tie bar and a cover plate. Evidence was then given of flange marks on tie bars in the fifth girder (north of the two rearmost carriages), the 'collision with girders' theory being duly modified to everything behind the tender having derailed.[121]

However, (it was countered) the girders would have been damaged by their fall regardless of its cause. They had had to be broken up with dynamite before they could be recovered from the bed of the Tay (but only after an unsuccessful attempt to lift the crucial girder in one piece which had broken many girder ties).[125] The tender coupling (which clearly could not have hit a girder) had also been found in the bottom boom of the eastern girder.[126] Two marked fifth girder tie bars were produced; one indeed had 3 marks, but two of them were on the underside.[127] Dugald Drummond, responsible for NBR rolling stock, had examined the wheel flanges and found no 'bruises' – expected if they had smashed up chairs. If the second-class carriage body had hit anything at speed, it would have been 'knocked all to spunks' without affecting the underframe.[note 29] Had collision with the eastern girder slewed the frame, it would have presented the east side to the oncoming brake van, but it was the west side of the frame that was more damaged. Its eastern footboard had not been carried away; the carriage had never had one (on either side). The graze marks were at 6–7 feet (1.8–2.1 m) above the rail, and 11 feet (3.4 m) above the rail and did not match carriage roof height.[129] Drummond did not think the carriages had left the rails until after the girders began to fall, nor had he ever known a carriage (light or heavy) to be blown over by the wind.[130]

The three members of the court failed to agree a report although there was much common ground:[131]

• neither the foundations nor the girders were at fault
• the quality of the wrought iron, whilst not of the best, was not a factor
• the cast iron was also fairly good, but presented difficulty in casting
• the workmanship and fitting of the piers were inferior in many respects
• the cross bracing of the piers and its fastenings were too weak to resist heavy gales. Rothery complained that the cross-bracing was not as substantial or as well-fitted as on the Belah viaduct;[132] Yolland and Barlow stated that the weight/cost of cross-bracing was a disproportionately small fraction of the total weight/cost of ironwork[133]
• there was insufficiently strict supervision of the Wormit foundry (a great apparent reduction of strength in the cast iron was attributable to the fastenings bringing the stress on the edges of the lugs, rather than acting fairly on them)[133]
• supervision of the bridge after completion was unsatisfactory; Noble had no experience of ironwork nor any definite instruction to report on the ironwork
• nonetheless Noble should have reported the loose ties.[note 30] Using packing pieces might have fixed the piers in a distorted form.
• the 25 miles per hour (40 km/h) limit had not been enforced, and frequently exceeded.

Rothery added that, given the importance to the bridge design of the test borings showing shallow bedrock, Bouch should have taken greater pains, and looked at the cores himself.[134]

According to Yolland and Barlow "the fall of the bridge was occasioned by the insufficiency of the cross-bracings and fastenings to sustain the force of the gale on the night of December 28th 1879 ... the bridge had been previously strained by other gales".[135] Rothery agreed, asking "Can there be any doubt that what caused the overthrow of the bridge was the pressure of the wind acting upon a structure badly built and badly maintained?"[134]

Yolland and Barlow also noted the possibility that failure was by fracture of a leeward column.[135] Rothery felt that previous straining was "partly by previous gales, partly by the great speed at which trains going north were permitted to run through the high girders":[134] if the momentum of a train at 25 miles per hour (40 km/h) hitting girders could cause the fall of the bridge, what must have been the cumulative effect of the repeated braking of trains from 40 miles per hour (64 km/h) at the north end of the bridge?[136] He therefore concluded – with (he claimed) the support of circumstantial evidence – that the bridge might well have failed at the north end first;[137] he explicitly dismissed the claim that the train had hit the girders before the bridge fell.[137]

Yolland and Barlow concluded that the bridge had failed at the south end first; and made no explicit finding as to whether the train had hit the girders.[135] They noted instead that apart from Bouch himself, Bouch's witnesses claimed/conceded that the bridge failure was due to a shock loading on lugs heavily stressed by windloading.[138] Their report is therefore consistent with either a view that the train had not hit the girder or one that a bridge with cross-bracing giving an adequate safety margin against windloading would have survived a train hitting the girder.

Yolland and Barlow noted "there is no requirement issued by the Board of Trade respecting wind pressure, and there does not appear to be any understood rule in the engineering profession regarding wind pressure in railway structures; and we therefore recommend the Board of Trade should take such steps as may be necessary for the establishment of rules for that purpose."[139] Rothery dissented, feeling that it was for the engineers themselves to arrive at an 'understood rule', such as the French rule of 55 psf (2.6 kPa)[note 31] or the US 50 psf (2.4 kPa).[141]

Rothery's minority report is more detailed in its analysis, more willing to blame named individuals, and more quotable, but the official report of the court is a relatively short one signed by Yolland and Barlow.[142] Rothery said that his colleagues had declined to join him in allocating blame, on the grounds that this was outside their terms of reference. However, previous Section 7 inquiries had clearly felt themselves free to blame (Thorpe rail accident) or exculpate (Shipton-on-Cherwell train crash) identifiable individuals as they saw fit, and when Bouch's solicitor checked with Yolland and Barlow, they denied that they agreed with Rothery that "For these defects both in the design, the construction, and the maintenance, Sir Thomas Bouch is, in our opinion, mainly to blame."[143]

No further judicial enquiries under Section 7 of the Regulation of Railways Act 1871 were held until the Hixon rail crash in 1968 brought into question both the policy of the Railway Inspectorate towards automated level crossings and the management by the Ministry of Transport (the Inspectorate's parent government department) of the movement of abnormal loads. A Section 7 judicial enquiry was felt necessary to give the required degree of independence.[144] The structure and terms of reference were better defined than for the Tay Bridge inquiry. Brian Gibbens, QC, was supported by two expert assessors, and made findings as to blame/responsibility but not as to liability/culpability.[145]

The Board of Trade set up a 5-man commission (Barlow, Yolland, Sir John Hawkshaw, Sir William Armstrong and Stokes) to consider what wind loading should be assumed when designing railway bridges.

Windspeeds were normally measured in 'miles run in hour' (i.e. windspeed averaged over one hour) so it was difficult to apply Smeaton's table[146] which linked wind pressure to current windspeed

${\displaystyle P_{t}=0.005(V_{t})^{2}\,}$

where:

${\displaystyle P_{t}}$is the instantaneous wind pressure (pounds per square foot)

${\displaystyle V_{t}}$is the instantaneous air velocity in miles per hour

By examination of recorded pressures and windspeeds at Bidston Observatory, the commission found[147] that for high winds the highest wind pressure could be represented very fairly,[note 32] by

${\displaystyle P_{m}=0.01(V_{h})^{2}\,}$

where:

${\displaystyle P_{m}}$is the maximum instantaneous wind pressure experienced (pounds per square foot)

${\displaystyle V_{h}}$is the 'miles run in hour' (one hour average windspeed) in miles per hour

However, they recommended that structures should be designed to withstand a wind loading of 56 psf (2.7 kPa), with a safety factor of 4 (2 where only gravity was relied upon). They noted that higher wind pressures had been recorded at Bidston Observatory but these would still give loadings well within the recommended safety margins. The wind pressures reported at Bidston were probably anomalously high because of peculiarities of the site (one of the highest points on the Wirral.[149][150]): a wind pressure of 30–40 psf (1.4–1.9 kPa) would overturn railway carriages and such events were a rarity. (To give a subsequent, well documented example, in 1903 a stationary train was overturned on the Levens viaduct but this was by a 'terrific gale' measured at Barrow in Furness to have an average velocity of 100 miles per hour (160 km/h), estimated to be gusting up to 120 miles per hour (190 km/h).[148])

The current Tay Bridge at dusk, with the masonry of one of Bouch's piers silhouetted against the sunlit water

A new double-track Tay Bridge was built by the NBR, designed by Barlow and built by William Arrol & Co. of Glasgow 18 metres (59 ft) upstream of, and parallel to, the original bridge. Work started 6 July 1883 and the bridge opened on 13 July 1887. Sir John Fowler and Sir Benjamin Baker designed the Forth Rail Bridge, built (also by Arrols) between 1883 and 1890. Baker and his colleague Allan Stewart received the major credit for design and overseeing building work.[note 33] The Forth Bridge had a 40 mph speed limit, which was not well observed.[152]

Bouch had also been engineer for the North British, Arbroath and Montrose Railway, which included an iron viaduct over the South Esk. Examined closely after the Tay bridge collapse, the viaduct as built did not match the design, and many of the piers were noticeably out of the perpendicular. It was suspected that the construction had not been adequately supervised: foundation piles had not been driven deeply or firmly enough. Tests in 1880 over a period of 36 hours using both dead and rolling loads led to the structure becoming seriously distorted and eight of the piers were declared unsafe.[153][154] Condemning the structure, Colonel Yolland also stated his opinion that "piers constructed of cast-iron columns of the dimensions used in this viaduct should not in future be sanctioned by the Board of Trade."[155] It had to be dismantled and rebuilt by Sir William Arrol to a design by W. R. Galbraith before the line could be opened to traffic in 1881.[153][156][157] Bouch's Redheugh Bridge built 1871 was condemned in 1896, the structural engineer doing so saying later that the bridge would have blown over if it had ever seen windloadings of 19 psf (0.91 kPa).[158]

A column retrieved from the bridge

The locomotive, NBR no. 224, a 4-4-0 designed by Thomas Wheatley and built at Cowlairs Works in 1871, was salvaged and repaired, remaining in service until 1919, nicknamed "The Diver"; many superstitious drivers were reluctant to take it over the new bridge.[159][160][161][162] The stumps of the original bridge piers are still visible above the surface of the Tay. Memorials have been placed at either end of the bridge in Dundee and Wormit.[163]

A column from the bridge is on display at the Dundee Museum of Transport.

On 28 December 2019, Dundee Walterfronts Walks hosted a remembrance walk to mark the 140-year anniversary of the Tay Bridge disaster.[164]