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6.3 Tacoma Narrows suspension bridge failure

Such over-design could not be sustained for long and bridge designers gradually pared back their margins of safety. There is elegance and economy in having the lightest structure compatible with function. But history has a habit of repeating itself.

In 1940 a new suspension bridge with a central span of 2800 feet was built over the Tacoma Narrows in the United States. It was soon noticed the bridge deck was prone to oscillate in certain winds. The vertical amplitude of the oscillations
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6.2 Forth Bridge

When the Forth was eventually bridged in 1890 it marked a new dimension in bridge construction. The main crossing is 5330 feet long and has a headroom above high water of fully 157 feet. It consists of three huge double cantilevers fabricated from steel with a maximum height above high water of 361 feet. The bridge contains 58 000 tons of steel, of which 4200 tons are just rivets. The steelwork has an external area of 145 acres and it is a full-time job for a gang of 29 painters to protect th
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6.1 New Tay Bridge

So the collapse of the bridge was probably caused by premature fracture of the lugs, perhaps aided by fatigue (Input 10). Once the wind braces had been lost, the stability of the piers was drastically reduced because each trio of columns became separated (Figure 47). It only needed a further small sway to cause toppling, because of the shift in centre of gravity of the piers.

The collapse of the Tay Bridge sent shock
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Myths persist

Many myths still surround the Tay Bridge disaster, the most pervasive being it was brought down by wind action alone. Rothery's report (see Paper 3) should dispel that particular myth, in addition to the numerous examples shown in this unit of the way the structure had deteriorated by the time of the storm in late 1879.

Click 'View document' below to open Paper 3 (35 pages, 39 MB).

Another m
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5.15 Further investigation is possible

There are still many mysteries that surround the Tay Bridge disaster, largely because so little was recorded at the time of construction. For instance, questions remain about the details of reject rates for the castings, and modifications made to the first designs of the piers and their component parts.

Although enlargement of the BoT set of pictures has helped clarify the various failure modes described by Henry Law and others at the enquiry, it has also revealed yet more mysteries. Wh
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5.14 Questions remain and myths persist

So ended the enquiry, with reports that condemned the design and construction of the bridge. However, the speed of the enquiry – only 6 months – left many gaps in the evidence. They included:

  • a detailed survey of the damage to the cast-iron piers that fell into the river, pinpointing the exact position of casting and other defects;

  • exploration of the way the original design was made, and the modifications to the design after construc
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5.13 Conclusion of the BoT enquiry

The BoT enquiry issued two reports at the end of the enquiry, one authored by the chair, Mr Rothery, the other by the two other assessors. The Rothery report is Paper 3, linked below. They agreed about most of the issues in contention, as follows (Paper 3, page 47 of report).

  1. There is no evidence to show that there has been any movement or settlement in the foundations of the pier
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5.12 Pole and Stewart report

Apparently prepared using the same methodology as Law, Pole and Stewart produced a report that calculated the loads at various points in the bridge under live locomotive loads and wind loading at various pressures. Stewart was employed by Bouch to perform the original design calculations for the bridge, while Pole was brought in as an independent expert. He had extensive experience of use of different materials in bridges, and indeed, had written a standard text book for engineers on the subj
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5.11 Further evidence on stability

Given the importance of establishing the nature of the stability of the bridge, further witnesses were called at a later stage in the enquiry to shed some light on the problem. If Mr Noble had observed chattering of the joints in the tie bars, had similar phenomena been observed earlier?

The key witnesses were the engineers in charge of erecting and finalising the structure before it was opened in May 1878, Major-general Hutchinson, the BoT inspector who approved the structure for publi
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5.10 Bridge stability

Any fracture of the diagonal wind brace tie bars could allow substantial lateral movement at the top of the piers. If these tie bars had already been injured by the previous train to cross the bridge, it would have only taken a little extra effort to complete the process as the mail train arrived over each pier supporting the high girders. Once the wind braces had failed completely, and the struts fractured at their connections each pier would behave as two separate supporting structures.


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5.9 Mechanical tests by David Kirkaldy

In order to determine which of the several parts of the joint were weakest, and gain some idea of the scatter in strength, David Kirkaldy was employed by Henry Law to test various samples he had collected from the bases of the fallen piers. David Kirkaldy had a good reputation for accurate and rigorous mechanical testing of materials using a large tensometer he had designed and built in London (see Input 9, linked below).

Click 'View document' below to open Input 9


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5.8 Design problems

Table 7 summarises the many design problems of the piers uncovered by Mr Law and his team. We have already seen the numerous fractured lugs in the remains of the bridge, shown in Figure 29. Was the weakness of the lugs somehow associated with their shape?

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5.7 Fitment flaws

The secondary category of defects observed by Law and his team refer to defects of fitment of the columns and braces together during construction of the bridge. He noted many bolt holes had been deliberately enlarged, but why this was necessary remains unclear, especially as the bolts were 0.125 inch smaller than the holes. Perhaps burrs or points in the holes needed removal before the bolts would fit correctly. The quadrants also came in for criticism for their poor fit to the columns, and i
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5.6 Casting defects

The first class of defect would have been inferred from examination of fractures in the cast-iron columns, where, for example, the wall thickness would be exposed for measurement. However, some of the casting defects he mentioned in his testimony – and which were to gain some notoriety both in the popular press accounts of the enquiry and in later accounts – are difficult to describe in detail because he did not specify where they were found in the debris, or how exactly they had contribu
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5.5 Evidence of Henry Law

Henry Law's report is brief and to the point, and includes a substantial appendix giving detailed calculations of the effects of wind pressure on the structure (not included in Paper 1). Further information on his inspection of the remains – the two standing piers, the twelve wrecked piers the high girders and the train within – was given during his testimony before the enquiry.

Law was able to examine the extant remains in considerable detail, and noticed numerous defects in the br
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5.4 Expert evidence: an overview

The second part of the enquiry was devoted to analysis of the disaster. There were three engineers appointed: Mr Henry Law for the enquiry, and Dr William Pole and Mr Allan Stewart acting on behalf of the NBR. In addition, Mr Law collected samples of columnar material and wrought iron straps, bolts and struts for mechanical testing, as well as many broken parts to be shown as exhibits at the enquiry. He asked Mr David Kirkaldy to test the samples using a hydraulically operated tensometer.


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Loosening of tie bars

On Monday, 19 April, when the sitting had been moved to Westminster, such comment received dramatic but indirect support from the man put in charge of maintaining the fabric of the bridge after completion and up to the disaster, Mr Noble. Although much of his time was spent examining the pier foundations, which involved measuring the depth of water, questions were asked about the piers:

11,404. Leaving the foundati
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Bridge oscillations

Testimony was taken from the many workers employed during construction and painting of the structure just after completion. Their evidence was more compelling, especially from painters working at the top of the high girders piers during passage of trains, as well as during windy weather. They were painting the cast iron of the piers during the summer of 1879. In the main, they reported feeling strong sideways as well as vertical motion:

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5.2 Eye-witness testimony

Their first main aim was to question local witnesses, including several who claimed to have seen the fall itself. One especially impressive eye witness was Alexander Maxwell, who lived on Magdalen Green, near the north end of the bridge. He was examined by Mr Trayner, counsel for the enquiry:

942. You are an engineer? – Yes

943. You live with your father, who is an ex-baillie of this town at Magdalen Green,
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