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Tay Bridge disaster

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# 4.8 Photographs showing the detail: standing pier 28

The final part of the survey deals with the two standing piers connected to the lower girders left after the high girders section fell during the disaster. The whole of pier 28 is shown in Figure 34, and two close-ups of the columns are shown in Figures 35 and 36.

Figure 34 Standing pier 28, looking south, about a week after the disaster
Figure 35 Damaged tie bars and struts of pier 28 and the debris pile on the platform
Figure 36 Damage to tie bars and struts of pier 28. In the centre of the top-most tier, a diagonal tie bar has been bent into a U shape. While just below it a strut has been bent after an impact and hangs near the column

Adopting the same procedure as we have used already, we will focus attention on the diagonal tie bars, although other damage must be examined for its relevance to the failure. In fact, it is the collateral damage that ought to be discussed first because both standing piers show damage that probably occurred towards the end of the collapse rather than at the beginning.

While Figure 34 is a useful starting point for the survey, it is important to see the pier in perspective. It is shown from two entirely different angles in Figures 20 and 21. That some damage was caused when the high girders were wrenched from the pier seems evident from examination of the top of the pier in Figure 34 (bent diagonals in the high girders section), and Figure 20 shows a gap between the girders and the top of the pier. With careful inspection, Figure 20 shows some of the bracing diagonals not in the critical north or south faces have also been broken. The cross braces facing the camera in Figure 34 show substantial damage, although there appears to be little damage to the braces on the corner faces.

The close-ups in Figures 35 and 36 help to clarify the situation. There is a large pile of debris on the platform, with bigger lumps of metal than in the case of piers 1 or 3. If it is debris that has fallen from the very top, it could have created substantial damage in the fall. Most of the cross tie rods appear hanging down in the centre section, and were probably broken by such a fall.

The top of the pier shows great damage (Figure 36), including three broken struts and one of the top diagonal tie bars has actually been bent double. The latter could have been caused by a large girder turning in its fall, and the strut just below shows a severely bent portion at its far end, which would support this suggestion.

Many of the tie bars below are broken at their lower single lugs and are hanging free. Indeed, the damage extends down to the lowest tier. One of the lowest diagonal tie bars is out of position – judging by the irregularity in pattern of the receding tie bars. Most of the east-facing diagonal tie bars on both sides of the centre of the pier appear to be broken at their lower lugs.

So, what happened at this pier? The girders were supported here only by resting freely on a roller bearing plate, and they were not fixed against lateral movement (Figure 10). It is known the girders remained substantially intact during and after the fall (Figure 22), and must have been wrenched sideways from the standing pier. As they were pulled off the pier, they pulled other parts of the structure off, which fell vertically and caused much of the collateral damage seen in Figure 34. This would probably have included the cast-iron rollers and the plate on which they rested on the columns very near the top of the girders (Figure 37).

The suggestion is reinforced by examination of the north pier, where even more collateral damage has occurred. It had a similar structure with a roller bearing plate, and most of the centre section supports were destroyed. Nevertheless, the damage to the tie bars and struts shows the connections to the columns were the weakest, no matter how that damage was caused. Most have failed by fracture of lugs, the connections cast into the structure of the columns.

Figure 37 Top of one of the brick piers of the old Tay bridge showing the cast-iron roller bearings in situ before the pier was demolished