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Lessons from the Story of the Box Girders

Image Showing the collapse of the box girder

Approximately 50 years ago, the west gate box-girder bridge in Melbourne, Australia collapsed under construction killing over 30 people in what is now widely regarded as Australia’s worst construction accidents ever. Prior to this failure, several other failures of box girder bridges were recorded. The first was the Fourth Danube bridge over the Danube river in Vienna Austria. This 426m long box girder bridge, in fact, did not actually collapse but was hanging in the air and appeared kink and distorted.

Next, was the Milford Haven bridge in Wales, also a box girder bridge with seven spans. While still under construction, on 2 june, 1970, one of it central spans suddenly buckled and collapsed leaving four fatalities. Subsequently, there were several other failures, infact two box girder bridges collapsed within the space of two months. However, out of all these failures, one stood out, the west gate bridge, it was the most catastrophic and had the highest fatality. (see featured image).

The West Gate Bridge

The west gate bridge in Melbourne Austraila had 8 lanes and was 2.6km long. It had 67m long concrete approach spans, and five continous steel box girders, totaling a span of 848m. The lengths of the box girder had trapezoidal Sections made up of three cells, supported by cables as they stretch over the Yarra River.

The design concept was made in the UK by Freeman, Fox & Associates Consulting – The same consultants on the british Milford Haven Bridge. When construction commenced on the West Gate in April 1968, the world was yet to even witness the first of the significant box girder collapse.

There were difficulties from the onset of the project, widespread labour strikes became recurrent such that the steel contractor infact had to be replaced in early 1970 for delays. Shortly after, came the news of the first collapse, Millford Haven, 2000km away in Wales. Interestingly, Millword Haven was designed by the same design company as the West-gate bridge. Freeman, Fox & Partners is on record to have thought the failure to be a once in a lifetime occurrence, although they undertook steps to improve the West Gate Work. This was true, considering the fact that a different construction method was adopted – the free cantilering erection method. However investigation into the failure later proved the chosen method of construction was in fact detrimental.

Causes of Failure

The contractor fabricated half of each span on the field, just imagine cutting each span down its length, leaving two spans of half-width. In addition, each of these half-spans was then raised up 50m in the air and slide into place (Figure 1). The contractor thus halved the load for each lift-although doubling the number of lifts. By the time of the Milford Haven collapse, the east and west spans of the West Gate Bridge, each 112m long, were ready for erection. Then, one of the eastern half-spans developed a problem. It was built but when it was separated at ground level from its temporary braces, it suddenly developed a buckle on the top free flange – the flange that would pass down the centerline of the bridge when attached to the other half-span.

Diagram Showing Box Girder Half Span being lifted into position
Figure 1: Diagram Showing Box Girder Half Span Lifted into Position

The buckle occurred because of the decision to lift each half-span separately. While the free flange was stiffened longitudinally and transversally, there was a problem with both sets of stiffeners (Figure 2). The transverse stiffeners didn’t have the necessary stiffness to restrain the longitudinal stiffeners, and this lack of restraint resulted in the longitudinal stiffeners and it bridged a gap of 318mm between the points where one stiffener ended and the other began. Additionally, the plate was set off from the stiffeners, and this eccentricity induced, coupled with a smaller splice plate region and the distance between the stiffeners, produced a point of weakness in the longitudinal stiffeners at every joint.

Details of picture showing stiffners to plate of box girder
Figure 2: Details of Stiffners to Plate

So, there was a buckle in the flange plate now, but instead of lowering the span back onto its brackets and removing the buckle when it was still at ground level, the decision was made to continue with the lift and then attempt to remove the buckle when the span was in its final position – at a height of 50 m. But this buckle was significant – 380mm – and once the span was placed in position, there was no way it could be unloaded. Despite all of this, the lift went ahead.

Now they were faced with the dilemma of straightening the buckle at a height of 50m above ground. The decision was to remove the bolts from some of the transverse splices in the top flange, practically removing the flanges ability to resist compression stresses. Thus, with the stresses relieved, they could let the plates slide over each other and then flatten out the buckle. Once flattened, new holes were drilled or existing holes were widened in the overlapping plates, and new bolts installed¹.

Attention then turned to the western span. Something had to be done before the buckling would happen. They decided to stiffen the flange itself with an extra longitudinal stiffener, and they also added cross beams running diagonally from the top free flange back to the bottom flange. The arrangement indeed worked and buckling was avoided. However, when they opted to connect both spans, they noticed a vertical gap of 115mm between them. Whilst they were faced with the same issue in the eastern span, they were able to remove it with the help of a hydraulic jack. But in the case of the western span, the 115mm gap was to much a distance for the jack to close. So, they opted to place large concrete blocks weighing about 51tonnes on one half of the span to close the gap. This worked and the gap was closed. However, after some time the entire upper flange suddenly buckled – this was what they’ve been trying to prevent all along. While there was sufficient capacity to prevent buckling during the lift, the extra loading from the 51 tonnes concrete block was to much load on the girder.

Left with no further choice, they had to consider the method used to fix the eastern gate. However, there was a significant difference between the eastern and western spans. Due to the additional load from the 51 tonnes concrete block on the western span, the buckling was significantly higher than that of the eastern span. As they began to remove bolts, the stresses in the remaining parts began to increase, with every bolt removal the stress increased, and there was a reduction in the section net area. A total of 37 bolts were removed, by then the bridge could take no more, the net section failed and the remaining bolts sheared. A mechanism has ensued (Figure 3).

Diagram Showing how failure of the box girder hapened.
Figure 3: Dynamic of Box Girder Collapse

The left hand of the span started to drop while the load shifted to the right hand half since it was partially connected. The the entire span collapsed 50m to the ground killing 35 people (Figure 4).

Workers Inspecting Bridge After Collapse
Figure 4: Workers Inspecting Bridge After Collapse

Lessons from Failure

As with all structural failures, the collapse spark global outrage and there was now very pressing need for answers. What followed was an intense period of lessons and learnings in bridge design. The need for answers became extremely urgent. Rightly so because in the U.K alone 49 steel box girders were already in the construction stages with another 30 in the design stage³.

To say that the events from 1969 up until 1973 were an unprecedented series of bridge collapse would almost be an understatement. Five box girder bridge failures resulting in the death of 56 people have already occured. The rapid collapse of the bridges was stark. The Fourth Danube bridge in November 1969, followed by Milford Haven seven months later, followed by the West Gate bridge four months later, followed by the Rhine river bridge 13 months later and lastly the Zeulenroda bridge 21 months later.

These failures inadvertently showcased how difficult it is to undertake fast and comprehensive investigations that can be disseminated back into the profession to arrest a flow of failures. Because, even though in the light of the Millford collapse, the design team took steps to improve the West Gate bridge, this wasn’t enough to arrest an endemic misunderstanding of bridges built from thin steel plates.

Investigation on the Fourth Danube bridge revealed that the bridge failed largely due to temperature effects¹. During its construction, both sides of the centre span cantilevered towards one another. On the afternoon of 6 November 1969, the two cantilevers met in the middle and were joined. But the warm temperatures during the afternoon had caused the spans to deflect more and they had to be shortened by 15mm at the top. Then in the evening, the temperature dropped, which placed the top flange in tension. As the temperature drop continued, tension in the top flange increased, which then placed the entire bottom flange in compression. Sadly, the original plan was to lower the two inner supports once the cantilevers had been joined, which would have prevented this behaviour, but it had been decided to undertake this lowering the following day.)

The Milford investigation team revealed that Milford collapse was initiated by an inadequatly stiffened diaphragm¹. Designers at the time were unaware of this issue, this is evident in the fact that the design codes at the time reflected a lack of this knowledge.

The west gate collapsed on the other hand happened due to a variety of issues as explained in the preceding sections. But on top of the list, was the lack of understanding of the behaviour of stiffening plates. The Rhine river bridge collapse was caused by the buckling of its compression flange halfway, whilst the zeulenroda bridge collapsed due to insufficient flange plates and longitudinal stiffners.

In the wake of this events and after four known failures. What followed was a Royal commission in Australia into the West Gate Bridge failure and the other bridge failures. The committee after investigations and research produced an interim report with new set of design rules and workmanship guidance. Efforts of this research went into practice, with existing bridges strengthened.

It is 50 years now and to a great extent, the technical issues have been learnt, and the underlying issues are now fully addressed by modern design codes. But has any ethical lessons been learnt, maybe or maybe not. This days bridge failures occur far to regularly, most commonly during construction. Usually there are technical issues and this needs to be understood and cannot be overlooked. However, in almost every case it’s almost always the ethical and procedural factors that hold the answer to the reason why the failure could not be averted.

For instance, in the case of the Fourth Danube bridge the lowering of the inner support could’ve being done much earlier but instead it was postponed for the next day. Legend has it, that ” never put off until tommorow what can be done today”. The consequence of not lowering the inner supports on time resulted in their inability to salvage the situation. Sir Alec Merrison said in presenting the Committee’s conclusions in 1973: “No amount of writing of design codes and writing of contracts can in the end be guaranteed to prevent the results of stupidity, carelessness or incompetence. But one can do a great deal to discourage these vices and that must be done.”

In conclusion, failures will always be a part of human endeavour because humans are part of it. Structural engineering like all other professions cannot really advance without it failures³. This is a sad reality. The testing of design assumptions and methods happen in real world and public view, sometimes with tragic consequences.

Ensuring that these lessons are learnt as they were with the stories of the box girder bridges will certainly not bring back the dead or take the pain left behind by failures away. However, engineers can learn from it in this age of generational amnesia where younger engineers are likely to dismiss these stories as interesting but irrelevant and inapplicable.

Also, See: The Collapse of the Malahide Viaduct

References

1. State of Victoria, West Gate Bridge Royal Commission (1971) Report of Royal Commission into the failure of West Gate Bridge [Online]

2. Merrison Committee (1973) Inquiry into the basis of design and method of erection of steel-box girder bridges, London: HMSO

3. Brady S. (2016) West Gate Bridge collapse – he story of the box girders. The Structural Engineer, 94 (10), pp. 26–28.

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Published by Omotoriogun Victor

A dedicated, passion-driven and highly skilled engineer with extensive knowledge in research, construction and structural design of civil engineering structures to several codes of practices

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