Are our Standards up to standard?

In my previous article Bridge Engineering Excellence, Need for Improvement? I raised several questions regarding the prevalent working practices in the light of recent bridge collapses during construction. ?In this article I ask whether the design and construction Standards being used are appropriate, and correctly applied.

Engineers are always pushing the boundaries of what is possible. With advances in materials, design and construction methods, the unsupported length of bridge spans is ever increasing. For example, the Russky bridge, Vladivostok (Russia), at 1104m main span, holds the record for the longest span cable stayed bridge, but at 2023m main span, the world record for the longest span for any bridge type is currently held by the 1915 Canakkale suspension bridge (Turkey).

Such is the appetite for long span crossings among bridge engineers, that the Zhangjinggao Yangtze River bridge (China) has been designed with a main span of 2300m. Currently under construction, the bridge will hold several records including the longest span suspension bridge and the tallest towers. The main cables are reportedly made of galvanized aluminium alloy high strength steel wires of tensile strength 2200 MPa. To the best of my knowledge, this is the highest strength yet used for a bridge suspension cable.

To address the issue of suitability of design and construction Standards, I’d like to take us back to 1970 and remind us of the fatal collapses of the Milford Haven bridge (UK) and the West Gate bridge (Australia) following which the UK Merrison Committee was set up. Its findings published in the Report of the Committee [1], included the following.

• We would suggest that an error which may have led to many of the design defects found in British bridges is the excessively mechanical use of codes of practice, even if they are directly applicable to the structures being designed (as BS 153 is not in the case of steel box girder bridges), are unlikely to result in a satisfactory design in the hands of a designer lacking the experience to appreciate?and allow for the peculiarities which each individual structure exhibits.
• A certified independent check should be carried out to establish that the permanent design complies with the relevant parts of the Design Rules and is in all respects an adequate one.

It became clear that the design Standards that existed at the time of design of the Milford Haven and West Gate bridges were not intended for the design of steel box girder bridges, and even if they were, were not applied correctly.

The UK Merrison Committee developed Interim Design and Workmanship Rules (IDWR) for the design of steel box girder bridges. Later these comprehensive rules were simplified into BS5400 and then into EN1993. Engineers considering the design of future major structure, or charged with revising EN1993, should re-examine, and understand the basis and research on which these Codes are based.

I raise the following questions for the designers of megastructures of tomorrow.

• ?Are the current design and construction Standards entirely appropriate for the structures being designed?
• How are the scale effects, for example, ?of ever-increasing span lengths being addressed?
• Is the design process subjected to an appropriate level of supervision and scrutiny by experienced engineers?
• Are the designs and construction methods subjected to independent checking by suitably qualified staff with relevant experience?
• Use of ever-increasing high strength steels in cables

Only a few years ago, the typical GUTS for suspension bridge wire was around 1500MPa. This has been increasing quite rapidly, with the Zhangjinggao Yangtze River bridge reportedly designed for 2200MPa. I feel sure that the designers will have addressed all relevant aspects in depth. However, to promote discussion in the long span bridge design community,

I raise the following points:

• What level of research and testing for the long-term performance has been carried out for such high strength steel wires?
• What is the impact of hydrogen embrittlement (manufacturing process and environmental exposure) on high strength steel cables?
• What is the impact on the fatigue life of cables, in particular stay cables and suspenders?

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I should like to conclude this short article on a positive note.

As part of the highway improvement programme on the Caribbean island of Aruba, a new arch bridge (see photo) was built across the Spanish Lagoon to ease traffic congestion on the Green Corridor Project. ?The client specified, not only an independent check, but also required independent certification for the entire project, particularly so for the Spanish Lagoon bridge. The bridge comprises a steel-concrete composite deck suspended from the steel arch, which is tied both longitudinally and transversely by replaceable external post-tensioning tendons.

Prof Farooq, MD, Bridge Experts Global,

(formerly a bridge director of Mott MacDonald, independent certifiers for the Aruba project).

Bridge Experts Global is a UK based independent consultancy providing specialist services worldwide.

www.bridgeexperts.co.uk

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References

[1] Inquiry into the Basis of Design and Method of Erection of Steel Box-Girder Bridges, Report of the Committee, London, 1973 (Her majesty’s Stationery Office, SBN 11 550279 3)

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