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CROSS Safety Report

Concern about bridge hangers

Report ID: 222 Published: 1 July 2012 Region: CROSS-UK

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Overview

A reporter is concerned that a failure in recent years involving the cast connections at each end of vertical bars, which connected to a bow string arch at one end and to a bridge deck at the other, may be in use elsewhere.

Key Learning Outcomes

For the construction team and design engineers:

  • Connections can often be the weak link in structures and attention to detail is required

  • Manufacturer’s requirements and guidance for the installation and storage of fixings should be followed

  • Consider having representatives of the manufacturer attending site to train operatives on best practices. They can help raise industry standards.

  • Where fixings are key components and part of the quality assurance procedure consider carrying out site testing to ensure their strength capacity

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A reporter says there was a failure in recent years involving the cast connections at each end of vertical bars, which connected to a bow string arch at one end and to a bridge deck at the other.

The reporter’s view is that these components are potentially of a similar design to those in use elsewhere. If this assumption is correct, other structures, be they bridges, stadia, or other buildings using the components may be at a higher risk of failure than anticipated. The reporter believes it would be worth establishing if this issue is correct by undertaking some further work and analysis. The reason for writing is that the reporter speculates that other failures may have some similarities.

The reporter hopes that CROSS may be able to take this forward, and recent problems perhaps will be a catalyst to investigating and therefore starting a risk reduction process.

Expert Panel Comments

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Tension systems normally work at high stress and this is only justified if the system is ductile because the true elastic stress in any part of it might be quite different from that predicted by design. This can come about from a number of causes, for example cable structures are not entirely elastic; so introducing some element of unpredictability in stress calculations. This situation is tolerable provided the system as a whole can deform and permit load redistribution. The need for this capability is particularly relevant because any single hanger failure on any part of the system raises the possibility of a progressive collapse as hangers adjacent to a failed one might get severely overloaded.

Modes of failure

To assure cable ductility as a whole it is normally desirable that the end terminations should be stronger than the tension members themselves since that avoids excessive yield demand on short lengths and permits the favourable long displacement failure mode of hanger stretch itself. But in turn that demand leads to a need to have high confidence in end connector capacity. It is particularly important that the mode of failure of the connector is yield not fracture since a fracture is instantaneous and prevents there being warning of failure by prior excessive deformation. The global protection against failure is absorption of excess stress by ductility before fracture.  Anything that undermines the ability to stretch, or the ability to cope with local overstress, undermines this fundamental philosophy.

Connectors therefore require care in their design and manufacture. As they are standard items it is possible that any fault will affect an entire batch and not just a single item which raises the possibility of the system having a common mode of failure, thus increasing the risk of catastrophe.

The risk of fracture

In some of the reports received, the hangers have been made from thick steel. With thick cast steel, a potential mode of failure is fracture which is extremely serious as a fracture can be instantaneous. The risk of fracture increases if the steel lacks toughness (Charpy) and lacks ductility. Achieving adequate toughness in thick steel can be problematical and heat treatment may be required. There is always tensile stress on hangers but within castings there may additionally be high residual stress as a consequence of differential cooling.

Stress concentrations and defects (which can give rise to acute stress concentrations) raise the risk further. Furthermore, fracture risk is higher in cold weather. Within connectors there are always going to be stress concentrations. For example, pin misalignment is a potential source. The stress concentrations should be controlled by elimination of casting defects as much as possible. It is essential to ensure the properties of ductility and toughness which permit plastic stress redistribution of stress concentration without facture.

Specification and testing

Overall, there must be a proper specification for the essential properties (not just concentrating on strength) and proper testing and checking systems to ensure that the components are fit for purpose. The engineering link between defect tolerance and service acceptability is extremely complex and expert advice may be needed. This Report emphasises the need to ensure that what has been provided is what was required. In addition to radiography, ultrasonic examination may be used to good effect for defect discovery.
In summary the key points to are to:

  • Identify what are safety critical tension structures

  • Take great care with analysis and design

  • Specify the required properties of the end connections and use expert advice as necessary

  • Specify the tests that will be carried out on completed components and state the required acceptance criteria.

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