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CROSS Safety Report

Estuary bridge - failure of stainless steel tie bars

Report ID: 522 Published: 1 October 2015 Region: CROSS-UK

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Overview

Concerns were raised after a principal inspection of a bridge found that numerous tie bars connecting the original bridge to strengthened footways had failed.

Key Learning Outcomes

For asset owners and managers:

  • This report emphasises the value of regular inspections and maintenance to ensure structures remain safe

  • The surface finish and regular washing of stainless steel components has a major effect on their durability and can be critical to their successful application in a marine environment

For civil and structural design engineers:

  • It is important that the correct grade of stainless steel is used for marine environments

  • While stainless steel may be specified, there can still be a risk of corrosion. Further protection measures may be required against stress corrosion cracking and other failure modes.

  • Be aware that it can be difficult to identify stress corrosion cracking from in-situ visual inspection and that sudden failures can occur in tensioned elements

  • If regular washing of stainless steel components is required then this should be highlighted in the operation and maintenance manual

Full Report

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In 2002, the footways of an important estuary bridge were widened, says a report from the owner of the bridge. The works replaced the cantilevered footways of the structure with footways supported on steel girders which in turn were supported on steel columns.

The strengthened footways and the original structure are connected through a keyed concrete joint and pairs of 32mm diameter stainless steel tie bars at the third points of each span (two pairs per span), giving a total of 90 tie bars across the whole structure. The tie bars are post-tensioned to pin the structure together and prevent a separation of the original and new structure in the event of an accidental impact of a vehicle into the bridge parapets (Figure 1).

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Figure 1: bridge section highlighting location of tie bars

Tie failure identified during principal inspection

A principal inspection of the structure was completed early in 2015 and it was seen that 12 ties had failed through bar fracture (Figure 2), and four by coupler fracture. Over-tensioning of the bars on installation was ruled out on the basis of records, whilst transverse thermal expansion and differential settlement were also ruled out.

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Figure 2: fractured tie bar

It was thought that the failure of the tie bars could have been from overstressing of the tie bars due to unanticipated effects of the structural form of the bridge, or under design of the tie bar durability in a harsh marine environment.

Stress corrosion cracking likely mode of failure

The tie bars were exposed to chlorides from sea spray and the marine atmosphere. As they were sheltered from rain, they would not be naturally cleaned with fresh water so there had been a build-up of chlorides and rust staining. Examination of samples showed that there was a consistent pattern of red rust along the length of the bars. In the area of fracture, there was a particularly large patch of corrosion on the bottom surface with deep pitting. The small size of the final fracture region indicated that the load at the time of failure, the 225kN prestress, was not excessive.

The tie bars were exposed to chlorides from sea spray and the marine atmosphere. As they were sheltered from rain, they would not be naturally cleaned with fresh water so there had been a build-up of chlorides and rust staining

The coupler showed a red rust patch on the outside bottom with some minor pitting evident. Several branch cracks were noted. Micro-examination of the failed components showed the steel to be free from any manufacturing or processing defects and that failure had initiated at pitting of the surface of the components. Branched cracks and corrosion products indicate that stress corrosion cracking was the most likely mode of failure.

Several key learning points identified

The reporter lists the following key learning points:

  • It is essential that the correct grade of stainless steel is used for marine environments

  • Just because a steel is stainless does not mean that it will not corrode. Further protection measures may be required against stress corrosion cracking and other failure modes.

  • It is difficult to identify stress corrosion cracking from in-situ visual inspection

  • Sudden failures can occur in tensioned elements

  • The surface finish and regular washing of stainless steel components has a major effect on their durability and can be critical to their successful application in a marine environment

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

As with report 524 which discussed the spalling concrete falling from a motorway bridge, this is an important report from a major infrastructure owner about a potentially serious situation and is very welcome. Apart from the recommendations about stainless steel in a marine environment, it further emphasises the value of regular inspections.

Advice about the use of the material can be obtained from the British Stainless Steel Association (BSSA). There are a number of references to stress corrosion cracking available such as Stress Corrosion Cracking from the National Physical Laboratory.

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