News

Structural safety alert - tension systems and post-drilled resin fixings in spotlight

SCOSS has this week published a new alert following the recent collapse of tension structures in buildings and tunnels resulting in fatalities and serious injuries. SCOSS chief executive Alastair Soane explains.

Recent structural disasters such as the lining failures in the Boston Big Dig tunnel (2006), Japan’s Sasago tunnel (2012), and the Balcome rail tunnel in the UK (2011) which was a near miss have highlighted fixing system failures as the common problem.

Investigations highlighted that fixing bolts were poorly installed, there was degradation in the fixings over time, or that issues that should have been found by inspection had ben missed.

Recently CROSS (Confidential Reporting on Structural Safety) Newsletters have also published reports of a number of heavy ceiling failures in cinemas and other venues which could have caused tragedy. Yet another group of failures has occurred in swimming pools due to stress corrosion of stainless steel hangers.

Alastair Soane explains:

Frequently it is the coming together of more than one element of bad practice that either causes a failure or turns a simple failure into a significant event. For example a fixing may fail due to poor selection or poor installation, and if the supported structure has insufficient redundancy the failure of one fixing leads to the progressive collapse of the whole.

All of these hanger failure incidents were serious and the ones in Boston, Sasago and Balcome have raised concerns about the reliability of older post-drilled resin fixings. Beyond that, it is considered that there should be concerns over hanging systems as a generic group.

Sudden, catestrophic, and progressive failures have occurred. The safety of any hanging or tension system may depend on the capability of the hanger to exhibit ductility to share uncertain loading. For safety, fixings ought therefore to have a substantial load factor and this is particularly important when there might be fatigue.

There are many structures incorporating tension systems in safety critical situations. Some may not have been inspected for years and there could have been deterioration which has not been seen. Some of these older fixings may be at risk of failure, and it is not safe to assume that simply having had a long service means the fixings are secure.

It is recommended by SCOSS that where there are hanging structures supported by anchors that have been in place for some time then they should be inspected by a competent person and a number of criteria are given in the Alert. Should the results of an inspection give rise to safety concerns, especially where load redistribution possibilities are absent, then steps should be taken to get specialist advice on testing, repair, strengthening, or replacement. 

Resin bonded anchors may be at risk when there are poor installation practices. The reasons for most problems are known and plenty of good advice is available on the selection, installation, and testing of new anchors. The most important issues are that personnel must be properly trained and competent and that resin manufacturer’s instructions must be followed.

Some resin formulations are known to suffer from creep and are unsuitable for long term use. However all resin anchor systems that have been awarded a European Technical Approval (ETA) will have been subjected to a sustained load test, along with many other tests, in order to validate their suitability.

In 2012 a new British Standard, BS 8539:2012 Code of practice for the selection and installation of post-installed anchors in concrete and masonry, was published to address the causes of identified failures and includes many of the recommendations made in a previous SCOSS Alert.

The BS sets out the roles and responsibilities of all those involved in the use of fixings from manufacturers and specifiers through distributors and contractors to installers and testers from before the selection stage to commissioning. SCOSS views the implementation of this code of practice as fundamental to reducing, and potentially eliminating, fixing failures.

Work is underway (2014) on Eurocode DD CEN/TS 1992-4-1:2009 Design of fastenings for use in concrete. The Construction Fixings Association has a range of guidance in the “8539 Toolkit” and other sources of information are given in the Alert.

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For further details visit the SCOSS website - click here