Structural Safety in Buildings

Welcome to the first in a series of technical articles that will add to your CPD and engineering knowledge through looking at the wide ranging issues involved in the industries important to those involved in the SOE. Our first article looks at the Structural Safety in Buildings, what effects this can have on the engineers involved, and what steps can be taken to avoid incidents from occurring again.

There are numerous ways in which a build could collapse; incorrect design processes, age and corrosion, environmental effects, overheavy loads and material deficiencies are just a few examples of the potential problems faced by engineers.

The main fear with any build would be the failure of the supporting structure and resulting collapse. The consequences of such an event are obvious; with potential for injuries and deaths should members of the public be within the build at the time. The reputation of those involved in the construction and manufacture would also be at a high risk, as well as any cost implications of such a catastrophic failure. It is therefore high in the priorities of any engineer involved in the construction sector to ensure that any and all methods to reduce the potential for failure are looked into.

An example of a structural failure during the construction was during the build of the Gerrards Cross Tunnel in 2005. A short section of the tunnel (30m) built for the construction of a supermarket collapsed mid-build, although the reasoning for this was not immediately clear. The initial investigation suggested that the backfilling operation on the tunnel was the main cause of the collapse, as engineers during this made specific comments on the difference in fill levels on the archway.

Copyright Guy Gorton

The collapsed tunnel. Copyright Guy Gorton

Copyright Guy Gorton

The collapsed tunnel. Copyright Guy Gorton

It was suggested that the units failed to make a hinge correctly within the concrete section. As a result, the downwards rotation caused the build to fail due to a lack of lateral restraint and exacerbated by the difference in backfill levels on either side. The poor weather conditions at the time were also deemed to be a potential attributing factor, as they would have increased the vertical load acting upon the archway and potentially reduced the passive pressure from either side of the arch as well.

As it was a major rail line that had since been closed due to the tunnel collapse, there were significant delays for 2 months to allow the site to be cleared. Checks were made to the remaining standing parts of the tunnel, with many tonnes (approx 27k) of backfill and some of the remaining standing sections also destroyed and removed in order to ensure that it did not collapse a second time. A new contractor team were also brought into the project, who took a more cautious approach to the build, using precast concrete arch sections as a formwork for one long continuous arch. Piled wall were also installed on the sides of the tunnel, with a lighter fill material used to solidify the build.

Copyright Guy Gorton

The cleared site ready for construction to restart. Copyright Guy Gorton

Copyright Guy Gorton

The new method of rebuilding the tunnel involved layered construction. Copyright Guy Gorton

The lessons learnt from this collapse focused on the construction of tunnel systems and the effects that severe weather can cause. In the example at Gerrards Cross, the failure to correctly backfill the structure was deemed as the main cause of the collapse, and as such a different method of constructing the tunnel was sought. The overall construction of the supermarket was completed in 2010, which meant that the project took a total time of 14 years from initial application, including a 2 year delay from the initial tunnel collapse and restarting the build.

Copyright Guy Gorton

An overview of the nearly completed tunnel. Copyright Guy Gorton

Copyright Guy Gorton

The completed build. Copyright Guy Gorton

With the advancement of technology in recent years there have been a few additions and changes implemented into the methodology of reducing the number of potential failures within the construction industry. The addition of CAD and CAM systems such as Autodesk or Solidworks have allowed for more stringent computer based testing to be done prior to the build; which in turn means a reduction of potential failures encountered during the real-life build.

Of course, these programmes are not 100% fail-proof and there are often some issues that occur which cannot be entirely predicted, such as in the example of the Millennium Bridge in London. It is always imperative that the engineers involved in the project do not solely rely on the data provided by the CAD program and ensure that their own legal and professional responsibilities are not ignored. For further information on the current laws and regulations concerning the role of an engineer operating in construction, we would recommend looking at The Construction (Design and Management) Regulations 2015 from the HSE website.

Copyright HSE

Copyright HSE

The potential problems of structural failure are clear for all to see, and it is up to the engineer to ensure that these are avoided through correct planning and procedures. This involves using CAD programs but not being over-reliant, to ensure that professional standards are kept to the highest level, and that the safety of the building is not compromised.

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For further images on the Tunnel Collapse incident, please take a look at Meadway Park.co.uk, the website of Guy Gorton who was kind enough to provide many of the photos on this article.