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CROSS Safety Report

Building extension causes snow drifting failure

Report ID: 736 Published: 1 April 2018 Region: CROSS-UK

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Overview

The reporter feels that structural engineers are failing to realise the implications of creating a valley or an upstand and the potential for snow drift to occur.

Key Learning Outcomes

For civil and structural design engineers:

  • Be aware of the implications of creating a valley or an upstand, particularly for building extensions that can create a snow drift scenario

  • Existing structural elements including foundations may be affected if there is an increase in load due to snow drift

  • The Institution of Structural Engineers (IStructE) Technical Guidance Note (Level 1, No.5) provides helpful guidance on the derivation of snow and snow drift loads

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A reporter raises concerns about an issue that they feel many engineers fail to recognise and consider. A client had instructed the extension to a standard steel portal framed single bay pitched roof factory. The extension was to house overhead travelling cranes. The extension was some 3m higher than the original thus creating an abrupt change in height.

The following error and failure occurred:

  • The structural engineer failed to realise that this created the potential for drift snow accumulation in the end bays of the existing building (Figure 1)

  • No check in accordance with BS6399-Part3:1988 was therefore carried out

  • Consequently, no localised strengthening of the end bays of the existing building was implemented

  • Building control’s independent checker, a Chartered Structural Engineer, also failed to identify the issue and allowed building control to approve the submission

  • Heavy snow combined with gales caused significant snow build-up to occur at the interface.

  • The roof elements in the end bay collapsed and damaged the penultimate bay

  • Both bays had to be stripped and replaced together with additional secondary members

For many years the reporter has acted as an independent checker and conservatively ‘guestimates’ that of all such submissions (i.e. involving extensions to buildings), at least 50% of the structural engineers failed to realise the implications of creating a valley or an upstand and failed to check existing structural elements including foundations for the increased load.

The reporter feels at least 50% of the structural engineers failed to realise the implications of creating a valley or an upstand and failed to check existing structural elements including foundations for the increased load

Image
Figure 1: snow drift loading

Expert Panel Comments

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A significant cause of failures is simply overlooking a potential hazard.

A second general cause of failures is poor change control and there are many classic cases of this. Failure to consider drifting snow is a common problem and not always limited to extensions. If the build-up of a drift does not cause collapse or excessive deflection, it can cause water ingress through the walls and windows at the back of the drift.

It is an alarming statistic that so many engineers overlook what seems an obvious consideration and that it takes a good fall of snow to remind them. In general terms, this type of problem is associated with inadequate professional training. The recent ‘Beast from the East’ storm in the UK (March 2018) resulted in heavy snow falls and significant drifting across large parts of the country.

Structures may also be designed for snow drift loading to Eurocode 1- Actions on structures - Part 1-3: General actions - Snow loads

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