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736 Building extension causes snow drifting failure

Report ID: 736

Published: Newsletter No 50 - April 2018

Report Overview

The reporter feels that structural engineers are failing to realise the implications of creating a valley or an upstand and are failing to check existing structural elements including foundations for the increased load.

Report Content

Here is an issue, says a reporter, that many engineers fail to recognise and consider:

  • Client instructs the extension to a standard steel portal framed single bay pitched roof factory to house EOTs (electric overhead travelling cranes).
  • The extension is some 3m higher than the original thus creating an abrupt change in height.
  • The structural engineer failed to realise that this creates the potential for drift snow accumulation in the end bays of the existing building.
  • No check in accordance with BS6399:Part3:1988 is therefore carried out.
  • Consequently, no localised strengthening of the end bays of the existing building is implemented.
  • Building Control’s independent checker, a Chartered Structural Engineer, also failed to identify the issue and allowed Building Control to approve the submission (alarming!).
  • 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.

Comments

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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Images

Snow drift loading (IStructE TGN-L1-N5)



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