The timber and trussed rafter sectors have responded to an ‘alert’ in the Structural Engineer magazine highlighting instances of poor structural timber design.
The alert was issued by the Standing Committee on Structural Safety of the Institution of Structural Engineers and was based on a letter from Dr John Bellamy of Timber Engineering Systems Ltd.
It maintained that, in some drawings from consulting engineers for non-trussed rafter roofs, principal timbers were found to be unsupported. They also undersized timbers and specified inadequate space at joints.
It also stated that engineers have been known to leave wind girder design to trussed rafter fabricators, but ‘only a handful of the latter employ designers with structural engineering training’. It added that in buildings with ‘exposed timber frame or trusses’ joints are ‘rarely designed by consulting engineers’.
Trussed Rafter Association secretary Peter Grimsdale expressed ‘disappointment’ at the alert’s inferences about truss designers. ‘UK Standards make it clear that wind girders, required to support masonry walls, not the roof, are the responsibility of the building designer, not the trussed roof designer. The truss industry invests heavily in expert software which enables specialist designs, such as wind girders, to be carried out provided correct loading information is supplied.
‘Nevertheless the respons-ibility for wind girders remains with the building designer. The SCOSS article seems to duck this responsibility by impugning the reputation of highly regarded truss designers.’
TRADA managing director Andrew Abbott said his organisation took construction safety very seriously and would be talking to SCOSS. But he added that ‘interpretation of the engineer’s intentions has not suddenly become a lot worse’. He also pointed out that the issue of UK timber engineering expertise is being addressed through the new Centre for Timber Engineering and the ‘Timber Design Knowledge for Professionals of the Future’ project. The latter is backed by TRADA, the DTI and universities and will develop an ‘electronic resource’ for engineering training.
