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CROSS Safety Report

Timber truss

Report ID: 62 Published: 1 February 2007 Region: CROSS-UK

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Overview

The reporter has been dealing with an application where the engineer involved discovered proprietary timber girder trusses with some glued joints that have come loose.

Key Learning Outcomes

For the construction team:

  • Any alterations to structural elements should be approved by the designer prior to works being carried out on site

  • Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design

  • It is important to ensure that the requirements of BS EN 14080:2005: Timber structures - Glued Laminated Timber - Requirements are met fully when fabricating trusses

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The reporter is a Chartered Structural Engineer working in building standards. They have been dealing with an application where the engineer involved discovered proprietary timber girder trusses 400mm deep and spanning 7.5m @ 1.2m centres with glued joints. Some of these joints have come loose. 

There are apparently no mechanical fixings to the joints, with diagonal members being glued between the pairs of top and bottom chord members, and vertical members being glued at the end to the face of the top and bottom chords. It is some of the vertical members that have come loose, and others are completely missing.

Some members have also been deliberately removed to allow for the passage of ducting. The girder is made up of effectively two girders of 30mm thick timber fixed either side of plywood webs which are present near supports. Outwith the ply web zones there are vertical and diagonal members, as referred to above.

Some members have also been deliberately removed to allow for the passage of ducting

It is not known whose system this is, other than it appears to be a manufactured proprietary product. At a guess it dates from the 1970s. Remedial works have been instructed, involving changing the girders to ply web beams.

Expert Panel Comments

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A useful report illustrating problems that may occur when relying upon glue in older timber trusses. The removal of structural members, by those unqualified to assess the effects is, unfortunately, too common, and may indicate that timber structures are modified because it is comparatively easy to do so without realizing the potential consequences.

The reporter does not state whether there was moisture present but it is known that moisture can cause non-resistant glues to deteriorate. It is thought that this was a contributory factor in the collapse of an ice rink roof in Germany in January 2006 when there were 15 fatalities. It is very important to ensure that the requirements of BS EN 14080:2005: Timber structures - Glued Laminated Timber - Requirements are met fully when fabricating trusses.

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