CROSS Safety Report
Risk in notching timber studs
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Overview
During the construction of a block of apartments a reporter found a couple of locations on an external fourth floor wall where the load-bearing studs had been notched well past permissible depths.
Key Learning Outcomes
For the construction team:
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Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
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Alterations should not be made to structural members without approval from the design engineer
For civil and structural design engineers:
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Consider putting notes on drawings indicating maximum depths of cut-out, or prohibited zones of cut-out for timber studs
Full Report
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During the construction of a block for student accommodation a reporter found a couple of locations on an external fourth floor wall where the load-bearing studs had been notched well past permissible depths (Figure 1). The site was on a coastal location so subject to strong wind loading which caused the reporter’s firm considerable concern as there was only 35mm depth of the stud left.
It had not been shown on any drawing how & where joists could be notched. Whilst the firm believes this practice is not widespread it is, in his opinion, imperative that the structural engineer designing any timber works should specify the zones in which drilling and notching can take place, and the maximum sizes for these notches. Also, the contactor should make all subcontractors especially the electricians and plumbers aware of these requirements.
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This is not an isolated case and highlights the need for designers to anticipate and consider the full demands of construction. This form of construction will require some form of ‘notching’ or other means of passing through the wall structure. The structural engineer should recognise this at the design phase and consider the consequences.
Notes on drawings to maximum depths of cut-out, or prohibited zones of cut-out is one way of proceeding. Other common examples are the weakening of block work walls by excess chasing out for services, the weakening of timber floors by excessive notching to carry services such as central heating pipes, and the cutting of ‘unauthorised’ holes in concrete and steel members.
Design codes and recommendations such as the NHBC Standards give notching and drilling limitations for this type of work. Part P competent electricians should also be aware of the structural effects of notching structural timber as part of their training for registration into a competent persons scheme. However, because timber frames are often procured as a system there are usually no full structural drawings as such. As usual it must be said that no changes should be made without the approval of the designer.