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CROSS Safety Report

Stability of timber frame construction

Report ID: 51 Published: 1 February 2007 Region: CROSS-UK

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Overview

A reporter who has many years design experience in timber frame construction raises concerns about a common defect they have experienced when checking calculations.

Key Learning Outcomes

For civil and structural design engineers:

  • Competent supervision of design by experienced personnel can allow less experienced engineers to develop a feel for the right solution

  • Consider appointing a single entity (or Chartered Engineer) to have overall control of the design of the stability system

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The reporter is a structural engineer with many years design experience in the field of timber frame construction both as a designer and a checker. When checking calculations, they have noted that a common defect appears to be related to the design for overall stability. Stability is provided by timber framed racking walls sheathed in OSB (Orientated Strand Board) providing in plane resistance to lateral loading.

Often the presence of large numbers of doors and windows means that external walls have insufficient racking resistance to provide overall stability on their own. Commonly, internal walls are also sheathed in OSB to provide the required lateral load resistance.

Racking resistance

Although these internal walls are designed in accordance with the British Standard in calculating the racking resistance of the wall panel, attention is not given to the overall stability, i.e. the resistance of the wall to global sliding or rotation.

Since internal walls are not secured by holding down straps against uplift, as in the case of external walls, and since they often carry little or no vertical loading, their resistance to sliding and, in particular, overturning is very limited.

Potentially dangerous buildings

The reporter has seen several cases of buildings which would be potentially dangerous being proposed by experienced designers. One solution has involved steel frames being introduced into timber frame construction with special details developed to anchor internal walls to foundations and so prevent uplift and rotation.

Of further concern, in one recent case, the engineer responsible for the design of the structure admitted that it could not be justified by calculation but appeared to indicate that his client, a timber frame house supplier, was unwilling to modify to the design unless this inconsistency was picked up by a checking engineer. This statement would seem to imply a lax attitude to designing for overall stability.

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

The importance of adequate timber design is a matter of continuing interest to CROSS. This case, and the Colindale Fire in NW London in July 2006 when a timber framed building under construction was destroyed by fire in less than 9 minutes, highlight that there are particular hazards with timber frame construction.

CROSS has identified from other sources that the issue of robustness is a major concern. This example illustrates a number of points: lack of understanding of structural concepts, failure to appoint one competent person with overall responsibility for stability and robustness (as required by codes of practice), and a failure to take an adequate stand on matters of safety in the face of influences from lay sources.

The implication that a client would not modify an unsuitable design unless this was noticed by a checker is a serious matter. If something is known to be questionable it cannot be left unresolved and in a court situation the design engineer could be deemed to have been negligent.

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