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CROSS Safety Report

Concerns over risky new buildings?

Report ID: 632 Published: 1 October 2017 Region: CROSS-UK

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Overview

A reporter is becoming increasingly concerned about some of the new 'exciting' structures and buildings that are being produced, with particular concern about the use of cantilevers in buildings.

Key Learning Outcomes

For civil and structural design engineers:

  • Cantilevers are often safety critical because of their lack of redundancy and should be adequately tied back and supported

For all built environment professionals:

  • Proper records of a structure should be retained that clearly identify the primary structure and its critical areas. This can ensure future engineers can safely adapt or demolish the structure.

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A reporter says firstly may they congratulate CROSS on the work that is done for structural safety and allied subjects. Secondly, they are becoming increasingly concerned about some of the 'exciting' structures and buildings that are being produced. They are particularly concerned about the use of cantilevers in buildings.

It is obvious where a cantilever exists in a stadium or similar conspicuous and simple structure and what is balancing what, is reasonably clear. But they see them appearing in buildings and are not sure how well the implications of the counterbalancing ties and struts are understood.

For example, there is a newish building in a major UK city that has a very pronounced cantilever at the front. This must be tied down further back in the building by one or more tension members that could be of a relatively small cross-section. What is to stop someone just cutting through it in - say 30 years’ time - without realising its importance?  

Theur spur for this note was the photo on the front of a major engineering Journal which they are sure meets all necessary structural safety criteria, but looks risky, especially against lateral load. The Standing Committee on Structural Safety (SCOSS) invitation to engineers to use 'reflective thinking', which they do from time to time, makes them question some modern structures/buildings; particularly over their lifetime (50 years plus?) and through numerous alterations. They ask if they are worrying unnecessarily.

Expert Panel Comments

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The reporter is right to be concerned. CROSS has had reports of cantilevers where the cantilever designer has ceased their design at the end of the cantilever and just assumed the supporting structure to be adequate. Cantilevers are often safety critical because of their lack of redundancy.

On larger structures, it is to be hoped that designers have paid adequate attention to robustness and that proper records have been retained so that future engineers can adapt or demolish safely. For many buildings it will be obvious what is the primary structure and that unconsidered removal of this would be reckless. However, in certain building types this is not the case and it may be prudent to indicate in some way critical areas of the structure.

On larger structures, it is to be hoped that designers have paid adequate attention to robustness and that proper records have been retained so that future engineers can adapt or demolish safely

Industry guidance in this regard would be welcome. A further area of risk is where the structure is distributed e.g. timber panel and metal stud type construction where it is less obvious what is working as structure and what is just a partition. This is equally true for buildings without cantilevers. In the same way that post tensioned tendons on the soffit of slabs are marked to stop people drilling into them perhaps there should be industry guidance on the marking of principle load paths on these types of structures.

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