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The need to pin down

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Many structures are escaping engineering analysis because they do not conform to common definitions, says John Carpenter.

Structures occur in all shapes and sizes and in unexpected places.

Lay people may not always be aware of this.

What should have been an enjoyable day out on Sunday 23 July at Riverside Park, Chester-le-Street turned into a nightmare when an unusual inflable structure of some 2,000m 2 in plan and some 4m to 5m high - a work of art named Dreamspace - broke free of its anchors, resulting in two deaths (NCE 27 July).

Although the causes of this incident have not yet been confirmed, it highlights some important considerations.

These are of general applicability, but apply specifically to the leisure and entertainment industry that continually seeks or uses new formats, technology and attractions.

While we all recognise buildings, bridges and the like as structures, and expect appropriate structural engineering input, 'structures' also manifest themselves in less expected formats - advertising hoardings, large outdoor television screens (a TV screen structure that collapsed in Birmingham in June this year was some 11m high), and bouncy castles.

Not many people would classify the latter as a structure. But bouncy castles can be subject to significant wind actions and need appropriate attention.

Wind actions tend to be critical because of the area presented. Their lack of permanence also introduces significant structural issues related to erection, operational and maintenance. This is a feature of temporary structures generally and sets them apart from permanent structures.

Ensuring that the structure does not overturn or lift off is essential. Dreamspace was not a bouncy castle, but was of the same type of construction. It was very much larger in plan size and used very light materials.

There is a recurring theme in structural engineering: extrapolation. Safe structures derive from working within established envelopes of experience, usually expressed by codes of practice and British Standards, but also testing or experience in use.

The risk profile increases when one moves outside this established zone of confidence without realising the implications. Movement of this type - making things bigger - might occur in small increments over a period of time, such that their significance is not appreciated, or as one large change. The former appears to have been the case with Dreamspace. The latter is what happened at Ronan Point in east London - which collapsed in 1968 following an explosion in one of its flats - when experience abroad of this form of construction up to five storeys, was extrapolated in the UK to 22 storeys.

We do not yet know whether the tragic event at Chester-le-Street was foreseeable or whether the risks were managed appropriately. However, it does emphasise the need for constant vigilance and is a reminder of some critical structural characteristics.

John Carpenter is secretary of the Standing Committee on Structural Safety.

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