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CROSS Safety Report

Progressive collapse of an old mill building during a fire

Report ID: 122 Published: 1 October 2008 Region: CROSS-UK

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Overview

An old mill building that was constructed around 1830 collapsed unexpectedly in a fire.

Key Learning Outcomes

For civil and structural design engineers:

  • Be aware that buildings that have performed adequately during their life may hide vulnerable details which make their behaviour unpredictable during extreme events

  • If working on older multi-storey buildings be aware of the potential for sudden progressive collapse and routinely raise the risks to contractors and the wider project team

  • Be aware that Part A of the building regulations, and particularly the section on disproportionate collapse would apply in the case of change of use

Full Report

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This report concerns the unexpected collapse of part of a twenty-five bay, five storey mill following a fire. The mill was constructed around 1830. It had external masonry walls and internal cast iron columns with brick arch floors. The floors were supported on cast iron beams.

The original part had concrete floors whilst a later addition, abutting the original with straight vertical joints in the walls, had timber floors. The floor beams were tied with rods at the level of the arch soffit. 

Fire spread through lift shaft

There was a stone clad roof on timber truss roof supports. It is believed that the seat of the fire was at the base of a lift shaft. The fire then travelled up the shaft and took hold of the roof structure. By the time the fire had been controlled the roof had collapsed and sections of the parapet had fallen on the front and rear elevations.

Risk of outwards collapse

In making the structure safe a long reach grab was used to reduce the height of the top storey walls which posed a risk of collapse outwards. This included an attempt to reduce the height of the freestanding gable wall. 

During this process some of the masonry from the gable wall fell inwards onto the section with timber floors. This started a chain reaction of failure that led to the sudden collapse of the five-storey section. 

During this process some of the masonry from the gable wall fell inwards onto the section with timber floors. This started a chain reaction of failure that led to the sudden collapse of the five-storey section

The straight joint in the masonry walls probably contributed to the sudden collapse.  Onlookers, including the reporter, were surprised by the speed and extent of the collapse. Although no one was injured there was considerable damage to neighbouring houses.

Expert Panel Comments

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This is a case of progressive failure in which an impact on the top floor of a typical mill structure dating from the 19th century resulted in the cascading collapse of 5 storeys. Whilst the fire caused the initial problems there is no indication that it had weakened the part that fell.

Whilst the collapse was initiated by the attempt to reduce the height of the unrestrained gable, if this had not been done it could have failed in any event from wind. The subsequent collapse could have had potentially fatal consequences.

Structural robustness of old mill buildings

Buildings such as this were not designed with robustness in mind and there was no structural continuity and little in the way of bracing apart from façade walls. They were constructed, not always very well, for specific industrial purposes. They are now used for a variety of functions including conversion into residences.

Part A of the building regulations, and particularly the section on disproportionate collapse would apply in the case of change of use. The Institution of Structural Engineers is preparing a report on ‘Robustness and progressive collapse’, and CROSS are concerned that the concepts are not well understood in the industry.

Four helpful references for working on older buildings

Buildings that have performed adequately during their life may hide vulnerable details which make their behaviour, unpredictable when subjected to events. Those involved with damaged buildings and those undergoing alteration should consider this. Developers, designers and contractors, must be aware of the potential for catastrophic progressive collapse of some older multi-storey buildings.

Developers, designers and contractors, must be aware of the potential for catastrophic progressive collapse of some older multi-storey buildings

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