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CROSS Safety Report

Collapse of Large Panel System (LPS) buildings during demolition

Report ID: 186 Published: 1 April 2010 Region: CROSS-UK

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Overview

In 2009, numerous progressive and unexpected collapses occurred during the demolition of Large Panel System (LPS) tower blocks.

Key Learning Outcomes

For clients and the construction team:

  • Where demolition works are required it is good practice to provide knowledge and history of the structure, including form, materials of construction, structural interactions, and location

  • Be aware that structures may have been susceptible to poor detailing and construction, as well as degradation over time which may need to be accounted for prior to demolition

  • There is always the legal need for a good management system to avoid excessive collapse which could introduce unnecessary risks to operatives, to people who may be nearby, and to surrounding buildings

Full Report

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In 2009, a local authority group was engaged in a scheme for the demolition of two thirteen storey Large Panel System (LPS) tower blocks which had been manufactured by a company, now out of business, in the 1960s. The report that follows was sent by a senior member of the group.

A high reach demolition rig with a concrete nibbling attachment was used to reduce a corner of the first building to ground level. The same method was used for five storeys of the opposite corner, but then the wall panels and floor slabs forming the bottom eight floors collapsed progressively and unexpectedly.

After precautionary measures were taken, work recommenced to remove the damaged portion of the structure which was now considered to be unsafe. However, this also suffered progressive collapse after the removal of panels from the top. The method of demolition was then reviewed, and a new system was adopted of reducing four storeys at a time across the rest of the block working around the building in a circular pattern. This was successful.

A consultant with considerable experience in LPS buildings worked with the local authority’s in-house buildings investigations team to analyse the second block. However, evidence gathered in the early stages led to such concern that the investigation was abandoned on safety grounds. The group then worked with their contractors and the client to determine a methodology and control measures for the demolition of the second block in early 2010.

After taking down three or four storeys in one corner, there was an uncontrolled collapse affecting the complete corner bay over its remaining height. Of considerable concern was the very clean shear failure across the slab/cross wall panel junction indicating a significant lack of connection (mechanical tying) between floor panels and the cross walls forming the structural cells.

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Figure 1: progressive collapse of building one corner
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Figure 2: shear failure across the slab/cross wall panel junction
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Figure 3: progressive collapse of building two corner

Expert Panel Comments

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There are two issues here; firstly, the demonstration of the lack of robustness in these particular structures when exposed to demolition loading, and secondly, the nature of the demolition process adopted. It certainly shows that the forces involved during demolition should nowadays be taken into account by designers, and CROSS has had previous reports of failure during demolition (which can be found on the website).

There is always the legal need for a good management system to avoid excessive collapse which could introduce unnecessary risks to operatives, to people who may be nearby, and to surrounding buildings. Learning from this example, the system of work for demolishing similar LPS structures should take into account that sudden progressive collapse may occur.

The pictures are very reminiscent of the Ronan Point collapse in 1968 which was the subject of a Public Enquiry in 1968 and guidelines on strengthening similar LPS tower blocks were later published by BRE.

The lack of adequate ties between walls and floors and between adjacent floor panels shown in the photographs means that the buildings would not comply with current Building Regulation A3, but at the time of construction, the need for horizontal and vertical tying through mechanical anchorages, or other suitable measures, was not appreciated. CROSS, along with others, will be further reviewing robustness in large buildings.

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