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CROSS Safety Report

Disproportionate collapse assessment of large panel system buildings

Report ID: 885 Published: 1 January 2020 Region: CROSS-UK

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Overview

A reporter has become concerned by the approach being taken by some engineers to assess the risk of disproportionate collapse due to fire in large panel system (LPS) buildings.

They suggest that structural engineers have not included the risk of collapse due to fire in their reports, asserting that this risk would be addressed by a fire engineer.

The reporter also states that reports have rated the risk from fire as ‘low’. This has been done without supporting evidence and, most likely, without the necessary competence to reach that conclusion.

Key Learning Outcomes

For clients and all built environment professionals:

  • Condition assessment reports should be completed by an engineer competent to assess all risks, or by a suitably skilled team that includes structural and fire engineers

  • Designers should recognise that fire is a risk to structural integrity

  • This report shows that it would be helpful to reconsider existing guidance, or to have further guidance, on the assessment of LPS buildings

  • It is good practice for LPS buildings to have a recorded structural history. Their condition and continued structural integrity should be monitored.

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In the wake of the Grenfell Tower fire, there has been increased interest in the safety of residents of high-rise residential buildings. This is particularly true from within the social housing sector, says a reporter. The fire safety of cladding has, with good reason, been the major focus of this interest.

Renewed concerns of LPS buildings post Grenfell

However, not long after the Grenfell Tower fire, renewed concerns were raised regarding the risks of disproportionate collapse of LPS buildings. A gas explosion at the Ronan Point tower block in 1968 caused the disproportionate collapse of a substantial portion of the structure, with associated loss of life.

This disaster led to the structural retrofit of many LPS buildings in the UK, lots of which remain in use today. On 5 September 2017, the Ministry of Housing, Communities and Local Government (MHCLG) sent letters to local authorities and housing associations regarding LPS buildings. They noted (in part) that:

‘Whether or not a gas supply is installed, it is important with all large panel system buildings that their structural history is known, and that their condition and continued structural integrity are understood and monitored’

The reporter says that a number of local authorities and housing associations subsequently commissioned Chartered Structural Engineers to undertake assessments of LPS buildings under their control, and to provide the necessary assurances.

Disproportionate collapse due to fire

The reporter has become aware of multiple examples of such assessments being undertaken. They have had sight of some disproportionate collapse assessment reports issued by Chartered Structural Engineers to their clients.

On this basis, the reporter has become concerned by the approach being taken by some of these engineers to assess the risk of disproportionate collapse due to fire in LPS buildings.

The reporter has become concerned by the approach being taken by some of these engineers to assess the risk of disproportionate collapse due to fire in LPS buildings

They essentially ignore such risks by invoking one of two justifications:

  1. That risks associated with disproportionate collapse due to fire need not be carefully considered by Chartered Structural Engineers because a Chartered Fire Engineer is also engaged by the client. Assessing risks associated with disproportionate collapse due to fire should be assessed (independently) by the Chartered Fire Engineer

  2. That the Chartered Structural Engineers are able to assess the risks associated with disproportionate collapse due to fire as ‘low’. This is based purely on their professional experience (without any examples or evidence given) and the fact that similar justifications have previously been accepted by approving authorities

In the reporter’s view, for the first case, the reasoning is flawed. It is unrealistic to expect Chartered Fire Engineers to have the requisite detailed understanding of structural design and structural mechanics to undertake the necessary assessments.

The reporter feels that this work very clearly falls within the professional remit of the Chartered Structural Engineer. They believe that it is unacceptable, unethical, and unprofessional to pass such important work off to individuals who cannot (in most cases) reasonably be expected to be competent to do it.

Should justifications be supported with calculations or analysis?

A Chartered Structural Engineer is expected to use reasonable skill and care in performing their professional duties. In the second case, the reporter believes that vague justifications based on experience without any supporting technical evidence clearly does not meet the expected professional standards.

The reporter would expect some (even minimal) technical justifications to be given and backed up with calculations or analysis. Otherwise, they question the justification for accepting a fee for this work.

The reporter adds that simply because an unsupported technical justification has previously been accepted by approving authorities is no justification for taking the same approach again. The reporter believes obtaining approvals based on precedent without an evidence base is unbecoming of a Chartered Structural Engineer. In their opinion, this should be strongly condemned.

The reporter speculates that Chartered Structural Engineers may be using either of the two justifications above because:

  • Performing a defensible systematic risk assessment of disproportionate collapse due to fire in an LPS building is likely to be extremely difficult

  • Many structural engineers may not have the required competence to carry one out

  • Structural engineers may wish to shed the resulting liability

A collaborative approach to disproportionate collapse assessments

In either case, the reporter considers it unethical and unprofessional to take this LPS assessment work on in the first place. They feel strongly about any attempt to shed liability for this onto a Chartered Fire Engineer.

The reporter's expectation in such cases would be that the Chartered Structural Engineer works alongside a Chartered Fire Engineer. This would allow them to:

  • Assess the risks of fire initiation, growth, and spread

  • Consider a range of credible design fire scenarios

  • Use this information to undertake a systematic assessment of the structural risks associated with disproportionate collapse due to these scenarios

Expert Panel Comments

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The report of the inquiry into the collapse at Ronan Point raised concerns over the safety of LPS buildings in fire. In particular, the effects of fire on the structural behaviour of the building as a whole. It was recommended that the regulations of the time were amended to include this.

It appears that subsequent consideration of the issue concluded that provided adequate tying was present, progressive collapse due to thermal movements was unlikely. If this is the case, then for an existing building where the adequacy of the tying is in doubt, clearly the recommendations of the inquiry with respect to fire are still valid.

However, the original quality of tying may not be known, and corrosion may have had an effect after 50 years, so caution is needed.

How a collaborative approach can improve safety

There are difficulties when the assessment requirements cross two technical boundaries, which in this case is between a Chartered Structural Engineer and a Chartered Fire Engineer. The assessment of the risk of disproportionate collapse due to fire in LPS buildings is not a routine task.

There will not be many engineers who have relevant experience to carry out this task. Few structural engineers will have the sufficient understanding of fire, and likewise, few fire engineers will have the necessary understanding of structural behaviour.

CROSS therefore supports the reporter’s view that that a Chartered Structural Engineer should work alongside a Chartered Fire Engineer to conduct this assessment.

It is unacceptable if an assessment of the risk of disproportionate collapse due to fire in LPS buildings is not conducted due to the reasons presented by the reporter. As a minimum, a Chartered Structural Engineer should highlight the need for such an assessment even if they feel it is outside of their scope or expertise.

A systematic risk assessment for high-risk buildings

Many LPS buildings will be Consequence Class 3 in accordance with Table 11 in Approved Document A. These buildings require a systematic risk assessment taking into account all the normal hazards that may reasonably be foreseen, together with any abnormal hazards.

Designers should not forget that responsibility for their design is theirs alone. Just because an approving authority has accepted their design does not absolve them of responsibility. Approving authorities do not have responsibility for design, nor is their agreement any guarantee of safety.

Designers should not forget that responsibility for their design is theirs alone. Just because an approving authority has accepted their design does not absolve them of responsibility

The reporter is correct to say that it is dangerous to use previous examples as precedent without fully understanding all the circumstances of why they have been accepted. In previous projects, there may have been mitigating factors which are not present in the project under consideration.

The Institution of Structural Engineers (IStructE) and the Building Research Establishment (BRE) have published the following:

This report shows that it would be timely to reconsider existing guidance, or to have further guidance, on the assessment of LPS buildings.

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