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885 Disproportionate collapse assessment of Large Panel System (LPS) buildings

Report ID: 885

Published: Newsletter 57 - January 2020

Report 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 LPS buildings.

Report Content

In the wake of the Grenfell Tower fire, there has been increased interest in the safety of residents of high-rise residential buildings, particularly 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.

However, not long after the Grenfell Tower fire, renewed concerns were raised regarding the risks of disproportionate collapse of Large Panel System (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 led to the structural retrofit of many LPS buildings in the UK, many of which remain in use today. On 5 September 2017, MHCLG issued letters to local authorities and housing associations regarding LPS buildings, noting (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. The reporter has become aware of multiple examples of such assessments being undertaken and has 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 - 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, and that assessing risks associated with disproportionate collapse due to fire should be assessed (independently) by the Chartered Fire Engineer; or
  2. that the Chartered Structural Engineers are able assess the risks associated with disproportionate collapse due to fire as “low" 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, as 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, and 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.

In the second case, the reporter believes that vague justifications based on ‘experience’ without any supporting technical evidence clearly do not meet the expected professional standard for Chartered Structural Engineers to use reasonable skill and care in performing their professional duties. 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. Obtaining approvals based on precedent without evidential basis is unbecoming of a Chartered Structural Engineer, and should, in the reporter's opinion, be strongly condemned.

The reporter speculates that Chartered Structural Engineers may be using either of the above two justifications because performing a defensible systematic risk assessment of disproportionate collapse due to fire in a LPS building is likely to be extremely difficult, and that many structural engineers do not have the required competence to do so. Structural engineers may also wish to shed the resulting liability.

In either case, the reporter considers it unethical and unprofessional to take this LPS assessment work on in the first place, and particularly to 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 to assess the risks of fire initiation, growth, and spread, and to then use this information to undertake a systematic assessment of the structural risks associated with disproportionate collapse due to a range of credible design fire scenarios.

 

Comments

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

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 and there will be few engineers who have relevant experience. 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 the 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.

Many LPS buildings will be Consequence Class 3 in accordance with Table 11 in Approved Document A and therefore will 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 any responsibility. Approving authorities do not have responsibility for design, nor is their agreement any guarantee of safety.

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 IStructE have published a Practical guide to structural robustness and disproportionate collapse in buildings and a Manual for the systematic risk assessment of high-risk structures against disproportionate collapse. BRE have also published a Handbook for the structural assessment of large panel system (LPS) dwelling blocks for accidental loading.

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

 

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View other CROSS reports published in Newsletter 57


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