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CROSS Safety Report

Fire protection to car park steel frame

Report ID: 1007 Published: 26 August 2022 Region: CROSS-UK


Overview

A reporter is concerned about the fire protection of a steel framed car park above a retail unit.

Key Learning Outcomes

For car park designers:

  • Be aware that a multi storey car park structure designed to a fire resistance rating of 15 minutes may not satisfy the functional requirement of the building regulations
  • Structures should be designed so that in the event of a fire they remain in a safe and stable condition to allow firefighters to safely enter the building, while also ensuring the safety of the people around the structure
  • Be aware of past failures and take them into account when designing or altering similar structures. Refer to the SCOSS Alert Fire in Multi-Storey Car Parks for further information. 

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A reporter is concerned about the levels of structural fire protection in multi-storey car parks. This concern was specifically raised about the possibility of a car park that was to have no structural fire protection and was to be constructed above two levels of retail units.

The reporter has worked on many car parks in the construction stages to completion. Recently, they worked on a development in a UK city where the steel frame of the lower retail floors was constructed with fire protected steelwork, but the seven floors of the car park above the retail unit had no such protection to the loadbearing steelwork.

The steelwork to all floors, including the entrance on a circle ramp, had no protection and the structural elements were just galvanised steel columns and beams. The stair cores were protected but the main steelwork to all floors was bare.

The reporter is of the mind that, on a new build development, the structure of the car park above the retail area should have at least a minimum of a 30 minute fire resistance rating.

Expert Panel Comments

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The panel recognises the potential for concern regarding the topic of car parks, and CROSS has published the SCOSS Alert on Fire in Multi-Storey Car Parks in the past. 

The conventional technical guidance in Approved Document B (ADB) for 15 minutes fire resistance for the steelwork in open sided car parks can intuitively 'feel' inadequate. The rules for fire resistance of car parks are largely based on the assumptions that single-car fires, rather than multi-car fires, occur and the idea that a fire will be less intense in well ventilated spaces. This was based on tests on cars from the 60s that had very little combustible material on the outside, were smaller, and tended to have a lower fuel load. It should be noted however, that it is guidance which has been in place for many years, and so presumably there are many car parks built to that standard. To the best of the panel’s knowledge there are no significant incidents of fires in open sided car parks which caused major structural failure. The possibility of multi car fires was demonstrated by research in 2010 (BRE report BD2552), and in real events such as the Liverpool Arena car park fire which also highlighted the risk of fire spread to other cars and adjacent structures.

the fire risks within car parks are changing

On the other hand, the fire risks within car parks are changing. The nature of the fire load is evolving and this can affect the fire size and heat release rate. In addition, the increased use of electric vehicles (EV) could also affect the risk of fire spread to multiple cars due to the characteristics of EV fires (especially the directional jet flames), invalidating the assumption of single-car fires. There are also concerns about the likelihood of dead spots where heat can accumulate, for example in corners or complicated geometries, a fact which will locally undermine the assumption of a well ventilated fire. These changing conditions might suggest that the historic guidance might be inappropriate in some cases, which is why this topic was included in the call for evidence for the revision on ADB and is therefore under review. 

Protecting people about the building

Safety requires a look at the whole. The Building Regulations through Regulation 8 clarify that they cover 'the purpose of securing reasonable standards of health and safety for persons in or about buildings (and any others who may be affected by buildings, or matters connected with buildings)'. In this case, whilst it may be reasonable to expect an open sided carpark to be evacuated relatively quickly, it seems unlikely that the retail floors below would be if failure of the carpark compromised the retail structure below. This would be defined by any consideration of the protection to the structure below the car park levels and whether it is designed as a crash deck in the event of collapse and its capable of shedding the volumes of water that would be used in the event of a fire. If no such consideration is provided, then the structure would not protect the people within the building and so would not comply with the regulations – regardless of how ADB was interpreted. One potential (conservative) approach could be that the whole structure should have protection to the level of the greatest need. Similarly, the building appears to be relatively tall with 7 floors of car parking on top of retail floors. It is not known to CROSS what structures were surrounding the car park. If this is in a densely built area then these could represent the greater fire risk if there is a possibility of the fire spreading from a multi-car fire in the car park to adjacent buildings, or that a structural failure could also impact adjacent buildings. This again indicates a failure to meet the building regulation requirements of protecting people about the building.  

Electric vehicles (EV)

Another overlap with CROSS report on battery fires, is the fact that vapour cloud explosions have been reported with EVs and this reinforces the view of some parties that ADB does not consider the risk posed by modern vehicles, and certainly does not consider the risk from EVs. This leads to the consideration that car parks containing EVs maybe should not be considered a 'common building situation' as described in the Manual to the Building Regulations.

Obvious concerns also exist over firefighter safety when entering these structures, and it should be acknowledged that if the Officer in Charge of an Incident has any doubts to the safety of the structure, then they may well decide to not commit to the structure and fight the fire defensively.

Absolute reliance on codified provisions can potentially be inadequate as an approach, especially when the codes are conceived for circumstances that differ from the reality at hand. What is needed is a full rational understanding by the designer of the risks and their control measures.

Additional guidance on car parks in fire can be found on the website of the Steel Construction Institute, and in the publication by the Institution of Structural Engineers (IStructE) titled “Design recommendations for multi-storey and underground car parks” (Fourth edition). The IStructE guide is currently under review, and it is the panel’s view that, until any updated guidance is available, there is a need for caution with the figure of 15 minutes of fire resistance rating and it is suggested that a competent fire engineer is employed when this is proposed without meaningful consideration.

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