Car parks in fire

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For the purposes of determining fire resistance requirements, car parks can be either open or other. Open car parks are above ground and have large openings in the facade for natural ventilation. They have relatively low fire resistance requirements whereas those for other car parks are typically consistent with those for commercial buildings of the same height.

[top]Open car parks

The definition of an open car park is set out in Approved Document B [1], Section 11.3 as being a car park which is above ground and in which each storey shall be naturally ventilated by permanent openings at each car parking level, having an aggregate vent area not less than 1/20th of the floor area at that level, of which at least half, 1/40th, should be provided between two opposing walls. The same definition operates in Scotland, Wales and Northern Ireland. In most instances, open car parks can be constructed without fire protecting the structural steel. In a small number of situations, it may be necessary to fire protect the steel or to increase the size/grade if it is to be unprotected.

[top]Unprotected steel

Unprotected steel in car park 1.png

Unprotected steel in an open car park

Across the United Kingdom, those responsible for preparing documents such as Approved Document B [1] recognise that there is a low risk of fire spread and ample opportunity for smoke and hot gases to be dissipated in open car parks when the minimum ventilation criteria are met. Consequently, the structural frame is required to have only 15 minutes fire resistance when the car park is open and less than 30 metres in height (18 metres in Scotland). Also across the UK, it is accepted that hot rolled structural sections and tubes under full design load in fire conforming to the following criteria are deemed to have 15 minutes inherent fire resistance:


In practice, the great majority of hot rolled structural sections meet these criteria. The small number of sections which have higher section factors may do so if they are less than fully loaded (see below).

It should be noted that Section 2.1.1 in Scottish Technical Handbook 2[2] states that open car parks up to 18 metres in height require 30 minutes fire resistance (a period defined in that document as ‘short’). However, this is overriden in Section 2.D.3, which states that sections conforming to the above limitations on section factor will be deemed to meet the required fire resistance criteria.


[top]Structural steel not achieving 15 minutes inherent fire resistance

The data on limiting section factors to achieve 15 minutes fire resistance can also be expressed as a function of load ratio as defined in BS 5950 Part 8[3] or utilization factor as defined in the fire Eurocodes. In the British Standard, a fully loaded beam or column is normally assumed to have a load ratio of 0.6. Assuming that the load ratio is 0.6, most hot rolled beams and columns will achieve 15 minutes fire resistance without fire protection. The exceptions, and the maximum load ratios which will allow these sections to achieve 15 minutes inherent fire resistance, are provided.

UKB sections not achieving 15 minutes fire resistance when fully loaded: British Standard design
UKB section size Section factor
(Am/V)
Maximum load ratio to achieve 15 minutes fire resistance
Non-composite 100% shear connection (Dovetail deck or filled voids)
127x76UKB13 280 m‐1 0.53 0.41
152x89UKB16 270 m‐1 0.53 0.41
178x102UKB19 260 m‐1 0.55 0.43
203x102UKB23 235 m‐1 0.64 0.5
203x133UKB25 245 m‐1 0.54 0.42
254x102UKB22 280 m‐1 0.47 0.37
254x102UKB25 250 m‐1 0.58 0.45
305x102UKB25 280 m‐1 0.49 0.38
305x102UKB28 250 m‐1 0.59 0.47
356x127UKB33 250 m‐1 0.59 0.46
406x140UKB39 240 m‐1 0.59 0.46
UKC sections not achieving 15 minutes fire resistance when fully loaded: British Standard design
UKC section size Section factor
(Am/V)
Slenderness ratioa Maximum load ratio to achieve 15 minutes fire resistance
152UKC23 305 m‐1 81 N/A, Choose next in the section range
152UKC30 235 m‐1 78 0.36
152UKC37 195 m‐1 77 0.45
203UKC46 200 m‐1 58 0.51

a Assumes an effective length of 3 metres. The failure temperature is a function of the slenderness ratio

The limiting utilization factors for Eurocode design are given below for non-composite beams and columns. These have been calculated using programmes which are available here. These are generally lower than those calculated using British Standards. That is because BS EN 1991-1-2[4] includes a methodology for calculation of temperature rise in fire which is much more severe than that on which the data in BS 5950 Part 8[3] is based. This means that the sections are assumed to get hotter more quickly.

UKB sections not achieving 15 minutes fire resistance when fully loaded: Eurocode design
UKB section size Section factor
(Am/V)
Maximum utilization factor to achieve 15 minutes fire resistance (non-composite)
127x76UKB13 280 m‐1 0.29
152x89UKB16 270 m‐1 0.29
178x102UKB19 260 m‐1 0.3
203x102UKB23 235 m‐1 0.33
203x133UKB25 245 m‐1 0.34
254x102UKB22 280 m‐1 0.27
254x102UKB25 250 m‐1 0.3
305x102UKB25 280 m‐1 0.26
305x102UKB28 250 m‐1 0.29
356x127UKB33 250 m‐1 0.29
406x140UKB39 240 m‐1 0.3
UKC sections not achieving 15 minutes fire resistance when fully loaded: Eurocode designb
UKC section size Section factor
(Am/V)
Maximum utilization factor to achieve 15 minutes fire resistance
152UKC23 305 m‐1 0.28
152UKC30 235 m‐1 0.37
152UKC37 195 m‐1 0.47
203UKC46 200 m‐1 0.5

b Assumes a length of 3 metres and an effective length factor of 0.5.

[top]Exceptional circumstances

Certain situations exist where the deemed to satisfy criteria for structural steelwork in an open car park may not apply:

  • Where one element of structure gives supports or carries or gives stability to another, the fire resistance of the supporting element should be no less than the minimum period of fire resistance of the other element (irrespective of whether that other element is loadbearing). An example of this is where a column gives stability to the open car park and also to a compartment wall surrounding an escape stairway. In that case, the supporting column should have the same fire resistance as the compartment wall.
  • If the building is also used for any other purpose, the part forming the car park is a separated part and the fire resistance of any element of structure that supports or carries or gives stability to another element in another part of the building should be no less than the minimum period of fire resistance of the elements that it supports. An example of this is where a column gives stability to an open car park but also to an adjoining wall in another, separated building, the column must have the same fire resistance as the steelwork in the other building.


[top]Other car parks

Basement car park.png

Basement car park

Approved Document B[1] requires 60 minutes fire resistance in car parks over 30 metres in height. Where a car park is above ground but not defined as an open car park, 30 minutes fire resistance is required for buildings up to 5 metres in height; 60 minutes fire resistance is required for buildings between 5 and 18 metres in height and 90 minutes fire resistance is required for buildings between 18 and 30 metres in height. For basement car parks, the requirement is for 60 minutes fire resistance if the depth is less than 10 metres and 90 minutes where the depth is greater than 10 metres.

In Scotland, Scottish Technical Handbook 2[2] requires basement car parks and open car parks over 18 metres in height to have 60 minutes fire resistance. In Northern Ireland, Technical Booklet E[5] generally follows the requirements of Approved Document B[1] with the exception that open car parks over 30 metres in height are not allowed and all car parks over that height have 120 minutes fire resistance .

[top]References

  1. 1.0 1.1 1.2 1.3 Approved Document B (Fire safety, Volume 2 – Buildings other than Dwellinghouses), 2006 Edition incorporating 2007, 2010 and 2013 amendments. Department for Communities and Local Government
  2. 2.0 2.1 Scottish Technical Handbook, 2016, Part 2 - Fire. The Scottish Government
  3. 3.0 3.1 BS 5950-8: 1993, Structural use of steelwork in buildings. Code of practice for fire resistant design. BSI
  4. BS EN 1991-1-2: 2002, Eurocode 1. Actions on Structures. General actions. Actions on structures exposed to fire. BSI
  5. Technical Booklet E, Fire safety, Building Regulations (Northern Ireland) 2012, Department of Finance and Personnel of the Northern Ireland Government, 2012

[top]Resources


Member fire design tools:

[top]Further reading

[top]See also

[top]CPD