Last edited 15 Dec 2021

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Business Sprinkler Alliance Institute / association Website

BS 9999 Automatic sprinkler design compensation benefits

This article was originally published as part of The impact of automatic sprinklers on building design, an independent report produced by WSP, sponsored by the Business Sprinkler Alliance (BSA), published in September 2017.

1 Introduction
2 Building layout / means of escape
3 Fire fighting access
4 Passive structural fire protection
5 Fire spread and compartmentation
6 References
7 Related articles on Designing Buildings

Introduction

BS 9999 provides an advanced and flexible approach in building design; it describes a risk-based approach where designers can take into account design benefits such as the inclusion of automatic sprinklers.

Within BS 9999, buildings are rated on their risk to life in the event of a fire by assigning risk profiles to classify the building in terms of occupant characteristics (Table 4-2) and fire growth rates (Table 4-3).

Table 4-2 BS 9999 occupant characteristics

Occupancy characteristic

Description

Examples

Occupants who are awake and familiar with the building

Office and industrial premises

Occupants who are awake and unfamiliar with the building

Shops, exhibitions, museums, leisure centres, other assembly buildings, etc.

Occupants who are likely to be asleep:

Ci [A]

Long-term individual occupancy

Individual flats without 24 h maintenance and management control on site

Cii [A]

Long-term managed occupancy

Serviced flats, halls of residence, sleeping areas of boarding schools

Ciii

Short-term occupancy

Hotels

D [B]

Occupants receiving medical care

Hospitals, residential care facilities

[A] Categories Ci and Cii are included for completeness within this table but are covered in more depth in BS 9991.
[B] Currently occupancy characteristic D, medical care, is dealt with in other documentation and is outside the scope of this British Standard.

Table 4-3 BS 9999 fire growth rates

Category	Fire growth rate [A]	Fire growth parameter [C] kJ/s³	Description	Typical examples [B]
1	Slow	0.003	Evenly distributed low level fire load, small discrete packets of fuel or material of limited combustibility [D]	Reception areas, concourses (without concession outlets) and halls with limited fire load such as sports stadia and foyers
2	Medium	0.012	Evenly distributed low to mid-level fire load comprising a mix of combustible materials	Offices, lounges, classrooms, auditoria, seating areas, galleries and car parks [E]
3	Fast	0.047	Stacked combustibles (on or off racking and shelving but excluding high rack storage), some small quantities of materials other than materials of limited combustibility [D] (or where larger quantities are stored in separate fire resisting enclosures), process, manufacturing or storage of combustible materials	Shop sales areas [F], workshops, factories and small storage buildings
4 [G]	Ultra-fast	0.188	Medium to large quantities of materials other than materials of limited combustibility[D], high racked storage, flammable liquids and gases or where rapid uncontrolled fire growth could occur	Warehousing [H] processing plants and car parks [E] utilizing a car stacker or similar method where there is no fire separation between stacked cars

[A] These categories are related to the fire growth rate and not the ultimate potential fire size.
[B] These are examples only and may be varied according to the specifics of the building/room contents.
[C] This is discussed in PD 7974-1.
[D] Limited combustibility is defined in 3.77 and includes for this purpose materials also defined in 3.85 as non-combustible.
[E] Includes both open and non-open sided car parks.
[F] Includes covered shopping complexes and department stores as well as high street shops and premises for personal services, delivery and collection of goods for cleaning/repair/treatment either carried out by staff or by members of the public themselves. Combustibility, quantity and how goods are displayed should also be taken into account and the risk category amended accordingly.
[G] See Table 4. This category is unacceptable unless a sprinkler system is installed.
[H] This is a worst case assumption. Combustibility, quantity and the way in which goods (including packaging) are stored should be taken into account and the risk category amended accordingly.

Automatic sprinkler systems can provide an efficient means of fire control within a building compartment. Consequently, the installation of an automatic sprinkler system throughout a building allows the fire growth rates that are determined to be reduced by one category. E.g. an industrial premises containing stacked cardboard boxes and wooden pallets, and occupants who are familiar with the building receives a risk profile of A3. The inclusion of an automatic sprinkler system reduces the risk profile to A2.

The extent of fire safety measures and design features required in a building is proportional to the risk profile. By reducing the risk as a result of installing an automatic sprinkler system, fewer additional safety measures are required.

Where only part of a building contains automatic sprinklers, only the areas that are covered may be subject to a risk category reduction. Corridors and linking spaces from sprinklered rooms should also be covered by sprinklers, or separated by fire resisting construction.

Building layout / means of escape

Reducing the risk profile (by incorporating automatic sprinklers) allows more time for escape by occupants. Consequently longer travel distances (Table 4-4), narrower door widths (Table 4-5) and narrower stair widths (Table 4-6) are allowed to achieve the same level of fire safety.

Table 4-4 BS 9999 maximum permissible travel distances

Travel distance when minimum fire protection measures are provided [A]

Risk profile

Travel distance, in metres (m)

Two-way travel

One-way travel

A4 [B]

Not applicable [B]

B4 [B]

Not applicable [B]

C3 [B]

C4 [B]

Not applicable [B]

Where exact travel distances are not known, direct distances should be taken as two thirds of the travel distance.

E.g. A building rated B3 has a maximum permissible one-way travel distance of 16 m and two-way travel distance of 40 m. The provision of automatic sprinklers reduces the risk profile to B2, allowing one-way travel distances up to 20 m and two-way travel distance up to 50 m.

Table 4-5 BS 9999 Minimum permissible door widths

Door widths when minimum fire protection measures are provided

Risk profile

Minimum door width per person (mm)

3.3

3.6

4.6

A4 [A]

Not applicable [B]

3.6

4.1

6.0

B4 [A]

Not applicable [B]

3.6

4.1

C3 [A]

6.0

C4 [A]

Not applicable [B]

E.g. A building with a risk profile B2 must contain doors with a minimum width of 4.1 mm per person. The installation of automatic sprinklers reduces the risk profile to B1 allowing minimum door widths of 3.6 mm per person.

Table 4-6 BS 9999: minimum permissible stair widths

Door widths when minimum fire protection measures are provided

Risk profile

1 floor

2 floors

3 floors

4 floors

5 floors

6 floors

7 floors

8 floors

9 floors

10+ floors

3.90

3.40

2.95

2.45

2.15

2.00

1.80

1.70

1.50

1.40

4.50

3.80

3.25

2.75

2.45

2.20

2.00

1.90

1.70

1.60

5.40

4.60

4.00

3.50

3.10

2.80

2.60

2.30

2.10

2.00

A4 [A]

–

4.20

3.60

3.10

2.60

2.30

2.10

1.90

1.80

1.60

1.50

4.80

4.00

3.40

2.90

2.60

2.30

2.10

2.00

1.80

1.70

7.00

6.00

5.30

4.60

4.20

3.70

3.40

3.10

2.80

2.60

B4 [A]

–

4.20

3.60

3.10

2.60

2.30

2.10

1.90

1.80

1.60

1.50

4.80

4.00

3.40

2.90

2.60

2.30

2.10

2.00

1.80

1.70

C3 [A]

7.00

6.00

5.30

4.60

4.20

3.70

3.40

3.10

2.80

2.60

C4 [A]

–

E.g. a two-storey building in planning is to have a risk profile of A2, but the designers decide to include an automatic sprinkler system throughout. The risk profile is reduced to A1, either allowing an increase in the number of occupants permitted to use the stair or a reduction in the overall stair size (from 3.8mm/person to 3.4mm/person). The impact of this on a scheme will vary case-by- case.

Where two or more stairways are provided, it should be assumed that one of them might not be available due to fire or smoke and must be discounted unless:

The stairs are protected by a lobby; or
The stairs are protected by a smoke control system designed in accordance with BS EN 12101-6; or
Automatic sprinklers are installed throughout the building.

For phased evacuation, two floors are used for the stair width calculations; the following table compares the improvements for different risk profiles:

Table 4-7 Impact of sprinklers on the variation of stair widths in BS 9999

Risk profile

Min. stair width factor (2 floors) [mm/ppl]

Reduction [%]

Non- Sprinklered

Sprinklered

A2: Offices

3.80

3.40

11%

A3: Teaching Labs

4.60

3.80

17%

B2: Restaurants

4.00

3.60

10%

B3: Libraries

6.00

4.00

33%

C2: Hotels

4.00

3.60

10%

For example; a seven-storey office building (risk profile A2) with open-plan area of 1,200m² per floor (200 ppl/floor).

Non-sprinklered – Simultaneous evacuation strategy: the minimum width of the stairs would be 1,200ppl x 2.2mm = 2,640mm
Sprinklered – The risk profile reduces to A1. Simultaneous evacuation strategy: the minimum width of the stairs would be 1200ppl x 2.0mm = 2,400mm.
A reduction of 9% (240mm) in the width of stair is achieved when sprinklered. If multiplied by 10m (typical staircase length per storey) this gives an additional 2.4m²/floor of useable floor space.

Another example is a six-storey library building (risk profile B3) with floor area of 1,²400m per floor (200 ppl/floor).

Non-sprinklered – Simultaneous evacuation strategy: the minimum width of the stairs would be 1,000ppl x 4.2mm = 4,200mm.
Sprinklered – The risk profile reduces to B2. Simultaneous evacuation strategy: the minimum width of the stairs would be 1,000ppl x 2.6mm = 2,600mm.
A reduction of 38% (1,600mm) in the width of stair is achieved when sprinklered. If multiplied by 10m (typical staircase length per storey) this gives an additional 16m²/floor of useable floor space.

Fire fighting access

BS 9999 provides the same concession as ADB in regards to the impact of providing automatic sprinklers on the number of firefighting shafts and basement smoke ventilation discussed earlier (see Design benefits of automatic sprinkler systems granted under approved document B.)

Passive structural fire protection

Elements of structure should have a minimum period of fire resistance to maintain the integrity and insulation of the building so that occupants can escape. The resistance required depends on the risk profile of the building but principally provides the same guidance as ADB.

Fire spread and compartmentation

BS 99994 states that radiation emitted from a building is halved by the action of automatic sprinklers and allows separation distances to be halved and unprotected areas to be doubled if automatic sprinklers are installed.

Compartments should be limited in size according to the risk profile of the building (Table 4-8). The provision of automatic sprinklers reduces the risk profile by one category and reduces the limits on compartment sizes.

Table 4-8 BS 9999 – maximum permissible building height and floor area

Maximum dimensions of compartments

Single storey

Multi storey

Risk profile

Maximum floor area m²

Height of top floor m

Maximum area of any floor m²

No limit

<30 greater than or equal to 30

No limit 4,000

No limit

<18 18 to 30 greater than or equal to 30

14,000 4,000 Not acceptable

A4 [A]

Not applicable [A]

No limit

<18 No limit

8,000 4,000

2,000

<30 greater than or equal to 30

2,000 Not acceptable

B4 [A]

Not applicable [A]

No limit

C3 [B]

No limit

Not applicable

C4 [A]

Not applicable [A]

[A] These categories are outside the scope of the standard.
[B] Risk profile C3 is unacceptable under many circumstances unless special precautions are taken.