Technical Note | Fire compartmentation and fire barriers

Words by AJ Hutchinson, Thursday 02 March 2017

AJ Hutchinson (MCIAT) is a Senior Architectural Technologist with BSBG and member of the CIAT Middle East committee, and this is his technical note on fire compartmentation and barriers.

Fire compartmentation is considered a vital part of any fire safety design approach, and is a form of passive fire protection because it doesn’t typically react or change in any fire conditions. Compartmentation is essentially sub-dividing a building into different ‘cells’ to help restrict or contain the spread of fire, heat and smoke to or from another part of the same building or an adjoining building for a specified time.

This protects building occupants and allows them time to escape, while also providing opportunity to the local firefighting services to act and reduce potential property damage in the event of a fire. The cells or compartments are separated from one another by full height walls and floors made of fire-resistant materials and construction.

Compartment sizes

Sizes of compartments are determined by a number of factors such as:
1) The height of the building – Low Rise, Mid-rise, High-rise.
2) The use of the building – Residential, Commercial, Public, Mixed.
3) Occupancy levels.
4) Availability of a sprinkler system.

Weaknesses

One possible weakness within fire compartment walls comes with the need for things to pass through them, whether this relates to the requirement for people (voids for doorways) or building services (electrical cables and mechanical ductwork), these create openings and therefore weaknesses within the compartment walls. Specialist attention needs to be given to these breaches. Firestopping is a term given to various components used to seal openings in compartmentation walls.

Each method used will differ depending on the type and size of the wall opening. Doors within compartment walls, as well as other openings (Windows, MEP pipes & ducts; see below) should have a similar fire resistance to the walls or floors they penetrate. Any joints between fire-separating elements should be fire stopped (seal openings in fire compartmentation) to maintain continuity of resistance.

Fire dampers are installed in the ductwork of heating, ventilation and air conditioning systems which pass through compartment walls and will automatically close on the detection of heat. With reference to NFPA 5000 – 2015 guidelines, a fire door within a 60 minute compartment wall needs to be 60-minute fire resistant; however, a door within a 120 minute compartment wall only needs to be 90-minute fire resistant.

Cables/Pipes

High expansion Intumescent Sealant is an easy to apply sealant. It expands at temperatures above 135°C sealing around various penetrations and services to prevent the passage of smoke and flames. Plastic pipe collars surround a pipe at the penetration of the wall and consist of a corrosion resistant power coated steel sleeve that contains a flexible graphite-based intumescent liner. Under fire conditions, the intumescent material expands, filling the void left by the burnt out plastic pipe.

Ducting

Fire dampers are products used in heating, ventilation and air conditioning ducts to prevent the spread of fire inside the ductwork through fire-resistant walls and floors. When a rise in temperature is detected by a thermal element (which melts at temperatures higher than ambient but low enough to indicate the presence of a fire), the damper closes. Fire dampers can also close via an electrical signal from a fire alarm system, which utilises detectors within the building management system. Unlike the intumescent sealant or firestop collars, inspection and testing is required one year after installation, and then every four years thereafter (NFPA Standards).

Cavity barriers

Ventilated rainscreen cavities require a continuous air gap, and within this circumstance the void is required to remain open. In this case, high density rock fibre with intumescent coating on both sides and an intumescent strip should be installed. In the event of a fire, the intumescent strip will expand and seal the ventilation gap stopping the spread of flames and fire within the cavity. Cavity barriers are a form of fire separation, placed within areas where fire and smoke could spread and go undetected (for example, within cavities behind the main façade). This is of high importance since many buildings have concealed spaces and voids within roofs, floors and walls.

It is a Dubai Civil Defence requirement to incorporate mechanically fixed cavity barriers into façade design at every floor horizontally and vertically to restrict flame spread laterally. There are a number of different products on the market for use in different circumstances.

Materials

There is a large selection of materials available on the market for different applications and use. Rockwool provide fire barrier slabs and firestop systems for wall, roof, ceiling and floor cavities, intumescent expansion joint sealant for some wall details, pipe collars and sleeves for piping and ablative coating for cables and cable trays. Other reputable international manufacturers are Pilkington Glass, Firefly, and Envirograf to name just a few.

The required fire-resisting timescale (30 mins to 180 mins) or where it’s being applied, can dictate the material selection and thickness requirements. All fire stopping materials have to be Class A when tested to NFPA 255 or ASTM E 84 or UL 723; Class 0 when tested to BS 476 Part 6 and 7, Class A2, d0 when tested to EN 13501-1 or Class 1 when tested to FM 4880, as set out by the Dubai Civil Defence.

Rate of smoke spread

As an example to highlight the rate that smoke can engulf a room you're standing in and dramatically reduce visibility if penetrations or cavities are not fire stopped adequately - if a square room measuring 6m x 6m x 3m has a pencil hole between compartments, it will take less than four minutes for the room to fill with smoke to the extent that you cannot see your hands 500mm in front of you.[1]

Suspended ceilings/Raised access floors

Fire slabs are aluminium foil faced slabs or battens with reinforcing steel mesh used to close voids of different depths above ceilings or below floors. Intumescent sealant is used in combination with the foil faced slabs and applied to the butt joints and perimeter of the barriers. A Fire Barrier Slab with an aluminium foil face from Rockwool can be used to close voids of up to 1m in height and 20m in length, supported by just the top of a masonry block wall.

Extended drops of up to 10.5m are easily catered for by the Rockwool Fire Barrier Systems with 25mm galvanised wire mesh stitched with wire, to one face, to produce a flexible fire barrier. When installed within ceiling voids in conjunction with fire dampers for MEP ductwork; the dampers must be supported independently of the fire barrier. Where there are areas within ceilings that will be accessed regularly for maintenance purposes, a fire rated door set and frame should be included within the design so the integrity of the fire barriers is not disturbed.

Combustible and non-combustible

Combustible materials are those that readily ignite and burn; wood and plastic products being great examples. Materials will be characterised by the flame spread rating and heat release rate. A non-combustible material is one that is not capable of undergoing combustion under specified conditions set out by ASTM E 176. Fire resistance is typically associated with an assembly construction and therefore considers the performance of a number of materials that would be incorporated in a wall, roof or floor. The assembly rating is for resistance to fire penetration, and the exposed materials for information regarding flame spread.

Further Reading:
NFPA 286 – Standard Methods of Fire Tests for Evaluating Contribution of Wall and Ceiling Interior Finish to Room Fire Growth
NFPA 288 – Standard Methods of Fire Tests of Horizontal Fire Door Assemblies installed in Horizontal Fire Resistance-Rated Assemblies
NFPA 5000 – Building Construction and Safety Code
UAE Fire and Life Safety - Code of Practice / Fire Stopping, Exterior Wall Cladding / Curtain wall and Roofing Systems
ASTM E84 – Standard Test Method for Surface Burning Characteristics of Building Materials
BS 476 – Fire Tests on Building Materials and Structures [1] http://www.asfp.org.uk/webdocs/ASFP%20launch%20Fire%20Risk.pdf

AJ Hutchinson

Lead Architectural Technologist

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