Fire protection in buildings
9.4 Building construction and design – preventive and protective measures
Designing and maintaining a ‘safe’ building is the fi rst step to ensuring reasonable levels of fi re safety for the building and its occupants in the event of a fi re. Without a safe building, all the efforts of management to provide systems that prevent fi res occurring or ensure emer-gency evacuation will be of little or no effect. The fun-damental features that provide a basis for both building and life safety in all buildings are:
➤ Elements of structure
➤ Compartmentation
➤ Internal linings.
9.4.1 Elements of structure
An element of structure is defi ned as being part of a building, which supports the building, i.e. a load-bearing part, for example:
➤ Any part of a structural frame (beams and columns)
➤ Any load-bearing wall (other than part which is only self-load-bearing)
➤ A fl oor or any element that supports a fl oor.
A roof structure is not considered as an element of structure unless the roof provides support to an element of structure or which performs the function of a fl oor.
The elements of structure should continue to function in a fi re. They should continue to support and maintain the fi re protection to fl oors, escape routes and access routes, until all occupants have escaped, or have been rescued. In order to achieve this load bearing elements of structure are required to have a demonstrable standard of fi re resistance.
The degree of fi re resistance for any particular building depends upon its use, size and location.
Elements of structure are required to have specifi c fi re resistance in order to:
➤ Minimise the risk to the occupants, some of whom may not evacuate the building immediately
➤ Reduce the risk to fi re fi ghters who may be engaged in fi re fi ghting or rescue operations
➤ Prevent excessive fi re damage and collapse of the building
➤ Prevent excessive transfer of heat to other buildings and structures.
Junctions
In a building that is designed to offer some resistance to fi re it is crucial that the joints between the various elem-ents of structure do not present any weak spots in the fi re protection.
It is therefore critical that the junctions of all of the elements of structure that form a compartment are formed in such a way that fi re is prevented from passing through the join for a period at least equal to the period of fi re resistance of any of the elements it joins. In
some situations, for example roofs in terraced buildings and buildings that abut each other, the compartment walls extend through the roofs and walls forming the compartment.
9.4.2 Compartmentation
Compartmentation is the subdivision of the building into compartments. Each compartment separated from others Figure 9.3 Elements of structure
Structure enclosing protected shaft
Columns or beams
External wall Lower floor not part of structure
Any loadbearing wall Any part of
structural frame Floor
Roof members not elements of structure
Any Gallery Separating or compartment wall
Figure 9.5 Party wall between two adjacent buildings Figure 9.4 Party wall between terraced properties
Fire protection in buildings
by walls and/or fl oors, thereby restricting the growth and spread of fi res in buildings.
Effective compartmentation limits the extent of damage caused by the heat and smoke from a fi re, which, in turn, will have direct and signifi cant implications for the business continuity and resilience of occupants of the building.
Compartmentation is also used as a means of preventing fi re spread between adjacent buildings.
Compartmentation can be achieved horizontally within a fl oor area or vertically between fl oors. Compartmentation is also used to create areas of relative safety for occupants escaping from fi re.
Horizontal compartmentation
In a single storey building or on any one level of a multi-storey building, compartmentation can be applied:
(a) To meet travel distances requirements (see later) (b) To enclose specifi c fi re hazards
(c) To assist progressive horizontal evacuation (d) To assist a phased evacuation
(e) To separate areas of different:
➤ Occupancy
➤ Risk category
➤ Standards of fi re resistance or
➤ Means of escape
(f) Where individual compartments are too large and exceed the limit for the standard of fi re resistance proposed
(g) Where it is desirable or necessary for the occupants to stay in a building involved in a fi re for as long as possible, for operational or safety reasons, e.g. an air traffi c control centre; intensive therapy unit, the control centre of an oil rig.
Vertical
In multi-storey buildings, each storey of any non-domestic building should be a separate compartment. Each compartment should be capable of sustaining the total destruction of the compartment involved without permitting the fi re to spread to other fl oors. This vertical compartmentation also protects occupants of the build-ing who might have to pass the storey involved in fi re while escaping. Finally vertical compartmentation also provides a degree of protection to fi re fi ghters working on storeys immediately above or below the fi re.
Fire resistance
Compartment walls and other elements of structure are normally required to have a degree of resistance to fi re.
Obviously any element of structure that was unable to Figure 9.6 Example of the horizontal subdivision within a fl oor which limits fi re spread and protects the means of escape
Final exit Fire door
Fire-resisting wall SC
SC
SC
SC
SC
SC
SC
SC SC
Self-closing
resist the passage of a fi re would rapidly collapse. There have been some notable instances where the lack of fi re resistance in the structural elements of buildings has led to rapid fi re spread and rapid collapse of a building resulting in signifi cant loss of life.
It is therefore vital that the structural elements within all buildings are designed and constructed in a way that:
➤ Limits and contains fi re spread
➤ Ensures structural stability for appropriate periods of time
➤ Ensures adequate means of escape in case of fi re.
Figure 9.8 Structural beams having undergone fi re tests Figure 9.7 Vertical compartmentation in a multi-storey building
There is a British Standard test for fi re resist-ance contained in BS 476 Fire resistresist-ance of elements of structure. To pass the test, ele-ments of structure must maintain their integ-rity and stability when exposed to fi re for a given period of time.
Part 20 of BS 476 categorises the elements of construction into three main groups:
(a) Load-bearing elements that have a fi re resistance
(b) Non-load-bearing elements that have a fi re resistance
(c) Elements that make a contribution to the fi re resistance of a structure.
The Building Regulations Part B – Section 2 outlines the minimum periods of fi re resistance for the structural elem ents of buildings. The period of fi re resistance is given in time. The minimum period that an element of structure can have is 30 minutes; this can be raised to 4 hours in certain circumstances.
9.4.3 Classes of building construction
The Building Regulations categorise buildings into one of the following into three classes of construction:
Class ‘A’ – complete non-combustible construction, i.e. elements of structure, fl oors, walls. Supporting structure of brick or concrete
Class ‘B’ – traditional construction, i.e. non-combustible walls with combustible fl oors
Class ‘C’ – combustible construction, i.e. timber fl oors and walls.
Fire protection in buildings
Purpose groups
The degree of fi re resistance and other measures that Approved Document B may ‘require’ for the main elements of construction depends to a great extent upon the purpose to which the building is put. Buildings are categorised into seven ‘purpose groups’:
1. Residential domestic 2. Residential institutional 3. Offi ce
4. Shop and commercial 5. Assembly and recreational 6. Industrial
7. Storage (including warehouses and car parks).
Appendix 9.1 provides a more detailed breakdown of the types of premises in each purpose group.
In addition to the use of the building, the degree of fi re resistance for any element of structure, including compartment walls and fl oors, is the size of the building;
the particular dimensions that building control offi cers, designers and builders need to consider are:
➤ Height of the building
➤ Total fl oor area
➤ Volume of each compartment.
9.4.4 Materials of construction
Fire resistance is often achieved in buildings due to the inherent qualities of the building materials used.
Architects and builders select a variety of materials for both esthetic and practical reasons. Some of the com-mon materials used in construction are:
➤ Brick and concrete
➤ Steel
➤ Plasterboard
➤ Glass
➤ Steel sandwich panels.
Brick and concrete
When using brick or concrete blocks for construction adequate fi re resistance is achieved by ensuring the joints at walls and ceilings are sound and providing suf-fi cient vertical stability by the provision of piers and/or corners. When mass concrete is used it is reinforced with steel which provides the necessary stability.
Steel
Steel is used because it is light, strong and to a degree fl exible. The major disadvantage of using steel for the Figure 9.9 Class A – complete non-combustible
con-struction, i.e. concrete or brick fl oors and walls
Figure 9.10 Class B – traditional construction, i.e. non-combustible walls with non-combustible fl oors
Figure 9.11 Class C – combustible construction, i.e. tim-ber fl oors and walls
elements for structure is that it has a low melting point and will lose 60% of its strength at temperatures in the region of 600ºC. The temperatures in fi res in buildings often reach 1000ºC and therefore it is important that the steel components of a building are protected against the heat from any fi re to prevent early collapse of the struc-ture. Methods for protecting structural steel include:
➤ Encasing in concrete
➤ Enclosing in dry lining material, e.g. plasterboard
➤ Coating with cement-based materials
➤ Coating with intumescent materials.
Plasterboard
Plasterboard achieves its fi re resistance because it is made from non-combustible material, commonly gyp-sum. A wall made from a 12 mm thickness of plas-terboard which is adequately sealed at the joints will achieve 30 minutes’ fi re resistance. The disadvantage of plaster board is that it has little strength or load-bearing cap acity. Its durability relies on the strength of its sup-porting structure (normally wooden or metal stud work) and its protection from mechanical damage.
Glass
The use of glass in buildings is becoming more wide-spread with the development of glass production tech-nology which has resulted in glazing that has a variety of specifi c applications, for example:
➤ In internal doors as vision panels
➤ As internal and external doors
➤ As partitions and compartment walls
➤ In roofs, fl oors and ceiling
➤ In escape and access corridors.
The stability of glass elements of structure relies totally on the systems that support the glass, for example the beading, seals and fi xings used.
When assessing the fi re resistance of glass it is important to fi nd evidence of its compliance to the required fi re resistance. Fire resisting glazing should be marked with a permanent stamp which indicates at least the product name and manufacturer. The mark should be entirely visible and legible.
It should be noted that there are many different proprietary types of fi re resisting glass available, many of them with similar sounding names. The main glass types are as follows:
➤ Non-insulating glasses:
➤ Integral wired glass
➤ Laminated wired glass
➤ Monolithic ‘borosilicate’ glass
➤ Monolithic ‘soda-lime’ glass
➤ Laminated clear ‘soda-lime’ glass
➤ Ceramic glass
➤ Laminated safety ceramic glass
➤ Insulating glasses:
➤ Intumescent multi-laminated soda-lime glass
➤ Intumescent ‘gel-fi lled’ glass
➤ Partially insulating glasses:
➤ Intumescent laminated glass
➤ Radiation control glasses:
➤ Coated monolithic ‘soda-lime’ glass.
The glazing system requirements for each of these glasses are very different and any change in the glass type without a change in the glazing system has the Figure 9.13 Example of a building with an all glass exterior Figure 9.12 Typical building materials
Fire protection in buildings
potential to reduce the performance as low as 10% of the required level in many cases, i.e. 3 minutes instead of 30 minutes. It is critical that the method of installa-tion and the material and design of the construcinstalla-tion being glazed fully complies with the glass manufactur-er’s recommendations.
Steel sandwich panels
Lightweight sandwich panels are being increasingly used in buildings; they are often constructed with combustible plastic core material which is included to provide ther-mal insulation. Lightweight sandwich panels combine the strength of the external material with the insulation properties of the inner core. Therefore they have become popular as a building material that enables simple and rapid erection. Unfortunately this type of panel has been implicated in the rapid fi re spread and early collapse of a number of large buildings. It is crucial that any cavities or concealed spaces that may be created when using sandwich panels are adequately protected against con-cealed, internal fi re spread.
9.4.5 Concealed spaces
Concealed spaces in buildings provide easy routes for fi re to escape both horizontally and vertically. Fire
spreading in the concealed spaces in a building presents signifi cant risks due to the fact that it can develop spread without being detected. Concealed spaces may also allow a fi re to move through fi re compartmenta-tion. Access to concealed spaces is always, by their very nature, limited, therefore, even if a fi re is discovered before it has developed suffi ciently to affect other parts of the building, the fi re service is often faced with dif-fi culties in bringing it under control.
Concealed spaces are found in numerous locations in a building including:
➤ Roof spaces
➤ False ceilings
➤ Service risers
➤ Behind decorative panelling
➤ Cavity walls
➤ Floors
➤ Raised fl oors for computer suites.
It is for those reasons that cavities in buildings should always be provided with barriers that resist the con-cealed spread of fi re (see Fig. 9.14). It is particularly important that cavity barriers are provided at those loca-tions where the cavity passes through a compartment wall or fl oor.
wall with cavity
wall with cavity wall with cavity
compartment floor
cavity barrier
cavity barrier
cavity barrier
cavity barrier a. SECTION
fire-resisting wall
fire-resisting door
c. PLAN
floor
suspended ceiling
fire-resisting wall (but not a compartment wall)
alternatively if it is a compartment wall carry wall up to underside of floor b. SECTION
d. PLAN
Figure 9.14 The provision of cavity barriers
Typical locations where cavity barriers should be in position are as follows:
➤ At the junction between an external cavity wall and an internal compartment wall or fl oor
➤ In any ceiling void where a long corridor (over 12 m) has been subdivided to limit fi re or smoke spread
➤ Between a compartment wall and the underside of the fl oor above, i.e. above any false ceiling.
Every cavity barrier must be constructed to provide at least 30 minutes’ fi re resistance and should be tightly fi t-ted and mechanically fi xed in position in such a way so as not to be affected by the:
➤ Movement of the building due to subsidence
➤ Collapse, as a result of a fi re, of any services pene-trating the barrier
➤ Failure in a fi re of any construction into which they abut.
It is often the case that during alterations to a building or its services, fi res are started in concealed cavities and develop and spread without easily being detected.
Alterations that require a fi re resisting enclosure are often made without consideration of their effectiveness, being negated due to the presence of a concealed cavity.
9.4.6 Openings in compartmentation
Compartmentation is vital for the safety of the building and its occupants. However, a building, which has com-partment walls and fl oors with no openings, has very limited use. There are always occasions when a com-partmentation wall or fl oor must be ‘breached’ in order for occupants to move around and the building to be fi t-ted with services. It is when these necessary breaches are made in compartmentation that it is absolutely vital that the compartmentation maintains its integrity against fi re spread.
The Building Regulations Approved Document limits the openings permitted in a compartment fl oor or walls to those for:
➤ Doors and shutters with the appropriate fi re resistance
➤ The passage of pipes, ventilation ducts and other services
➤ Refuse chutes of non-combustible construction
➤ Atria designed in accordance with specifi c rules
➤ Fully enclosed protected shafts
➤ Fully enclosed protected stairways.
The two types of openings in compartment walls and fl oors that are the most diffi cult to manage once a building is occupied are doors with the appropriate fi re resistance
and the passage of pipes, ventilation ducts and other services. Time and time again breaches in compartmen-tation that occur as a result of inadequate fi re risk man-agement have allowed relatively small fi res to spread and develop in tragic proportions.
Breaches of compartmentation are found to routinely occur by fi re doors being faulty or being wedged open and contractors’ works involving the routing of new services through compartment walls or fl oors being carried out without the necessary reinstatement of the integrity of the compartment.
Fire doors and shutters
Fire doors – fi re doors are provided not only to allow passage through a fi re compartment wall but also to protect persons escaping from the heat and more import-antly smoke generated from a developing fi re.
In addition to the fi re resistance of the door and its assembly, it is also vital, in order to protect escape routes and prevent smoke damage to the building, that the door limits the spread of smoke. This is achieved by ensuring that the door effectively self-closes and is fi tted with two types of seal; an intumescent strip that expands when it becomes hot and forms a fi re resist-ing seal around the door and a cold smoke seal which is normally in the form of bushes or a felt material, which prevents the movement of cold smoke.
The fi re resistance of doors is certifi ed through a testing procedure laid down in BS 476. Fire doors can only achieve a certifi ed rating in conjunction with its assembly, i.e. door frame hinges, door handles, glazing, etc. It can be understood therefore that the fi re resistance on any particular door relies as much on the entirety of the completed assembly as on the door itself.
A Code of Practice for fi re doors with non-metallic leaves (BS 8214:1990) states that all fi re doors should be Figure 9.15 A typical fi re resisting door