External Wall Fire Spread - Coursework Example

Table of Contents Table of Contents 1 Introduction 2 Functional requirements of external fire spread and space separation 2 Areas where Space Separation needs to be considered 4 Boundaries 4 Relevant boundary 5 Notional boundary 6 Methods of Assessing Space Separation and Allowable Unprotected Areas 6 How Space Separation between Buildings on the Same Site is taken into Account 8 Alternative Methods or Solutions for Increasing the Allowable Unprotected Area 12 Conclusion 13 Introduction The main aim of safety in a building is facilitate the relocation of the occupants in case of emergency such as fires. Adjacent buildings needs adequate space separation that usually meets the standards set in fire safety building regulations. The external wall can provide sufficient fire resistance from one building to another taking into accounts its height. Some regulations which address fire safety include BR 187, Bs 9999, approved document B etc. The risk of fire spread between adjacent buildings may not be eradicated completely, but the objective of calculating adequate separation distance between buildings is to make sure that the fire ignition in one building is delayed from spreading to the adjacent building to allow fire service to arrive and prevent the fire spread (Muckett and Furness, 2007). To ensure safety of the occupants in case of an emergency such as fire, design standards and codes has to be followed. The codes set out minimum requirements for the protection of the occupants and the contents in the building. The correct building materials used to minimize the fire ignition, the rate of fire spread and establish safe separation distances between adjacent buildings (Great Britain, 2006). The standards set out the requirements for the construction materials and building layout for various building types. It covers the fire safety, precautions, prevention and the required procedures to prevent fire spread. The prevention deals with prevention of the fire ignition to the spreading to adjacent areas. Functional requirements of external fire spread and space separation The requirements for space separation and exterior spread of fire are set out in the rule B4 of the building regulations. It requires that the building’s external walls should effectively resist fire spread over the walls and from one building to the adjacent buildings, taking into account the position, the use and the height of the building. The provision also requires the roofing of the building to be resistant to fire spread over the roof and from one building to the adjacent buildings, considering the position and the use of the building (Great Britain, 2006). The possibility of fire spread between adjacent buildings depends on: The separation of the buildings The intensity and size of the ignited fire. Fire protection provided on each side The potential risk on occupants in adjacent buildings. Thus the provision requires the construction of the external elevation to minimize the risk of fire spread to other buildings. This is made possible through the provisions of the excepted distance between the building walls and the boundaries depending on the extent of the unprotected area in the façade (Muckett and Furness, 2007; Great Britain, 2006). Section 13 of the approved document B4 provides guidance on external wall fire resistance and to minimize the likelihood for the ignition and the spread of fire on the external wall. Flammable materials are required to be limited to 1m from the boundary for some buildings. Section 14 provides for limited openings and unprotected places so as to minimize the spread of fire through radiation. Flammable cladding is taken to be unprotected and must be minimized considering the boundary distance. In order to prevent the spread of fire between adjacent buildings on both side of the boundary, B4 requires the following actions to be carried out (Great Britain, 2006). a) The external walls should be constructed such that the risk caused by external fire ignition as well as the spread of fire on the wall surface should be restricted. This is made possible through the constructions of walls with materials of low rates of flame spread. b) The amount of thermal radiation which passes through the wall is reduced by minimizing the size of the unprotected area on the wall surface by considering the distance to the boundary. c) The roof must be constructed such that the risk of penetration or spread of the flame through the roof due to external ignition fire source is reduced. These measures are implemented depending on the distance of the building to the boundary, the height and the use of the building. In case of fire occurrence, the fire spread between buildings may depend on the external wall fire resistance, the space between the buildings, the severity of fire and the risk exposed to the occupants in another building (Great Britain, 2006; Read & Construction Research Communications, 1999). Areas where Space Separation needs to be considered The space separation depends on the assumptions, of which some may differ from the specific case, but when put together they provide reasonable space separation. The amount of the unprotected areas in the building like areas with flammable materials and openings that does not provides enough protection against the risk of fire. The roofing is not covered under this section unless the roofing is constructed with a horizontal deviation of 700. In other words, the roofing is treated as a wall if it is too steep or if the angle to the horizontal is more than 700. The assumptions are as follows. (a) The fire intensity depends on the purpose or the use of the building, although it can be decreased using automatic sprinklers. (b) The size of fire depends on fire compartmentation on the building, such that if there is fire occurrence in one compartment, it does not spread out of the compartment. (c) Recreation, residential and assembly purposes represent a greater life risk compared to others. (d) The radiation which goes through an external fire resistant wall is minimal and may be neglected. (e) There is a building of the same height with the building in question on the opposite side of the boundary, and is at the same distance to the boundary. If case the amount of the unprotected wall is required, if will be advisable to construct smaller compartments (Stephenson, 2001). Boundaries Boundary distance is used instead of building to measure the separation distance. The separation distance is thus used to calculate allowable size of the unprotected areas, disregarding the building site or if there is an adjoining building or unprotected areas in it. An external wall is considered to be facing the boundary if it forms an angle of 800 or less, like as shown below. Normally, only the distance to the site boundary is taken into account. But if the boundary of the site adjoins the area where more development may not take place like roads, then some part of the adjoining area is taken to be within the boundary in this section (Carlsson, 1999). Relevant boundary This is the boundary that is parallel to the wall that coincides with the wall or with an angle of less or equal to 800 to the given side of the building. Thus relevant boundary may one of the following. a) The center line of the river, railway, road or canal that is adjacent to the boundary. b) The boundary of the building in which it is located Notional boundary formed where two buildings are in the one site (Stephenson, 2001). B4 requirements state that relevant boundary that can be used to determine the space of separation is located on the space between two buildings. The designers will use the distance to calculate the size of the unprotected area in the exterior wall (Carlsson & Lunds universitet, 1999). Notional boundary Notional boundary is established when a boundary becomes relevant. It applies to recreational, assembly and residential group of buildings for buildings on the same site. Notional boundary applies where one of the buildings has been present and the other is a new one (Stephenson, 2001). Methods of Assessing Space Separation and Allowable Unprotected Areas Calculation methods There are two methods set out in the approved document B4 used to calculate the allowable unprotected space in the external wall. The methods are used for the building whose location is greater than 1 m from the boundary line. The aim for calculating the unprotected area is to ensure that the boundary is located at a distance of not less than half the distance though which the radiant heat flux of 12.6kW/m2 from the unprotected areas, assuming the air remains still (Carlsson, 1999). The radiant heat flux emitted from the unprotected area is assumed to be as follows. a) The radiation is 84kW/m2 for recreational, assembly, residential and offices. b) The radiation is 168kW/m2 for storage, commercial, industrial and non-residential purpose groups like as shown below. Figure: Mirror image concept (Carlsson, 1999) Geometric method The elevation is seen as a rectangular drawn around unprotected areas like windows and doors. A table is used to obtain the relevant boundary for the rectangular surface. A trace of a plan in obtained by drawing other building elevations, such that the area outside the trace is not included in the calculations. A boundary inside the trace required more considerations for example by changing the elevation or the plan. In this method, the external wall is considered to be one plane, such that if the some parts are recessed or set back the resulting value will be an approximate (Read et al., 1991). This method can be divided into the following steps. 1) Establish the part of the building that is unprotected 2) Determine the reference point from which to measure the boundary distance 3) Establish the degree of the exposure to the fire risk because of the unprotected area 4) Establish minimum distance to the boundary due to the exposure in the last step. 5) Find out the area exposed which may require less or greater distance to the boundary, before calculating the final distance of the relevant boundary (Read et al., 1991). For example: Calculation for JB Firth building Unprotected area = 83.22 m2 From the table in BR 187, the distance to the boundary of JB building shall be 2.5m Calculation of Maudland building Unprotected area = 143.8 m2 From the table in BR 187, the distance of the boundary of Maudland building shall be 10.5m. How Space Separation between Buildings on the Same Site is taken into Account Notional boundary is assumed such that two adjacent buildings have the required space separation. Usually one of the buildings is an existing construction. It assumed to be present where. a) Either one or both of the buildings in question are assembly, recreational or residential purpose group. b) Two or more buildings are constructed on the same site but with different management or ownership. c) The current builing is assumed to a new building with unprotected area and fire resistance on the wall that is facing the notional boundary. d) The notional boundary is located in an area with the regulations, with the required unprotected area. e) When the notional boundary becomes a relevant boundary (Carlsson, 1999). In case the space separation has not been taken into account, compartmentation may be considered. Space separation between the two buildings may be taken into account. X is the space separation for Maudland building, and Y complies with the space separation for building JB firth. The location of the new building is located is set taking into account the notional boundary, that should also be set considering the size of the unprotected area. But if the two buildings are to be constructed, the boundary between the two can be assumed to be located at the middle of the space separation between the buildings and its location is located accordingly. Calculations on how allowable unprotected areas are determined The elevation of the two building Calculation for JB Firth building with different scenario: 1. If the building has fire compartment and fire resistance on the opening of external wall such as windows and doors. The openings are protected. Therefore, distance to the boundary of JB building shall be 1.0m 2. If the building has fire compartment and no fire resistance on the opening of external wall such as windows and doors. In this case the opening of external wall such as windows and doors are is considered to be an unprotected area. Area for windows and doors (unprotected area) = 3.65 × 0.95 × 6 = 20.805m2 From the table in BR 187, the distance to the boundary of JB building shall be 2.5m 3. If the building with no fire compartment and no fire resistance on the opening of external wall such as windows and doors. Area for windows and doors (unprotected area) = 20.805×4=83.22m2 From the table in BR 187, the distance to the boundary of JB building shall be 2.5m 4. If the building with no fire compartment and fire resistance on the opening of external wall such as windows and doors. The openings are protected. Therefore, distance to the boundary of the building shall be 1.0m Calculation for Maudland building: - 1. If the building has fire compartment and fire resistance on the opening of external wall such as windows and doors. The openings are protected. Therefore, distance to the boundary of Maudland building shall be 1.0m 2. If the building has fire compartment and no fire resistance on the opening of external wall such as windows and doors. In this case the opening of external wall such as windows and doors are is considered to be an unprotected area. Area for windows and doors (unprotected area) = (1.8 × 1.7 × 10) + (3.5 × 1.7) = 30.6 + 5.95 = 36.55m2 From the table in BR 187, the distance to the boundary of Maudland building shall be 10.5m 3. If the building has no fire compartment and no fire resistance on the opening of external wall such as windows and doors. Area for windows and doors (unprotected area) = 36.55 × 4=146.2m2 From the table in BR 187, the distance to the boundary of Maudland building shall be 10.5m 4. If the building has no fire compartment and fire resistance on the opening of external wall such as windows and doors. The openings are protected. Therefore, distance to the boundary of Maudland building shall be 1.0m Alternative Methods or Solutions for Increasing the Allowable Unprotected Area Sections on the wall that is less resistant to fire than the recommended level is considered unprotected. It is the same case for fire resistant wall that has been coated with combustible materials of more than 1mm thickness (Carlsson, 1999). In buildings fitted with sprinklers, it is assumed that the fire spread will be minimal. Therefore, the unprotected area size can be doubled, provided that the distance to the boundary is kept constant. The sprinklers are assessed to make sure that it meets the BS 9251 regulations for domestic and residential purposes. More amounts of unprotected openings may also be also permitted on the external walls facing a street with a space separation of over 15 feet (Building regulation, 2000, vol. 1, 46). Conclusion It has been shown that there are various regulations that address fire spread between adjacent buildings. Various ways for minimizing or delay of spread of fire to allow fire fighters to arrive has been discussed. The methods include provision of adequate space separation between one building to the other and constructing the external walls with fire resisting materials. Fire resistance required for external walls depends on the usage, the size and height of the building. The provisions made in the Approved Document B4 are aimed at reducing the combustibility of the exterior wall. The advantage of provision of codes for space separation is that anyone can determine the require space separation without necessarily waiting for an experienced engineer to do it. However, some of the methods applied require experienced individual. A compartment provide physical barrier to fire spread, and thus the compartment should withstand the fire. A sprinkler may prevent the rate of heat released, and can has been used to control fire. Attention has been given to the ways for determining the distance to the boundary. Different methods predict approximately the same distance. However, some methods do not cover some situations like recessed walls. Most methods assume the situation, and boundary distance becomes will be overstated. This does not cause problems in small buildings, but it become an issue in large buildings. References Carlsson, E., & Lunds universitet. (1999). External fire spread to adjoining buildings: A review of fire safety design guidance and related research. Lund: Lunds universitet. Building regulation, 2000, vol. 1, 46, section 9 External fire spread: Building separation and boundary distances: Fire Research Station. (2007). Bracknell: IHS BRE Press. Great Britain. (2006). Approved document J: 2002 edition: Guidance and supplementary information on the UK implementation of European standards for chimneys and flues. London: TSO. Read, R. E. H., & Fire Research Station. (1991). External fire spread: building separation and boundary distances. Fire Research Station, Building Research Establishment. Muckett M., Furness A., 2007. Introduction to Fire Safety Management, Routledge, Read, R. E. H., & Construction Research Communications. (1999). External fire spread: Building separation and boundary distances. Watford: Construction Research Communications. Stephenson J., 2001. Building Regulations Explained: 2000 Revision, Taylor & Francis, Great Britain., Wales., & Great Britain. (2006). Building regulations and fire safety: Procedural guidance. Norwich: Stationery Office. Read More