Fire Spread: Low Fire Load Density - Assignment Example

Table of Contents 1.0 Introduction 3 1.1 Boundaries 4 1.2 Enclosing rectangles methods 9 1.2.1 Stage 1:Establishing unprotected area 9 1.2.2 Stage 2 Plane of reference 10 1.2.3 Stage 3 Determining area of exposure hazard 12 1.2.4 Stage 4: Establishing minimum distance 14 1.3 Finding unprotected area from boundary position 14 1.4 JB Firth Building calculations 15 1.5 Maudland Building calculations 15 2.0 Calculation of distance considering intensity of radiation at any point 16 2.1 Elevation and openings 16 2.2 JB Firth Building calculations 19 2.3 Maudland Building calculations 21 References 23 1.0 Introduction There are two methods which are useful in giving distances that exhibit desirable accuracy for most applications. Design of these methods it done in such a manner to ensure that there is an overestimate of the distances. In case the overestimate is considered as being undesirable then need arise to have a more refined calculations are (Ashton,1960). The assumption made when considering boundary distance is that a building will be required to be further away from the boundary if openings and the unprotected areas provided in the walls external to the building cover a high percentage. The methods involve some design features which are common to them which are as follows: First is the determination of should be regarded as being unprotected area with relation to space separation. This involve taking into consideration any part of the wall that is likely to contribute some radiation during the fire and these include the parts on the external walls will low fire resistance including windows and doors. There is also need to take into consideration combustible cladding as it may ignite and contribute radiation at some point during the fire. The second factor that has to be considered is making a decision on how much of the elevation of the building is to be put into consideration when making the calculations. If there is no division of the building into compartments, then there will be consideration of all the unprotected areas. For the case involving division of a building into compartments then the assumption made is that the spreading of fire does not spread to go beyond the area in which it started. With the possibility of each compartment being treated as a separate source of radiation in calculation, the wall area where the compartment is largest is given priority when it comes to taking the areas into account. Openings that form part of stairways external enclosures are normally not put into consideration in performing the calculation where the stair way a protected shaft Where the building under consideration is of low fire load density, then the assumption would be that the unprotected areas would have radiation of low intensity with chances of reduction in boundary distance and this results to having two figures being shown in a table when considering the method of Enclosing Rectangle (Table 1, Appendix 2) 1.1 Boundaries Use of distance to a boundary as opposed to the distance to another building when measuring separation distance, allows the calculation of the allowable ratio of unprotected area without there being a concern of another building being on the neighbouring site and the extent of the unprotected area in the building suppose there is any building. If a wall makes an angle of 800 or a smaller angle with a boundary then it qualifies to be regarded as facing the boundary. In normal circumstances what is put into consideration is the distance to actual boundary but in exceptions where a boundary is joined to a space where the chance of development is nil for example a building bordering a road then it would fallow that part of adjoining space would be included within relevant boundary. A relevant boundary is that boundary that is faced by a building wall and may be actual boundary or notional boundary(Figure 1 gives illustration of the boundaries). In circumstances where there is consideration of distances to other buildings that are within the same site then there is assumption of existence a notional boundary between the said buildings. The existence of a notional boundary comes to play in the case where either or both buildings under consideration fall under Residential or Assembly and Recreation Purpose Groups or in circumstances where there is construction of more than one building on a site with their operation being under different organizations(Approved Document B). Figure 1 Method 1 in Approved Document B is noted to be only applicable to a building that is intended for use for blocks flats or other residential purposes where the relevant boundary is 1m or more. The conditions required include the building not exceeding 3 storey with regards to height and not going beyond 24m lengthwise. The sides of the building would have conformed to separation space requirements suppose the distance from the side of the building measured starting from relevant boundary as well as the unprotected area fulfill conditions in diagram 46 in Approved Document B. While calculating the unprotected area the area that fall within limits of diagram figure 2 are to be excluded. The parts of side of building that surpasses maximum unprotected area required are to be fire resisting. Figure 2 Method 2 in Approved Document B finds application in a building or compartment that qualifies any use with the building being at least 1m from the relevant boundary. The building height is restricted to 10m with the exception where there is a car park that is open sided that falls under Purpose Group 7 clarified in the document. For a building that goes beyond 10m the method that is found in BRE report with regards to spreading of external fire will come into use. The sides of the building will have met requirements separation space where the distance measured building side to relevant boundary as well as to the unprotected area do not go beyond the limits stipulated in Table 1 (table 15 in document). Just as in method 1 any point lying the building side of building that goes beyond the required unprotected area need to be fire resisting. Table 1 Methods 1 and 2 of calculating separation distance has the aim of ensuring that the separation of a building from a boundary should be half the distance or more that resulting to the intensity of thermal radiation that is delivered from the entire unprotected areas in the wall under consideration being 12.6kw/m2 when it is assumed that the level of radiation from the unprotected area would be 84kw/m2 for the case of residential office or Assembly and Recreation Purpose Groups or for a case involving one sided multi-storey car park that fall under Purpose Group 7(b) of Approved document B or 168kw/m2 in the case where the buildings belongs to Industrial, Shop and Commercial, storage or Other non-residential Purpose groups. 1.2 Enclosing rectangles methods This in one of the methods that is given in BRE report and it is also known as geometric method. The method involves viewing elevations with drawing of rectangles all around the unprotected areas, with a table being used in establishing of the minimum boundary distance depending on the size of the rectangle with respect to the proportion on the unprotected area. Then there is repetition of the procedure for all the elevations of the building under consideration resulting to obtaining a trace on the plan of the building. If the boundary surrounding the building is found to not to be within the trace, there will be no need for further calculation, but in case it is inside the trace at any point, it is important to have a more detailed examination of the building before undertaking of any alteration to the plan or elevation (Bergstrom M. et al., 1957). In the basic method there is assumption that the external enclosure would lie on a single plane, but having large parts of it recessed or having a setback would result to the calculated distance being overestimated. There can be modification of this method to a certain degree to carter for the overestimate of this but plans that are much more complicated may call for using the protractor method. 1.2.1 Stage 1:Establishing unprotected area In stage 1 the unprotected areas is established and it calls for consulting relevant Building Regulations or supporting technical documents with regards to building parts that needs consideration and those areas qualifying being put out of consideration when making the required calculations. 1.2.2 Stage 2 Plane of reference There are two stages in looking in to this stage: Part 1: it involves establishing the reference plane that will pass as being the most desirable building side that is being considered and that complies with the following requirements: it touches part of the side if not all parts; no passing within the building notwithstanding the level of extension even though it is allowable for it to pass through projections such as coping and balcony; and crossing of the relevant boundary is not allowable. In normal circumstances it would be desirable to have the reference plane being approximately in parallel with relevant boundary and in the case involving building side all being in one plane then this side would be considered as being the reference plane. For matters being simplified , any elevation part that is set back behind the reference plane of with the distance not in excess of 1.5m, then there can be assumption of no set back in making the calculation. In case where we have elevations parts a set back as either break in elevation or a recess having length that goes beyond 1.5m being used as plane of reference then it would call for stage 5 guidelines be put into consideration. Figure 1 Part b: this involves a reference plane and perpendicular to it, having project lines that marks the unprotected areas that are 800 or less with regards to the reference plane as can be observed in Figure 2 Figure 2 1.2.3 Stage 3 Determining area of exposure hazard Two parts are considered in this stage Part a: Exposure hazard area is taken from the table as being the smallest rectangle which upon elevation would be found to be equal or next greater in terms of both the height and width, in comparison to a rectangle enclosing the entire relevant unprotected areas that fall on the building side under consideration or compartment of the building whose projection is on reference plane and is referred to as an enclosing rectangle as can be seen in Figure 3 Figure 3 When making a choice of enclosing rectangle height, this is to be drawn from the compartment that is able to give the greatest exposure hazard. In the case where there is no horizontal subdivision of the building by compartment floors, then the requirement is taking into account rectangle height that is enclosing the entire unprotected areas. This is extended to a building that has no vertical sub-division that involve use of walls, and it would then be required that the whole width to be taken into overall enclosing rectangle. Part b: the entire unprotected area less the areas that may not be included because of conditions set in stage 1 is given as enclosing rectangle percentage to which they are part which gives the unprotected percentage. 1.2.4 Stage 4: Establishing minimum distance After the determination of the parts of elevation that are to be put into consideration , it can be accomplished through utilization of it table 1 to determine the required distance from the building to the relevant boundary or to determine maximum unprotected area allowed in a specific boundary location. The table 1 in Approved Document B sets out the boundary distance bearing in mind a variety of heights and widths of rectangles as well as the unprotected areas percentages that lie within these rectangles. It is worth noting that the table factors in the fire load density (which may be normal or low), that the distance is related to reference plane and not building side unless in a situation where there is coincidence of the two and as well as unprotected percentage lies in-between figures give in the table then interpolation is required. 1.3 Finding unprotected area from boundary position By use of Table 1(Appendix 2) the size of unprotected area that is allowable for a specific distance from the relevant boundary is established. In case the total area of unprotected area of the total area in relation to enclosing rectangle size goes beyond the unprotected percentage allowed for the distance, then it would call for modification of the design up to the point of establishment of enclosing rectangle as well as unprotected percentage allowable with to distance of the boundary. There would then follow a repeat of the excess to include all building sides that fall in a distance of 1m from any point on the relevant distance. 1.4 JB Firth Building calculations The unprotected area which consists of the openings = 83.22m2 Total area =12x28.8m=345.6m2 Percentage =  The distance is found to be 6.4m from BR 187 considering that the building has normal radiation 1.5 Maudland Building calculations The unprotected area which consists of the openings = 143.8m2 Total area =12x28.8m=345.6m2 Percentage =  The distance is found to be 10.5m from BR 187 when building is considered having normal radiation. 2.0 Calculating distance considering intensity of radiation at any point 2.1 Elevation and openings Elevations where there is a number of windows would involve the problem being reduced to that of a single radiator with the single radiator being the rectangle that fully encloses all the openings in the elevation and this would be assumed to have a lower radiation intensity, where the reduction factor is the ratio of total area of all openings versus the area of the enclosing rectangle. This means that for the case where there is area of opening of 50% of enclosing rectangle where the building is of normal fire load , then it would result in reduction in intensity from 4 cal cm–2s–1 to 2 cal cm–2s–1 this being a 50% reduction. Then there would be calculation of configuration factor so as to come up with the separation distance. Considering elevation given in figure4a it can be observed that ABCD encloses the openings with the area of these openings being 50% of the area ABCD. A radiator that is equivalent is given by figure4b where Io = 2.0 cal cm–2s–1.  = 0.15 Figure 4a Figure 4b The value of  will take a maximum value on a line that is normal to the centre of the rectangle whereby the point on this normal where we have  having a value of 0.15 at the minimum separation distance. At a point P  is a summation of  value for each of the rectangles considered separately as can be observed in figure and with the rectangle being identical to each other we have for ABFP’ And with reference to figure A=10 N=3 at =0.0375,  and this gives c=44ft 2.2 JB Firth Building calculations The unprotected area which consists of the openings = 83.22m2 Total area =12x28.8m=345.6m2 Percentage =  Considering the building having high emission of 2 cal cm–2s–1 and factoring in the percentage of unprotected area  =0.96 cal cm–2s–1.  = 0.31 Now considering only ¼ of the elevation With ¼ of elevation the width of area will be 6m=A while the length will be 14.4=NA N =14.4/6=2.4 From diagram at =0.08 for N=2.4 NA/C =1.02 But NA=14.4 14.4/c=1.02 /1.02 =14.11ft = 4.3m 2.3 Maudland Building calculations The unprotected area which consists of the openings = 143.8m2 Total area =12x28.8m=345.6m2 Percentage =  Considering the building having normal emission of 4 cal cm–2s–1 and factoring in the percentage of unprotected area  =1.64 cal cm–2s–1.  = 0.18 Now considering only ¼ of the elevation With ¼ of elevation the width of area will be 6m=A while the length will be 14.4=NA N =14.4/6=2.4 From diagram at =0.045 for N=2.4 NA/C =0.9 But NA=14.4 14.4/c=0.9 /0.9 =16ft = 16x0.305m = 4.88m References Ashton L. A. and Malhotra H. L.(1960). External walls of buildings. Part 1. The protection of openings against spread of fire from storey to storey. Joint Fire Research Organization F R Note No 436/1960. Bergstrom M, et al.(1957). Fire research, some results of investigations. Statens Provningsanstalt Meddelande 122. Stockholm, 1957. Bevan R. C. and Webster C. T. (1950). Radiation from building fires. National Building Studies TechnicalPaper No 5. London,. HM Stationery Office. Hamilton D. C. and Morgan W R.(1952). Radiant interchange configuration factors. US National Advisory Committee for Aeronautics TechnicalNote 2836. McGuire J H. (1953)Heat transfer by radiation.Department of Scientific and Industrial Research and Fire Offices’ Committee Fire Research Special Report No 2. London,. HM Stationery office. Simms D L, et al (1955). A preliminary examination of the byelaw requirements for structural fire precautions in dwelling houses. Joint Fire Research Organization FR Note No 223/1955. \ Appendix 1 Appendix 2 Read More

If a wall makes an angle of 800 or a smaller angle with a boundary then it qualifies to be regarded as facing the boundary. In normal circumstances what is put into consideration is the distance to actual boundary but in exceptions where a boundary is joined to a space where the chance of development is nil for example a building bordering a road then it would fallow that part of adjoining space would be included within relevant boundary. A relevant boundary is that boundary that is faced by a building wall and may be actual boundary or notional boundary(Figure 1 gives illustration of the boundaries).

In circumstances where there is consideration of distances to other buildings that are within the same site then there is assumption of existence a notional boundary between the said buildings. The existence of a notional boundary comes to play in the case where either or both buildings under consideration fall under Residential or Assembly and Recreation Purpose Groups or in circumstances where there is construction of more than one building on a site with their operation being under different organizations(Approved Document B).

Figure 1 Method 1 in Approved Document B is noted to be only applicable to a building that is intended for use for blocks flats or other residential purposes where the relevant boundary is 1m or more. The conditions required include the building not exceeding 3 storey with regards to height and not going beyond 24m lengthwise. The sides of the building would have conformed to separation space requirements suppose the distance from the side of the building measured starting from relevant boundary as well as the unprotected area fulfill conditions in diagram 46 in Approved Document B.

While calculating the unprotected area the area that fall within limits of diagram figure 2 are to be excluded. The parts of side of building that surpasses maximum unprotected area required are to be fire resisting. Figure 2 Method 2 in Approved Document B finds application in a building or compartment that qualifies any use with the building being at least 1m from the relevant boundary. The building height is restricted to 10m with the exception where there is a car park that is open sided that falls under Purpose Group 7 clarified in the document.

For a building that goes beyond 10m the method that is found in BRE report with regards to spreading of external fire will come into use. The sides of the building will have met requirements separation space where the distance measured building side to relevant boundary as well as to the unprotected area do not go beyond the limits stipulated in Table 1 (table 15 in document). Just as in method 1 any point lying the building side of building that goes beyond the required unprotected area need to be fire resisting.

Table 1 Methods 1 and 2 of calculating separation distance has the aim of ensuring that the separation of a building from a boundary should be half the distance or more that resulting to the intensity of thermal radiation that is delivered from the entire unprotected areas in the wall under consideration being 12.6kw/m2 when it is assumed that the level of radiation from the unprotected area would be 84kw/m2 for the case of residential office or Assembly and Recreation Purpose Groups or for a case involving one sided multi-storey car park that fall under Purpose Group 7(b) of Approved document B or 168kw/m2 in the case where the buildings belongs to Industrial, Shop and Commercial, storage or Other non-residential Purpose groups. 1.2 Enclosing rectangles methods This in one of the methods that is given in BRE report and it is also known as geometric method.

The method involves viewing elevations with drawing of rectangles all around the unprotected areas, with a table being used in establishing of the minimum boundary distance depending on the size of the rectangle with respect to the proportion on the unprotected area. Then there is repetition of the procedure for all the elevations of the building under consideration resulting to obtaining a trace on the plan of the building.

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