A Comparative Analysis of the Use of Different External Fire Spread Calculation Methods

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2 A Comparative Analysis of the Use of Different External Fire Spread Calculation Methods (1)Declan Thomson, (2)Paul Currie, and (2)Jianqiang Mai (1)Hoare Lea and Partners (6th Floor Royal Exchange, Manchester, UK, M2 7FL) (2)School of Engineering, University of Central Lancashire (Preston, Lancashire, UK, PR1 2HE)

3 History Great Fire of London 1212 Claimed thousands of lives and destroyed a vast proportion of the city. Great Fire of London 1666 Destroyed 80% of the city but killed only 5 people (it is assumed). Despite the need to reduce the risk of fire spread between buildings as far as reasonably practicable, it is recognised that the risk cannot be eliminated entirely. Today in England, Building Regulation B4(1) states: The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building.

4 The Study Aim: To critically review the methods of calculating external fire spread requirements using national and international guidance documents. Objectives: 1. Establish the meaning of boundary distances in terms of UK and international guidance documents. 2. Identify the assumptions and input factors which each assessment method is based. 3. Establish whether the mirror image principle remains valid. 4. Apply the assessment methods to a variety of studies to determine their applicability and the difference in results obtained from the assessment. 5. Determine the applicability of the methods for achieving compliance with UK based Building Regulations.

5 How the Study Was Undertaken An extensive literature review was undertaken looking into the background research on which the space separation assessments are based. A review of the guidance documents used in the UK (e.g. Approved Document B, the Technical Handbook) was carried out to determine the differences in each countries requirements. The external fire spread assessments recommended by the National Fire Protection Association (NFPA) and International Building Code (IBC) have been compared to UK methods.

6 Background The Research The majority of research on the subject was undertaken after the Second World War at the Fire Research Station in the UK and during the St. Lawrence Burns tests in Canada. The assessments are based on the pilot ignition of oven dried wood which, in the UK, is considered to occur at 12.6kWm -2,however, in the US this value is considered to be 12.5kWm -2. The actual value is around 12.56kWm -2.

7 Background UK Guidance It was established that buildings with a high fire load density over 25kgm -2 (wood equivalent) and a lack of adequate ventilation would radiate at 168kWm -2. These are considered to be shops, warehouses and industrial type buildings. Buildings with a low fire load density (i.e. below 25kgm -2 ) provided with sufficient ventilation to sustain the fire would radiate at 84kWm -2. These are considered to be offices, assembly and recreation buildings, and residential buildings.

8 Background Flame Projection In the UK the additional effects created by flame projection are considered to be insignificant and, as such, are discounted from the assessment. However, NFPA 80A recommends that an additional 1.5m is added to the space separation assessment to account for flame projection.

9 The Measurements Relevant Boundary

10 The Measurements Mirror Image Principle In the UK, the assessment methods are based on a mirror image principle (i.e. it is assumed that the building on the other side of the relevant boundary with the exact same shape and number of openings). However, this is never the case and, it is considered by the BRE that people have forgotten this assumption. So what's the issue?

11 The Measurements Unprotected Areas and Enclosing Rectangles In the UK unprotected areas are those which do not achieve the minimum period of fire resistance required for the external walls of the building. The majority of methods are based on the principle of enclosing rectangle (i.e. a rectangle is drawn around the unprotected areas to determine an area of assessment).

12 The Methods Method 1 The Simple Geometry Method BR 187 Enclosing Rectangle BRE s Alternative Method NFPA 80A NFPA 5000 IBC

13 The Simple Geometry Method (Technical Method 1 (AD-B) Handbook) Method The Simple 1 is Geometry restricted to method use on residential proposed in buildings the Technical not more Handbook, 24m in width recommends and not that more the than unprotected three storeys area can above be 6 times ground. the separation distance and discounts the 5.6m 2

14 The Enclosing Rectangle Method (BR 187) The BR 187 enclosing rectangle method is considered to be the go to method for UK fire engineers.

15 BRE s Alternative Method During work undertaken with the Forestry commission, Scotland, BRE concluded that the simple geometry method did not compare well with other calculation methods and, therefore, proposed an alternative: U = 2.5A 2 Where: A = separation distance and U = The unprotected area. However, this method has not been incorporated into the 2014 edition of BR 187.

16 BRE s Alternative Method In accordance with the BRE research, this method can be used when assessing small residential buildings. However, in this study, the field of applicability has been increased to all buildings to determine its use in practice. As such, an alternative formula has been proposed for high fire load assessments. U =1.1A 2

17 NFPA 80A NFPA 80A follows a table based approach, however, unlike the UK guidance, the assessment is measured to the adjacent building. Severity Percentage of Openings Width/Height or Height/Width (Whichever is greater) Guide Number (Multiply by lesser dimension, and add 1.5m to obtain building to building separation Light Moderat e Severe

18 NFPA 5000 This method is table based, wherein the separation distance required is determined, based on the percentage of unprotected openings on the façade. However, the document provided no background information or proposed alternative to its external fire spread assessment. It is noted that the method permits the façade of any building to be 100% non-fire rated over 3.05 for low hazard buildings (e.g. schools) and 9.14m for high hazard buildings (e.g. shops).

19 Maximum Area of Exposing Building Face (ft 2 ) Horizontal Separation (ft) > >

20 IBC This method is table based, wherein the separation distance required is determined, based on the percentage of openings per storey. Similar to NFPA 5000, the document provided no background information or proposed alternative to its external fire spread assessment. It is noted that the method permits the façade of any building to be 100% non-fire rated over 9.14m.

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22 Assessments Five assessments were undertaken: Three low fire load (i.e. below 25kgm -2 ) tests; and Two high fire load (i.e. above 25kgm -2 ) tests.

23 Assessments Low Fire Load Tests 1 and 2 Test 1 The building façade is considered to be fire rated and there is no provision of compartment walls or floors. Each window is 2.25m 2 (i.e. 1.5m x 1.5m) and the door is 5m 2 (i.e. 2m x 2.5m) and, therefore, the total unprotected area is 34.25m 2. Test 2

24 Assessments Low Fire Load Test 1 Separation distance to the notional boundary (m) NFPA 80A* 3.75 NFPA IBC 6.1 BR187 (ENCLOSING RECTANGLE) 2.7 BRE ALTERNATIVE METHOD 3.7 SIMPLE GEOMETRY METHOD (SCOTLAND)

25 Assessments Low Fire Load Test 2 Separation distance to the notional boundary (m) NFPA NFPA 80A* 80A* NFPA5000 NFPA IBC IBC BR187 BR187 (ENCLOSING (ENCLOSING RECTANGLE) RECTANGLE) BRE BRE ALTERNATIVE ALTERNATIVE METHOD METHOD SIMPLE SIMPLE GEOMETRY GEOMETRY METHOD METHOD (SCOTLAND) (SCOTLAND)

26 Assessments Low Fire Load Tests 3 The purpose of this test is to assess the space separation requirements when considering a large non-fire rated construction. The building is 27m in height and 50m in length and, therefore, the unprotected area 1350m 2.

27 Assessments Low Fire Load Test 3 Separation distance to the notional boundary (m) NFPA 80A* SEVERE NFPA 80A* NFPA NFPA 80A* 80A* NFPA 80A* MODERATE NFPA5000 NFPA5000 NFPA NFPA 80A* LIGHT IBC IBC NFPA5000 IBC BR187 BR187 (ENCLOSING (ENCLOSING RECTANGLE) RECTANGLE) IBC BR187 (ENCLOSING RECTANGLE) BRE BRE ALTERNATIVE ALTERNATIVE METHOD METHOD BR187 (ENCLOSING RECTANGLE) SIMPLE SIMPLE BRE GEOMETRY GEOMETRY ALTERNATIVE BRE ALTERNATIVE METHOD METHOD METHOD METHOD (SCOTLAND) (SCOTLAND)

28 Assessments High Fire Load Tests 1 The building façade is considered to be fire rated and there is no provision of compartment walls or floors and, as such, the windows are considered to be the only unprotected areas. On this basis, the total unprotected area is 96m 2.

29 Assessments High Fire Load Test 1 Separation distance to the notional boundary (m) NFPA 80A* SEVERE NFPA NFPA 80A* 80A* NFPA 80A* MODERATE NFPA5000 NFPA5000 NFPA 80A* LIGHT IBC IBC NFPA5000 BR187 BR187 (ENCLOSING (ENCLOSING RECTANGLE) RECTANGLE) IBC BRE BRE ALTERNATIVE ALTERNATIVE METHOD METHOD BR187 (ENCLOSING RECTANGLE) SIMPLE SIMPLE BRE GEOMETRY GEOMETRY ALTERNATIVE METHOD METHOD METHOD (SCOTLAND) (SCOTLAND)

30 Assessments High Fire Load Tests 2 This test is considered to represent a warehouse/industrial process facility with a 100% unprotected façade.

31 Assessments High Fire Load Test 2 Separation distance to the notional boundary (m) NFPA 80A* SEVERE NFPA NFPA 80A* 80A* NFPA 80A* MODERATE NFPA5000 NFPA5000 NFPA 80A* LIGHT IBC IBC NFPA5000 BR187 BR187 (ENCLOSING (ENCLOSING RECTANGLE) RECTANGLE) IBC BRE BRE ALTERNATIVE ALTERNATIVE METHOD METHOD BR187 (ENCLOSING RECTANGLE) SIMPLE SIMPLE BRE GEOMETRY GEOMETRY ALTERNATIVE METHOD METHOD METHOD (SCOTLAND) (SCOTLAND)

32 Conclusions The BR 187 enclosing rectangle method generally calculates the shortest separation distance to the notional boundary. The BRE s alternative method to the simple geometry method appears to be applicable to all enclosing rectangle assessments providing a result which is suitable for use in practice as a rough assessment, provided a detailed assessment is conducted at a later date. The IBC and NFPA 5000 methods, do not provide background details defining how the methods have been derived. Although the results are generally comparable where a short separation distance is required, this method quickly becomes hazardous where large distances are required. The results of the NFPA 80A assessment provide a good comparison to the UK assessments. However, due to the lower incident radiation used and addition of flame projection, this method will always be more onerous. Although it would be acceptable to use this method in the UK, to avoid legal issues, it is recommended that this method is only used for buildings on the same site.