Guidelines on Fire Resisting Construction for Roof Structures

SEB GUIDELINESSEBGL-OTH1 Guidelines on the Fire Resisting Construction for Roof Structures (2nd Edition) STRUCTURAL ENGINEERING BRANCH ARCHITECTURAL SERVICES DEPARTMENT July 2013 Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/- Page 1 of 8 File code : SEBGL-OTH1 - 1 -Page 1 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013 Contents 1. 2. 3. 4. 5. 6. Introduction .................................................................................................................. 3 Roof not an Element of Construction ......................................................................... 4 Protection of Adjoining Buildings ............................................................................... 5 Single-storey Building .................................................................................................. 6 Other Cases Requiring Fire Resisting Construction ........................................... 6 Discussion on Non-combustible Roof ......................................................................... 6 References Copyright and Disclaimer of Liability This Guideline or any part of it shall not be reproduced, copied or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without the written permission from Architectural Services Department. Moreover, this Guideline is intended for the internal use of the staff in Architectural Services Department only, and should not be relied on by any third party. No liability is therefore undertaken to any third party. While every effort has been made to ensure the accuracy and completeness of the information contained in this Guideline at the time of publication, no guarantee is given nor responsibility taken by Architectural Services Department for errors or omissions in it. The information is provided solely on the basis that readers will be responsible for making their own assessment or interpretation of the information. Readers are advised to verify all relevant representation, statements and information with their own professional knowledge. Architectural Services Department accepts no liability for any use of the said information and data or reliance placed on it (including the formulae and data). Compliance with this Guideline does not itself confer immunity from legal obligations. Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/Page 2 of 8 File code : SEBGL-OTH1 - 2 -Page 2 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013 1. 1.1 Introduction Should roof structures, for instances steel roof trusses at an Indoor Recreational Centre or a timber pitched roof at an adaptive re-used old building, require fire resisting construction? Buildings shall be constructed so that, in case of fire (Whyte and Powell-Smith 1982): (a) (b) (c) they will resist collapse for a sufficient period of time to allow evacuation of the occupants; the occupants are able to reach a place of safety; the spread of fire within and between buildings is kept to a minimum . 1.2 The first requirement is met by providing adequate fire resisting construction to the elements of construction of the buildings. The second is met by providing adequate means of escape. The third is dealt by compartmentation within the building and measures for protection of adjoining buildings. In Hong Kong, in order to ensure compliance of the fire safety requirements, in particular the above ones, under Regulations 41(1), 41A, 41B, 41C and 41D in Building (Planning) Regulations and Regulation 90 of Building (Construction) Regulations. Buildings Department published the following three code of practice over the years: (a) the Code of Practice for Fire Resisting Construction 1996 (the “FRC”); (b) the Code of Practice for the Provision of Means of Escape in Case of Fire 1996 (the “MOE”); and (c) the Code of Practice for Means of Access for Firefighting and Rescue 1995 (the “MOA”). In 2011, Buildings Department issued the Code of Practice for Fire Safety in Buildings 2011 (the “FS Code”), which consolidates and replaces the requirements of the MOE Code (now Part B of the FS Code), the FRC Code (now Part C of the FS Code) and the MOA Code (now Part D of the FS Code). 1.3 1.4 Fire resisting construction of an element of construction is measured by the fire resistance rating (FRR), which is the degree of resistance, in terms of period of time, against one or more of the following criteria, depending on the type of element or components, as per Table C2 of the FS Code: 1) 2) 3) stability, i.e. to avoid collapse of load-bearing elements (Figure 1(a)), integrity, i.e. to resist fire penetration and inhibit spreading (Figure 1(b)); and insulation, i.e. to prevent transfer of excessive heat such that the unexposed surface of a fire resistant construction should not be heated excessively and cause further ignition (Figure 1(c)). FRRs are designated by three terms, to represent the makeup of the element of construction, i.e. X/Y/Z, where X is stability fire resistance rating (minutes), Y is integrity fire resistance rating (minutes), and Z is insulation fire resistance rating (minutes). Table C1 of the FS Code gives the required values of the FRR of the element of construction according to the use class of the building and the compartment volume considered. Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/Page 3 of 8 File code : SEBGL-OTH1 - 3 -Page 3 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013 Figure 1 Failure modes of construction elements during fire (Source: Wang 2002) 2. 2.1 Roof not an Element of Construction “Element of construction” is defined in the FS Code Part A Section 3 as: (a) any floor, beam, column or hanger; (b) any load-bearing wall or load-bearing member other than a member forming the roof or part of the roof; (c) any required staircase including the landings and supports thereto. It is clear from above that the “element of construction” does not include member forming the roof or part of the roof. Therefore a roof is not considered as an “element of construction” and there is no fire resisting construction requirement for it. This interpretation is applicable irrespective of the compartment volume bounded by the roof. 2.2 It is also noted that the FS Code mainly follows the old UK Building Regulations 1976: Part E in which it stipulates: “Element of structure means(a) any member forming part of the structural frame of a building or any other beam or column (not being a member forming part of a roof structure only); (b) a floor including a compartment floor, other than the lowest floor of a building; (c) an external wall; (d) a separating wall; (e) a compartment wall; (f) structure enclosing a protected shaft; (g) a loadbearing wall or loadbearing part of a wall; and (h) a gallery.” Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/- Page 4 of 8 File code : SEBGL-OTH1 - 4 -Page 4 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013 2.3 The present UK’s Building Regulations 2010 (in fact, the last few editions) is even clearer. Paragraph 8.4 of the Approved Document B-Fire Safety issued under the UK Building Regulations 2010 excludes, among other items, “structure that only supports a roof” from the definition of “element of structure”. The difference with the FS Code is that the condition “ONLY” is added to the exclusions for “element of structure” so as to differentiate from cases where: “the ( roof ) structure is essential for the stability of an external wall which needs to have fire resistance.” The case is particularly relevant to buildings having so called “boundary conditions”, which means that the buildings are situated relative to the site boundary in such a way that the walls are serving as a barrier to prevent the spread of fire to adjacent properties. Where a column is built into or supports a boundary wall, it shall have the same degree of fire resisting construction as that required for the wall. If the roof beams/rafter is an integral part of the column, as in the case of portal frame, one may consider that the roof beam is not only forming part of a roof structure but also serves to provide portal action for the column, fire resisting construction has then to be considered. However, if the foundation and its connection to the columns of the portal frame have been designed to transmit the overturning moment caused by the collapse of roof rafters in a fire, while allowing the external wall to continue to perform its fire barrier function, only the columns & walls, not the roof rafters, require to be fire protected. Details of the calculation method can be found in Fire and steel construction: the behaviour of steel portal frames in boundary conditions published by SCI (Newman 1990). This design method is also referred by Clause 8.5 of BS 5950: Part 8 (BSI 2003). 3. Protection of Adjoining Buildings The FS Code has the following requirements: (a) Clause C5.2: Buildings on the same site are regarded as adjoining buildings if they are less than 1.8 m apart. To ensure protection against spread of fire to adjoining buildings, any part of any such building within 1.8 m of any such adjoining building should be enclosed by imperforate external walls and roofs having the same FRR as that of the internal elements of the construction. Clause C5.3: Any part of a building within a distance of not more than 900 mm of a common boundary with an adjoining site should be enclosed by imperforate external walls and roofs having the same FRR as that of the internal elements of the construction. (b) It should be noted that only portion of the roof needs fire resisting construction even for protection of adjoining buildings. However, this portion of fire resisting construction includes roofing material (e.g. metal sheeting), purlins as well as any truss/beam elements. Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/- Page 5 of 8 File code : SEBGL-OTH1 - 5 -Page 5 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013 4. Single-storey Building For free standing single-storey buildings that setback from site boundary or adjoining buildings, Clause C4.3 of the FS Code allows the steelwork construction to be unprotected if the building does not exceed 7000m3 in volume and 7.5m in height, and the setback from site boundary or adjoining buildings is by not less than 6m width of clear unobstructed open space. 5. Other Cases Requiring Fire Resisting Construction There are, however, the following cases where the roof members require fire resisting construction: (a) (b) (c) (d) roof forming part of an exit route as described in Clause C12.4 of the FS Code. roof performing the function of a floor, e.g. vehicle parking or accessible roof garden. roof used or intended to be used as a refuge floor or part of a refuge floor, should have an FRR of not less than that of the storey below (Clause C12.3). for single-staircase buildings, if the level of the highest floor is more than 13m above ground level, the roof, though might not be designed as a refuge floor, should still have flat area for refuge purpose as stipulated in Clause B6.1(d) of the FS Code and the roof should have an FRR of not less than that of the storey below as per Clause C12.2 of the FS Code. 6. 6.1 Discussion on Non-combustible Roof Clause C12.1 of the FS Code stipulates that “all roofs, together with the members forming the roof structure, should be constructed of non-combustible materials complying with Part E [of the FS Code].” Part E of the FS Code deems materials to be non-combustible if they can comply with one of the following the tests on noncombustibility: a) b) c) BS 476: Part 4 (BSI 1970), BS EN 13501-1 (BSI 2007) and BS EN ISO 1716 (BSI 2010), or BS EN ISO 1182 (BSI 2010a). 6.2 Clause C12.1of the FS Code indeed causes much difficulty for timber roof. The corresponding clause of the Approved Document B-Fire Safety issued under the UK Building Regulations 2010 does not have such non-combustibility requirement on the roof structure but rather control the use near a relevant boundary of roof coverings to give adequate protection against the spread of fire over them. It should be noted that the roof covering is used to describe constructions which may consist of one or more layers of material, but does not refer to the roof structure as a whole. For instance, the roof covering with slates or tiles supported by timber rafters can be used without subject to minimum clearance distance from relevant boundary, if such covering has Performance Designation “AA (National Class)” or “BROOF(t4) (European Class)” The Performance Designation is defined in Appendix A to the Approved Document B-Fire Safety, which includes BS 476: Part 4 (BSI 1970), BS 476: Part 11 (BSI 1982) and BS EN 13501-1 (BSI 2007). Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/- Page 6 of 8 File code : SEBGL-OTH1 - 6 -Page 6 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013 Similarly, the US NFPA 220: Standard on Types of Building Construction (NFPA 2012) only requires non-combustible construction for all structural elements (including roof) for Type I buildings, which are usually high-rise and/or large sized. Most of timber roof structures being found in historical buildings maintained by ArchSD are Type V construction (Figure 2), where non-fire rated construction is allowed. This is a more practical approach especially for those heritage buildings and yet fulfils the objectives of fire safety. Figure 2 Typical Type V construction (Source: http://online.missouri.edu/) 6.3 It can be argued that, as long as the requirements of Clause C5 (Prevention of Fire Spread between Buildings) of the FS Code are satisfied, there is a control of the spread of fire and the protection of adjoining buildings. Hence, the requirement of “non-combustible materials” is not necessary. As the requirements of noncombustible construction has been specified by the FS Code, the Practice Guidebook on Compliance with Building Safety and Health Requirements for Adaptive Re-use of and Alteration and Addition Works to Heritage under the Buildings Ordinance issued by Buildings Department (BD 2012) clarifies this requirement by requiring the timber members for roof (similar to timber staircase used as a means of escape within fire resisting walls) to be protected with fire retardant treatment to achieve a Class 1 surface spread of flame when tested in accordance with BS 476: Part 7 (BSI 1977). Project officer should therefore note that full fire-rated protection in the form of intumescent coating, fire protection boards or sprayed coating are not required for timber roof. Such fire retardant paint to BS 476: Part 7 has been specified in Clause 21.14 of General Specification for Buildings 2012, and if fire retardant is considered necessary, project officer should follow this clause. The cost of the fire retardant paint is about $27.80/m2 on new surfaces and $31.00/m2 for redecoration works according to the Schedule of Rates Schedule of Rates for Term Contracts for Building Works (2010 Edition) published by ArchSD. Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/- Page 7 of 8 File code : SEBGL-OTH1 - 7 -Page 7 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013 References BSI (1970), BS 476: Part 4: Fire tests on building materials and structures – Noncombustibility test for materials (London: BSI). BSI (1982), BS 476: Part 11: Fire tests on building materials and structures – Method of assessing the heat emissions from building materials (London: BSI). BSI (1987), BS 476: Part 7: Fire tests on building materials and structure - Method of test to determine the classification of the surface spread of flame of products (London: BSI). BSI (2003), BS 5950: Part 8: Structural use of steelwork in building - Code of practice for fire resistant design (London: BSI). BSI (2007), BS EN 13501-1: Fire classification of construction products and building elements - Classification using data from reaction to fire tests (London: BSI). BSI (2010), BS EN ISO 1182: Reaction to fire tests for products - Non-combustibility test (London: BSI). BSI (2010a), BS EN ISO 1716: Reaction to fire tests for products - Determination of the gross heat of combustion (calorific value) (London: BSI). Buildings Department (2011), Code of Practice for Fire Safety in Buildings 2011 (Hong Kong: Building Authority). Buildings Department (2012), Practice Guidebook on Compliance with Building Safety and Health Requirements for Adaptive Re-use of and Alteration and Addition Works to Heritage under the Buildings Ordinance (Hong Kong: Buildings Department) (available: www.bd.gov.hk/; accessed: 23 July 2012). Davis, L. (1992), Guide to the Building Regulations 1991 for England and Wales (Oxford: Butterworth Architecture). Department for Communities and Local Government (2013), Approved document B - Fire safety (London: NBS/RIBA Publishing). Elliott, D.A. (1983), An introduction to the fire protection of steel (Croydon: Constrado). Newman, G.M. (1990), Fire and steel construction: the behaviour of steel portal frames in boundary conditions (Ascot: SCI). NFPA (2012), NFPA 220: Standard on Types of Building Construction (Quincy, MA: National Fire Protection Association) (available: www.nfpa.org; accessed: 20 July 2012). Wang, Y.C. (2002), Steel and composite structures, behaviour and design for fire safety (London: Spon Press). Whyte, W.S. and Powell-Smith, V. (1982), The Building Regulations: Explained and illustrated for residential buildings (London: Crosby Lockwood Staples). Structural Engineering Branch, ArchSD Guidelines on the Fire Resisting Construction for Roof Structures Edition No./Revision No. : 2/- Page 8 of 8 File code : SEBGL-OTH1 - 8 -Page 8 of 8 KTL/CTW/MKL First Edition: January 2008 Second Edition: July 2013