Highlights of The Code of Practice for Fire Safety in Buildings 2011.pdf

Information Paper

Highlights of

T he C ode of P r acti cti ce for F i r e S afet fety in in B Bui uild ldii ngs 2011

STRUCTURAL ENGINEERING BRANCH ARCHITECTURAL SERVICES DEPARTMENT June 2012

Structural Engineering Branch, ArchSD Information Paper – Paper – CoP CoP for FS in Buildings 2011 Issue No./Revision No. : 1/-

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File Code: FS Code.doc MKL/SCF/CHM First Issue Date : June 2012

1.

Introduction

1.1

Regulations 41(1), 41A, 41B, 41C and 41D in the Building (Planning) Regulations and Regulation 90 of the Building (Construction) Regulations. specify requirements for buildings to be designed in such a way that they exhibit an acceptable level of performance in the event of fire. Over the years, Buildings Department and Fire Services Department have issued the following codes on the performance requirements complying the statutory requirements: a) the Code of Practice for the Provision of Means of Escape 1996 (the “MOE Code”); b) the Code of Practice for Fire Resisting Construction 1996 (the “FRC Code”); c) the Code of Practice for Means of Access for Firefighting and Rescue 2004 (the “MOA Code”); d) the Code of Practice for Minimum Fire Service Installations and Equipment ; and e) the Code of Practice for Inspection and Testing and Maintenance of Installations and Equipment .

1.2

In September 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

The Statutory Compliance Checking Unit (the “SCCU”) of ArchSD has uploaded a soft copy of the FS Code with highlights of the changes onto ArchSD Intranet at the following URL: http://asdiis/cmbiis/cmbiis_a/circulars/SCCU_Corner/Code_of_Practice_f or_Fire_Safety/Code%20of%20Practice%20for%20Fire%20Safety%20in %20Buildings%202011%20(amendments%20highlighted%20in%20color ).pdf The SCCU has further provided detailed comparison the FS Code with the MOE, FRC and MOA together with summary of key points in powerpoint form and recorded videos of the briefing sessions arranged by Buildings Department in the following URL: http://asdiis/cmbiis/cmbiis_a/circulars/SCCU_Corner/Code_of_Practice_f or_Fire_Safety/Code_of_Practice_for_Fire_Safety.htm

1.4

Section 2 of this paper will highlight:

a) the major changes in the FS Code that is most relevant to structural engineers; b) the fire resistance rating of hollow-block floors; and c) a summary of the percentage of reinforcement to be provided for reinforced concrete wall according to the fire resistance rating in the FS Code.

Structural Engineering Branch, ArchSD Information Paper – CoP for FS in Buildings 2011 Issue No./Revision No. : 1/-

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2.

Major Changes in the FS Code

2.1

Fire Resistance Period

2.1.1 In the FS Code Part A, the term “fire resistance period (FRP)” in the FRC Code has been replaced by “fire resistance rating (FRR)”, which means the period of time that a building element is capable of resisting the action of fire. Moreover, FRRs are now 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). 2.1.2 The FS Code Table C1 has revised the FRRs and fire compartment limitations in the FRC Code Table 2, and the FRR criteria for some elements of construction in the FRC Code Table 3. 2.2

Roof Structure

2.2.1 The definition of “element of construction” in the FS Code Part A remains unchanged, and a member forming the roof or part of the roof is not an element of construction. However, in Clause 12.2 of the FS Code (same as Clause 1.31 of the FRC Code) still states that “all roofs, together with the members forming the roof structure, should be constructed of non-combustible materials”, despite in drafting of the FS Code, ArchSD raised our reservation on this clause. 2.2.2 The requirement of non-combustible materials for roof structure imposes an undue burden to those old or heritage buildings with timber pitched roof. ArchSD is currently handling a few such heritage buildings, many of them being with timber truss/purlins/battens supporting roof tiles. The timber truss/purlins/battens cannot meet such “non-combustible” requirement. However, 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 is ensured already. Hence, the requirement of “non-combustible materials” is not necessary. The corresponding clause at the Approved Document B - Fire Safety issued under the UK Building Regulations 2000 does not have such noncombustibility 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. This is a more practical approach especially for those heritage buildings and yet fulfills the objectives of fire safety. 2.2.3 The view of Buildings Department was that for such heritage buildings, fire engineering approach will be appropriate, if it is decided not following the noncombustibility requirement. However, as member forming the roof or part of the roof is not an element of construction, there is no concern on “stability”, “integrity” or “insulation”, whilst the purpose of a fire engineering study, if performed, is to assess the performance of the roof in fire scenario against these criteria and therefore not needed. Further information can be obtained from SEB Guidelines SEBGL-OTH1 Guidelines on the Fire Resisting Construction for Roof Structures (URL: http://asdiis/sebiis/2k/resource_centre/). Structural Engineering Branch, ArchSD Information Paper – CoP for FS in Buildings 2011 Issue No./Revision No. : 1/-

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2.3

FRR of Construction and Building Materials

2.3.1 The FS Code Tables E3-E7 (extract as follows) set out minimum dimensions of different building components for different construction and building materials for different FRR, which are the same as the FRC Code Tables A-F. There are no changes on the minimum thickness (including cover for rc construction) for structural elements (slabs, beams, walls, stairs, columns, etc). 2.3.2 Hollow-block Floors 2.3.2.1 Same as that in FRC Code, the minimum dimensions or cover to reinforcement for hollow-block floor s (or “hollow tile floors”) are not mentioned in the FS Code. However, hollow-block floors were commonly used from the 1950s to 1970s in Hong Kong. They were constructed by placing precast clay or cement sand blocks (Figure 1) on formwork, and concrete is then cast to form ribs spanning in one direction ( Figure 2). Its advantages are its lightweight, the excellent sound insulation and thermal insulation. The clay or cement sand blocks were not usually included in the design, and hence the topping can be very thin (may be of 50mm). Similarly, the width of the ribs can be as small as 50mm. Such floor construction is seldom used nowadays. However, in renovation projects, project officer has difficulties in assessing the adequacy of its FRR, as there is no provision in the FS Code on this specific type of floor construction. Applying the minimum thickness for solid floor construction in the FS Code Table E4, the topping can hardly achieve a FRR of 60 mins (minimum 100mm), and applying the minimum width for beam in the FS Code Table E6, the width of the ribs can again hardly achieve a FRR of 60 mins (minimum 200mm).

Figure 1 Precast Clay or Cement Sand Block

Figure 2 Section of Hollow-block Floor Structural Engineering Branch, ArchSD Information Paper – CoP for FS in Buildings 2011 Issue No./Revision No. : 1/-

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2.3.2.2 It seems that ignoring the contribution of the clay blocks in fire resistance is not a correct assumption, as one of the advantages of hollow-block floors is its good thermal insulation of the hollow blocks. As a matter of fact, the then Building (Construction) Regulations 1985 states that the minimum thickness of the topping (excluding non-combustible finish) for hollow-block floor for FRR of 30 mins, 60 mins, 120 mins and 240 mins to be respectively 65mm, 75mm, 90mm and 125mm. CP 110:Part 1:1972 also specifies the overall depth (excluding non-combustible finish) for such floor for FRR of 30 mins, 60 mins, 120 mins and 240 mins to be respectively 100mm, 110mm, 160mm and 190mm. It further specifies the minimum width of the ribs for FRR of 30 mins, 60 mins, 120 mins and 240 mins to be respectively 50mm, 70mm, 90mm and 125mm. 2.3.2.3 The current provision of minimum dimension for hollow-block floors can be referred to BS 8110-2:1985, which recommends that such type of floor construction can be treated as solid construction in calculating its fire resistance by including the contribution of the cement sand or clay blocks using the effective thickness te given by the following equation: te



h

ξ



t f

where h is the overall actual thickness of slab;  is the proportion of solid material per unit width of slab; and tf is the thickness of non-combustible finish. In some cases, without carrying out detailed measurement of the hollow blocks to calculate , the topping plus the non-combustible finish may already be able to achieve the required FRR. Koon (2010) further considered that in addition to non-combustible plaster, tiles and floor screed may also be included as the cover to the reinforcement. Koon (2010) also recommended the same approach can be used to resolve the requirements of cover for structural elements in other structural forms (e.g. beam-and-slab construction). The following example illustrates the calculation of the effective thickness te: Consider a hollow-block floor with a topping of 75mm, and ribs of width 80mm and depth of 350mm at 500mm c/c as shown in Figure 3. The thickness of the clay blocks is 20mm. There is also cement sand floor screed of 25mm.

Figure 3 Typical Hollow-block Floor


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 = the proportion of solid material per unit width of slab =

1

360  235 500  350

= 0.517

tf = the thickness of non-combustible finish = 25mm h = the overall actual thickness of slab = 350mm Hence, the effective thickness te  h  ξ  t f 

350 

0.517



25 =277mm

For a FRR of 60 mins, the minimum thickness as specified in the FS Code Table E4 is 100mm, and hence the effective thickness of the hollow-block floor slab far exceeds the minimum requirements. Indeed, even ignoring the contribution of the floor screed, the effective thickness of the hollow-block slab is 241mm, which already exceeds the specified 100mm. Alternatively, ignoring the contribution of the clay blocks, the effective thickness of the hollow-block slab including the floor screed is 100mm, which also meets the specified 100mm.


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2.4

Minimum Percentage of Reinforcement in Reinforced Concrete Walls

2.4.1 Table 1 and Table 2 show respectively the recommended thickness and cover of reinforced concrete wall with different FRRs and different percentages of reinforcement as stipulated in the FS Code and BS 8110-2: 1985, and they tally with each other. Structural Engineering Branch, ArchSD Information Paper – CoP for FS in Buildings 2011 Issue No./Revision No. : 1/-

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Table 1: Minimum thickness and cover of rc wall in the FS Code

Reinforced Concrete Wall

Minimum thickness in mm (excluding plaster) for FRR of 240 mins 120 mins 60 mins

(a) Containing not less than 1% of 180 100 75 vertical reinforcement Concrete cover to main 25 25 15 reinforcement (b) Containing less than 1% of 240 160 120 vertical reinforcement Concrete cover to main 25 25 25 reinforcement * where finished with 13mm gypsum plaster on each side, the thickness may be reduced to 100mm (Sources: The FS Code Table E.2)

Table 2: Minimum thickness and cover of r.c. wall in BS 8110

Reinforced Concrete Wall Containing over 1.0% reinforcement Containing with 0.4% to 1.0% reinforcement

Minimum dimensions (in mm) excluding any combustible finish for a fire resistance of: 240 mins 120 mins 60 mins Thickness 180 100 75 Cover 25 25 15 Thickness 240 160 120 Cover 25 25 25

(Sources: BS8110 Table 4.6)

2.4.2 SEB has incorporated the above minimum requirements ( Table 3) in the SEB Standard Drawings no. SD/008 and /009 (URL: http://asdiis/sebiis/2k/MAIN%20DOC/std_draw/index.htm), and Explanatory Notes on the Code of Practice for Structural Use of Concrete 2004 (URL: http://asdiis/sebiis/2k/MAIN%20DOC/info_paper/upload/cop2004_ep.pdf ) further contains the corresponding discussion on the minimum requirements. Project officer should note that the above minimum provisions are conservative, as Eurocode 2 now specifies a smaller minimum thickness ( Table 4) irrespective of the cover and percentage of reinforcement for non-load bearing partition walls.


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Table 3(a): Minimum reinforcement for non-load bearing partition wall

Table 3(b): Minimum reinforcement for load bearing wall

Table 4: Minimum thickness of wall in E urocode 2 Minimum thickness (in mm) for a fire resistance of: 240 mins 120 mins 60 mins 175 120 80 (Sources: Eurocode 2 Table 5.3)

2.5

Fire Engineering Approach The FS Code Part G now replaces APP-87: Guide to Fire Engineering Approach and dedicates a full section providing guidance and methods on using the fire engineering approach. Fire engineering approach (or termed as “Alternative Solution” in the FS Code) is a performance based method, which is to ensure that an “equivalent level” of safety of the building environment is not eroded without following the “Prescriptive Requirements” in the FS Code. For the structural aspects, SEB is now preparing guidelines on the behaviour of fire and the structural behaviour of structural steel, reinforced concrete, composite structure and timber at elevated temperature. Part I of this set of guidelines has been promulgated and is available in SEB Resource Centre (URL: http://asdiis/sebiis/2k/resource_centre/).


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References

Approved document B - Fire safety (2007) (London: NBS/RIBA Publishing) (available: http://www.planningportal.gov.uk/buildingregulations/approveddocuments/part b/; accessed: 2 March 2012) BSI (1972), CP 110:Part 1:1972 - Structural Use of Concrete: Code of Practice for Design and Construction (London: BSI). BSI (1985), BS 8110-2: 1985 - Structural Use of Concrete: Code of Practice for Special Circumstances (London: BSI). Koon, C M (2010), “Structural Appraisal of Reinforced C oncrete Buildings with Historic Values,” Presented at Seminar on Concrete Damage Assessment, Concrete Repair and Concrete Mix Technology, Hong Kong, China, 2 February 2010.


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Highlights of The Code of Practice for Fire Safety in Buildings 2011.pdf

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