A Brief on Performance- Based Design for Building Fire Safety

Transcription

1 International Conference on Safety & Security Management and Engineering Technology 2009 (ICSSMET2009) December 2009, Taiwan A Brief on Performance- Based Design for Building Fire Safety Professor W.K. Chow Research Centre for Fire Engineering Department of Building Services Engineering Area of Strength: Fire Safety Engineering The Hong Kong Polytechnic University Hong Kong, China Founding President, Society of Fire Protection Engineers - Hong Kong Chapter President, Asia-Oceania Association for Fire Science and Technology TaiwanDec09A.ppt

2 1. Introduction 2. Need for Performance-based Design (PBD) 3. Performance-Based Design in Hong Kong 4. Common Problems in PBD 5. Needs of Research, Education and Training 6. Conclusions 2

3 1. Introduction Construction industry in the Far East is developing rapidly. There are many new architectural features: highrise, well-sealed, deep plan and frame structure. Fire safety features must be designed to the satisfaction of the local authorities as it is a key area to be considered seriously. This topic has drawn public attention as a consequence to serious accidental fires occurred recently. People started to aware of the hidden problem on fire safety and ask the question: Are they sufficiently protected while staying inside a building? 3

4 Why Fire Protection? Accidental fire Arson fire Terrorist attack fire Natural disaster fire Fire Safety Objectives to be Clarified e.g. Accidental small fire in an empty room without fireload in a building or a train? 4

5 Accidental fire : The Big Garley Building Fire

6 Well-sealed Structure Bus Fire: Burnt completely within 10 minutes 6

7 2. Need for Performance- Based Design 7

8 Many highrise buildings packed together Victoria Harbour Hong Kong 8

9 Over 60 levels for Residential Buildings! Kowloon West The Arch (two 60-storey and two 54-storey residential building with 1,060 flats and a clubhouse at the 40/F) 9

10 Building Characteristics in Hong Kong Highrise building Reinforced concrete Protected structural steel Materials not combustible Lightweight structure : (Thermal expansion and creeping?) High occupant loading 10

11 New Architectural Features Examples Extensive use of glass Provision of natural ventilation Internal building void Good thermal insulation materials Fire safety issues Prescriptive codes Conflicting with environmental aspects 11

12 The prescriptive codes, basically on the fire resistance construction (FRC), the means of escape (MoE) for occupants, the means of access (MoA) for fire fighting, and the fire services installation (FSI), Are they sufficient for providing fire safety in some buildings with special designs? 12

13 Fire Codes in Hong Kong BS5588? MoE Codes MoA Codes FRC Codes FSI Codes Local Codes: most of them failed to satisfy the prescriptive codes. 13

14 Can those codes apply to supertall building? Any research support? Where are research on supertall buildings in the world? Might be OK for these! Compliance with codes? 14

15 IFC Hong Kong : No Big Fires Just a little bit of LUCKS! 15

16 Four Generations of Fire Codes in UK Something set up after the Great London fire, then: Highrise 1960 Shopping Mall 1970/80 Atrium 1980/90 (BS 5588) PBD : 1990/2000 (BS 7974) A method only, not codes! None for supertall Buildings! 16

17 3. Performance-based Design in Hong Kong Fire Engineering Approach since

18 There is no engineering performancebased fire codes (EPBFC) yet. The authorities (i.e. BD) accepted that Fire Engineering Approach FEA for passive fire safety design since PBC design following MoE, MoA, FRC codes New project FSI design following FSI code FEA Design revised Y Submission to BD Submission to FSD Design revised Y FEA? N Opportunities for revision? N Approved? Approved? N Opportunities for revision? Y Evaluation by fire consultant N Y Y N Considered by FSC, BD Design rejected Design rejected Opportunities for revision? N Approved? Y N Design rejected Proceed to construction Proceed to installation Figure 1: Process for approving fire safety designs 18

19 FEA is particularly useful for buildings of special hazards requiring individual design considerations. Common approaches - Deterministic approach: Not yet - Stochastic approach: No - Equivalence to existing codes: Always But doing this without strong scientific fire research for local safety provision is impossible. 19

20 Fire Safety Provisions Authority: - Asking for too much? - Over 20 MW design fire? Client: - Arguing for too little? MW in big hall. After fire: - Who are going to the site? - Who pay for the damage? - Who are the First Respondents. 20

21 Compartmentation [From The Magazine of Engineers, Australian Vol. 79, No. 12, p. 52, Dec 2007] 21

22 No big fires in PBD research in local environment yet! - When? - What happens after this? What will be general public feel if they know some PBD projects was only supported by a 1-page Report! Research is needed! 22

23 Basic Concept in Fire Engineering Approach or Performance-Based Fire Codes EPBFC Hazard Assessment : Physical only, with engineering judgment Scenarios: identification S 1, S 2,, S N Consequence to scenarios: Engineering tools such as fire models are needed. Hazard Assessment + Stochastic Modelling Risk Analysis Risk level R 1, R 2,, R N Example: Risk cost model For each scenario, estimate a cost C i of consequence (Life cost is difficult to estimate) R i = C i Frequency of occurrence 23

24 Performance-Based Design in Elsewhere UK : BS 7974 USA : NFPA 5000 (New standards might not be applicable to others) 24

25 BS 7974 Application of fire safety engineering principles to the design of buildings -- Code of practice Published Documents PD Guide to design framework and fire safety engineering procedures Design approach QDR Comparison with criteria Reporting and presentation PD (Sub-system 1) Initiation and development of fire within the enclosure of origin PD (Sub-system 2) Spread of smoke and toxic gases within and beyond the enclosure of origin PD (Sub-system 3) Structural response and fire spread beyond the enclosure of origin PD (Sub-system 4) Detection of fire and activation of fire protection systems Design approach Acceptance criteria Analysis Data References PD (Sub-system 5) Fire service intervention PD (Sub-system 6) Evacuation PD Probabilistic risk assessment 25

26 Time Critical damage to property /business suppression Available safe egress time (ASET) t esc Escape time t evac t a t pre premovement time Recognition Evacuation time t trav Travel time t det time Response time Tenability limit Ignition Alarm Detection Evacuation Suppression Containment Venting extinguishment Example of the time line comparison between fire development and evacuation/damage to property (For quantitative analysis) 26

27 Others: in USA NFPA 5000: Building Construction and Safety Code, 2006 Edition 27

28 Application of the Fire Engineering Approach in Hong Kong Local approval of fire safety design and inspection of the building upon completion are held responsible by the Buildings Department (BD) and Fire Services Department (FSD). Normally, the building design shall be submitted to the BD to check against all fire aspects for approval; and the requirement and installation of fire protection system shall be determined by the FSD. 28

29 4. Common Problems in PBD Accepted by government, might not be accepted by insurance. Many examples in other places such as Australia, South Korea. Need to do very detailed studies, Life Safety! Not just submitting a 1-page report. Human resource is the key problem. 29

30 A concern to watch Karaoke fire Top One After sorting out criminal cases, there are court cases on challenging the: Building owner Building management Karaoke owner Registered FSI contractor How to pay the compensation? Who will be responsible? The Authority? The Designer? The Owner? Court cases after a fire? 30

31 Top One Karaoke Fire 31

32 Knowledge of fire science and engineering was applied by both FSD and BD in assessing the fire safety designs. Fire models, both zone and field, were applied where necessary. But for Modelling: How reliable a model is? Validation? Verification? Demonstration? Still Arguing in SFPE 2007 Meeting Rubbish IN Rubbish OUT V&V necessary! 32

33 Two Views on V&V: Empty room with a fire V&V Room with combustibles. 33

34 Very few validated data. Fire models and evacuation models are not demonstrated to work. Research is needed, but take a long time to upgrade. Full-scale burning tests to support the argument! Burn an agreed scenario to see! 34

35 Arguments on some hot issues Application of Computational Fluid Dynamics CFD, only for studying the fire induced flow. - No realistic combustion chemistry Orange = Apple? Use of zone model - Simulating smoke movement in a long and narrow corridor. - A two-layer zone model was commonly used for quick estimate. Evacuation model - Studying the evacuation patterns. - The psychological effects of local citizens were also queried. - Field measurements on the evacuation pattern and total evacuation time should require to justify the result. - Compare with fire drills results? 35

36 Design fire and full-scale burning tests - There had been lots of arguments on taking the heat release rate of a design fire as, say 1 MW in an atrium! - The heat release rate has to be assessed in order to answer a common question: How big is a fire? A database on heat release rate should be developed for determining a design fire. Different values were used by the designers for different purposes in the past in designing fire protection systems for terminal halls; shopping malls; atria; and train compartments. The same value was accepted in a project, but rejected in another similar project. This is due to the lack of a database for local products. 36

37 Big fires in an atrium? 37

38 Fire Field (or CFD) Model In the past decade, the technique of computational fluid dynamics (CFD) (or called field modelling technique for the case on fire simulation) has been developed and used to some extent for design purpose. This is good for predicting fire environment in big enclosures such as an atrium or a tunnel. Although the computing time required was very long, the problem has been improved with efficient numerical schemes and fast computers. Three-dimensional simulation can now be executed successfully in Pentium personal computers. 38

39 Can handle 3 Million cells in a pentium. The time required to get the predicted results is now highly shortened because of the development of efficient computing schemes and high speed computers. CPU time for 3 x 10 6 cells only a few days using FDS. That is a great advancement compared with the past years when even a two-dimensional fire simulation would take days of CPU time in a mainframe. 39

40 Field models had been applied to simulate the fireinduced field of flow, temperature and smoke concentrations within an enclosure. At the moment, the flow and temperature field can be simulated successfully by taking the burning object as a heat source. To the best of knowledge, combustion effects of the burning process had not been simulated successfully apart from using the simple chemical reacting system approach, although attempts were made to use the flamelet model to develop a chemistry library, and use empirical formula on the burning rate. 40

41 The field model is a natural convection problem good only for predicting the flow field driven by buoyancy at distance away from the fire source. Flame spread process cannot be simulated realistically without studying the combustion chemistry. But the predicted results are good enough for assessing performance of some fire protection systems. 41

42 The thermal power resulted by a fire in an enclosure is of the order of MW, such as that produced by burning a polyurethane foam sofa. The resultant flow is turbulent with the use of turbulence models. Different approaches were proposed to deal with the problem: Reynolds Averaging of Navier-Stokes Equation (RANS) Large Eddy Simulation (LES) 42

43 Discussion on the Use of Field Models There are many reasons why the field model is so attractive. The predicted microscopic picture on the thermal environment described by the velocity vector diagram, the temperature, pressure and smoke concentration contours are useful for deriving the relevant macroscopic parameters for engineering purposes. The calculated vertical distribution of the air temperature is useful in studying the thermal sensitivities of sprinkler heads. Also, the air entrainment rates for a fire plume can be calculated from the predicted horizontal velocity components. 43

44 The most attractive thing is the graphical presentation of the geometry, temperature fields and velocity vectors. All these are good reasons for promoting the use of CFD technique for fire simulation. However, there are still lots of developmental works and careless application of the technique would lead to getting wrong results. It is obvious that there must be problems in simulating a complicated physical system described by a set of partial differential equations using a set of relatively simple linear equations derived from the finite difference method, or the control volume method for most cases. 44

45 There are at least three parts to consider even for simulating the fire induced air flow in a simple building with rectangular shape: The physical principles behind the turbulence have to be reviewed. Current turbulence closure models might give many parameters for fine tuning. This becomes even more complicated when combustion processes of the burning object or evaporation effects of a sprinkler water spray are studied. The schemes for discretizating the set of equations have to be considered carefully if finite difference method is used. The control volume method together with those different schemes might not be the only solution. The algorithms in solving the system of pressure-linked equations have to be reviewed. 45

46 Treatment of the boundary conditions would be very important. For relatively simple structure like the NBS fire chamber, the computing domains have to be extended to outside for determining the neutral plane height. The orifice coefficient for a vertical vent is an obvious example. Results have to be compared and be consistent with those derived from simple theory. More importantly, experimental work designed for studying field models must be performed for verifying the results, tuning the parameters concerned in the physical equations and deriving new solution methods. 46

47 Selection of parameters. - Physical parameters such as C1, C2, C3, C in RANS k- model; and CS in the sub-grid scale model in using LES such as those appeared in turbulence models; - Numerical parameters for stability and convergence such as relaxation factors, convergence criteria, residuals and time steps; and - Parameters concerned in the boundaries, orifice coefficient of the free openings and how long the computing domain has to be extended to outside from an opening. Values of all these parameters should be selected very carefully, say through numerical experiments on trial and error for simulating different problems. 47

48 5. Needs of Research, Education and Training Research Vs Development Fire physics Fire chemistry Upstream Turbulence Modelling Numerical Works Midstream Fire Engineering Sprinkler Design Downstream Get the occupant permit? Practical world 48

49 The need for accredited, education and training BEng first degree Accreditation by whom? Syllabus: Model Curriculum (MC) by IFASS BEng(Hons) in Building Services Engineering with specialism in fire engineering MC + local needs Fire subjects + projects in BEng in BSE MSc programmes PhD research projects CPD programmes 49

50 6. Conclusions Many questions asked in PBD/FEA. Input database reliable? Hazard assessment tool reliable? Trust of results, demonstrations only? All ABSENT! Education & Training also necessary. 50

51 As raised by Professor Hirano in 7AOFST. Fire safety problems in the Far East should be studied carefully, not necessary same as those in other advanced areas! Hong Kong 51

52 Who will support research on PBD? Government research laboratories: None in Hong Kong yet! University: Budget cut! Professional Body (HKIE: only $150,000 reviewing on all codes (energy, fire, air quality ) Industry - Support if able to solve their problems - Consultancy based - Needs space and resources - Who will run the tests? 52

53 Still depends on Higher Education Institutions Some resources, though not too much: - Laboratory Spaces - Equipment Students: MSc/PhD Staff: Downstream application 53

54 Create a profession! Job market! 54