case study for · analysis of possible fire scenarios • positon a • these dead-ends made smoke...

Case Study for Performance-Based Design

Edgar C.L. Pang, C.H. Cheng, Arthur K.K. Wong, N.K. Fong and W.K. Chow

Research Centre for Fire EngineeringDepartment of Building Services Engineering

The Hong Kong Polytechnic University Hong Kong, China

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Abstract

• A performance-based fire safety strategy report is presented in this case study under Hong Kong practice for a building with internal inter-connected spaces of mixed-use, consisting of carparks on the lower floors and office space on the upper floors.

• Fire Engineering Approach in Hong Kong has been applied and results showed that the tenability criteria can be maintained for occupants to evacuate.

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Content• Introduction• Assumptions Made and Related Estimations• Requirements from Local Codes• Design Fires for Carpark• Analysis of Possible Fire Scenarios• CFD Simulations on Fire Scenarios• Discussion / Conclusion

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Introduction

• Case Study 1 – Underground Carpark was selected to study in this report. The given objectives and some background information of the building are as follows:

• Objectives of the case study• Safeguard occupants from injury due to fire and smoke until they reach a safe

place. • Safeguard fire fighters while performing rescue operations or attacking the fire. • Limit the threat to the structure.

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Introduction• Background information of the underground carpark

• Two story underground carpark under office or residential building.• Maximum dimensions: 133 (L) x 129 (W) m (details as shown in Figure

1 with the corresponding measurements in cm).• Net floor area: around 14,000 m2.• Floor to ceiling height: 2.9 m with smooth ceiling.• Only one layout plan is given without any indication for the main

entrance/exit of the carpark.• Except the main entrance/exit of the carpark, 6 staircases were

indicated in Figure 1 with 1425 mm in width.• Two design options: with and without sprinkler system for the carpark.

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Assumptions Made and Related Estimations

• Since there is no clear indication for the up and down or in and out along the driveway for the ease of accessing the usable floor areas of each floor, the travel distance and the smoke flow patterns through the floor openings.

• Figure 1 is modified as Figures 2 and 3 as the basis for formulating this case study report incorporating the following assumptions.

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• Estimated floor areas for basement 1 (B1) and basement 2 (B2) are both 14,000 m2.

• Ground floor (G/F) is also added in Figure 4 to indicate the communication and the connection of related facilities between B1 and G/F.

• Floor to floor height is taken to be 3150 mm with 250 mm slab thickness to fit in proper number of steps for staircases.

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• The slope of ramps and the bends at both ends of the ramps connecting B2 to B1 and B1 to G/F is assumed to be 1:10 as far as possible.

• The spaces under the ramps at B2 are used as plant rooms.• The spaces under the half-landings and staircases at B2 are

used as refuge places.• Unless indicated in Figures 2 to 4, there is no marking for lanes

and allowing by two way travel especially for dead ends.

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Requirements from Local Codes• Two codes should be complied in Hong Kong with as listed below:

• Code of Practice for Fire Safety in Buildings 2011 (FS Code) [1]• Codes of Practice for Minimum Fire Service Installations and Equipment and

Inspection, Testing and Maintenance of Installations and Equipment April 2012 (FSI Codes) [2]

• The corresponding requirements are listed in the following sections:• 3.3 Classification and Requirement from FS Code• With reference to Subsection A7 of FS Code, the proposed use falls into

group 7 classification and the corresponding definition is highlighted in Appendix A.

• 3.3.1 Requirement of Means of Escape• Based on Subsection B4 of FS Code as shown in Appendix B, the occupancy factor

is 30 m2 /person and the corresponding occupant load for each floor is 467 person with a total of 934 persons for the whole carpark.

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Requirements from Local Codes• According to Table B4 from FS Code as shown in Appendix C, • it is required to have two exit routes for this carpark which

should be not less than 1.05 m in width with 3 m minimum total width while six exit routes which are not less than 1.4 m in width with total width more than 8.5 m are provided in Figures 2 and 3. The given design complies with the exit width requirement of FS Code.

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Requirements from Local Codes• There are dead-ends at the lower right hand corner in Figures

2 and 3 and • the maximum travel distance from the dead-ends to the escape

route is much larger than the requirement of 18 m as stated in Subsection B11 of FS Code as shown in Appendix D. The other parts of the carpark having more than one direction of escape also do not comply with the travel distance requirement of 36 m as highlighted in Appendix D.

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Requirements from Local Codes• The discharge values of each given staircase are 315 and 585

and the total discharge values of 6 staircases are 1890 and 3150 for non-sprinkler protected and sprinkler protected building respectively according to Subsection B12 of FS Code in Appendix E.

• Even though the given discharge values should be multiplied by 0.8 for going upwards based on clause B12.6 of FS Code in Appendix E which are much greater than the total occupant load of this carpark.

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Requirements from Local Codes• Requirement of Fire Resisting Construction• With reference to Table C1 from FS Code in Appendix F, 60 minutes

fire rating is required with maximum floor area 10,500 m2 for group 7 classification carpark but 240 minutes fire rating is further required for the construction elements of basement according to Clause C14.1 from FS Code in Appendix F.

• The wall thickness in Figure 1 is 250 mm which can provide 240 minutes fire rating if constructed by solid bricks of clay, concrete or sand lime without plaster as stated in Table E2 in Appendix F.

Also, 120 minutes fire rated fire shutters should be installed at ramps connecting upper floors and within each floors in order to comply with the maximum floor area requirement.

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Requirements from Local Codes• Further to fire rating and compartmentation mentioned above,

smoke vent or smoke extraction system should be provided for basement based on Clause C14.2 to C14.4 of FS Code in Appendix G.

• Since the locations of smoke vents and fire shutters would affect possible smoke flow patterns which would be marked after detailed CFD simulations in later sections.

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Requirements from Local Codes• Requirement of Means of Access•• According to Table D1 from FS Code in Appendix H, there are

non-compliances on having a “fireman’s lift” and “firefighting and rescue stairway” every 60 m at lower left region of Figure 1 which the same area of non-compliance is mentioned in section 3.3.1.

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Requirements from Local Codes• Requirement from FSI Codes• With reference to FSI Codes, this carpark is classified as

“basements with total area exceeding 230 m²” and “garage”.

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Requirements from Local Codes• On top of fire shutters, smoke ventilation systems, fireman’s lifts and firefighting and

rescue stairway mentioned in previous sections, the following fire services installations are required that some provisions can be excepted in accordance with the statements extracted from FSI Codes in Appendix I [2].

• Emergency lighting• Exit sign• Fire alarm system• Fire detection system• Fire hydrant/hose reel system• Portable hand-operated approved appliance• Sprinkler system• Static or dynamic smoke extraction system• Ventilation/air conditioning control system

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Requirements from Local Codes• Therefore, sprinkler system should not be an option but

compulsory for this carpark in Hong Kong. “Automatic actuating devices” have to be installed as an alternative for no sprinkler system design in which automatic foam systems could be one of the choices.

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Requirements from Local Codes• Summary for Non-compliances to Local Codes and Related

Problems• The non-compliances found in the given layout are mainly due to

the dead-end region around the lower right corner of Figure 1 where there is no fireman’s lift with long travel distance for evacuees and firemen as mentioned in previous sections.

• Due to the large plan area, proper allocation of smoke extraction points under ceiling becomes the key point in this project in order to manifest the efficacy of smoke control with the help of fire shutters for compartmentation.

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Design Fires for Carpark• Assumption

• Since this carpark is used by the occupants in the commercial or residential units above, no heavy goods vehicles or similar high fire load items would be allowed on top of the limitation due to low headroom of this carpark.

• The fire sizes of vehicles under consideration are reviewed in two ways – from literature and local fire scene before the final selection of the design fire size for the fire scenes in this case study.

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Design Fires for Carpark• Fire Sizes from Literature

• There is no fire size specified in the local code but it refers to other overseas design guides and references [1] with a presumption that sprinkler system is installed to control the fire growth confined within a vehicle.

• From NFPA 502, the design fires for passenger car and multiple passenger car are in the range of 5 MW to 20 MW [3] while the heat release rate (HRR) curves from some overseas experiments [4] are shown in Appendix J as reference which are quite similar to the possible fire scenes in this case study. Both oversea references are consistent.

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Design Fires for CarparkFire Sizes from Local Fire Cases

The local code claims that the fire size for free burning vehicle should be similar to a burning vehicle with sprinklers [1].

Therefore, the design fire sizes can be estimated by reviewing flame size [5] based on local vehicle fire cases.

A private vehicle was burned down in 8 minutes on 3 July 2015 [6-9] and the corresponding calculated fire size is about 8 MW with reference to the flame size evaluated from the photos of local newspapers.

A similar situation happened again on 25 August 2015 [10].

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Design Fires for Carpark• Another LPG taxi exploded in a garage on 27 April 2015 and

caused a fire claiming lives and damages to a building [12]. • Although there is no picture released about the flame size to

estimate the possible fire size for that case, the fire size is large enough to abandon a six-storey building.

• Such fire cases [13,14] may be due to improper maintenance but those vehicles or fire cases should be included in this case study since there is not obvious “identification” to screen out them not being driven into the carpark under consideration.

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Design Fires for Carpark• Fire Sizes Selected• With reference to Sections 4.1 and 4.2, the fire sizes for the

cases with and without sprinkler are discussed here. • The minimum fire size is 5 MW and another selected fire size is

8 MW for the case with sprinkler in operation according to the specified statement in the local code.

• 20 MW is the minimum value for the case without sprinkler.

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Analysis of Possible Fire Scenarios• Selection of Fire Scenarios• There are four fire locations a, b, c and d selected at the two

carpark floors B1 and B2, giving 8 locations totally labeled as S1a, S1b, S1c and S1d at B1 and S2a, S2b and S2c at B2 in Figure 5 respectively.

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Analysis of Possible Fire Scenarios• Positon a • These dead-ends made smoke and heat easily accumulate and build

up than elsewhere in the basement. • Hence, the structural stability and the tenability limits would be

violated faster and the prompt action and proper functioning of the smoke exhaust system would be very crucial.

• Also, the exhaust points of smoke management system should not have any adverse effect on the evacuees and firemen, because it is just next to two vehicular accesses and escape routes of the carpark at ground level.

• Smoke can be contained and reenter the carpark even though smoke can exhaust through underground ducts in short distances.

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Analysis of Possible Fire Scenarios• Position b• The only evacuation points and vehicular accesses for the dead-ends

would be blocked and people could be trapped there without any help from outside.

• Therefore, proper fire safety provisions should be installed to keep the dead-ends to be tenable before the fire is put out or some portable devices and personal protective equipment (PPE) should be provided to keep people to survive or even escape from the fire scene.

• However, the capability of sufferers and the corresponding impacts should also be taken into account especially for infants and the elderly.

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Analysis of Possible Fire Scenarios• Position c• For this deepest planned position in the carpark area, smoke can spread

out under ceiling but dragged down over certain distance due to loss of buoyancy which would confine the visibility of the evacuees and firemen.

• Proper pressure profiles for various scenarios to extract the smoke to maintain the visibility become essential while it is not possible to have smoke exhaust duct installed vertically in driveways nor parking spaces.

• Thus, main smoke exhaust ducts must locate under the slab and turn vertically at the clearance alongside the boundary of the carpark.

• The corresponding losses in air ducts or buoyancy should be studied clearly in order to achieve the expected pressure profiles.

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Analysis of Possible Fire Scenarios• Position d • This is near the center of the carpark and smoke and heat

easily accumulate and build up affecting the occupants escaping from the dead ends.

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Analysis of Possible Fire Scenarios• With and Without Sprinkler• As mentioned in Section 3.2, it is a must to have sprinkler or

similar automatic firefighting provisions for the carpark under consideration and the concerns become:

• how to promote the stability of smoke layer for the ease of extraction and minimize the possible draw backs due to accidental discharge.

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Analysis of Possible Fire Scenarios• Stability of Smoke Layer • According to local regulations, sprinkler system is a must unless with

some alternative provisions as mentioned in Section 3.2 and the instability of smoke layer due to the dragging effect from discharging firefighting agent is unavoidable.

• Hence, the fire should be effectively controlled or suppressed by the fighting agents with minimum the discharge locations and/or dragging force.

• One of the ways to limit the discharge locations is by zonal application which numerous zones are designed within the premises and the firefighting agents are controlled by detectors and discharge valves.

• Such design would detect a fire faster than traditional sprinkler system and the region of discharge are limited by earlier suppression before the fire further developed.

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Analysis of Possible Fire Scenarios• Accidental Discharge• On top of using cross-zone detection criteria to enhance the

reliability of detection system, the damage is subjected to how small the zone is and the size of each zone is as small as a car.

• Although there would be hundreds or even thousands of zones, it can minimize the disturbances to the adjacent areas and easier to resume normal operation.

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CFD Simulations on Fire Scenarios • CFD Studies• This is a report on studying fire incidents in a two-storey

underground carpark under six fire scenarios with fires at different locations (S1a, S1b, S1c, S1d, S2a, S2b, S2c, S2d each referring to one fire location) using the Fire Dynamics Simulator version 6.0

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CFD Simulations on Fire Scenarios • Model• The two floors of the carpark are simulated separately, with

their inter-connections (car pathways, staircases) taken as boundary conditions in the simulation.

• Otherwise the two floors are assumed to be independent. The original floor plan of the carpark is given in Figure 6.1.

• The location of fires S1a, S1b, S1c, S1d (floor B1) and S2a, S2b, S2c, S2d (floor B2) are also shown in Figure 6.1.

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CFD Simulations on Fire Scenarios • Simulation of the fire scenario arising from a burning car on level B1

(upper floor of a 2-storey underground carpark) is carried out using FDS. • For simulation purpose, the carpark is taken as composed of three

rectangular boxes. • All boxes are of 2.56 m in height but the cross-sections are of different

dimensions. • The first box has a cross section of 128 m in length and 72 m in width, the

second 64 m in length and 40 m in width and last one is of 128 m in length and 16 m in width.

• The geometry of the carpark is shown in Figure 6.2.

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CFD Simulations on Fire Scenarios • A Cartesian coordinates system is assigned with the x, y and z

directions shown in Figure 6.2, with the x-y plane on the floor level. The first box is divided into 144, 256 and 16 cells in each of the x, y and z directions, the second, 80, 128 and 16 cells and the third, 32, 256 and 16 cells.

• Carpark layout with exit locations is shown in Figure 6.3.

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• Summary of CFD results• The selected results at location S1d are shown from Figures 6.4

to 6.14, while those at locations S1a, S1b and S1c are shown from Figures 6.15 to 6.17.

• As a demonstration, the activation times corresponding to fire scenarios S1a for the smoke detectors, heat detectors, and sprinklers are given in Table 6.2a, 6.2b and 6.2c, respectively.

• A summary of the important values for temperature, smoke pattern and smoke velocity simulation is shown from Tables 6.3 to 6.7.

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CFD Simulations on Fire Scenarios • Remarks • On both floors B1 and B2, the highest temperature reached (350°C)

comes from fire source at the “dead-end”, that is, at S1a and S1b. The corresponding temperature (170°C) is lower for fire source at S1c and S2c, which are located near the centre of the floor. This is because the “dead-end” is more obscured.

• The maximum smoke velocity is around 4 m/s for fire sources in Box 3 on both floor, but is lower when the fire source is located near the centre of B1 or B2. This because the limited freedom of smoke movement for fire in Box 3 speeds up smoke movement.

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CFD Simulations on Fire Scenarios • When the fire source is located in Box 3, the spreading of

smoke to other boxes take more time than when the fire source is located near the floor centre.

• The temperature and smoke field are essentially the same for both floors. The differences between scenarios S1a, S1b, S2a, S2b are small, where the fire sources are located in Box 3. When the fire source is near the floor centre (S1c, S2c), the highest temperature reached is lower.

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• A series of simulations including both fire and evacuation at fire location S1d are also conducted.

• The details of fire size, fire location, grid size and evacuation grid settings are shown in Table 6.4 to Table 6.6.

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Discussion

• Besides simulating the temperature and smoke spread inside the carpark, Fire Dynamics Simulator with Evacuation: (FDS + Evac) [21] is also able to simulate the evacuation movement of people from various locations within the carpark. This is very important in assessing the required safe egress time (RSET) in the carpark. FDS + Evac allows the user to create a 3-D model of a building by using a number of CAD- designed floor plans connected by staircases.

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Discussion

• The user defines ‘final’ exit of the design routing, and the software will automatically calculate all travel distance and route throughout the carpark space.

•• The body dimensions and the unimpeded moving speeds of the

default population types in FDS + Evac are shown in Table 7 •

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Discussion

Body type Rd (m) Rt/Rd Rs/Rd dt/Rd Speed (m/s)

Adult 0.255 ± 0.035 0.5882 0.3725 0.62751.25

±0.30

Male 0.270 ± 0.020 0.5926 0.3704 0.62961.35

±0.20

Female 0.240 ± 0.020 0.5833 0.3750 0.62501.15

±0.20

Child 0.210 ± 0.015 0.5714 0.3333 0.66670.90

±0.30

Elderly 0.250 ± 0.020 0.6000 0.3600 0.64000.80

±0.30

Table 7: Unimpeded moving speeds of the default population types [21]

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Discussion

• In this model, the percentage distribution of different body types is as follows:

•• 100 % Adult (50% male and 50% female).• Total number of occupants on each floor considered in the simulation is

based on Table B1 in the Code of practice for Fire Safety in Buildings 2011.

• Based on the carpark design and staircase as shown in Figure 6.3.

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Discussion

• Simulation results are:• The evacuation simulation results are generated in the form of

snapshot files to provide a better understanding of the simulation as shown from Figures 7.1 to 7.3.

• Discharge time • The time required for the last occupant to pass through the exit and

entering into the staircases. Total time for evacuation for 467 occupants is approximately 125 s.

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• The provision of a safe design of means of escape system is to allow the occupants to escape from the fire area to a place of safety under a tenable condition.

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Discussion• The required safe egress time (RSET) is composed of a number of

components including the detection time, pre-movement time and evacuation time based on PD7974-6 [15]. RSET is given by:

• tRSET = ∆tdet + ∆ta+(∆tpre + ∆ttrav) (1)• where

• ∆tdet is the time from ignition to detection by an automatic system or the time for the first occupant to detect fire cues;

• ∆ta is the time from detection to a general alarm;• ∆tpre is the pre-movement time for the enclosure or building occupants;

and• ∆ttrav is the travel time of the enclosure occupants or building occupants.

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Discussion

• In the current simulation, the total required safe egress time (RSET) is based on the assumptions that ∆tdet + ∆ta+∆tpre = 40 s and the calculated ∆ttrav.

• The results for different number of occupants inside the carpark are shown in Figure 7.1.

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Discussion

• Based on the simulation results, it is found that occupants may be trapped at the dead end location if the fire occurs in S1b, S1d, S2b and S2d location.

• A protected corridor is proposed to provide an alternative path for the occupants in the dead end location as shown in Figure 7.4.

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Discussion

• For the alternative design solution with fire curtains, the hot air plume from the fire compartment cannot affect the upper floor of the carpark.

• With the installation of sprinklers, the fire size can be limited to 5 MW or below. Then the ASET is greater than RSET.

• However, there are concerns [16,17] in using such timeline analysis.

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Conclusion

• Fire Engineering Approach has been applied and results showed that the tenability criteria can be maintained for occupants to evacuate.

• However, there are concerns about such approach as reported• Occupants will have sufficient time for evacuation i.e. ASET

longer than RSET when a longer pre- movement time for non-fire floor is taken as 1 minute in accordance with PD7974-6

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Conclusion

• The entire analysis has incorporated various safety factors which include the following:

• The fire size is calculated from ultra-fast fire growth.• Warning system with flashlights is used to alert the occupants for

reducing the RSET.

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Conclusion

• Fire Safety Management is important too• Being a carpark, an inspection, maintenance and testing of the fire

safety systems will be conducted at least once every 12 months. Clear signage must be provided to indicate the exit and dead end locations.

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References• [1] Buildings Department, Code of Practice for Fire Safety in Buildings

2011 (October 2015 version), Hong Kong Special Administrative Region.• [2] Fire Services Department, Codes of Practice for Minimum Fire

Service Installations and Equipment and Inspection, Testing and Maintenance of Installations and Equipment, April 2012, Hong Kong Special Administrative Region.

• [3] NFPA 502, Standard for Road Tunnels, Bridges, and Other Limited Access Highways, 2014 Edition, National Fire Protection Association.

• [4] C. Mayfield and D. Hopkin, Design Fires for Use in Fire Safety Engineering, IHS BRE Press, 2011.

• [5] B. Karlsson and J. G. Quintiere, Enclosure Fire Dynamics, Chapter 4, page 52.

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References• [6] Hong Kong Daily News, “BMW burnt down in 8 minutes”, 4

July 2015 – In Chinese.• [7] Sing Tao Daily, “BMW burnt down in 8 minutes”, 4 July

2015 – In Chinese.• [8] Ming Pao Daily, “BMW burnt down in 8 minutes”, 4 July

2015 – In Chinese.• [9] Apple Daily, “BMW burnt down in 8 minutes”, 4 July 2015 –

In Chinese.• [10] South China Morning Post, “Yet another Ferrari

proves far too hot to handle”, 25 August 2015.

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References• [11] South China Morning Post, “Taxi gas leak behind garage blast”, 28 April

2015.• [12] South China Morning Post, “Taxi’s gas tank still intact after fire” 29 April

2015.• [13] W.K. Chow, “Gas explosion in residential buildings to watch”, Department of

Building Services Engineering, The Hong Kong Polytechnic University, January 2015. Available at: http://www.bse.polyu.edu.hk/researchCentre/Fire_Engineering/Hot_Issues.html

• [14] Y. Huo, Y.W. Ng and W.K. Chow, “A study on gas explosions in buildings: LPG as fuel or for air-conditioner?”, CLIMA2016, 22-25 May 2016, Aalborg, Denmark – Accepted to present.

• [15] PD 7974-6: 2004 The application of fire safety engineering principles to fire safety design of buildings – Part 6: Human factors: Life safety strategies –Occupant evacuation, behavior and condition (Sub – system 6).

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http://www.bse.polyu.edu.hk/researchCentre/Fire_Engineering/Hot_Issues.html

References• [16] V. Babrauskas, J.M. Fleming, B.D. Russell, “RSET/ASET, a flawed concept for fire

safety assessment”, Fire and Materials, Vol. 34, pp. 341-355 (2010). • [17] W.K. Chow, “Letter to the Editor: Comment on ‘RSET/ASET, a flawed concept for

fire safety assessment’ by V. Babrauskas, J.M. Fleming and B.D. Russell, Fire and Materials, Vol. 34, pp. 341-355 (2010)”, Fire and Materials, Vol. 37(3), pp. 257-258 (2013).

• [18] W.K. Chow, “Performance-based approach to determining fire safety provisions for buildings in the Asia-Oceania regions”, Building and Environment, Vol. 91, p. 127-137 (2015).

• [19] W.K. Chow, “A discussion on tall building fire safety in the Asia-Oceania regions”, Keynote speech, The 10th Asia-Oceania Symposium on Fire Science and Technology, Tsukuba, Japan, 5-7 October 2015.

• [20] Fire Research Division, Fire Dynamics Simulator User’s Guide, NIST Special Publication 1019, National Institute of Standards and Technology, U.S.A.

• [21] T. Korhonen and S. Hostikka, Fire Dynamics Simulator with Evacuation: FDS+EvacTechnical Reference and User’s Guide, VTT Technical Research Centre of Finland, Finland.

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Thank you !

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case study for · analysis of possible fire scenarios • positon a • these dead-ends made smoke...

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