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Structural response of unprotected floor assemblies in realistic residential fire scenarios
Bénichou, N.; Su, J. Z.; Bwalya, A. C.; Lougheed, G. D.; Taber, B. C.; Leroux, P.
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St ruc t ura l re sponse of unprot e c t e d floor a sse m blie s in re a list ic re side nt ia l fire sc e na rios
N R C C - 5 2 7 0 9
B é n i c h o u , N . ; S u , J . Z . ; B w a l y a , A . C . ;
L o u g h e e d , G . D . ; T a b e r , B . C . ; L e r o u x , P .
J u n e 2 0 1 0
A version of this document is published in / Une version de ce document se trouve dans:
SIF'10, Michigan State University, East Lansing, MI, USA, June 2-4, 2010, pp. 1-8.
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STRUCTURAL RESPONSE OF UNPROTECTED FLOOR ASSEMBLIES IN REALISTIC RESIDENTIAL FIRE SCENARIOS
N. Bénichou1, J.Z. Su, A.C. Bwalya, G.D. Lougheed, B.C. Taber, and P. Leroux
ABSTRACT
This paper presents the results of a full-scale experimental program undertaken to study the impact of basement fire scenarios on the structural integrity of unprotected floor assemblies. The experiments utilized fast-growing fires to challenge the structural integrity of the unprotected floor system above the basement, which may provide an egress route for occupants. A range of floor assemblies were used in the experiments. The results show that the type of joist used and the ventilation conditions have the most impact on the time to structural failure of the floor assemblies. The results also helped in establishing the sequence of fire events and their impact on the ability of occupants to escape during a fire.
INTRODUCTION
With the advent of new materials and innovative products for use in the construction of houses, there is a need to understand their potential impacts on occupant life safety under fire conditions. To address this need, the National Research Council of Canada undertook a research study to understand the factors that affect the life safety of occupants in single-family house fires from the perspective of tenability for occupants and structural integrity of structural elements as egress routes. The research sought to establish the typical sequence of events, such as smoke alarm activation, onset of untenable conditions, and structural failure of floor assemblies using specific fire scenarios in a full-scale test facility. The experimental studies focused on basement fires and the unprotected floor assembly located over a basement. A range of floor systems, including solid wood joist, wood I-joist, steel C-joist, and wood truss assemblies were used in the experiments. This paper focuses on the structural response of the test floor assemblies and the time of loss of structural integrity of these assemblies.
N. Bénichou, J.Z. Su, A.C. Bwalya, G.D. Lougheed, B.C. Taber, and P. Leroux, National Research Council of Canada, Ottawa, Ontario, Canada, K1A 0R6
EXPERIMENTAL PROGRAM Full-Scale Test Facility
A test facility was designed to represent a typical two-storey single-family house with a basement. The floor layout of the facility is shown in Figure 1. Each storey had a floor area of 95 m2 and a ceiling height of 2.4 m. The basement was partitioned to create a fire room representing a 27.6 m2 living area. An exterior window opening measuring 2.0 m wide by 0.5 m high and located 1.8 m above the floor was provided in the south wall of the fire room. A 0.91 m wide by 2.05 m high doorway opening located on the north wall of the fire room led into a stairwell enclosure. At the top of this stairwell, a 0.81 m wide by 2.05 m high doorway led into the first storey. The first storey had an open-plan layout. A floor assembly was constructed directly above the fire room. A 0.89 m wide by 2.07 m high doorway led to the exterior. The second storey had a corridor and two bedrooms.
Figure 1. Three-storey facility.
Fire scenarios
Bench-, medium- and full-scale fire tests were conducted in order to select a fuel package and fire scenarios for use in the experiments. A simple fuel package was developed to fuel a fire that simulated a basement living area fire [1]. The fuel package consisted of a mock-up sofa and wood cribs; see Figure 2. Full-scale fire scenario tests were conducted in the facility using the fuel package to investigate the effect of fuel quantity and ventilation on fire development [2]. The placement of the fuel package in the basement is illustrated in Figure 3. Two fire scenarios were selected for use in experiments with the unprotected floor assemblies: using a closed or open door to the basement from the first floor.
Figure 2. Fuel package. Figure 3. Layout of the fuel package.
Floor Assemblies and Instrumentation
A range of floor systems, including solid wood joist, wood I-joist, steel C-joist, metal plate wood truss and metal web wood truss assemblies were used in the full-scale fire experiments (see Table 1). For each experiment, a floor assembly was constructed on the first storey directly above the basement fire compartment. A single layer of 15.1-mm thick oriented strand board (OSB) was used for the subfloor of all assemblies. The span of the assemblies was chosen based on the load and the type of framing. An imposed load of 0.95 kPa (i.e., half of the imposed load of that prescribed by the National Building Code of Canada [3]) was applied on each floor assembly.
Various measurement devices were used in the experiments, including thermocouples to measure temperatures in the test facility and on the test floor assemblies, flame-sensing devices, floor deflection devices, smoke alarms, smoke density and gas measurement devices, and video cameras. Details of the design, construction and instrumentation are provided in a series of reports [4-9].
Table 1. Fire Tests with Unprotected Floor Assemblies.
Unprotected assemblies Open door Closed door
Solid wood joist (235 mm deep) UF-01 UF-02
Wood I-joist A (302 mm deep) UF-03 UF-09
Steel C-joist (203 mm deep) UF-04 N/A
Metal-plate wood truss (305 mm deep) UF-05 N/A
Wood I-joist B (302 mm deep) UF-06/UF-06R/UF-06RR N/A
Metal web wood truss (302 mm deep) UF-07 UF-08
Test Procedure
• The mock-up sofa was ignited.
• The fire room’s exterior window opening was opened when the temperature measured at the opening reached 300°C.
• The exterior door on the first storey was opened at 180 s after ignition and left open, simulating a situation where some occupants escaped leaving the exterior door open.
• The tests were terminated when excessive flames penetrated through the floor assembly and/or structural failure of any part of the floor assembly occurred.
EXPERIMENTAL RESULTS Fire development
Figure 4 shows the temperature profiles measured at the centre of one quadrant of the basement fire room. The polyurethane foam used for the mock-up sofa dominated the initial fire growth. The fast development of the fire from ignition to attainment of the first temperature peak was consistent for all of the tests. Following this initial stage, the effects of ventilation became more pronounced and the fire became wood-crib-dominated and also involved the unprotected floor assemblies.
Basement NW quadrant. Time (s) 0 200 400 600 800 1000 T e m perature ( ºC) 0 200 400 600 800 1000 FS-1 UF-01 UF-03 UF-04 UF-05 UF-06 UF-06R UF-06RR UF-07 Basement NW quadrant Time (s) 0 200 400 600 800 1000 T e m p er ature ( ºC) 0 200 400 600 800 1000 FS-4 UF-02 UF-08 UF-09
(a) Open doorway (b) Closed doorway
Figure 4. Temperature profiles in the fire compartment at 2.4 m height.
The temperatures at the 2.4 m height exceeded 600°C at approximately 120 s, indicating that the basement fire compartment reached flashover. Figure 4(a) indicates that under the full ventilation conditions (open basement doorway) the fire scenario provided a very reproducible fire exposure to the floor assemblies in all experiments. As shown in Figure 4(b), under the limited ventilation conditions (closed basement doorway), the fire scenario also provided a relatively severe and consistent fire exposure to the floor assemblies. The results from the fire scenario tests (FS-1 and FS-4) [2] in the fire room are also included in Figures for reference. Flame Penetration
A floor system provides an egress route for occupants and its structural integrity directly impacts the safe evacuation of the occupants. Figure 5 shows exemplar data plots, which are representative of the results for tests with the engineered floor assemblies. The temperatures shown in Figure 5(a) are from measurements by nine thermocouples (TC) under insulated pads on top of the OSB subfloor. A rapid
increase in temperature indicates that the floor was being significantly breached. The subsequent rapid decrease in temperature was due to the termination of the experiment by extinguishing the fire with water. Under standard fire resistance testing, the temperature failure criterion is defined as a temperature rise of 140°C on average of the nine TCs or a temperature rise of 180°C at any single point [10].
a) Thermocouples under insulated pads on top of the subfloor Time (s) 0 100 200 300 400 500 Temp eratur e (ºC) 0 100 200 300 400 500 600 700 800
b) Flame sensors at 3 tongue and groove joints of the subfloor Time (s) 0 100 200 300 400 500 Vo lts 0.0 0.5 1.0 1.5 2.0
c) Floor deflections at 9 points on top of the floor assembly Time (s) 0 100 200 300 400 500 D e fl e c ti o n ( m m ) -700 -600 -500 -400 -300 -200 -100 0
Figure 5: Exemplar plots of measurements for Test UF-06R.
Flame penetration through a floor assembly is considered to be an initial indicator of failure of the assembly, and is also a failure criterion in standard fire resistance testing [10]. Flame penetration could also impact the ability of occupants to evacuate. Flame-sensing devices were placed at joints on the unexposed side of the OSB subfloor to detect flame penetration through the floor. As shown in Figure 5(b), the flame-sensing devices produced noticeable voltage spikes, an indication that the devices are being struck by flames that is penetrating through the assembly. Deflections
The floor deflection was measured at nine points in the central area of the test assembly. Figure 5(c) shows examples of the deflection measurements. The sharp increase in deflection is an indication that structural collapse had occurred.
Figure 6 shows a comparison of the floor deflection near the centre of all of the test assemblies prior to structural failure. The steel C-joist assembly produced the highest deflection rate, followed by metal-web wood trusses and metal-plate wood trusses. The solid wood joist assemblies produced the lowest deflection rate. There
were three distinct patterns of failure of the test floor assemblies. In UF-01 and UF- 02, the subfloor failed, with most of the solid wood joists charred but still in place at the end of the tests. In UF-03, UF-05, UF-06R, UF-06RR, UF-07 and UF-08, the assemblies with wood I-joists or wood trusses structurally deflected and then broke at the mid-points and the floor assemblies collapsed in the form of a “V” shape. In UF-04, UF-06 and UF-09, the assemblies with steel C-joists and wood I-joists structurally deflected and then they completely collapsed into the basement.
a) Tests with open basement doorway Time (s) 0 100 200 300 400 500 600 700 D ef lect ion (mm ) -350 -300 -250 -200 -150 -100 -50 0 UF-01 UF-03 UF-04 UF-05 UF-06 UF-06R UF-06RR UF-07
b) Tests with closed basement doorway Time (s) 0 100 200 300 400 500 600 700 D ef lec ti on ( m m) -350 -300 -250 -200 -150 -100 -50 0 UF-02 UF-08 UF-09
Figure 6. Floor deflection near the centre of assemblies prior to structural failure.
Failure times
Table 2 shows the times to failure for the assemblies based on temperature measurements, flame penetration and floor deflection, and confirmed by visual observations. The times to reach structural failure for the wood I-joist, steel C-joist, metal plate and metal web wood truss assemblies were 35-60% shorter than that for the solid wood joist assembly. As shown by the results from the three replicate tests with Type B wood I-joist assembly (Tests UF-06, UF-06R and UF-06RR), the times to structural failure were repeatable. Having a closed door to the basement limited the air available for combustion and delayed the time for the test assemblies to reach structural failure (50-60% longer than with the open basement doorway).
Table 2. Summary of Structural Failure Times of the Assemblies (in seconds). UF-01 UF-02 UF-03 UF-04 UF-05 UF-06 UF-06R UF-06RR UF-07 UF-08 UF-09 740 1200 490 462 469 382 380 414 325 474 778 DO DC DO DO DO DO DO DO DO DC DC DO: Door Open DC: Door Closed.
Sequence of events
The two scenarios (door open or closed) were designed to better understand how the structural integrity and tenability conditions would affect the ability of occupants on the upper storeys to escape a single family house in the event of a
basement fire. Reference [11] provides the chronological sequence of the fire events in the full-scale experiments – fire initiation, smoke alarm activation, onset of untenable conditions, and structural failure of the test floor assembly. For the experiments with the open basement doorway the untenable conditions generally occurred within 180 to 240 s after ignition and the structural failure of the test floor assemblies occurred after the untenable conditions were reached. With the presence of the closed door to the basement, the structural failure of the solid wood joist assembly (Test UF-02) and wood I-joist Type A assembly (Test UF-09) also occurred well after untenable conditions were reached. However, one case (UF-08, scenario with the closed door) showed that structural failure occurred concurrently with the onset of untenable conditions.
CONCLUSIONS
Two basement fire scenarios were used in the full-scale fire experiments to meet the objectives of the research. Overall, the fire scenario with the open basement doorway was more severe than the fire scenario with the closed basement doorway to the structural integrity of the unprotected floor assemblies and the life safety of occupants on upper storeys.
In all of the experiments, structural failure of the test floor assemblies occurred. The time to structural failure was characterized by an increase in floor deflection and was usually accompanied by considerable flame penetration through the assemblies and an increase in compartment temperature above the floor assemblies. The times to reach structural failure for the wood I-joist, steel C-joist, metal plate wood truss and metal web wood truss assemblies were 35-60% shorter than that for the solid wood joist assembly. It was also found that having a closed door to the basement limited the air available for combustion and delayed the times to structure failure for the assemblies (50-60% longer than with the open basement doorway).
There was structural deflection of all floor assemblies prior to their structural failure. The steel C-joist floor assembly produced the highest deflection rate, followed by metal-web and metal-plate wood trusses. The solid wood joist assemblies produced the lowest deflection rate. For all engineered floor assemblies, the structure failure occurred either in the form of complete collapse into the basement or in the form of a “V” shaped collapse due to joist or truss failure.
In general, untenable conditions occurred before structural failure of unprotected floor assemblies. However, the time gap between the onset of untenable conditions and the structural failure of the floor assembly was smaller for the engineered floor assemblies than it was for the solid wood joist assembly used in the experiments. There was one case where structural failure of the unprotected assemblies was concurrent with untenable conditions.
ACKNOWLEDGEMENTS
The following organizations provided valuable financial and technical support to the research as the project partners: Canada Mortgage and Housing Corporation, Canadian Automatic Sprinkler Association, Canadian Wood Council, Cement Association of Canada, City of Calgary, FPInnovations - Forintek Division, North American Insulation Manufacturers Association, Ontario Ministry of Community Safety and Correctional Services/Office of the Fire Marshal, Ontario Ministry of Municipal Affairs and Housing, Wood I-Joist Manufacturers Association.
The authors would like to acknowledge contributions of G. Proulx, A. Kashef, C. McCartney, J.R. Thomas, D. Carpenter, G. Crampton, E. Gibbs, C. McCartney, M. Ryan, M. Wright, J. Henrie, R. Monette and R. Rombough in the project.
REFERENCES
1. Bwalya, A.C., Lougheed, G.D., Su, J.Z., Taber, B.C., Bénichou, N., Kashef, A. (2007) Development of a fuel package for use in the fire performance of houses project. In: Fire and Materials Conference, San Francisco, CA, January 29, 2007.
2. Su, J.Z., Bwalya, A.C., Lougheed, G.D., Bénichou, N., Taber, B.C., Thomas, J.R. (2007) Fire Scenario Tests in Fire Performance of Houses Test Facility - Data Analysis, Research Report 210, National Research Council of Canada, Ottawa, ON
3. Canadian Commission on Building and Fire Codes (2005) National Building Code of Canada, National Research Council of Canada, Ottawa, ON.
4. Bénichou, N., Su, J.Z., Bwalya, A.C., Lougheed, G.D., Taber, B.C., Leroux, P., Kashef, A., McCartney, C., Thomas, J.R. (2009) Fire Performance of Houses, Phase I, Study of Unprotected Floor Assemblies in Basement Fire Scenarios, Part 1 - Results of Tests UF-01 and UF-02 (Solid Wood Joists), Research Report 246, National Research Council of Canada, Ottawa, ON.
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6. Bénichou, N., Su, J.Z., Bwalya, A.C., Lougheed, G.D., Taber, B.C., Leroux, P., Kashef, A., Thomas, J.R. (2009) Fire Performance of Houses, Phase I, Study of Unprotected Floor Assemblies in Basement Fire Scenarios, Part 3 - Results of Test UF-04 (Light-Gauge Steel C-Joists), Research Report 248, National Research Council of Canada, Ottawa, ON.
7. Su, J.Z., Bénichou, N., Bwalya, A.C., Lougheed, G.D., Taber, B.C., Leroux, P., Kashef, A., Thomas, J.R. (2009) Fire Performance of Houses, Phase I, Study of Unprotected Floor Assemblies in Basement Fire Scenarios, Part 4 - Results of Test UF-05 (Metal-Plate-Connected Wood Trusses), Research Report 249, National Research Council of Canada, Ottawa, ON. 8. Bénichou, N., Su, J.Z., Bwalya, A.C., Lougheed, G.D., Taber, B.C., Leroux, P., Thomas, J.R.
(2009) Fire Performance of Houses, Phase I, Study of Unprotected Floor Assemblies in Basement Fire Scenarios, Part 5 - Results of Tests UF-06, UF-06R and UF-06RR (Wood I-Joists B), Research Report 250, National Research Council of Canada, Ottawa, ON.
9. Su, J.Z., Bénichou, N., Bwalya, A.C., Lougheed, G.D., Taber, B.C., Leroux, P., Thomas, J.R. (2009) Fire Performance of Houses, Phase I, Study of Unprotected Floor Assemblies in Basement Fire Scenarios, Part 6 - Results of Tests UF-07 and UF-08 (Metal-Web-Connected Wood Trusses), Research Report 251, National Research Council of Canada, Ottawa, ON. 10. Underwriters' Laboratories of Canada (2007) Standard Methods of Fire Endurance Tests of
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