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Baleshan, Balachandren&Mahendran, Mahen (2017)
Experimental study of light gauge steel framing floor systems under fire conditions.
Advances in Structural Engineering, 20(3), pp. 426-445.
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Experimental Study of LSF Floor Systems under Fire Conditions
Balachandren Baleshan1 and Mahen Mahendran2
1PhD Researcher, School of Civil Engineering and Built Environment, Queensland University
of Technology, Brisbane, QLD 4000, Australia
2Professor, School of Civil Engineering and Built Environment, Queensland University of
Technology, Brisbane, QLD 4031, Australia
Abstract: Cold-formed steel members can be assembled in various combinations to provide cost-efficient and safe light gauge floor systems for buildings. Such Light gauge Steel Framing (LSF) floor systems are widely accepted in industrial and commercial building construction. LSF floor systems must be designed to serve as fire compartment boundaries and provide adequate fire resistance. Floor assemblies with higher fire resistance rating are needed to develop resilient building systems for extreme fire events. Recently a new composite panel system based on external insulation has been developed for LSF floors to provide higher fire resistance rating under fire conditions. This paper presents the details of an experimental investigation of LSF floors made of both the conventional (with and without cavity insulation) and the new composite panel systems under standard fires. Analysis of the fire test results showed that the thermal and structural performance of externally insulated LSF floor system was superior than conventional LSF floors with or without cavity insulation. Details of experimental results including the temperature and deflection profiles measured during the tests are presented along with the joist failure modes. Such fire performance data can be used in the numerical modelling of LSF floor systems to further improve the understanding of their fire behaviour and to develop suitable fire design rules. Keywords: Fire tests, LSF floor systems, Lipped channel sections, Fire resistance rating, Composite panels
2Corresponding author’s email address: [email protected], Phone number: 61 73138 2543,
1. Introduction
Light Gauge Steel Framing (LSF) floor systems made of cold-formed steel lipped channel section (LCS) joists are commonly used in the building industry (Alfawakhiri, 2001, Sakumoto et al. 2003 and Sultan et al. 1998). However, these thin, cold-formed steel sections heat up quickly under fire conditions, resulting in rapid reduction to their strength and stiffness. Gypsum plasterboard linings provide protection to steel joists during building fires by delaying the temperature rise in the cavity. The fire resistance of a floor is its ability to remain stable under exposure to fire. It is usually expressed in terms of its fire resistance rating (FRR), which is the length of time the floor can stay exposed to a standard fire resistance test without losing its load-bearing or fire separating functions. It is important to develop and use LSF floor systems with enhanced FRR as part of resilient building systems to avoid disasters during extreme fire events. FRR of LSF floor systems can be increased by adding more plasterboard sheets to the steel joists (the traditional method). However, innovative fire protection systems are essential without simply adding more plasterboard sheets, which is inefficient.
Many research studies (Alfawakhiri, 2001, Outinen et al., 2001, Sakumoto et al. 2003, Klippstein, 1978, Gerlich, 1995, Feng et al. 2003, Feng and Wang 2005, Gunalan et al. 2013, Keerthan and Mahendran, 2013, Ariyanayagam and Mahendran, 2014 and Kesawan and Mahendran, 2015) have been undertaken on the fire behaviour of LSF wall panels. In comparison, such work on LSF floor panels has been limited. Sakumoto et al.’s (2003) experimental study on LSF floor panels has shown that the use of interior cavity insulation improved their fire resistance rating. However, Alfawakhiri (2001) and Sultan et al. (1998) found that floor assemblies without cavity insulation provided higher fire resistance compared to cavity insulated assemblies. Hence past research was unable to conclude the effects of this
traditional approach of using cavity insulation. Recently Kolarkar and Mahendran (2008) developed a new composite panel system, where the insulation was sandwiched between two plasterboards and placed outside the steel frame, and suggested that the fire resistance of LSF floors and walls can be improved considerably. Such innovations in the plasterboard and insulation systems, steel joist configurations and construction methods have the potential of increasing the FRR of LSF floor systems.
This research investigated the structural and fire performance of LSF floor systems with the new composite panel system using full scale fire tests. In this experimental study under standard fire conditions, both the conventional (with and without rock fibre cavity insulation) and the new composite panel systems were tested. In the new composite floor system, a composite panel including rock fibre insulation between two plasterboards was used on the ceiling side of the steel frame. The LSF floor specimens were first loaded to pre-determined values, and then exposed to standard fire conditions on the ceiling side (plasterboards). Table 1 gives the details of the three full scale floor specimens used in this study. This paper presents the details of the experimental study into the thermal and structural performance of the chosen LSF floor assemblies and the results.
2. Test Specimens
Test specimens were built using four joists, two tracks, two layers of plasterboard and one layer of plywood. The floor area was more than 5 m2 (2.4 m x 2.1 m) with a span of 2400 mm
and the floor specimen was simply supported at its two short sides. All the joists and tracks used were fabricated from galvanized steel sheets having a nominal base metal thickness of 1.15 mm and a measured yield strength of 612 MPa (G500 steel). The frames were made of four joists made of 180x40x15x1.15 mm lipped channel sections (LCS) as shown in Figures 1(a) and (b). Test frames were made by attaching the joists to the top and bottom tracks made
of 182x50x1.15 mm unlipped channel sections using 12 mm long self-drilling wafer head screws as shown in Figures 1(b) and (c).
Test steel frames were lined on the ceiling side (fire side) by two layers of 16 mm thick 1200 mm width x 2400 mm length gypsum plasterboards manufactured by Boral Plasterboard under the product name Firestop (850 kg/m3). The frames were lined on one side by a single
layer of 19 mm thick plywood board to simulate the sub-floor for Test Specimen 1. Two layers of plasterboards were used in the other two tests to avoid any burning of plywood. The plasterboards were placed across the steel joists and attached to the joists by 25 mm long drill point screws. The first layer of plasterboard was fastened at 200 mm centres in the field of plasterboard (Figure 2(a)) while the second layer was fixed in the same manner, but their joints were staggered by 200mm as shown in Figure 2 (b). The second layer of plasterboards was attached to the joists by 45 mm long self-drilling bugle head screws spaced at 200 mm centres in the field of plasterboard. A minimum edge distance of 10-15 mm was maintained for all the screws from plasterboard edges.
To prevent any heat loss from the edge of the floor, exterior joists were protected with plasterboards and insulation as shown in Figure 2(c). Plasterboards in the first layer were back blocked as specified in AS/NZS 2589.1 (SA, 1997). 200 mm wide back-blocking pieces were cut from 16 mm fire stop plasterboard to the required sizes and fixed between the joists to the ceiling plasterboards using 10 x 38 mm Type L Screws at 200 mm centres (Figure 2(d)).
All the exposed screw heads were covered with two coats of joint compound. The joints were sealed with 50 mm wide reinforced paper tape and covered with two coats of joint compound (Figure 2(e)). On the other side (ambient side), one layer of plywood board was fixed at right
angles to the joists as shown in Figure 2(f) for Test Specimen 1. For other specimens, two plasterboard layers were used on both sides.
K-type thermocouple wires were installed to measure the temperature variations across the width and along the length of test floor (over the plasterboard and joist surfaces). Figures 3 (a) to (d) show the attachments of thermocouples. On the joists, the thermocouple wires were attached to the hot flange, web and cold flange.
2.1. Test Specimen 1
The test steel frame was lined on the ceiling side (fire side) by two layers of plasterboard. The base layer of plasterboard on the fire side was attached first across the joists with its associated K-type thermocouples to the steel frame. The face layer of fire side plasterboard was fixed in the same manner. The floor panel was then turned over to fix the ambient side plywood. Before fixing this plywood board, all the thermocouples attached to the base layer plasterboard on the fire side were drawn to the ambient side through tiny holes drilled in the plywood boards. Figure 4 shows the assembled Test Specimen 1.
2.2. Test Specimen 2
In Test specimen 1, there was considerable smoke during the fire test, which was followed by plywood burning despite being on the ambient side. Hence plywood was not used in Test specimen 2. Two plasterboards were used on both sides with cavity insulation. Three 25 mm thick layers of Rock fibre insulation were used in the cavity space between the joists after fixing the two plasterboards on the fire side along with their associated thermocouples. Figure 5(a) shows the installation of the rock fibre cavity insulation. The cavities of the individual joists and tracks were also packed with rock fibre insulation to avoid the formation of air pockets within the floor cavity. Figure 5(b) shows the positioning of the base layer plasterboard on the ambient side to facilitate the passing of thermocouple wires through holes
drilled at appropriate places. After fixing the base layer plasterboard, the face layer plasterboard on the ambient side was fixed in a manner similar to the base layer.
2.3. Test Specimen 3
In this test specimen the new composite panel with external insulation was used. The base layer plasterboard on the fire side was attached across the joists with its associated thermocouples to the steel frame. Two 13 mm plasterboard strips were then fixed to the base layer plasterboards (Figure 6(a)) along the periphery of the panel giving a total depth of 26 mm. The space generated by these edge strips was filled with a single layer of 25 mm thick rock fibre mat (Figure 6(b)). Finally the face layer plasterboards were fixed on the fire side (sandwiching the insulation between the face and base layer plasterboards). The floor specimen was then turned over to fix the two ambient side plasterboards as shown in Figure 6(c).
3. Test Arrangement 3.1. Gas Furnace
A propane fired gas furnace with internal dimensions of 2.1 m width, 2.4 m height and 0.3 m depth and mounted on a carriage and wheel arrangement was used to conduct the fire tests. To start the test the carriage was moved forward to make contact with the large steel frame holding the test floor specimen, thereby completing the combustion chamber. On starting the furnace the specimen was exposed to heat from one side as desired. The furnace was designed to deliver heat in accordance with AS 1530.4 (SA, 2005) to develop the required standard fire time-temperature curve.
3.2. Loading Frame
A heavy steel frame was specially constructed to support the test floor specimens. It consisted of two columns firmly bolted to the strong floor and a universal beam connecting the two columns to form an ‘H’ shaped portal frame (Figure 7 (a)). The floor specimen was located within this portal frame with its ends simply supported by angle sections as shown in Figure 7 (b). The gas furnace only allowed test floor specimens to be set in a vertical position. Hence the transverse loads on the floor specimens were applied in a horizontal direction.
In order to simulate a uniformly distributed loading present in LSF floor systems, a load distribution system was developed (Figure 8(a)). Each load distribution unit consisted of a main spreader beam and two secondary spreader beams. At the ends of each secondary beam there were 180 mm x 180 mm loading plates to apply the loads to each joist. The spreader beams had bolted pinned connection at its centre and ends. Each load distribution unit with four loading points was able to load two joists at two loading points each. The main spreader beam was connected to the end of an SHS by using bolts as shown in Figure 8(b). Other side of the SHS was welded with a nut so that the loading cell end could be screwed into it. The load was applied using two hydraulic jacks that were connected to the load distribution units via load cells. A single pump was used to ensure equal loading to the joists.
3.3. Deflection and temperature measurements
To measure the out of plane deflections of floor specimens, 12 Linear Variable Displacement Transducers (LVDT) were used at 0.25L, 0.50L and 0.75L (where ‘L’ is joist length) along the length of each joist as shown in Figure 9. The transducers were attached to a series of wooden beams in front of the specimen.
K-type thermocouples were used to measure the temperature development across the floor specimens. The joist temperatures were measured along the interior joists at 0.25L, 0.50L and 0.75L. At each point three thermocouples were attached to measure the temperatures of the hot flange, web and cold flange, thus giving a total of nine thermocouples per joist. For exterior joists they were attached only at 0.5L. This gave a total of 24 thermocouples that allowed the determination of the average joist temperature and the temperature gradients across the joist cross-section and along the joist length.
To measure the plasterboard surface temperatures, three sets of 12 thermocouples were attached to these surfaces between the joists at 0.5L. Two more sets of thermocouples were attached at 0.25L and 0.75L height along the mid-vertical line assembly between the two interior joists. This gave a total of 44 thermocouples for Test Specimen 1 (Figure 10(a)). For Test Specimen 2 with cavity insulation, additional thermocouples were installed between the plasterboard surfaces on the ambient side at mid-height (Figure 10(b)), giving a total of 47 thermocouples. For Test Specimen 3 with external insulation, five more thermocouples were installed to measure the temperature across the insulation layers at mid-span thus giving a total of 52 thermocouples (Figure 10(c)). To measure the average temperature on the ambient side, five thermocouples were used, one at the centre of the area and one at the centre of each quarter section as mentioned in AS 1530.4 (SA, 2005). To measure the temperature at various other points on the ambient side an infrared gun was also used.
4. Test Method
Test floor specimen was installed within the heavy steel frame as shown in Figure 7. After proper positioning of the floor with furnace location, the top track was fastened to the top beam on each side. The furnace was then moved next to the test floor specimen and this completed the combustion chamber of the furnace with the test floor specimen forming the
fourth side of the chamber facing the burners. This arrangement ensured that only one face of the test specimen was exposed to fire conditions. The floor specimen width was 20 mm less than that of the furnace opening, which gave a gap of 10 mm on each side of the specimen. This gap was packed with Isowool, a non-restraining and non-combustible mineral fibre. A target load of 18 kN per jack (9 kN per joist) was applied to the specimen first using the two hydraulic jacks. This target load was determined based on a load ratio of 0.4 where the load ratio is the target load in the fire test to the ultimate failure load of the floor specimen at ambient temperature. The latter ultimate failure load was predicted to be 23 kN per joist based on AS/NZS 4600 (SA, 2005) design rules and numerical modelling (Baleshan and Mahendran, 2016a,b). Following the application of the target load, the furnace was started. During the fire test, the furnace temperature was regulated such that the average temperature recorded by the control thermocouples inside the furnace followed the standard cellulosic time-temperature fire curve based on AS 1530.4 (SA, 2005). The load of 18 kN per jack was maintained throughout the fire test. During the fire test the lateral displacements, the temperature readings from all the thermocouples and the furnace pressure readings were taken at intervals of 1 minute. The test was stopped immediately following the failure of the floor specimen and the time to failure recorded. Test specimen was assumed to have failed when the applied load could not be maintained. This was also confirmed by the measured load-displacement/time graph, which showed rapid load reduction.
5. Results of Test Specimen 1
5.1. Structural and Fire behaviour of Test Specimen 1
Smoke was observed after three minutes (Figure 11(a)) due to the burning of the plasterboard paper on the exposed side. This intensified after 10 minutes associated with steam escaping from the outer edges of the specimen. After the complete burning of the paper and the
conversion of water into steam from the plasterboard, there was steady burning with little or no smoke or steam although smoke and steam resumed with subsequent layers of plasterboard heating up from 30 to 40 minutes. After 70 minutes considerable smoke appeared because the exposed plywood boards started to burn (Figure 11(b)).
Lateral deformation of the floor specimen could be seen by 80 minutes and then the failure was sudden at 107 minutes with the load quickly dropping with the joists moving in the inward direction and bending the plywood on the ambient side. The measured ambient surface temperatures (< 90°C) were well below the insulation failure temperature of 140 °C throughout the test. The failure was due to the structural failure of the joists.
On rolling the furnace back to expose the plasterboard on the fire side, it was noticed that the exposed plasterboard pieces, especially at the top, had fallen off as it was the first layer of the fire side. All four joists failed near the top, due to the higher temperatures in the chamber at the top caused by the upward movement of hot air. Due to the plasterboard fall-off, the joists in this area not only lost their lateral support from the plasterboard on the fire side but were also severely exposed to higher temperatures. By visual inspection it was clear that Joist 2 was the first to fail. Lateral deflection graphs (Figure 14) showed that Joist 2 had the maximum deflection and also the sudden deflection change. The upper and lower tracks supporting the joists were relatively undamaged and were seen holding the joists firmly in place. The plywood boards on the ambient side were intact giving good lateral support to the compression flange of the joists and hence the flexural torsional buckling of joists was fully prevented throughout the test. When the exposed plasterboards were removed, the presence of local web buckling waves was seen along Joist 1, confirming the occurrence of local buckling of joists before the ultimate failure (Figure 12(c)). Figures 12 (a) and (b) show the failures of joists at their supports.
The cavity facing surface of plywood board on the ambient side was burnt and had built up a charred layer (Figure 11(b)) but the other surface of plywood was in good condition, thus maintaining the integrity of the floor. Insulation failure was also not detected as the temperature on the ambient surface was much lower than the limiting temperature given in AS 1530.4 (SA 2005) until the end of the test.
5.2. Time-Temperature Profiles of Test Specimen 1
Figure 13 presents the time-temperature profiles of plasterboard and plywood surfaces (see Table 1 for notations). It shows that the time-temperature profile of the furnace and the exposed face of the floor followed the standard time-temperature curve as defined by AS 1530.4 (SA, 2005).
5.3. Lateral Deflection Behaviour of Test Specimen 1
Figures 14 (a) and (b) show the lateral deflections of the floor during the fire test. Sudden increase in lateral deflection in these figures confirms the time of failure. Around this time the average temperatures across the hot flanges, webs and cold flanges were 450oC, 400oC and
340oC, respectively.
6. Results of Test Specimen 2
6.1. Structural and Fire Behaviour of Test Specimen 2
The initial response of Test Specimen 2 was similar to that of Test Specimen 1. Smoke was seen due to the burning of plasterboard papers which was different to Specimen 1 that had plywood on one side. Only the face layer of the fire side plasterboards had fallen off partially at the end of the test (Figure 15(a)). The rock fibre cavity insulation was almost fully intact with only the outer layer of insulation having lost its integrity at certain locations as seen in
Figure 15(b). On stripping the cavity insulation from the test floor, it was noted that both ambient side plasterboards remained in good condition.
Lateral deflection was more noticeable after about 65 minutes. This continued until the failure and resulted in failing towards the furnace. The lateral deflection was the largest in this test with cavity insulation compared with the other two tests. This was due to the higher temperature difference between the hot and cold flanges of the joists, which caused larger thermal bowing effects in this test. The failure was sudden with the load quickly dropping off at 99 minutes.
Flexural buckling about the minor axis and torsional buckling of joists were fully prevented by the lateral support offered by the double layers of plasterboard. Figure 16 shows the failures of joists near their supports in Test 2 including the local buckling in Joist 2.
6.2. Time-Temperature Profiles of Test Specimen 2
Figure 17 presents the time-temperature profiles of plasterboard surfaces. It shows that the time-temperature profile of the furnace and the exposed face of the floor followed the standard time-temperature curve as defined by AS 1530.4 (SA, 2005).
6.3. Lateral deflection behaviour of Test Specimen 2
Figures 18 (a) and (b) show the lateral deflections of the joists. The sudden increase in lateral deflection as seen in Figure 18(a) confirms the failure at 99 minutes. Around this time the average temperatures across the hot flanges, webs and cold flanges were 446oC, 198oC and
7. Fire Test Results of Test Specimen 3
7.1. Structural and Fire behaviour of Test Specimen 3
The behaviour of Test Specimen 3 was similar to that of Test Specimens 1 and 2. However, Test Specimen 3 with external insulation survived the fire for 139 minutes. Observations relating to smoke and steam were the same as in Test 2. From the beginning of the test, the specimen bent inwards and this continued until the end of the test when the specimen suddenly failed towards the furnace. Figure 19(a)-(c) shows the specimen soon after the fire test. Exposed plasterboard (Pb1) had fallen off in most areas. The external insulation had undergone overall shrinking, leading to the opening of the joints and exposing the base plasterboard layer (Pb2) (Figure 19(b)). On stripping the cavity insulation, it was noted that ambient side plasterboards remained in good condition except for some burning of the cavity facing surface of Pb3 in certain locations. The joists did not suffer from any torsional or flexural buckling about the minor axis. Figure 20 shows the failures of Joists 2 and 3 near their supports.
7.2. Time -Temperature Profiles of Test Specimen 3
Figure 21 presents the time-temperature profiles of plasterboard surfaces. It shows that the time-temperature profile of the furnace and the exposed face of the floor followed the standard time-temperature curve as defined by AS 1530.4 (SA, 2005).
7.3. Lateral deflection behaviour of Test Specimen 3
Figures 22 (a) and (b) present the lateral deflections of the joists. These figures showing a sudden increase in deflection near the end of the test confirm the specimen failure at 139 minutes. At this time the average temperatures across the hot flanges, webs and cold flanges were 340oC, 260oC and 210oC, respectively.
8. Discussion of Fire Test Results
Full scale fire test results are summarized in Table 1. It gives the fire resistance ratings (in minutes) of the three LSF floor specimens tested in this experimental study. This study provided the fire performance results for the LSF floor systems using both conventional (with and without cavity insulation) and external insulation systems. The results confirmed the superior performance of the LSF floor system using external insulation over cavity insulation (139 versus 99 minutes). Detailed results of time-temperature profiles and structural behavioural characteristics of joists obtained from this study can now be used in the numerical analyses of LSF floor systems (Baleshan, 2012). Following sections present some of the main findings by comparing the results of Tests 1 to 3.
8.1. Comparison of the fire performance LSF floor systems with external insulation, cavity insulation and no insulation
Average time-temperature profiles of joists from all three tests are compared in Figure 23 (a). It compares the variations in thermal responses of joists due to the different ways of using insulation. In the case of cavity insulated specimen, the average temperature plateau (second phase) of joists was seen to last only up to 60 minutes in comparison to 90 minutes in the case of externally insulated specimen. The average hot flange and cold flange temperature profiles of the interior joists are shown in Figures 23 (b) and (c), respectively. The hot flange temperatures in the cavity insulated specimen (Test 2) were seen to rise very rapidly with large temperature differences across the joist cross-sections due to the presence of cavity insulation. The hot flange temperatures of the externally insulated floor specimen (Test 3) on the other hand were seen to rise gradually with a small temperature difference across the joist cross-sections due to the faster transfer of heat by radiation across the empty cavity. This is probably because, in the cavity insulated specimens, the insulation is on the ambient side of the hot flange and thus is incapable of offering any protection to it. In the case of externally
insulated specimens, it is seen that the temperature profiles of the joists are well separated implying the effect of insulation on the joist temperatures. From Figures 23 (b) and (c), it is clear that external insulation offers the maximum protection to the joists.
The temperature difference between the hot and cold flanges of the joists during the fire test is shown in Figure 23(d). By 80 minutes the hot flange temperatures of the cavity insulated specimens had crossed 340oC with a temperature difference of over 250oC across the joist
cross-section whereas the hot flange temperatures in the externally insulated specimens at the same time were 128oC with a temperature difference of less than 50oC across the joist
cross-sections. At the end of the test the average hot flange temperatures of the joists for cavity insulated test specimen was about 455oC with a temperature difference of about 346oC across
the joist cross-sections, whereas in the case of externally insulated test specimen the average hot flange temperature was close to 381oC with a temperature difference of only about 128oC
across the joist cross-sections. These temperature measurements and differences show the superior fire performance of the externally insulated specimen compared with cavity insulated specimen.
The higher temperature differences across the joist cross-section in the cavity insulated specimen led to higher lateral deformations compared to the externally insulated specimen (Figure 24). After 80 minutes the lateral deflections in the cavity insulated specimen was close to 13 mm, compared to about 6 mm in the case of externally insulated specimen. By the end of 90 minutes, the lateral deflection in the cavity insulated specimen had crossed 20 mm whereas it was still less than 8 mm for the externally insulated specimens.
Figure 25(a) shows the time-temperature profiles of the Pb2-Cav surface for the specimens. The temperature profiles of the cavity insulated and non-insulated specimens were seen to be almost identical up to 60 minutes. Beyond 60 minutes, cavity insulation caused the
temperature profiles to rise sharply by blocking and redirecting the heat flow back to the cavity facing surface. Figure 25(b) shows the time-temperature profiles of the Ply/Pb3-Cav surface of the specimens. In this case the temperature profile increased dramatically for the non-insulated specimen whereas the temperatures were low for the cavity insulated specimens. The ambient side temperatures of all the floor specimens were observed to be below 90oC, ie. well below the insulation failure temperature of 140oC (Figure 25(c)).
The time-temperature profiles of the externally insulated floor specimens were found to be the most favorable. This is probably because, any insulation by virtue of their physical presence essentially serves the main function of eliminating the transfer of heat across the floor cavity by radiation and convection which essentially are the faster modes of heat transfer as compared to conduction. No cavity insulation can reduce the transfer of heat towards the cold flange by conduction along the metallic cross-section of the joist. Thus the cold flange picks up heat from the hot flange by conduction along the web, which would be the fastest mode of heat transfer in the case of cavity insulated specimens. Because of the very low conductivity of the cavity insulating material as compared to steel, most of the heat gets directed and channelled along and across the steel joists which act as the heat sink thus raising their own temperatures much faster than in the case of non-cavity insulated specimens, thus making the very presence of cavity insulation a threat to the survival of steel during fire conditions.
Externally insulated specimens on the other hand can offer a much higher level of protection to the joists as they are installed on the fire side of the joist, thus minimizing the transfer of heat by radiation (by virtue of their physical presence) and conduction (on account of their low conductivity). Rock fibre insulation when used externally was seen to give the maximum protection.
8.2. Joist Temperatures and Failure
The failure of the specimens was always by the structural failure of the joists and never by insulation or integrity failure. In the case of cavity insulated specimen, the external plasterboards collapsed prior to joist failure thus hastening the collapse of the floor specimen by exposing the steel frame to direct furnace heat.
The two interior joists recorded higher temperatures during the test. Table 2 gives a comparison of the thermal and structural responses of these interior joists (J2 and J3) at the end of 30, 60, 90 and 120 minutes. Joists of Specimens 1 and 2 reached higher temperatures compared to those in Specimen 3. This is because of the external insulation used in Specimen 3. The cold flange temperature values near the failure of the interior joists of Specimens 1 and 2 were 320oC and 105oC, respectively. The hot flange failure temperatures of these interior
joists are very close to each other (i.e. 489oC and 491oC for J2). For these joists (J2) the
temperature differences between hot and cold flanges were 143oC and 388oC, respectively.
This may mean that joist failure is mostly governed by the (maximum) hot flange temperature than the temperature difference between hot and cold flanges. Hence we can conclude that structurally similar LSF floor panels will fail once their joist reach a particular temperature and the fire resistance can be increased only by delaying the maximum temperature in the joists. This is confirmed by the increase in fire resistance time of Specimen 3, which was achieved by the delay in temperature rise in joists due to external insulation.
9. Conclusions and Recommendations
This paper has presented the details of three full scale fire tests of conventional LSF floor systems with and without cavity insulation, and the new composite panel system using external insulation, and the results. Test results have shown the superior fire resistance characteristics of the LSF floor system using the new composite panels. Temperature
measurements showed that structurally similar LSF floor panels will fail when their joists reach a critical temperature and the fire resistance can be increased only by delaying the temperature in the joists. This is confirmed by the higher fire resistance time of Test Specimen 3, which was achieved by the reduced temperature rise in joists by the use of external insulation. This study has shown that the use of cavity insulation led to poor thermal and structural performance of LSF floors. In contrast, the thermal and structural performance of externally insulated LSF floor system was superior than conventional LSF floors with or without cavity insulation. Details of the three fire tests and the results are presented and discussed in this paper. Details of experimental results including the temperature and deflection profiles measured during the tests are also presented along with the joist failure modes.
Acknowledgements
The authors would like to thank Australian Research Council for providing financial support to this project and Queensland University of Technology for providing fire testing facilities and the technical support to conduct this research project.
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12. Kesawan, S. and Mahendran, M. Predicting the Performance of LSF Walls Made of Hollow Flange Sections in Fire. Thin-Walled Structures Vol.98 Part A, pp.111-126. 13. Klippstein, K.H. (1978), Behaviour of Cold-formed Steel Studs in Fire Tests, American
Iron and Steel Institute, Washington, DC, USA.
14. Kolarkar, P. N. and Mahendran, M. (2008), Thermal Performance of Plasterboard Lined Steel Stud Walls, Proceedings of the 19th International Specialty Conference on Cold Formed Steel Structures 2008, St. Louis, Missouri, USA, pp.517-530.
15. Outinen. J., Kaitila, O. and Mäkeläinen, P. (2001), High-Temperature Testing of Structural Steel and Modelling of Structures at Fire Temperatures, Research Report, Helsinki University of Technology Laboratory of Steel Structures Publications TKK-TER-23, Espoo, Finland.
16. Ranby, A. (1999), Structural Fire Design of Thin Walled Steel Sections, Licentiate Thesis, Department of Civil and Mining Engineering, Lulea University of Technology, Stockholm, Sweden.
17. Sakumoto, Y., Hirakawa, T., Masuda, H. and Nakamura, K. (2003), Fire Resistance of Walls and Floors Using Light-Gauge Steel Shapes, Journal of Structural Engineering, pp.1522-1530.
18. Standards Australia (SA) (1997), Gypsum linings in residential and light commercial construction – application and finishing. Part 1: Gypsum Plasterboard, AS/NZS 2589.1, Sydney, Australia.
19. Standards Australia (SA) (2005), Methods for fire tests on building materials, components and structures, Part 4: Fire-resistance tests of elements of building construction, AS 1530.4, Sydney, Australia.
20. Standards Australia (SA) (2005), Cold-formed steel structures, AS 4600, Sydney, Australia.
21. Sultan, M. A., Seguin, Y. P. and Leroux, P. (1998), Results of Fire Resistance Tests on Full-Scale Floor Assemblies, Internal Report No. 764, Institute for Research in Construction, National Research Council of Canada, Ottawa, Ontario, Canada.
(a) LSF floor frame
(b) Joist and Track Sections (c) Screw connecting joist and track Figure 1: Details of LSF Floor Frames
15mm 15mm 40mm 180mm 40mm 50mm 182mm 50mm Track Joist Joist Track Track
Interior Joists Exterior Joists
Figure 2: Fixing of Plasterboard, Insulation and Plywood (c) Insulation layer next to
exterior joists
Insulation Plasterboards
(e) Protection of plasterboard joints (f) Fixing of plywood boards Back block pieces
(d) Details of back-blocking 200 mm
200 mm
(b) Staggered plasterboards (a) Screw spacing along the
(a) Joist (b) Plasterboard
Figure 3: Fixing of Thermocouples
Figure 4: Test Specimen 1 (c) Inside the cavity
fire side (d) Inside the cavity ambient side
(a) Rock fibre insulation in cavity
(b) Placing the ambient side plasterboard face layer Figure 5: Test Specimen 2 with Cavity Insulation Washers to hold rock fibre insulation
(a) Edge strips placed to hold the insulation
(b) After placing the rock fibre insulation
(c) After fixing the ambient side external layer Figure 6: Test Specimen 3 with External Insulation
Edge strips Sealed horizontal joint
(a) Gas furnace and Portal Frame
(b) Specimen Supports Figure 7: Fire Test Set-up
Angle section Angle section
Data logger
Universal beam Universal column
Diagonal struts Thermocouple wires
(a) Load distribution unit
(b) Load cell connected to hydraulic jack Figure 8: Loading System Spreader beams Load cell Loading plate Hydraulic Jack SHS Hydraulic jack
Figure 9: LVDTs for Deflection Measurements Wooden beam LVDT
Wooden beam LVDT
Ambient Side
Fire Side
a) Test Specimen 1 without insulation Ambient Side
Fire Side
b) Test Specimen 2 with cavity insulation Ambient Side
Fire Side
c) Test Specimen 3 with external insulation Figure 10: Thermocouple Locations
(a) Heavy smoke (b) Test Specimen after the Fire Test Figure 11: Fire Performance of Test Specimen 1
(a)
Joist 2 (b) Joist 3 Figure 12: Failure Modes of Joists in Test 1
Smoke Joist Joist Joist Joist
Figure 13: Average time - Temperature profiles of Plasterboard and Plywood Surfaces in Test Specimen 1
Figure 14: Lateral Deflection - Time Profiles of Test Specimen 1 at middle level (0.5L)
0 200 400 600 800 1000 1200 0 10 20 30 40 50 60 70 80 90 100 110 Tem per at ur e ( oC) Time (min)
AS 1530.4 Furnace FS Pb1-Pb2 Pb2-Cav Ply-Cav AS
0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 80 90 100 110 Lat er al D ef lect io n ( m m ) Time (min)
(a) Exposed plasterboards (b) Cavity insulation (Rock fibre) Figure 15: Fire Performance of Test Specimen 2
(a) Joist 2 (b) Joist 3 (c) Local web buckling along Joist 2 Figure 16: Failure Modes of Joists in Test 2
Figure 17: Average time - Temperature Profiles of Plasterboard Surfaces in Test Specimen 2
(a) at upper level (0.75L)
0 200 400 600 800 1000 1200 0 10 20 30 40 50 60 70 80 90 100 Tem per at ur e ( oC) Time (min) AS 1530.4 Furnace FS Pb1-Pb2 Pb2-Cav Pb3-Cav Pb3-Pb4 AS 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 90 100 Lat er al D ef lect io n ( m m ) Tmie (min)
(b) at middle Level (0.5L)
Figure 18: Lateral Deflection - Time Profiles of Test Specimen 2
Figure 19: Fire Performance of Test Specimen 3
0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 80 90 100 Lat er al D ef lect io n ( m m ) Time (min)
Joist 1 Joist 2 Joist 3 Joist 4
(b) Gap in insulation
(c) Screws bent on the fire (a) Partial collapse of fire side
(a) Joist 2 (b) Joist 3 Figure 20: Failure Modes of Joists in Test 3
Figure 21: Average time - Temperature Profiles of Plasterboard Surfaces in Test Specimen 3 0 200 400 600 800 1000 1200 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Tem per at ur e ( oC) Time (min)
(a) at upper level (0.75L)
(b) at middle level (0.5L)
Figure 22: Lateral Deflection - Time Profiles of Test Specimen 3
0 5 10 15 20 25 30 35 40 45 50 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Lat er al D ef lect io n ( m m ) Time (min)
Joist 1 Joist 2 Joist 3 Joist 4
0 5 10 15 20 25 30 35 40 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Lat er al d ef lect io n ( m m ) Time (min)
(a) Average-time temperature profiles of joists
(b) Average time-temperature profiles of hot flanges of interior joists
0 50 100 150 200 250 300 350 400 450 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Tem per at ur e ( oC) Time (min)
Test1-Avg J1 Test1-Avg J2 Test1-Avg J3 Test1-Avg J4 Test2-Avg J1 Test2-Avg J2
Test2-Avg J3 Test2-Avg J4 Test3-Avg J1 Test3-Avg J2 Test3-Avg J3 Test3-Avg J4
0 50 100 150 200 250 300 350 400 450 500 0 20 40 60 80 100 120 140 Tem per at ur e ( oC) Time (min)
(c) Average time-temperature profiles of cold flanges of interior joists
(d) Difference between hot and cold flange temperatures of joists
Figure 23: Average Time–Temperature Profiles of Steel Surfaces in Test Specimens 1, 2 and 3 0 50 100 150 200 250 300 350 0 20 40 60 80 100 120 140 Tem per at ur e ( oC) Time (min)
Test 1 Test 2 Test 3
0 50 100 150 200 250 300 350 400 450 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Tem per at ur e ( oC) Time (min)
Test1-J1 Test1-J2 Test1-J3 Test1-J4 Test2-J1 Test2-J2
Figure 24: Lateral Deflection - Time Profiles at Upper Level
(a) Average time-temperature profiles of Pb2-Cav surface
0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Lat er al d ef lect io n ( m m ) Time (min)
Test1-Top 1 Test1-Top 2 Test1-Top 3 Test2-Top 2
Test2-Top 3 Test3-Top 2 Test3-Top 3
0 100 200 300 400 500 600 0 20 40 60 80 100 120 140 Tem per at ur e ( oC) Time (min)
(b) Average time-temperature profiles of Ply/Pb3-Cav surface
(c) Average time-temperature profiles of ambient surface
Figure 25: Average Time - Temperature Plots of Plasterboard Surfaces in Test Specimens 1, 2 and 3 0 50 100 150 200 250 300 350 400 0 20 40 60 80 100 120 140 Tem per at ur e ( oC) Time (min)
Test 1 Test 2 Test 3
0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 140 Tem per at ur e ( oC) Time (min)
Table 1: Details of LSF Floor Test Specimens and Results Test
No. Floor Configuration Insulation Failure Mode Time (mins) Failure 1 Plywood Plasterboard None Structural 107 2 Plasterboard Plasterboard Rock fibre (Cavity insulation) Structural 99 3 Plasterboard Rock fibre (External insulation) Structural 139
Table 2: Thermal and Structural Response of Interior Joists
Note: HF – Hot Flange; CF – Cold Flange; L.D – Lateral Deflection
Test Specimen 1 Test Specimen 2 Test Specimen 3
Time (min) HF (oC) HF-CF (oC) L.D (mm) HF (oC) HF-CF (oC) L.D (mm) HF (oC) HF-CF (oC) L.D (mm) J2 J3 J2 J3 J2 J3 J2 J3 J2 J3 J2 J3 J2 J3 J2 J3 J2 J3 30 121 106 48 34 11 9.5 100 131 31 68 7 7 75 72 25 23 5 6 60 208 204 124 119 12 11 199 236 121 158 8 8 109 104 34 30 5 6 90 392 371 166 155 22 20 440 450 348 354 21 21 152 150 65 67 7 7 99 - - - 491 504 388 398 73 48 - - - - 107 489 464 143 147 48 32 - - - - 120 - - - 298 272 134 118 12 13 139 - - - 379 358 143 138 48 38 Plasterboard J1 J2 J3 J4 Pb1 Pb2 Pb3 Pb4 Pb4 Pb3 Pb2 Pb1 J1 J2 J2 J3 J4 J4 J3 J1 Pb1 Pb2
