EXPERIENCE WITH THERMO IMAGING CAMERAS ON FIRE TESTS

Transcription

1 EXPERIENCE WITH THERMO IMAGING CAMERAS ON FIRE TESTS JAN PAŠEK 1, FRANTIŠEK WALD 2, ANTONÍN UHLÍŘ 3 ABSTRACT The paper is focussed to the experiences with measurements by the thermo imaging cameras during the fire test on the 4 th floor of the testing steel-concrete structure in the Cardington BRE laboratory performed January 16, The work presents analyses of the development of the temperature in the connections and elements, readings of temperatures on the structure and observations of the buckling of the lower flange of primary beam. The prediction of the emissivity from the reported matrix is discussed. 1. INTRODUCTION The instrumentation used in the seventh large test in BRE Cardington laboratory performed January 16, 2003 included thermocouples, strain gauges and displacement transducers, see [1]. A total of 133 thermocouples were used to monitor the temperature of the connections, the steel beams within the compartment, the temperature distribution through the slab and the atmosphere temperature within the compartment, see Fig. 1. An additional 14 thermocouples were used to measure the temperature of the fire protected columns. In the exposed and un-protected elements (fin plate and end plate minor axis) nine high temperature strain gauges were used. In the protected columns and on the slab a total of 47 ambient strain gauges were installed. Twenty-five displacement transducers were attached along the 5 th floor to measure the vertical deformation of the concrete slab. An additional 12 transducers were used to measure the horizontal movement of the columns and the slab. Ten video cameras and two thermo-imaging cameras, FLIR 695 PM, recorded the fire and smoke development, the deformations and temperature distribution, see Fig. 1 [2]. The thermo-imaging cameras were focused primarily on the development of the temperature 1 Lecturer, Czech Technical University in Prague, Dept. Building Structures, CZ Praha, Czech Republic jan.pasek@fsv.cvut.cz 2 Professor, Czech Technical University in Prague, Dept. Steel & Timber Struct., CZ Praha, Czech Rep. wald@fsv.cvut.cz 3 Research assistant, Czech Technical University in Prague, Dept. Steel & Tim. S., CZ Praha, Czech Rep. antonin.uhlir@fsv.cvut.cz 082-1

2 at the connections. The cameras were recording the matrix of the measured temperatures on the structure. The perpendicular projection dimension of the observed points (pixels) on the structure were due to resolution of the camera detector matrix 320 x 240 pixels, applied lens of 6 viewing field and the distance between cameras and the measured surfaces of 20 m about 5 mm. The temperatures scanned by the cameras were corrected by the temperatures measured by the thermocouples at the beam-to-beam and beam-to-column connections as well as the atmosphere temperature to precise the temperature on the connection surface A N B C D E Fire compartment Window F Videocamera Thermo imaging camera located on the structure of hangar West view West view C441 C447 C450 C444 C442 C448 C451 C443 C445 C452 C446 C449 C D1.5 Position of thermocouples E1.5 at primary beam and connection Fig. 1 Positions of the thermo imaging cameras on the structure of hangar 2. TEMPERATURE The thermo imaging cameras enable to visualise the development of the surface temperatures during the natural fire. The measured surface temperatures matrix 320 x 240 points obtained from camera was calibrated by temperature values of the visible thermocouples to increase the accuracy, which is limited due to changes of the temperature dependent emissivity, temperature changes of surrounding surfaces, reflected temperature development dependent on the radiation of surrounding surfaces and flames and their emissivity and last but not least chemical changes of gas (atmosphere) in the compartment resulting into the changes of emissivity and transparency of gas. Fig. 2a illustrates the matrix of temperatures after 57 min of the experiment, corresponding to the maximum temperature measured by the thermocouple on the lower flange of the secondary beam, 1088 C. The matrix is visualised by colours (or gray on black and white copy) in whole compartment temperature range from 220 C to 1050 C. Because of better readability, in Fig. 2b the range is shortened from 850 C to 1050 C. Detailed temperature readings along lines 01 to 06 (marked on Fig. 2b) are illustrated in Fig. 2c and 2d; continuance of the lines 01+04, and is discontinued by the tested specimen hung in the view field of the camera detector

3 1050,0 C 1050,0 C LI01 LI03 LI02 LI04 LI06 LI a) 220,0 C b) 850,0 C C IR01 C IR01 li01 li02 li03 li04 li05 li06 c) Line Min Max... li01 901,0 C 967,4 C li02 926,0 C 991,0 C li03 906,6 C 985,0 C d) Line Min Max... li04 974,6 C 1022,2 C li ,4 C 1047,7 C li ,8 C 1041,2 C Note: Scale of greys on figures is different to visualise temperatures. Fig. 2 The visualisation of the beam maximal temperature, in 57 min of the experiment; a) thermogram with full matrix; b) the same record with limited temperature scale and marked measured lines; c) and d) the temperature profiles at the lines marked above 930,0 C C IR01 LI01 LI02LI03LI04LI li01 li02 li li04 li a) ,0 C b) Line Min Max... li01 860,3 C 884,9 C li02 858,0 C 885,3 C li03 844,3 C 860,3 C li04 853,5 C 884,0 C li05 853,5 C 887,6 C 597,1 C 523,1 C c) 390,0 C d) 391,4 C Note: Scale of greys on figures is different to visualise temperatures. Fig. 3 Imagine at a) maximal connection temperature in 63 min of experiment; b) the temperature profiles on lines in 63 min of experiment, where the fin plate reached 908,3 C; and c) in 92 min of experiment by the highest temperature differences; d) continuing cooling of the structure surfaces 082-3

4 The maximum temperature on the fin plate connection was measured after 63 min of the experiment. Fig. 3a illustrates the observed vertical lines positions in the connection; the influence of the developed fire flames temperature on total surface temperature detected by camera is clear. Fig. 3b shows the adequate measured temperatures at marked lines. On cooling the joints are hotter than the surrounding structure surfaces. The highest difference between the temperatures of the connection and the beam was reported after 92 min, see Fig. 3c. The continuing cooling of the connection and whole structure surfaces is reported on Fig. 3d. 3. LOCAL BUCKLING The experiment in BRE Cardington laboratory January 16, 2003 was oriented to the connections behaviour and their reliability. Two video cameras were placed behind the windows in the plasterboard walls. Both cameras failed due to burning of cables for lightning. The visibility in the wavelength radiation observed by the thermo imaging cameras allows reporting the mechanical behaviour during the test even in the phases with heavy smoke development. The used lens 6 view field and distance of cameras leads to visualized points of size about 5 x 5 mm, which enable to observe the behaviour of local flange. The development of the local buckling is shown in 25 min of fire at Figs 4a, in 35 min 4b and in 110 min 4c. The Fig. 4d shows the observed beam to beam fin plate connection after the experiment. a) b) c) d) Note: Scale of greys on figures is different to visualise contours. Fig. 4 The observed connection with visible local buckling of the lower flange of the primary beam a) in 25 min of fire b) in 35 min; c) in 110 min; and d) after cooling of the structure 082-4

5 4. EMISSIVITY S TEMPERATURE DEPENDANCY The thermo cameras measurement enables to calculate the temperature of observed surface based on the non-contact scanned infrared radiation emitted by this surface. The accuracy of prediction depends on correct estimation of external factors affecting the calculation; see [3] and [4]. One of the major parameters is the emissivity of the scanned surface ε. The non-contact temperature measurement is often utilised vice versa to calculate the emissivity based among others on exact contact technique measured temperature in the points of interest and surrounding surfaces as well as on atmosphere temperature. For the emissivity prediction the primary beam secondary beam connection surface without corrosion after its scorch and the measured point identical with position of thermocouple were used. The point C442 was chosen inside the structure (in the corner of primary and secondary beam connection) where the impact of the thermal radiation emitted by the surrounding surfaces on the point of interest temperature was estimated from 2 / 3 till 3 / 4, see Tab. 1. The gas temperature was measured, see [1]. The temperature dependent emissivity of the gas was expected from 0,42 to 0,29 according to [5]. The contamination of the air by the products of combustion changes rapidly the transparency of air and its permeability for thermal radiation detected by thermocamera. These affects were eliminated so that the emissivity analysis was done on the thermograms created on the structure after combustion during cooling part of experiment. Time, min Steel temperature θ a C, Thermocouple C442 Tab. 1 Estimation of the emissivity of steel structure Gas temperature θ g C, Thermocouple C525 Emissivity ε for 66 % radiation of surrounding surfaces Emissivity ε for 75 % radiation of surrounding surfaces Estimated emissivity ε ,65 0,56 0, ,63 0,54 0, ,61 0,50 0, ,60 0,48 0, ,58 0,47 0, ,57 0,46 0, ,56 0,45 0,53 5. CONCLUSIONS The thermo imaging cameras helped to visualise the development of the temperatures during the natural fire experiment. With support of measurements by thermocouples located in the observed field of the camera the surface temperatures were predicted with good accuracy. The non-contact measured matrix of temperatures showed the structural behaviour and its contours in cases where the visibility of the video cameras is limited. The calculation of the relative change of the position may enable to observe the deflections as well. For known temperatures of the structure surfaces, surrounding structures and atmosphere the surface emissivity of the structure during the natural fire may be observed with limited accuracy due to the complexity of radiation. The non-contact temperature measurements by thermo imaging camera brings a good experiences in development of fire safe connection of sandwich panels, see [6]

6 ACKNOWLEGEMENT This outcome has been achieved with the financial support of the Czech Ministry of Education, Youth and Sports, projects No. VZ MSM and MSM REFERENCES [1] Wald F., Silva S., Moore D. B, Lennon T., Chladná M., Santiago A., Beneš M.: Experimental behaviour of a steel structure under natural fire, The Structural Engineer, New Steel Construction, 3/2005, pp , ISSN [2] Wald F., Chladná M., Moore D., Santiago A.: Temperature distribution in a full-scale steel framed building subject to a natural fire; Steel and Composite Structures, Vol. 6, No. 2 (2006), in printing. [3] Madding R. P.: Emissivity measurement and temperature correction accuracy consideration, proc. Thermosence XXI., Vol. 3700, SPIE, 1993, pp [4] Pašek J., Svoboda J.: Physical aspects of the application of non-contact thermography in the analysis of external skin of buildings, Stavební obzor, 3/2004, pp [5] Ghojel J. I.: A new approach to modeling heat transfer in compartment fires, Fire safety journal 31, 1998, pp [6] Uhlíř A., Wald F.: On development of fire safe connection of sandwich panels, internal report, Czech Technical University in Prague, 2005, p