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Temperature of a partially embedded connection subjected to fire
Fire Safety Journal 54 (2012) 121–129
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Temperature of a partially embedded connection subjected to fire Jiˇrı´ Chlouba, Frantiˇsek Wald n Czech Technical University in Prague, Czech Republic
a r t i c l e i n f o
abstract
Article history: Received 29 October 2011 Received in revised form 7 August 2012 Accepted 13 August 2012 Available online 8 September 2012
When subject to fire, structural steel and connectors lose both their strength and stiffness. Structures expand when heated and contract on cooling. Furthermore, the effect of restrained thermal movement can introduce high strains in both the steel member and the associated connections. Fire tests on steel structures have shown that the temperature within the connections is lower compared to the connected steel members. The beneficial effect of the partial embedding of a connection in a concrete slab for its resistance in fire conditions was already intuitively utilised by structural engineers. Three sets of tests provide the measuring of the accuracy of the developed numerical and analytical model for heat transfer and temperature distribution in the partially embedded connections. The connection shadow factor is proposed further to allow the prediction of temperatures in connections from the calculated gas temperature during a post-flashover fire. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Fire design Composite steel and concrete structure Heat transfer Connection design Partially embedded connection
1. Introduction The fire resistance of structures has been traditionally verified by experiments on fire protected or unprotected elements. Currently, the test results are substituted for a member analysis of structural elements. Higher economy can be achieved by verifying the overall structural behaviour; particularly when accounting for the membrane action of the composite steel and concrete floors of multi-storey buildings. In such cases, structures may meet the requirements also being only partially fire protected, see [1]. There are two approaches to the fire design of steel connections. Using the first approach, fire protection is applied to the member and its connections. The level of fire protection is based on the protection of the connected members. In the design, the degree of efficiency is considered, which may vary between the connections and connected members. More detailed second approach is uses an application of the component procedure described in EN1993-1-8, see [2], together with a method for calculating the behaviour of welds and bolts at elevated temperatures. Using this approach, the connection moment, shear and axial capacity can be evaluated at elevated temperatures with a good level of accuracy by simple or advanced models of mechanical behaviour, see [3]. The accuracy of the design depends on prediction of the temperature of elements and of forces acting in the connection during the fire.
n
Corresponding author. Tel.: þ420 224354757; fax: þ420 233334766. E-mail addresses: [email protected] (J. Chlouba), [email protected] (F. Wald). 0379-7112/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.firesaf.2012.08.008
The thermal conductivity of steel is high. Nevertheless, because of the concentration of material within the joint area, differential temperature distribution should be considered within the joint. Various temperature distributions have been proposed or used in experimental tests by several authors, see [4]. In an attempt to quantify the temperature distribution within a joint, tests have been done on several joint typologies, see [5]. One example for larger beams, a web temperature similar to the bottom flange temperature, is provided while for smaller beams, a smaller web temperature is provided, see [6]. Additionally, the presence of the concrete slab above the joint causes a reduction in beam top flange temperatures, see [7]. A detailed description can be found in the literature [8,9]. The values proposed in EN1993-1-2, see [10], are in agreement with these experimental results. However, these values are based on a nominal standard fire curve, if the fire follows other curves it is necessary to analyse the particular case using a numerical or experimental study, see [11,12]. According to EN1993-1-2, the temperature of a joint may be assessed using the local massivity value A/V of the joint components. As a simplification, a uniform distributed temperature may be assumed within the joint; this temperature may be calculated using the maximum value of the ratios A/V of the adjacent steel members. For beam-to-column and beam-to-beam joints, where the beams support any type of concrete floor, the temperature may be obtained from the temperature of the bottom flange at midspan, see Fig. 1. The temperature of the joint components at its height hk may be determined, see Annex D of EN1993-1-2, as follows. The depth of the beam hb is less or equal to 400 mm
yk ¼ 0:88 y0 ½10:3ðhk =hb Þ
ð1Þ
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O
Nomenclature hb heq h k hnet kcon qf,k x A A/V Av
ac e
depth of the beam weighted average of window heights on all walls components at its height thermal flux connection shadow factor fire load density distance from upper surface of slab total area of enclosure local massivity value total area of vertical openings on all walls
y0 yg yh ym yx
s F
where y0 is the temperature of the lower flange of the beam at midspan. The depth of the beam is greater than 400 mm
pffiffiffiffiffiffiffi opening factor O ¼ Av heq =At coefficient of heat transfer by convection emissivity of surface temperature of the lower flange of the beam at midspan gas temperature the temperature of the bottom surface of slab element temperature the temperature in distance x in mm from bottom surface of the slab Stephan–Boltzmann constant configuration factor
2. Experiments 2.1. Header plate connection with a stiffener
yk ¼ 0:88y0 hk
hk is less than hb =2
ð2Þ
yk ¼ 0:88y0 ½10:2ðhk =hb Þ hk is greater than hb =2
ð3Þ
The above prediction is based on findings on structural configuration similar to Cardington frame, see [12], and is applicable for similar structures and fires only. Heat transfer reduction to the connection may be one of the possible solutions to enhance structural fire resistance, see [13]. One proposed method is to ensure sufficient resistance by embedding a part of the connection into the concrete slab. Direct contact with hot gases is eliminated and concrete creates a thermal protection in the connection. As a consequence to this structural solution, temperatures in the connection during a fire reach significantly lower values in comparison to the temperatures in the connections without embedding. A series of tests with partially embedded connections were carried out between 2007 and 2009 at the Czech Technical University in Prague, see [14]. These tests consisted of two fire tests in a furnace and on an experimental building. During these tests, the temperature distribution in the connection was measured to understand the effect of embedding in concrete. The most precise solution of heat transfer is a numerical model. The accuracy of the model relies on the accuracy of the input data, which are material properties and boundary conditions. The connection shadow factor is proposed further to allow the prediction of temperatures in the connections from the calculated gas temperature during a postflashover fire. A simple prediction model of the temperatures in the connection, which are calculated from the bottom flange midspan temperature, is measured and added to the temperature prediction in the concrete slab above the connection.
The first experiment with the embedded connection took place in the PAVUS testing facility in 2007. The specimen consisted of a composite steel and concrete beam with a cross section IPE 160 with a span of 3.0 m. The concrete C25/30 slab, 100 mm thick, was concreted in trapezoidal sheeting TR 50/250/0.75. The reinforcement diameter, 5.0 mm, with a distance of 150 150 mm was located 30 mm from the upper surface. The beam was connected to a supporting frame on both sides with a partially embedded header plate connection. The header plate connection, a 6 mm plate, bolts M16 class 8.8, was designed with an 8 mm plate stiffener on the top of the beam. The welded frame from cross sections HE 200 B was designed to simulate the plane stiffness of the composite floor. A view from the furnace on lower face of the specimen is shown in Fig. 2. In a four point bending test in a furnace, the beam was loaded by self-weight and a mechanical load by a hydraulic jack with a cross member to create two forces of 25 kN. The fire load followed the measured gas temperature during the Seventh Large Fire Test on a steel building in Cardington, see [15]. The temperatures were measured in the lower flange of the beam, the lower bolt, the header plate next to the lower bolt, the upper encased bolt and on the top of the concrete slab Fig. 3). Temperature development measured with thermocouples is shown in Fig. 4. The temperature profile measured at the time when the temperature in the lower flange of the beam reached the maximum is shown in Fig. 5. From the profile, it can be seen that the influence of the encasing is significant; when the temperature in the lower bolt reached 711 1C at 55 min, the upper bolt in the same time showed only 249 1C. Due to good thermal conductivity of the steel, the temperature of the lower bolt and of the header plate next to the bolt during the whole fire test was about the same.
Composite steel and concrete slab
0,62
hb
hk
0,75
0,88 hb< 400 mm Fig. 1. Thermal gradient within the depth of connection.
0,70
0,88
0,88 hb> 400 mm
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123
Slab surface 221 °C Upper bolt
249 °C Lower bolt 711 °C 726 °C Next to lower bolt
1050 °C
Lower flange Fig. 5. Temperature profile of the header plate connection with a stiffener in 55 min of the fire test. Fig. 2. View of specimen from the furnace.
Temperature, °C
Gas temperature in furnace Lower flange,TC13 End plate,TC02
1000 Upper bolt, TC03 800
End plate, TC04 Upper bolt, TC05
600
400
200 Upper surface, TC06
Fig. 3. Header plate connection with a stiffener.
0 0
Temperature, °C
30
60
90
120
150
Time min
Fig. 6. Measured temperature in the header plate connection without a stiffener.
Gas
1000
Lower flange Lower bolt
800
Next to lower bolt 600
400
200
Upper bolt Slab surface
0 0
30
60
90
120 Time, min
were changed to M12 grade 8.8. The stiffening plate was left out and the header plate was extended towards the lower flange of the beam to ensure sufficient shear resistance of the beam web. Other dimensions such as bolt quality remained unchanged. The beam was loaded by a mechanical load by a hydraulic jack to create a four point bending test with the forces of 30.2 kN. The fire load followed the measured gas temperature during the Cardington Fire Test, see [15]. The temperatures were measured on both connections. Temperature development was measured with thermocouples at the connection without a stiffener compared to gas temperature in Fig. 6. The temperature in the free part of the connection decreases linearly and in the embedded part nonlinearly. Fig. 7 shows the measured temperature profile at 56 min when the temperature in the lower flange of the beam reached the maximum.
Fig. 4. Temperature of the header plate connection with a stiffener.
2.3. Measured temperatures in connections on the experimental building 2.2. Header plate connection without stiffener The fire experiment performed in 2009 took place in the same horizontal furnace as the test in 2007. The setting of the experiment was similar. The beam and the supporting frame were the same; the only two changes were in the geometry of the connection to increase its ductility and simplify its detailing. To reach more balanced resistances of all the connection components, the bolts
One of the objectives of the fire test on the experimental building in Mokrsko, see [16], was the thermal and mechanical behaviour of the connections partially encased in a concrete slab. The structure was designed to allow simple as well as advanced modelling of today’s modern buildings. The experimental structure represents a part of a floor of an administrative building of 18 12 m and 4.00 m high. One half of the floor was covered by a composite slab supported
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Upper surface of the slab, TC06
P10-65x200 Plate
146 °C
35
90
35
Upper bolt, TC05
35 End plate, TC04 top part
287 °C
120
35 50
Bottom bolt, TC03
P10-155x160 Bolt 10.2
650 °C 709 °C
IPE 270-395 Beam
742 °C
HEB 180 Column
End plate, TC02 bottom part 1047 °C
Fig. 9. Geometry of the beam to column connection in the Mokrsko fire test 2008.
Lower flange, TC13 Fig. 7. Temperature profile in the header plate connection without stiffener in 56 min of fire test.
B
A 9 000 Window opening
N
C
9 000 Window opening
3 AS7 AS6 6 000
AS5 AS4
2 Cladding
Slab Slab
Beams with corrugated web CS2
Cellular beams
CS1 S5
AS3
S4 AS2
6 000 1
CS4 CS3
Hollow core panels
AS1 Sandwich panels
S3 S2 S1
Concrete wall
Door
+4,00
+0,00
Fig. 8. Fire compartment with description of the major floor and wall structures and connections in the Mokrsko fire test 2008.
by non-fire protected composite castellated beams with large openTM ings: ArcellorMittal Angelina , see Fig. 8. The composite slab on the castellated beams was designed with a span of 9–12 m and on the beams with corrugated webs with a span of 9–6 m. The deck was a simple trapezoidal composite slab with a thickness of 60 mm and the height over the rib of 120 mm with a sheeting of CF60, Cofraplus 0.75 mm, and the concrete of cubic strength 34 N/mm3 in 28 day reinforced by a smooth mesh ø 5 mm 100/100 mm; with 500 MPa strength and 20 mm of coverage. The castellated beams with sinusoidal shape openings were made from profile IPE 270 and its web height was 395 mm. The header plate connection with bolts M20 grade 8.8 and thicker header 10 mm were designed partially encased in the concrete slab with a stiffener at the top of the beam. The geometry of the connection is pictured in Fig. 9.
The fire load was made of rough battens from soft pine wood, a total volume of 15 m3 and the fire load reached 515 MJ/m2. Two window openings in the front wall, 2.43 4.0 m, provided air supply into the fire compartment. The gas temperature round the beams was measured during the test by jacketed thermocouples 3 mm in diameter at the level of the lower flange and the steel temperature by 2 mm thermocouples. A total of four connections were equipped, of which two were beam to column and two beam to beam connections. The temperature in these connections was measured in similar places as in the laboratory test—at the lower flange of the connected beam, at the web of the beam in one third of the height, at the lower bolt, at the header plate next to the lower bolt, at the upper flange, at the upper encased bolt and at the header plate next to the upper bolt, see Fig. 10. In Fig. 11 measured temperatures in the beam to column connection on beam AS4 are given. The upper bolt temperature reached its maximal temperature 157 1C at the 58. min; see Table 1, while the maximal temperature of the lower unprotected bolt was 520 1C at the 60. min. The temperature along the end plate height was close to the bolt temperatures. The maximal temperature was 201 1C at 61 min of fire and the lower part reached maximum 505 1C at 60 min. For the beam to beam connection at beam AS5 the temperatures are summarised in Fig. 12. At 62 min of the fire test, the temperature of the upper bolt was 198 1C and for the embedded end plate 253 1C, while the lower bolt had already reached 410 1C and the unprotected part of the end plate 397 1C. The differences in temperatures in the beam to column connection are lower due to the plated fire protection of the primary beams, while the central column was protected only 50 mm below the connection.
3. Numerical simulation of heat transfer 3.1. Model and shadow effect A numerical model for analysing the transfer of heat and mechanical behaviour was developed with the SAFIR code with the help of pre-processor GiD, see [17], to assist the preparation of an analytical prediction model by a sensitivity study of the fire scenarios, see [18]. Five material properties were applied: steel column, beam, end plate and bolt and concrete slab. The predefined values of the material properties were used. The brick elements used were from 200 mm3 to 35 100 mm3 in size. The mesh was refined to the direction of the heated surface. During the thermal analyses, all nodes have one degree of freedom,
J. Chlouba, F. Wald / Fire Safety Journal 54 (2012) 121–129
1000
125
Temperature, °C TC48
TC54
800 TC55
600
TC53
TC55
TC47
TC50 TC51 TC51
400
TC48 TC53
TC50
TC47
200 TC54
0 0
15
30
45
60
75
Time, min
Fig. 11. Measured temperature in beam to column connection during the Mokrsko fire test, beam AS4.
the non-heated upper slab surface the boundary conditions for gas were assumed at 20 1C. In case of one direction heat transfer, thermal energy is given by thermal flux which consists of gas convection and radiation and may be expressed in the form
hnet ¼ ac ðyg ym Þ þ Fesððyg þ273Þ4 ðym þ 273Þ4 Þ
ð4Þ
where ac is the coefficient of the heat transfer by convection, yg is the gas temperature, ym is the element temperature, F is the configuration factor, e is the emissivity of the surface, and s is the Stephan–Boltzmann constant. The connection surface itself and its components are not fully exposed to flames. The part of the heat transfer by conduction depends on the structural configuration and is neglected in the simplified prediction of the temperature in connections, as in members as well. The heat transfer by convection is influenced by the connection position and the ventilation of the fire compartment. The heat transfer by radiation depends on the position of the connection and flames during a fire. The described geometry of the fire compartment the fire configuration factor F may take into account the position of the connection. In this study, the influence of the position of the connection was taken as analogous and both contributions, heat transfer by convection and radiation, were simulated by one connection shadow factor kcon, which may reach values from 0.0 to 1.0. Eq. (4) for one direction heat transfer was modified to
hnet ¼ kcon ac ðyg ym Þ þ kcon esððyg þ 273Þ4 ðym þ273Þ4 Þ
Fig. 10. Thermocouples on (a) beam to beam and (b) beam to column connections in the Mokrsko fire test 2008.
temperature. There are heated elements in the fire compartment. The boundary condition at the heated surface was considered the heat transfer by convection and the heat transfer by radiation between the surface and the surrounding gas temperature. For
ð5Þ
In the numerical simulations, the value of the surface emissivity 0.70 and the value of the coefficient of heat transfer by convection 35 W/m2K were used. The FEM model of the connection consists of the composite steel and concrete beam with a connection and trapezoidal sheeting with a concrete slab. The size of the model was driven by good representation of the heating and cooling sections. Based on a sensitivity study, the width of the slab was selected as a beam flange plus 300 mm to both sides. The bolts are simulated as cuboid, see Fig. 13. For the model of the composite beam in a horizontal furnace, thermal insulation was applied as a separate material. The values of the connection shadow factors, see Table 2, were find empirically by calibration so that the numerical results agree with the experimental ones. The experiment with the header plate connection with a stiffener in a furnace was used to derive the presented values. The values were used to simulate the header plate connection without the stiffener tested in a furnace and on an experimental building with good accuracy. The low value of the shadow effect for beam reflects the lower flange influence. The values are also influenced by the simplified geometry of the bolt head and nut as rectangle, see Figs. 13, 14 and 17b.
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Table 1 Measured beam to column connection, beam lower flange at mid span and gas temperatures during the Mokrsko fire test at beam AS4. Time (min)
Temperature in 1C measured by thermocouples in connection
0 15 20 25 30 35 40 45 47 50 52 55 60
Gas
TC 47
TC 48
TC 49
TC 50
TC 51
TC 53
TC 54
TC 55
TC 43
TG08
TG9
10.0 134.1 211.7 360.2 476.0 585.1 658.3 715.5 727.4 748.3 758.7 751.4 749.9
10.9 193.4 283.5 434.4 536.2 622.7 670.4 726.1 735.8 772.9 785.8 769.0 762.6
10.4 163.3 239.3 384.4 486.1 581.3 633.1 693.7 702.7 734.5 750.4 738.4 731.8
9.8 96.2 142.6 252.2 276.2 345.1 395.6 446.2 463.3 479.2 493.8 500.6 520.0
10.2 79.3 115.2 157.8 205.9 304.6 364.0 422.6 440.1 467.3 482.5 490.3 504.8
9.6 26.1 32.8 77.9 87.4 87.3 95.7 119.6 132.7 148.0 157.7 167.3 195.5
9.6 23.6 38.2 73.5 93.9 100.4 101.2 114.1 129.7 142.8 135.5 136.6 156.0
10.4 99.1 152.2 237.9 303.3 395.5 466.8 523.3 538.1 557.8 570.4 574.4 587.8
13.8 326.4 457.5 606.4 683.8 780.7 781.6 853.1 918.5 919.3 939.8 907.2 916.2
20.3 497.1 546.2 742.4 798.4 819.4 851.8 942.5 966.9 947.4 961.5 923.0 931.0
20.1 534.2 625.6 756.2 827.8 851.7 859.7 922.2 935.9 901.7 922.6 889.6 887.7
Temperature, °C
1000
At mid span
Table 2 Connection shadow factor kcon from the fire test with header plate connection with a stiffener.
TC64 TC70
800
TC69
TC71
TC63 600
TC66
TC71
TC66
TC67
400 TC69 TC67 200
TC64
Component
kcon
Slab Bolts Frame/column Beam End plate
0.40 0.30 0.60 0.25 0.60
TC63
TC70 0 0
15
30
45
60
Time, min
Fig. 12. Measured beam to beam connection temperature during Mokrsko fire test, beam AS5.
Fig. 14. The distribution of temperatures on connection in 60 min for fire test with header plate connection with a stiffener.
Temperature,°C
Lower flage, numerical model Lower flage, experiment
1000
Lower bolt, model Lower bolt, experiment
800
Upper bolt, model Upper bolt, experiment
600 Fig. 13. Model of connection from the fire test with header plate connection with a stiffener.
3.2. Tests comparisons
400 200
Concrete surface, experiment Concrete surface, model
The distribution of temperatures on the header plate connection with a stiffener at 60 min of fire test is provided in Fig. 14. A comparison of the calculated and measured temperatures during the fire test with the header plate connection with a
0 0
15
30
45
60
75
90
105
120Time, min
Fig. 15. Comparison of numerically predicted and measured temperatures during the fire test with header plate connection with a stiffener.
J. Chlouba, F. Wald / Fire Safety Journal 54 (2012) 121–129
Temperature, °C
127
Flange, experiment TC13 Flange, model Lower bolt, TC03
1000
Lower bolt, model
800
TC06
Upper bolt, model
TC05
Upper bolt, TC05
600
TC03
400
TC13
200 Upper surface, TC06 Upper surface, model
0 0
30
60
90
120
150
Time, min
Fig. 16. Comparison of numerically predicted and measured temperatures during the fire test with header plate connection with a stiffener.
Fig. 17. Header plate connection during erection of experimental frame and its FEM representation.
stiffener shows a reasonable conservative prediction of temperatures, see Fig. 15. The difference in prediction is caused namely by the partial fire protection of the experimental frame, which simulates the overall structure. The model represents the cooling of the connection as well. 3.3. Test comparisons for header plate connections without a stiffener The numerical model of the header plate connection without a stiffener was similar to the model for the connection with a stiffener. The geometry was adjusted to reflect the longer free length of the connection. The comparison of the predicted temperatures of the major connection components to measured ones is in Fig. 16. The maximal calculated temperature of the bottom flange was 1058 1C and during the experiment the temperature reached 1047 1C. Also, the calculated maximal bottom bolt temperature 757 1C corresponds to the measured one, which was 729 1C. The top bolt predicted temperature 398 1C was not reached in the test; it measured 360 1C only. The agreement is good. The initial part of the simulation is affected by the difficulties of predicting water evaporation and missing its transport in the model. The model approved the values of connection shadow factor kcon derived for the previous test. The FEM model of transfer of heat to a partially embedded connection was applied to the measured results from the Mokrsko Fire Test on an experimental structure. The header plate connection during the erection of the experimental frame and its FEM representation is shown in Fig. 17. The distribution of the temperatures in the connection is shown in Fig. 18 with the help of the postprocessor code diamond at 60 min of the experiment.
Fig. 18. The distribution of temperatures on header plate connection in 60 min of the experiment in Mokrsko by numerical model code SAFIR.
A comparison of the predicted temperature for the upper flange and bolt with the measured specimens, see Table 1, is summarised in Fig. 19. The development of gas temperatures at the level of the lower flanges of beams at mid span is included. The predicted temperatures were higher, e.g. on the safe side, compared to the measured ones, e.g. the calculated temperature of the bottom bolt was 520 1C and the measured one 513 1C The difference in the prediction of the upper flange to the measured ones was round 25 1C, when calculated the maximal temperature is (was) 613 1C and measured 588 1C. The variance in temperatures of the upper bolt from 21 to 26 min reflects the crack
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J. Chlouba, F. Wald / Fire Safety Journal 54 (2012) 121–129
1000
Temperature, °C Gas, experiment, TG08
Temperature, °C
TC54
Beam lower flange, mid span, experiment
TC55
800
Gas, experiment, 600 TG09
Connection, model
Gas, experiment
1000 800
Upper flange, experiment, TC54
600
TG09
Upper flange, TG08 model
400
TC2
400
Upper bolt, model
200
Connection, TC2, experiment
200
Upper bolt, experiment TC54
0 0
0 0
15
30
45
60
75
90
30
60
90
120
150 Time, min
Time, min
Fig. 19. Comparison of the numerically predicted temperature for the upper flange and bolt to measured ones for header plate connection during the experiment in Mokrsko.
openings in the concrete slab on the beam support. Also the maximal calculated temperature of the lower flange of the beam, 831 1C, was in good agreement with the maximal measured one, 759 1C, see [11].
Fig. 20. Comparison of calculated, by analytical model, and measured temperatures of the bottom bolt during the fire test with header plate connection with a stiffener.
Temperature, °C
Beam lower flange, mid span, experiment
Gas, experiment
1000 800
TC8
600
4. Analytical model
400
4.1. Model
Connection, model
200
For the analytical prediction of temperature the model in Annex D of EN1993-1-2 Eq. (1)–(3) was selected, see [10]. The distribution of the temperatures in the composite slab is influenced by the transport of water and its evaporation. The simple conservative model of the temperature profile for a slab until 3% humidity was derived from the presented three experiments only and the FEM simulation of the connections of the tested geometry. The temperature is expressed as a function of the distance to the surface of the slab exposed to fire only
Connection, TC2, experiment
0 0
90
120
150 Time, min
Temperature, °C Gas, experiment
ð6Þ
where yx is the temperature in distance x in mm from the bottom surface of the slab and yh is the temperature of the bottom surface of the slab. It may be expected that this is the temperature of the upper flange of the beam hk ¼hb and the temperature in the connection embedded in slab on beams with the depth of the beam hb less or equal to 400 mm may be given as
yx ¼ 0:62y0 eð0:02xÞ
60
Fig. 21. Comparison of calculated, by analytical model, and measured temperatures of the upper embedded bolt with header plate connection with a stiffener.
1000
yx ¼ yh eð0:02xÞ
30
Lower bolt A, experiment
800 600
TC03 TC08
400 Lower bolt B, experiment
200
Lower bolt, model
ð7Þ
The bolt temperatures in this simple analytical prediction depends on their position in the plate only. The coefficients were derived for the experiment in a furnace with the header plate connection with a stiffener. Eq. (7) provides a prediction of temperature of the header plate connection without a stiffener tested in furnace and on experimental building with good accuracy. For different fire conditions, e.g. cooling, and different steels/concretes, the dimensions were not approved. 4.2. Comparison to tests The comparison of temperature predicted by a simple analytical model, Eq. (1) for the part of the plate directly exposed to fire and Eq. (7) for the part of the plate imbedded in concrete, gives very good results compared to laboratory tests. Fig. 20 shows, that for the bottom bolt during the fire test with a header plate connection with a stiffener, the calculated temperature during the heating phase until 68 min is conservatively higher at about 70 1C. During the cooling phase, the model slightly gives a lower temperature. The prediction of the top bolt, see Fig. 21, is less
0 0
30
60
90
120
150 Time, min
Fig. 22. Comparison of calculated, by analytical model, and measured temperatures of the lover bolt with header plate connection without stiffener.
successful. The calculated maximal temperature, 334 1C, is higher compared to the measured one, 276 1C. The shape of the curve follows the beam midspan temperature and the delay in cooling in the model is not included. The improvements for the simple model are available, see [19], but for this joint the configuration is not necessary. The test with the header plate connection without a stiffener is compared to the analytical simulation of the lover bolt in Fig. 22. The temperature 407 1C is obtained from the calculation at 56 min. The recorded test maximum is 359 1C reached at 77 min. This simple model gives a good and safe prediction. Fig. 23 shows the comparisons of the analytically calculated and measured temperatures of a beam to column connection in the Mokrsko Fire Test. The maximal temperature in the lower bolt of the beam to column connection, which is predicted from Eq. (1) at 60 min as 606 1C, is higher compared to the measured one,
J. Chlouba, F. Wald / Fire Safety Journal 54 (2012) 121–129
which may support the measurement of radiation to the connection by radiometers or by adiabatic thermometers. At the Mokrsko fire tests, the radiometers were used to measure the distribution of the radiation to the composite beam in midspan only.
Temperature, °C
1000 Lower flange, experiment
800
129
TC54 Lower bolt, model
600 TC50
400
Lower bolt, experiment
Acknowledgements
TC47
Upper bolt, model
200
ˇ The preparation of the paper was supported by Grant GACR P105-10-2159.
Upper bolt, experiment
0 0
15
30
45
60
75
90
Time, min
Fig. 23. Comparison of calculated, by analytical model, and measured temperatures of beam to column connection in Mokrsko fire test.
which reached 520 1C at 52 min only. The maximal temperature in the upper bolt of beam to column connection is predicted at 285 1C at 52 min. From Eq. (7), which is higher compared to the measured one, 157 1C at 58 min. The measured temperatures from 45 min of the experiment were affected by cracks in the thin slab and slow wind, see [16].
5. Summary The beneficial effect of the partial embedding of a connection in a concrete slab for its resistance in fire conditions was approved by three set of tests to provide the measuring of the accuracy of the developed numerical and analytical models for heat transfer and temperature distribution in the partially embedded connections. Due to a sharp decrease of the material properties of structural materials at elevated temperatures from 300 1C till 700 1C, namely of bolts and welds, the accuracy of the temperature prediction for the connection resistance and robustness is crucial, see [4]. The results of the calculations of different fire scenarios, see [14], proved a significant positive influence of the partial encasing of the connection in the concrete slab. The benefit is greater for fires with longer durations. The knowledge of temperature development in connection is the first step of the evaluation of connection resistance and robustness, which is taken into account in this contribution. The mechanical behaviour of the header plate connection at elevated temperature in case of fire exposure is developed based on the component method; see [3]. This method is applicable to partially embedded connections with the proposed prediction of the connection temperatures. The presented analytical method for the prediction of temperatures is based only on the tested geometry and the related FEM simulation. For different geometries further studies should be based on the separation of components of heat transfer to the connection,
References [1] B. Zhao, M. Roosefid, Connections of Steel and Composite Structures Under Natural Fire Conditions, RTD Project, RFCS, Final Report, 2009. [2] EN1993-1-8 Eurocode 3: Design of Steel Structures—Parts 1–8: General Rules—Design of Joints, CEN, 133 pp, 2005. [3] I. Burgess, A. Santiago, F. Wald, Component-Based Approaches to Steel and Composite Joints in Fire, in: Urban Habitat Constructions Under Catastrophic Events, The Authors and The Editors, Malta, 2008, ISBN:978-99909-44-40-2. ˇ ´ , Temperatures during [4] F. Wald, J. Chlouba, A. Uhlı´rˇ, P. Kallerova´, M. Stujberova fire tests on structure and its prediction according to Eurocodes, Fire Safety J. 44 (2009) 135–146. [5] Enhancement of Fire Resistance of Beams by Beam-to-Column Connections, Technical Report, The Steel Construction Institute, SCI Publication 086, 1990. [6] L.C. Leston-Jones, I.W. Burgess, T. Lennon, R.J. Plank, Elevated temperature moment-rotation test on steelwork connections, Proc. Inst. Civil Eng. Struct. Build. 122 (4) (1997) 410–419. [7] K.S. Al-Jabri, T. Lennon, I.W. Burgess, R.J. Plank, Behaviour of steel and composite beam-column connections in fire, J. Construct. Steel Res. 46 (1998) 1–3. [8] K.S. Al-Jabri, The Behaviour of Steel and Composite Beam-to-Column Connections in Fire, Ph.D. Theses, The University of Sheffield, December 1999. [9] R.M. Lawson, Behaviour of steel beam-to-column connections in fire, Struct. Eng. 68–14 (1990) 263–271. [10] EN1993-1-2, Eurocode 3: Design of Steel Structures—Parts 1–2: General Rules—Structural Fire Design, CEN, 78 pp., 2005. [11] K. Anderson, M. Gillie, Heat transfer modelling in connections, in: Proceedings of Fifths International Conference Structures in Fire, SiF08, May 2008, Singapore, pp. 229–237, ISBN:978-981-08-0767-2. [12] K. Anderson, M. Gillie, Investigation into methods for predicting connection, in: Application of Structural Fire Engineering, ASFE, 2009, Prague, Czech Republic, pp. 332–337, 978-80-01-04266-3, /eurofiredesign.fsv.cvut.czS. [13] J. Ding, Y.C. Wang, Temperatures in unprotected joints between steel beams and concrete filled tubular columns in fire, Fire Safety J. 44 (2009) 16–32. [14] J. Chlouba, Header Plate Connection with Improved Fire Resistance, Ph.D. Thesis, Czech Technical University in Prague, June 2011. ~ [15] F. Wald, L. Simoes da Silva, D.B. Moore, T. Lennon, M. Chladna´, A. Santiago, M. Beneˇs, L. Borges, Experimental behaviour of a steel structure under natural fire, Fire Safety J. 41 (2006) 509–522. [16] P. Kallerova´, F. Wald, Fire Test on Experimental Building in Mokrsko, CTU in Prague, Prague. 2009. 978-80-01-04146-8, URL: /fire.fsv.cvut.cz/ firetest_MokrskoS. [17] J.M. Franssen, A thermal/structural program modelling structures under fire, Eng. J. 42 (3) (2005) 143–158, URL:. [18] F. Hanus, J.M. Franssen, Distribution of temperature in steel and composite beams and joints under natural fire, in: Application of Structural Fire Design, Prague, pp. 467–473, 2011, ISBN:978-80-01-04798-9, /fire.fsv.cvut.cz/ASFE/ ProceedingsS. [19] A. Ticha´, The Fine Plate Connections in Fire, Ph.D. Thesis Czech Technical University in Prague, September 2010.
Contents lists available at SciVerse ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
Temperature of a partially embedded connection subjected to fire Jiˇrı´ Chlouba, Frantiˇsek Wald n Czech Technical University in Prague, Czech Republic
a r t i c l e i n f o
abstract
Article history: Received 29 October 2011 Received in revised form 7 August 2012 Accepted 13 August 2012 Available online 8 September 2012
When subject to fire, structural steel and connectors lose both their strength and stiffness. Structures expand when heated and contract on cooling. Furthermore, the effect of restrained thermal movement can introduce high strains in both the steel member and the associated connections. Fire tests on steel structures have shown that the temperature within the connections is lower compared to the connected steel members. The beneficial effect of the partial embedding of a connection in a concrete slab for its resistance in fire conditions was already intuitively utilised by structural engineers. Three sets of tests provide the measuring of the accuracy of the developed numerical and analytical model for heat transfer and temperature distribution in the partially embedded connections. The connection shadow factor is proposed further to allow the prediction of temperatures in connections from the calculated gas temperature during a post-flashover fire. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Fire design Composite steel and concrete structure Heat transfer Connection design Partially embedded connection
1. Introduction The fire resistance of structures has been traditionally verified by experiments on fire protected or unprotected elements. Currently, the test results are substituted for a member analysis of structural elements. Higher economy can be achieved by verifying the overall structural behaviour; particularly when accounting for the membrane action of the composite steel and concrete floors of multi-storey buildings. In such cases, structures may meet the requirements also being only partially fire protected, see [1]. There are two approaches to the fire design of steel connections. Using the first approach, fire protection is applied to the member and its connections. The level of fire protection is based on the protection of the connected members. In the design, the degree of efficiency is considered, which may vary between the connections and connected members. More detailed second approach is uses an application of the component procedure described in EN1993-1-8, see [2], together with a method for calculating the behaviour of welds and bolts at elevated temperatures. Using this approach, the connection moment, shear and axial capacity can be evaluated at elevated temperatures with a good level of accuracy by simple or advanced models of mechanical behaviour, see [3]. The accuracy of the design depends on prediction of the temperature of elements and of forces acting in the connection during the fire.
n
Corresponding author. Tel.: þ420 224354757; fax: þ420 233334766. E-mail addresses: [email protected] (J. Chlouba), [email protected] (F. Wald). 0379-7112/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.firesaf.2012.08.008
The thermal conductivity of steel is high. Nevertheless, because of the concentration of material within the joint area, differential temperature distribution should be considered within the joint. Various temperature distributions have been proposed or used in experimental tests by several authors, see [4]. In an attempt to quantify the temperature distribution within a joint, tests have been done on several joint typologies, see [5]. One example for larger beams, a web temperature similar to the bottom flange temperature, is provided while for smaller beams, a smaller web temperature is provided, see [6]. Additionally, the presence of the concrete slab above the joint causes a reduction in beam top flange temperatures, see [7]. A detailed description can be found in the literature [8,9]. The values proposed in EN1993-1-2, see [10], are in agreement with these experimental results. However, these values are based on a nominal standard fire curve, if the fire follows other curves it is necessary to analyse the particular case using a numerical or experimental study, see [11,12]. According to EN1993-1-2, the temperature of a joint may be assessed using the local massivity value A/V of the joint components. As a simplification, a uniform distributed temperature may be assumed within the joint; this temperature may be calculated using the maximum value of the ratios A/V of the adjacent steel members. For beam-to-column and beam-to-beam joints, where the beams support any type of concrete floor, the temperature may be obtained from the temperature of the bottom flange at midspan, see Fig. 1. The temperature of the joint components at its height hk may be determined, see Annex D of EN1993-1-2, as follows. The depth of the beam hb is less or equal to 400 mm
yk ¼ 0:88 y0 ½10:3ðhk =hb Þ
ð1Þ
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O
Nomenclature hb heq h k hnet kcon qf,k x A A/V Av
ac e
depth of the beam weighted average of window heights on all walls components at its height thermal flux connection shadow factor fire load density distance from upper surface of slab total area of enclosure local massivity value total area of vertical openings on all walls
y0 yg yh ym yx
s F
where y0 is the temperature of the lower flange of the beam at midspan. The depth of the beam is greater than 400 mm
pffiffiffiffiffiffiffi opening factor O ¼ Av heq =At coefficient of heat transfer by convection emissivity of surface temperature of the lower flange of the beam at midspan gas temperature the temperature of the bottom surface of slab element temperature the temperature in distance x in mm from bottom surface of the slab Stephan–Boltzmann constant configuration factor
2. Experiments 2.1. Header plate connection with a stiffener
yk ¼ 0:88y0 hk
hk is less than hb =2
ð2Þ
yk ¼ 0:88y0 ½10:2ðhk =hb Þ hk is greater than hb =2
ð3Þ
The above prediction is based on findings on structural configuration similar to Cardington frame, see [12], and is applicable for similar structures and fires only. Heat transfer reduction to the connection may be one of the possible solutions to enhance structural fire resistance, see [13]. One proposed method is to ensure sufficient resistance by embedding a part of the connection into the concrete slab. Direct contact with hot gases is eliminated and concrete creates a thermal protection in the connection. As a consequence to this structural solution, temperatures in the connection during a fire reach significantly lower values in comparison to the temperatures in the connections without embedding. A series of tests with partially embedded connections were carried out between 2007 and 2009 at the Czech Technical University in Prague, see [14]. These tests consisted of two fire tests in a furnace and on an experimental building. During these tests, the temperature distribution in the connection was measured to understand the effect of embedding in concrete. The most precise solution of heat transfer is a numerical model. The accuracy of the model relies on the accuracy of the input data, which are material properties and boundary conditions. The connection shadow factor is proposed further to allow the prediction of temperatures in the connections from the calculated gas temperature during a postflashover fire. A simple prediction model of the temperatures in the connection, which are calculated from the bottom flange midspan temperature, is measured and added to the temperature prediction in the concrete slab above the connection.
The first experiment with the embedded connection took place in the PAVUS testing facility in 2007. The specimen consisted of a composite steel and concrete beam with a cross section IPE 160 with a span of 3.0 m. The concrete C25/30 slab, 100 mm thick, was concreted in trapezoidal sheeting TR 50/250/0.75. The reinforcement diameter, 5.0 mm, with a distance of 150 150 mm was located 30 mm from the upper surface. The beam was connected to a supporting frame on both sides with a partially embedded header plate connection. The header plate connection, a 6 mm plate, bolts M16 class 8.8, was designed with an 8 mm plate stiffener on the top of the beam. The welded frame from cross sections HE 200 B was designed to simulate the plane stiffness of the composite floor. A view from the furnace on lower face of the specimen is shown in Fig. 2. In a four point bending test in a furnace, the beam was loaded by self-weight and a mechanical load by a hydraulic jack with a cross member to create two forces of 25 kN. The fire load followed the measured gas temperature during the Seventh Large Fire Test on a steel building in Cardington, see [15]. The temperatures were measured in the lower flange of the beam, the lower bolt, the header plate next to the lower bolt, the upper encased bolt and on the top of the concrete slab Fig. 3). Temperature development measured with thermocouples is shown in Fig. 4. The temperature profile measured at the time when the temperature in the lower flange of the beam reached the maximum is shown in Fig. 5. From the profile, it can be seen that the influence of the encasing is significant; when the temperature in the lower bolt reached 711 1C at 55 min, the upper bolt in the same time showed only 249 1C. Due to good thermal conductivity of the steel, the temperature of the lower bolt and of the header plate next to the bolt during the whole fire test was about the same.
Composite steel and concrete slab
0,62
hb
hk
0,75
0,88 hb< 400 mm Fig. 1. Thermal gradient within the depth of connection.
0,70
0,88
0,88 hb> 400 mm
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123
Slab surface 221 °C Upper bolt
249 °C Lower bolt 711 °C 726 °C Next to lower bolt
1050 °C
Lower flange Fig. 5. Temperature profile of the header plate connection with a stiffener in 55 min of the fire test. Fig. 2. View of specimen from the furnace.
Temperature, °C
Gas temperature in furnace Lower flange,TC13 End plate,TC02
1000 Upper bolt, TC03 800
End plate, TC04 Upper bolt, TC05
600
400
200 Upper surface, TC06
Fig. 3. Header plate connection with a stiffener.
0 0
Temperature, °C
30
60
90
120
150
Time min
Fig. 6. Measured temperature in the header plate connection without a stiffener.
Gas
1000
Lower flange Lower bolt
800
Next to lower bolt 600
400
200
Upper bolt Slab surface
0 0
30
60
90
120 Time, min
were changed to M12 grade 8.8. The stiffening plate was left out and the header plate was extended towards the lower flange of the beam to ensure sufficient shear resistance of the beam web. Other dimensions such as bolt quality remained unchanged. The beam was loaded by a mechanical load by a hydraulic jack to create a four point bending test with the forces of 30.2 kN. The fire load followed the measured gas temperature during the Cardington Fire Test, see [15]. The temperatures were measured on both connections. Temperature development was measured with thermocouples at the connection without a stiffener compared to gas temperature in Fig. 6. The temperature in the free part of the connection decreases linearly and in the embedded part nonlinearly. Fig. 7 shows the measured temperature profile at 56 min when the temperature in the lower flange of the beam reached the maximum.
Fig. 4. Temperature of the header plate connection with a stiffener.
2.3. Measured temperatures in connections on the experimental building 2.2. Header plate connection without stiffener The fire experiment performed in 2009 took place in the same horizontal furnace as the test in 2007. The setting of the experiment was similar. The beam and the supporting frame were the same; the only two changes were in the geometry of the connection to increase its ductility and simplify its detailing. To reach more balanced resistances of all the connection components, the bolts
One of the objectives of the fire test on the experimental building in Mokrsko, see [16], was the thermal and mechanical behaviour of the connections partially encased in a concrete slab. The structure was designed to allow simple as well as advanced modelling of today’s modern buildings. The experimental structure represents a part of a floor of an administrative building of 18 12 m and 4.00 m high. One half of the floor was covered by a composite slab supported
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Upper surface of the slab, TC06
P10-65x200 Plate
146 °C
35
90
35
Upper bolt, TC05
35 End plate, TC04 top part
287 °C
120
35 50
Bottom bolt, TC03
P10-155x160 Bolt 10.2
650 °C 709 °C
IPE 270-395 Beam
742 °C
HEB 180 Column
End plate, TC02 bottom part 1047 °C
Fig. 9. Geometry of the beam to column connection in the Mokrsko fire test 2008.
Lower flange, TC13 Fig. 7. Temperature profile in the header plate connection without stiffener in 56 min of fire test.
B
A 9 000 Window opening
N
C
9 000 Window opening
3 AS7 AS6 6 000
AS5 AS4
2 Cladding
Slab Slab
Beams with corrugated web CS2
Cellular beams
CS1 S5
AS3
S4 AS2
6 000 1
CS4 CS3
Hollow core panels
AS1 Sandwich panels
S3 S2 S1
Concrete wall
Door
+4,00
+0,00
Fig. 8. Fire compartment with description of the major floor and wall structures and connections in the Mokrsko fire test 2008.
by non-fire protected composite castellated beams with large openTM ings: ArcellorMittal Angelina , see Fig. 8. The composite slab on the castellated beams was designed with a span of 9–12 m and on the beams with corrugated webs with a span of 9–6 m. The deck was a simple trapezoidal composite slab with a thickness of 60 mm and the height over the rib of 120 mm with a sheeting of CF60, Cofraplus 0.75 mm, and the concrete of cubic strength 34 N/mm3 in 28 day reinforced by a smooth mesh ø 5 mm 100/100 mm; with 500 MPa strength and 20 mm of coverage. The castellated beams with sinusoidal shape openings were made from profile IPE 270 and its web height was 395 mm. The header plate connection with bolts M20 grade 8.8 and thicker header 10 mm were designed partially encased in the concrete slab with a stiffener at the top of the beam. The geometry of the connection is pictured in Fig. 9.
The fire load was made of rough battens from soft pine wood, a total volume of 15 m3 and the fire load reached 515 MJ/m2. Two window openings in the front wall, 2.43 4.0 m, provided air supply into the fire compartment. The gas temperature round the beams was measured during the test by jacketed thermocouples 3 mm in diameter at the level of the lower flange and the steel temperature by 2 mm thermocouples. A total of four connections were equipped, of which two were beam to column and two beam to beam connections. The temperature in these connections was measured in similar places as in the laboratory test—at the lower flange of the connected beam, at the web of the beam in one third of the height, at the lower bolt, at the header plate next to the lower bolt, at the upper flange, at the upper encased bolt and at the header plate next to the upper bolt, see Fig. 10. In Fig. 11 measured temperatures in the beam to column connection on beam AS4 are given. The upper bolt temperature reached its maximal temperature 157 1C at the 58. min; see Table 1, while the maximal temperature of the lower unprotected bolt was 520 1C at the 60. min. The temperature along the end plate height was close to the bolt temperatures. The maximal temperature was 201 1C at 61 min of fire and the lower part reached maximum 505 1C at 60 min. For the beam to beam connection at beam AS5 the temperatures are summarised in Fig. 12. At 62 min of the fire test, the temperature of the upper bolt was 198 1C and for the embedded end plate 253 1C, while the lower bolt had already reached 410 1C and the unprotected part of the end plate 397 1C. The differences in temperatures in the beam to column connection are lower due to the plated fire protection of the primary beams, while the central column was protected only 50 mm below the connection.
3. Numerical simulation of heat transfer 3.1. Model and shadow effect A numerical model for analysing the transfer of heat and mechanical behaviour was developed with the SAFIR code with the help of pre-processor GiD, see [17], to assist the preparation of an analytical prediction model by a sensitivity study of the fire scenarios, see [18]. Five material properties were applied: steel column, beam, end plate and bolt and concrete slab. The predefined values of the material properties were used. The brick elements used were from 200 mm3 to 35 100 mm3 in size. The mesh was refined to the direction of the heated surface. During the thermal analyses, all nodes have one degree of freedom,
J. Chlouba, F. Wald / Fire Safety Journal 54 (2012) 121–129
1000
125
Temperature, °C TC48
TC54
800 TC55
600
TC53
TC55
TC47
TC50 TC51 TC51
400
TC48 TC53
TC50
TC47
200 TC54
0 0
15
30
45
60
75
Time, min
Fig. 11. Measured temperature in beam to column connection during the Mokrsko fire test, beam AS4.
the non-heated upper slab surface the boundary conditions for gas were assumed at 20 1C. In case of one direction heat transfer, thermal energy is given by thermal flux which consists of gas convection and radiation and may be expressed in the form
hnet ¼ ac ðyg ym Þ þ Fesððyg þ273Þ4 ðym þ 273Þ4 Þ
ð4Þ
where ac is the coefficient of the heat transfer by convection, yg is the gas temperature, ym is the element temperature, F is the configuration factor, e is the emissivity of the surface, and s is the Stephan–Boltzmann constant. The connection surface itself and its components are not fully exposed to flames. The part of the heat transfer by conduction depends on the structural configuration and is neglected in the simplified prediction of the temperature in connections, as in members as well. The heat transfer by convection is influenced by the connection position and the ventilation of the fire compartment. The heat transfer by radiation depends on the position of the connection and flames during a fire. The described geometry of the fire compartment the fire configuration factor F may take into account the position of the connection. In this study, the influence of the position of the connection was taken as analogous and both contributions, heat transfer by convection and radiation, were simulated by one connection shadow factor kcon, which may reach values from 0.0 to 1.0. Eq. (4) for one direction heat transfer was modified to
hnet ¼ kcon ac ðyg ym Þ þ kcon esððyg þ 273Þ4 ðym þ273Þ4 Þ
Fig. 10. Thermocouples on (a) beam to beam and (b) beam to column connections in the Mokrsko fire test 2008.
temperature. There are heated elements in the fire compartment. The boundary condition at the heated surface was considered the heat transfer by convection and the heat transfer by radiation between the surface and the surrounding gas temperature. For
ð5Þ
In the numerical simulations, the value of the surface emissivity 0.70 and the value of the coefficient of heat transfer by convection 35 W/m2K were used. The FEM model of the connection consists of the composite steel and concrete beam with a connection and trapezoidal sheeting with a concrete slab. The size of the model was driven by good representation of the heating and cooling sections. Based on a sensitivity study, the width of the slab was selected as a beam flange plus 300 mm to both sides. The bolts are simulated as cuboid, see Fig. 13. For the model of the composite beam in a horizontal furnace, thermal insulation was applied as a separate material. The values of the connection shadow factors, see Table 2, were find empirically by calibration so that the numerical results agree with the experimental ones. The experiment with the header plate connection with a stiffener in a furnace was used to derive the presented values. The values were used to simulate the header plate connection without the stiffener tested in a furnace and on an experimental building with good accuracy. The low value of the shadow effect for beam reflects the lower flange influence. The values are also influenced by the simplified geometry of the bolt head and nut as rectangle, see Figs. 13, 14 and 17b.
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Table 1 Measured beam to column connection, beam lower flange at mid span and gas temperatures during the Mokrsko fire test at beam AS4. Time (min)
Temperature in 1C measured by thermocouples in connection
0 15 20 25 30 35 40 45 47 50 52 55 60
Gas
TC 47
TC 48
TC 49
TC 50
TC 51
TC 53
TC 54
TC 55
TC 43
TG08
TG9
10.0 134.1 211.7 360.2 476.0 585.1 658.3 715.5 727.4 748.3 758.7 751.4 749.9
10.9 193.4 283.5 434.4 536.2 622.7 670.4 726.1 735.8 772.9 785.8 769.0 762.6
10.4 163.3 239.3 384.4 486.1 581.3 633.1 693.7 702.7 734.5 750.4 738.4 731.8
9.8 96.2 142.6 252.2 276.2 345.1 395.6 446.2 463.3 479.2 493.8 500.6 520.0
10.2 79.3 115.2 157.8 205.9 304.6 364.0 422.6 440.1 467.3 482.5 490.3 504.8
9.6 26.1 32.8 77.9 87.4 87.3 95.7 119.6 132.7 148.0 157.7 167.3 195.5
9.6 23.6 38.2 73.5 93.9 100.4 101.2 114.1 129.7 142.8 135.5 136.6 156.0
10.4 99.1 152.2 237.9 303.3 395.5 466.8 523.3 538.1 557.8 570.4 574.4 587.8
13.8 326.4 457.5 606.4 683.8 780.7 781.6 853.1 918.5 919.3 939.8 907.2 916.2
20.3 497.1 546.2 742.4 798.4 819.4 851.8 942.5 966.9 947.4 961.5 923.0 931.0
20.1 534.2 625.6 756.2 827.8 851.7 859.7 922.2 935.9 901.7 922.6 889.6 887.7
Temperature, °C
1000
At mid span
Table 2 Connection shadow factor kcon from the fire test with header plate connection with a stiffener.
TC64 TC70
800
TC69
TC71
TC63 600
TC66
TC71
TC66
TC67
400 TC69 TC67 200
TC64
Component
kcon
Slab Bolts Frame/column Beam End plate
0.40 0.30 0.60 0.25 0.60
TC63
TC70 0 0
15
30
45
60
Time, min
Fig. 12. Measured beam to beam connection temperature during Mokrsko fire test, beam AS5.
Fig. 14. The distribution of temperatures on connection in 60 min for fire test with header plate connection with a stiffener.
Temperature,°C
Lower flage, numerical model Lower flage, experiment
1000
Lower bolt, model Lower bolt, experiment
800
Upper bolt, model Upper bolt, experiment
600 Fig. 13. Model of connection from the fire test with header plate connection with a stiffener.
3.2. Tests comparisons
400 200
Concrete surface, experiment Concrete surface, model
The distribution of temperatures on the header plate connection with a stiffener at 60 min of fire test is provided in Fig. 14. A comparison of the calculated and measured temperatures during the fire test with the header plate connection with a
0 0
15
30
45
60
75
90
105
120Time, min
Fig. 15. Comparison of numerically predicted and measured temperatures during the fire test with header plate connection with a stiffener.
J. Chlouba, F. Wald / Fire Safety Journal 54 (2012) 121–129
Temperature, °C
127
Flange, experiment TC13 Flange, model Lower bolt, TC03
1000
Lower bolt, model
800
TC06
Upper bolt, model
TC05
Upper bolt, TC05
600
TC03
400
TC13
200 Upper surface, TC06 Upper surface, model
0 0
30
60
90
120
150
Time, min
Fig. 16. Comparison of numerically predicted and measured temperatures during the fire test with header plate connection with a stiffener.
Fig. 17. Header plate connection during erection of experimental frame and its FEM representation.
stiffener shows a reasonable conservative prediction of temperatures, see Fig. 15. The difference in prediction is caused namely by the partial fire protection of the experimental frame, which simulates the overall structure. The model represents the cooling of the connection as well. 3.3. Test comparisons for header plate connections without a stiffener The numerical model of the header plate connection without a stiffener was similar to the model for the connection with a stiffener. The geometry was adjusted to reflect the longer free length of the connection. The comparison of the predicted temperatures of the major connection components to measured ones is in Fig. 16. The maximal calculated temperature of the bottom flange was 1058 1C and during the experiment the temperature reached 1047 1C. Also, the calculated maximal bottom bolt temperature 757 1C corresponds to the measured one, which was 729 1C. The top bolt predicted temperature 398 1C was not reached in the test; it measured 360 1C only. The agreement is good. The initial part of the simulation is affected by the difficulties of predicting water evaporation and missing its transport in the model. The model approved the values of connection shadow factor kcon derived for the previous test. The FEM model of transfer of heat to a partially embedded connection was applied to the measured results from the Mokrsko Fire Test on an experimental structure. The header plate connection during the erection of the experimental frame and its FEM representation is shown in Fig. 17. The distribution of the temperatures in the connection is shown in Fig. 18 with the help of the postprocessor code diamond at 60 min of the experiment.
Fig. 18. The distribution of temperatures on header plate connection in 60 min of the experiment in Mokrsko by numerical model code SAFIR.
A comparison of the predicted temperature for the upper flange and bolt with the measured specimens, see Table 1, is summarised in Fig. 19. The development of gas temperatures at the level of the lower flanges of beams at mid span is included. The predicted temperatures were higher, e.g. on the safe side, compared to the measured ones, e.g. the calculated temperature of the bottom bolt was 520 1C and the measured one 513 1C The difference in the prediction of the upper flange to the measured ones was round 25 1C, when calculated the maximal temperature is (was) 613 1C and measured 588 1C. The variance in temperatures of the upper bolt from 21 to 26 min reflects the crack
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1000
Temperature, °C Gas, experiment, TG08
Temperature, °C
TC54
Beam lower flange, mid span, experiment
TC55
800
Gas, experiment, 600 TG09
Connection, model
Gas, experiment
1000 800
Upper flange, experiment, TC54
600
TG09
Upper flange, TG08 model
400
TC2
400
Upper bolt, model
200
Connection, TC2, experiment
200
Upper bolt, experiment TC54
0 0
0 0
15
30
45
60
75
90
30
60
90
120
150 Time, min
Time, min
Fig. 19. Comparison of the numerically predicted temperature for the upper flange and bolt to measured ones for header plate connection during the experiment in Mokrsko.
openings in the concrete slab on the beam support. Also the maximal calculated temperature of the lower flange of the beam, 831 1C, was in good agreement with the maximal measured one, 759 1C, see [11].
Fig. 20. Comparison of calculated, by analytical model, and measured temperatures of the bottom bolt during the fire test with header plate connection with a stiffener.
Temperature, °C
Beam lower flange, mid span, experiment
Gas, experiment
1000 800
TC8
600
4. Analytical model
400
4.1. Model
Connection, model
200
For the analytical prediction of temperature the model in Annex D of EN1993-1-2 Eq. (1)–(3) was selected, see [10]. The distribution of the temperatures in the composite slab is influenced by the transport of water and its evaporation. The simple conservative model of the temperature profile for a slab until 3% humidity was derived from the presented three experiments only and the FEM simulation of the connections of the tested geometry. The temperature is expressed as a function of the distance to the surface of the slab exposed to fire only
Connection, TC2, experiment
0 0
90
120
150 Time, min
Temperature, °C Gas, experiment
ð6Þ
where yx is the temperature in distance x in mm from the bottom surface of the slab and yh is the temperature of the bottom surface of the slab. It may be expected that this is the temperature of the upper flange of the beam hk ¼hb and the temperature in the connection embedded in slab on beams with the depth of the beam hb less or equal to 400 mm may be given as
yx ¼ 0:62y0 eð0:02xÞ
60
Fig. 21. Comparison of calculated, by analytical model, and measured temperatures of the upper embedded bolt with header plate connection with a stiffener.
1000
yx ¼ yh eð0:02xÞ
30
Lower bolt A, experiment
800 600
TC03 TC08
400 Lower bolt B, experiment
200
Lower bolt, model
ð7Þ
The bolt temperatures in this simple analytical prediction depends on their position in the plate only. The coefficients were derived for the experiment in a furnace with the header plate connection with a stiffener. Eq. (7) provides a prediction of temperature of the header plate connection without a stiffener tested in furnace and on experimental building with good accuracy. For different fire conditions, e.g. cooling, and different steels/concretes, the dimensions were not approved. 4.2. Comparison to tests The comparison of temperature predicted by a simple analytical model, Eq. (1) for the part of the plate directly exposed to fire and Eq. (7) for the part of the plate imbedded in concrete, gives very good results compared to laboratory tests. Fig. 20 shows, that for the bottom bolt during the fire test with a header plate connection with a stiffener, the calculated temperature during the heating phase until 68 min is conservatively higher at about 70 1C. During the cooling phase, the model slightly gives a lower temperature. The prediction of the top bolt, see Fig. 21, is less
0 0
30
60
90
120
150 Time, min
Fig. 22. Comparison of calculated, by analytical model, and measured temperatures of the lover bolt with header plate connection without stiffener.
successful. The calculated maximal temperature, 334 1C, is higher compared to the measured one, 276 1C. The shape of the curve follows the beam midspan temperature and the delay in cooling in the model is not included. The improvements for the simple model are available, see [19], but for this joint the configuration is not necessary. The test with the header plate connection without a stiffener is compared to the analytical simulation of the lover bolt in Fig. 22. The temperature 407 1C is obtained from the calculation at 56 min. The recorded test maximum is 359 1C reached at 77 min. This simple model gives a good and safe prediction. Fig. 23 shows the comparisons of the analytically calculated and measured temperatures of a beam to column connection in the Mokrsko Fire Test. The maximal temperature in the lower bolt of the beam to column connection, which is predicted from Eq. (1) at 60 min as 606 1C, is higher compared to the measured one,
J. Chlouba, F. Wald / Fire Safety Journal 54 (2012) 121–129
which may support the measurement of radiation to the connection by radiometers or by adiabatic thermometers. At the Mokrsko fire tests, the radiometers were used to measure the distribution of the radiation to the composite beam in midspan only.
Temperature, °C
1000 Lower flange, experiment
800
129
TC54 Lower bolt, model
600 TC50
400
Lower bolt, experiment
Acknowledgements
TC47
Upper bolt, model
200
ˇ The preparation of the paper was supported by Grant GACR P105-10-2159.
Upper bolt, experiment
0 0
15
30
45
60
75
90
Time, min
Fig. 23. Comparison of calculated, by analytical model, and measured temperatures of beam to column connection in Mokrsko fire test.
which reached 520 1C at 52 min only. The maximal temperature in the upper bolt of beam to column connection is predicted at 285 1C at 52 min. From Eq. (7), which is higher compared to the measured one, 157 1C at 58 min. The measured temperatures from 45 min of the experiment were affected by cracks in the thin slab and slow wind, see [16].
5. Summary The beneficial effect of the partial embedding of a connection in a concrete slab for its resistance in fire conditions was approved by three set of tests to provide the measuring of the accuracy of the developed numerical and analytical models for heat transfer and temperature distribution in the partially embedded connections. Due to a sharp decrease of the material properties of structural materials at elevated temperatures from 300 1C till 700 1C, namely of bolts and welds, the accuracy of the temperature prediction for the connection resistance and robustness is crucial, see [4]. The results of the calculations of different fire scenarios, see [14], proved a significant positive influence of the partial encasing of the connection in the concrete slab. The benefit is greater for fires with longer durations. The knowledge of temperature development in connection is the first step of the evaluation of connection resistance and robustness, which is taken into account in this contribution. The mechanical behaviour of the header plate connection at elevated temperature in case of fire exposure is developed based on the component method; see [3]. This method is applicable to partially embedded connections with the proposed prediction of the connection temperatures. The presented analytical method for the prediction of temperatures is based only on the tested geometry and the related FEM simulation. For different geometries further studies should be based on the separation of components of heat transfer to the connection,
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