CC BY-NC-ND
0c®
CrossMark
Available online at www.sciencedirect.com
ScienceDirect
Procedia Engineering 210 (2017) 565-573
Procedia
Engineering
www.elsevier.com/loeate/proeedia
6th International Workshop on Performance, Protection & Strengthening of Structures under Extreme Loading, PROTECT2017, 11-12 December 2017, Guangzhou (Canton), China
Performance of T-Shaped Steel Reinforced Concrete Column under
High Temperature
Yuzhuo WANGab ,Ying HUANGb ,Chuanguo FUa—
a College of Civil Engineering, Shandong Jianzhu University, Jinan, Shandong Province,250101,CHN b College of Architecture Engineering, Shandong Xiehe University, Jinan, Shandong Province,250107,CHN
Abstract
With high strength and good seismic performance, T-shaped steel reinforced concrete column is widely used in high-rise building structure. Three T-shaped steel reinforced concrete columns were tested under high temperature and vertical load, to simulate fire effect. The test results indicate that failure characteristics, distribution of temperature field, vertical deformation characteristics and fire resistance were comparatively analyzed under different axial compression ratios and different eccentricity. The test also indicated that the cracks increased with the increase of axial compression ratio and eccentricity. The damages of web were severer than the flange. The cracks were mainly distributed on the eccentric side and mostly inclined cracks in specimen. The vertical expansion became more obvious as the eccentricity decreased. The fire resistance decreased as the axial compression ratio increased. Compared with, the fire resistance of large axial compression specimens (the axial compression ratio is 0.6) were decreased by 57% than small axial compression specimens (the axial compression ratio is 0.2). The fire resistance decreased by about 30min as eccentricity increased by 20mm.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of tine scientific committee of the 6th International Workshop on Performance, Protection & Strengthening of Structures under Extreme Loading
Keywords: Steel reinforced concrete columns; Fire resistance; Experimental Research; Axial compression ratio; Eccentricity
* Yuzhuo wang. Tel.: 13793173899. E-mail address: yuzhuowang@163.com
1877-7058 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 6th International Workshop on Performance, Protection &
Strengthening of Structures under Extreme Loading.
10.1016/j.proeng.2017.11.115
Introduction
The steel reinforced concrete special-shaped column with high space utilization, flexible layout and good economic benefits is widely used in practical engineering [1]. Because of its high steel content , it is necessary to study the fire resistance of steel reinforced concrete special - shaped columns due to the decrease of the bearing capacity of steel - reinforced concrete special - shaped columns under high temperature.
The current research results are: Eurocode[2] carried on the research and analysis of the SRC column hinged at the ends of the beam .The results showed that the fire resistance limit was related to the column size, the thickness of the protective layer of the steel and the thickness of the protective layer of the steel, and it was independent of the slenderness ratio. The fire load ratio was determined by its cross section. Wu Bo and Xu Yuye[3~7] completed the fire resistance test of 11 full-sized reinforced concrete special-shaped columns under the standard heating process of ISO834. Axial deformation and fire resistance of reinforced concrete columns under high temperature were analyzed in the influences of load ratio, mode of fire and section form on the failure modes[8]. The influence of different parameters on the fire resistance of reinforced concrete columns was numerically analyzed and the method of calculating the fire resistance was deduced.
The test, four kinds of steel reinforced concrete T - pillar specimen in the high temperature fire and vertical load coupling under the fire resistance, was carried out. The damage morphology, temperature field distribution, vertical deformation and fire resistance of steel reinforced concrete T-type columns under different axial compression ratios and different eccentricity factors were analyzed and studied. The relevant conclusions will promote to further understand the fire behavior of steel reinforced concrete T - type columns under high temperature.
1 Test design
1.1 Specimen design
Considering the influence of axial compression ratio and eccentricity, four steel reinforced concrete T column specimens were designed and tested under the coupling action of high temperature fire and vertical grading loading. The size of T-shaped specimen cross-section was 300 x 200 x 100mm, and calculated length was 600mm. Q235 hot-rolled steel was built-in and specimen steel content was 6.9%. In order to avoid the local failure of the specimen during the loading process, the enlarged column of which the size was 300 x 200 x 200mm was set in the upper and lower ends of the specimen. The steel frame consisted of four longitudinal sections and a number of horizontal webs, with a horizontal web spacing of 200 mm. The horizontal webs were equipped with diagonal rods, and steels were set deep into stigma to ensure the strength of the stigma and the integrity of the column and the column[9]. Specimen specific parameters and refractory limit were shown in Table 1, the specimen cross-section were shown in Figure 1, and the specimen steel skeleton and reinforcement diagram were shown in Figure 2.
Table 1. Component design parameters and test results
Specimen number Axial compression ratio Design load /kN Eccentricity /mm fire resistance/min
TZ-1 0.2 225.6 0 787
TZ-2 0.6 676.8 0 338
TZ-3 0.6 676.8 20 304
TZ-4 0.6 676.8 40 279
Fig.1. section of steel T-shaped section Fig.2. Steel skeleton and reinforcement diagram of test piece 1.1. 1.2 The mechanical properties of the material
The concrete was made of C25 ordinary commodity fine stone concrete. The concrete cubes of the standard size were made in the same batch of concrete. Three sets of three cubes were made in each group. After 28 days of natural curing, the concrete test cubes were tested to get the average compressive strength. The compressive strength and modulus of elasticity were shown in Table 2. Reinforcement was HPB300 grade steel with diameter of 8mm, mechanical performance indicators of steel were shown in Table 3.
Table 2. Concrete mechanical properties(N/mm2)
Test block number Compressive strength Average compressive strength Elastic Modulus Average elastic modulus
1 27.9 2.20x104
2 28.5 28.2 2.39x104 2.36x104
3 28.2 2.50x104
Table 3. Mechanical properties of steel(N/mm2)
Steel strength
Steel category Elastic Modulus Yield tensile
Strength/;; strengthfi
Chanel iron 2.04x105 330 450
Steel 2.05x105 520 630
1.2. 1.3 Test device and test process
1.3. 1.3.1 Test device
The test device was consisted of reaction frame, hydraulic equipment, electric heating furnace and data acquisition system. The specific load test devices were shown in Figure 3. The counterforce frame consisted of the left and right side columns and the upper and lower beams, and the crossbeam was fixed on the upright post through bolts. The vertical load was applied by the hydraulic jack that was upside down on the upper beam. Between the jack and the test piece, a pressure sensor was provided to measure the load on the specimen. The displacement meter was fixed on the stigma to measure the vertical and horizontal displacement of the specimen during the test. The upper part of the stigma was placed to prevent local damage to the stigma. Laying rockwoides between the steel plate and the stigma prevented the pressure of the pressure sensor from causing the stigma temperature to be high. High power electric heating furnace through the furnace inner resistance wire produced heat transfer to the furnace.
The height of the furnace was 500mm and the inner diameter of the furnace was 500mm. With the heating furnace supporting XMTD digital regulator can control the furnace temperature.
Fig.3. Schematic diagram of the loading device Fig.4. Load the system curve Fig.5. Test furnace temperature curve
1.4. 1.3.2 Lifting process and loading system
The test piece was suspended by the crane to the bottom beam of the furnace, so that the loading point was aligned with the center of the jack, connecting the thermocouple wire, installing the pressure sensor and the displacement meter, and sealing the upper and lower openings of the test furnace with rock wool. According to the experimental loading system to apply the vertical load, the vertical design load FN was determined according to the axial compression ratio, and the vertical loading load FN of each specimen was shown in Table 2. Preloaded before loading, preloaded 20% of the vertical design load, loaded 10min after unloading. The formal loading process used the hierarchical loading system to load the vertical design load in 10 stages. After loading for 2 min, the load was loaded step by step until the vertical design load is set. The loading curve is shown in Figure 4. When the set load was reached, the fire test was started, and the vertical design load on the test piece was kept constant during the heating process. The measured temperature rising curve of the test furnace was shown in figure 5.When the vertical compression of the specimen reached H / 100 (mm) or the vertical compression rate of the component exceeded 3H / 1000 (mm / min), the test was stopped(H was the specimen height). When the test specimen had a vertical displacement of 6 mm or the specimen vertical displacement descending rate reached 1.8 mm / min, the test was stopped.
1.5. 1.4 Measuring point arrangement
The internal temperature field and the furnace temperature were measured by thermocouple embedded in the specimen. The thermocouple was a nickel-chromium-nickel-silicon K-type thermocouple with a diameter of 1.2mm. The thermocouple was set in the vertical section of the special-shaped column. The test piece was set with 7 thermocouple measuring points. The measuring point 1 measured the surface temperature of the concrete, and the measuring points 2, 3, 4, 5 measured the temperature of the flange plate. The measuring point 6 measured the temperature of the web shaped steel, the measuring point 7 measured the temperature of the center area of the specimen, and the specific temperature measurement points were shown in figure 6.
As shown in Fig. 7, the vertical direction measured the displacement of "vertical 1" and "vertical 2" in the vertical direction. When the displacement value was positive, the member was elongated, and vice versa Component shortened. When the eccentric compression was vertical, the measurement point was the vertical load side, and the displacement at the center of the specimen taken the mean value of the two points. The horizontal direction measured the displacement of the X and Y axes in both directions, and the value was positive to indicate that the stigma moved toward the displacement meter.
Fig.6. Temperature measuring point
Fig.7. Displacement point arrangement
2 Specimen failure morphology analysis
The test was done in Shandong Union Institute of Structural Engineering Laboratory fire test furnace on the furnace. The test procedure was carried out according to the process of loading - constant temperature rise -destruction. When the temperature was raised to about 20 min, the test furnace started with slight water vapor. After 40min heating, a lot of water vapor emitted. After 110min heating, the water vapor began to decrease. After 140min heating, the water vapor disappeared. Overall, four test specimens, the time of water vapor emergence and disappearance was basically the same. When the specimen was exposed to fire, the surface of the concrete was milky white and gray. There were a lot of irregular cracks in the surface. The surface of axial compression specimens were mainly vertical fracture. In addition to vertical cracks, the eccentric side of the surface of the specimen was irregularly inclined. Parts of the specimen concrete were peeled and steel exposed. The damage
patterns of each specimen were as follow from figure 8 to 10.
(a) Positive (b) Back (c) Left side (d) Right side (a) Positive (b) Back (c) Left side (d) Right side
Fig. 8. Failure morphology of TZ-1
Fig. 9. Failure morphology of TZ-2
(a) Positive (b) Back (c) Left side (d) Right side Fig. 9. Failure morphology of TZ-3
(a) Positive (b) Back (c) Left side (d) Right side Fig. 10. Failure morphology of TZ-4
Comparison of the four specimens of the failure forms, concluded as follow:
(1) After the high temperature fire, the surface of the concrete T-shaped column specimen showed milky white, and the longer the refractory time, the more obvious the milky white phenomenon, the more cracks. This showed
that high temperature fire would reduce the tensile strength of concrete, damage the concrete properties of materials, and the longer the refractory time of specimens, the more obvious the damage of concrete properties.
(2) The crack of the axial compression specimen was vertical, the damage degree of the web was more serious than that of the flange, and the concrete falls off, and the middle part of the web had a large diagonal crack. With the increase of axial compression ratio, the cracking degree of specimens increased, and the specimens appeared the overall bending around the web. This was because the flange of the specimen had large amount of steel, large rigidity and relatively large bearing capacity, so the damage occurred mainly at the web position.
(3) The fractures of the eccentric compression specimen were mainly concentrated on the eccentric side, and the vertical cracks on the left side of the flange plate and the web were dense, and some concrete felled off. The eccentric side of the test piece had a diagonal crack extending downward from the stigma. This was because the eccentric loading flange bearing load increases, when the concrete cracked due to decreased capacity of fire, steel bear a larger load, which destroyed the bond slip between section steel and concrete and caused the specimen flange back through the inclined cracks. The non eccentric side cracks were less, and the horizontal cracks appeared on the right flange, which was caused by the splitting of the concrete in the non eccentric flange. With the increased of eccentricity, the crack increased with the increase of eccentricity, and the maximum bending deflection increased with the increase of eccentricity.
3 Measured temperature field analysis
The temperature rise curve of the test piece TZ-1 with time was shown in Figure12. Measuring point 1 in 0 ~ 70min was similar to the linear rise, 80min after the temperature reached the peak temperature and similar to that of the furnace temperature, then the temperature was almost unchanged. Warming curves of measured points 2, 3, 4 changed roughly the same trend. 0 ~ 40min temperature rise was slow, at 50min the measured temperature reached 100 °C lasting for about ten minutes and did not change. In time the temperature continued to rise steadily. In the 50 ~ 100min temperature of measured point 3 was higher than the measured point 2, 100min after the measured point 2 temperature was higher than the measuring point 3,0 ~ 300min measured point 4 temperature was lower than the measuring points 2,3. After 300min, the temperature of the measuring point reached the maximum and remained approximately unchanged. The measuring point 1 had the highest temperature, the second point of the measuring point 2, the third point of the measuring point and the lowest point of the measuring point 4. In 0 ~ 40min temperature of measuring points 5,6,7 grow relatively slow, 50min after entering the temperature level, measuring points in the 50 ~ 150min heating rate became faster, then gradually increased the rate of heating rate, the temperature reached the maximum after the basic remains unchanged. Measuring point 6 get the highest temperature, second the measuring point 5, the lowest point 7. The temperature distribution of TZ-2, TZ-3 and TZ-4 is similar to that of TZ-1, and is not described here.
The following indications can be obtained by the heating curve of specimens:
(1) It can be seen that the temperature rise curve of each test point was divided into five stages[10]. The first stage was the slow warming stage, at this time the heat of the electric heating furnace had not yet passed to the measuring points, the temperature changed slowly. The second stage was the temperature rising section, the heating rate of each measuring point was accelerated. The closer the temperature was to the surface of the specimen, the faster the heating rate was. The third stage was the horizontal section, when reaching the point temperature of 100 C , the internal measuring points appeared obvious level, since a large amount of water vapor in the concrete evaporated and absorbed heat, at high temperature, and the internal temperature was no longer rising. The fourth stage was the stage of stable heating up. At this stage, the temperature of each measurement point increased steadily, and the heating rates was first fast, and then slow. When the temperature was approaching the maximum, the heating rate slowed down. In the fifth stage, the test points reached the maximum temperature and remain unchanged after the test point. At this point, each measuring point reached the fire resistance limit.
(2) The temperature rise curve of the seven measuring points in the specimen showed that the heating rate of the surface of the test point was the fastest, and the maximum temperature reached at the earliest, and the temperature was the highest. This was because the temperature of the concrete had hysteresis, the internal concrete heating rate was lower than the surface, the crack development of the concrete has an impact on the temperature of the measuring point. The earlier the crack occurs, the more cracks and the faster the temperature of the measurement
point. At different eccentricity, the bigger the eccentricity was, the earlier cracks appeared at one side of the loading point, and the more cracks occurred, the faster the temperature at one side of the loading point, and the lower the fire resistance.
MiBl/nill
(a) Temperature curve of 1, 2, 3, 4 point (b) Temperature curve of 5, 6, 7 point Fig.12. TZ-1 measuring point temperature curve
4 vertical deformation and fire resistance limit
1.6. 4.1 vertical deformation
Figures 13 to 16 showed the curves of the axial deformation of the test pieces with time. when the displacement value is positive , the member is elongated. As can be concluded from the figure:
(1) The vertical deformation of the four specimens basically went through four stages[11]. The first stage of was the slow deformation stage, at this time the internal temperature of concrete was slow, the specimen had sufficient bearing capacity. The second stage was the initial expansion stage. At the initial stage of heating, the specimen expanded when heated and the vertical displacement increased. The smaller the axial compression ratio, the greater the expansion and the longer the duration since the large loads can counteract part of the specimen's thermal expansion force. The third stage was the compression deformation stage. As the temperature of the specimen increased, the strength of the material decreased and the bearing capacity decreased. The fourth stage was the failure stage. With the increase of the fire time, the bearing capacity of the specimen decreased abruptly, and the vertical deformation reached 6mm or decreased rapidly.
(2) The vertical displacement curve of TZ-1 and TZ-2 two axial compression specimens had displacement expansion phenomenon in the early stage of fire. That was, the thermal expansion of the specimen was greater than the load on the specimen. But when the axial compression ratio was different, the expansion time and expansion of the maximum displacement was different. The expansion time of TZ-2 was only about TZ-1 of 1/6, and the maximum displacement of expansion was only about TZ-1 of 1/4.It illustrated that the greater the axial compression ratio, the shorter the specimen expansion specimen and the smaller the expansion displacement.
(3) There was no obvious expansion phenomenon on the eccentric side of TZ-3 and TZ-4 two eccentric compression specimens. In the beginning of the fire, the displacement expansion of the non eccentric side was increased, and the larger the eccentricity was, the more obvious the non eccentric lateral displacement expansion was. The vertical displacement of the eccentric side was faster in the second stage, and played a controlling role in the failure of the specimen.
(a) Vertical displacement (b) Horizontal displacement (a) Vertical displacement (b) Horizontal displacement
Fig. 13. TZ-1 displacement curve of stigma Fig. 14. TZ-2 displacement curve of stigma
(a) Vertical displacement (b) Horizontal displacement (a) Vertical displacement (b) Horizontal displacement
Fig. 15. TZ-2 displacement curve of stigma Fig. 16. TZ-3 displacement curve of stigma
1. 7. 4.2 fire resistance
The fire resistance limit of each specimen was determined according to the time when the vertical displacement of specimen reached 6mmin figure. The specific fire resistance value was shown in table 1, which was can be seen that:
For the T-shaped SRC, the fire resistance of specimen TZ-1 is 787min, while TZ-2 is 338min, which reduced 449min compared with TZ-1, a significant decrease of 57%. It showed that the axial compression ratio would seriously affect the fire resistance of T-shaped SRC, and the higher the axial compression ratio of column was the lower the fire resistance was.
When the axial compression ratio of T-shaped SRC remained unchanged, corresponding the offset 0mm,20mm and 40mm, the fire resistance was 338min,304min and 279min, it showed that the eccentricity of each 20mm increased, the fire resistance limit shortened about 30min. Compared with the axial compression ratio, the influence of offset on fire resistance limit of specimen was slower.
5 Conclusion
Though the study of fire resistance tests on 4 T-shaped SRC in fire and vertical constant load, the failure modes, the temperature filed distribution, the vertical deformation and fire resistance were analysis, conclusions were as follows:
(1) The specimens of the axial compression were mainly damaged by vertical cracks. The oblique cracks were symmetrical in the "eight" shape symmetrical distribution on the back flap of the specimen. The eccentric compression member was mainly composed of the diagonal crack, and the inclined direction was generally consistent with the eccentric direction. The eccentric side is destroyed seriously, and there was horizontal crack at the big eccentric distance. When the specimen was eccentrically compressed, the overall bending occurred, and the maximum bending deflection increased with the increased of eccentricity.
(2) Under the action of high temperature, the eccentric compression specimen was heated faster along the loading point side, and the limit temperature was high. With the increase of eccentricity, the heating rate of specimens increased and the fire resistance decreased, which showed that the development of cracks would increase the temperature of the core area and reduce the fire resistance of the specimens.
(3) Under the action of high temperature, the axial deformation of concrete T column with small axial compression ratio and small eccentric distance experienced four stages, that was basically unchanged, slowly expanding and rising, gradually decreasing, and decreasing rapidly. Specimen with large axial compression ratio and large eccentricity, expansion phenomenon was not obvious, and even did not occur. The axial deformation generally experienced the three stage, i.e., basically unchanged, slowly decreasing, and linearly decreasing, reaching the fire resistance limit.
(4) The axial compression ratio was an important factor affecting the fire resistance of steel reinforced concrete T - pillar. The small axial - compression ratio steel - concrete T - shaped column had better fire resistance. The fire resistance limit was 787min when the axial compression ratio was 0.2, when the axial compression ratio increased to 0.6, the fire resistance limit reduced to 57%. The eccentricity had little influence on the fire resistance of SRC T
columns. When the axial compression ratio was 0.6, the eccentricity increased by about 20mm each, and the fire
resistance limit decreased by about 30min.
References
[1] Li Guo Qiang , Han L H , Lou G B. Fire resistance of steel structures and steel-concrete composite structures[M].China Building Industry Press. 2006:365-370.
[2] Eurocode4, 1994, Design of composite steel and concrete structures-part1-2: General rules-structural fire design [S]. ENV 1994-1-2:1994, European Committee for Standardization, Brussels.
[3] Xu Y Y, Study on fire resistance of reinforced concrete L shaped columns[J], journal of south china university of technology, 2007,35(7): 110115, 130.
[4] Xu Y Y, et al. Numerical simulation of mechanical properties of reinforced concrete special-shaped columns under high temperature[J]. China Civil Engineering Journal, 2006,39 (12): 48-53.
[5] Xu Y Y, et al. Study on fire resistance of reinforced concrete T shaped columns[J]. China Civil Engineering Journal, 2007, 40(3): 32-39.
[6] Xu Y Y, et al. Experimental study on reinforced concrete special-shaped columns at high temperature[J]. Journal of Building structures, 2007,28(5): 24-31.
[7] Xu Y Y, et al. Study on fire resistance of reinforced concrete ^-shaped columns[J]. Engineering Mechanics, 2008, 25(3): 167-173.
[8] Fu Ch G, Wang G Y, Wang Y Zh. Experimental study and theoretical analysis of fire resistance performance of reinforced concrete frame joints[J].Journal of Southeast University(Natural Science Edition), 2010.40(4):822-828.
[9] Xu M, Xiao D H, Ch Zh F. Experimental study on fire resistance of reinforced concrete columns strengthened with fiber[J]. Journal of Disaster Prevention and Mitigation Engineering, 2013.33(4):369-374
[10] Zhu Ch M, Meng F Q, He B J. Experimental study on fire resistance of steel tubes filled with reinforced concrete columns. Journal of Building Structures. 2016.37(3):36-43
[11] Zhou L R, Mao X Y, Zhang X. Experimental study on fire resistance of reinforced concrete columns strengthened with angle steel[J]. China Civil Engineering Journal, 2017.50(3):35-50.
