Fire Resistance of Reinforced Concrete T-Beams with Circular Web Openings
Abstract
:1. Introduction
2. Experimental Program
2.1. Material Properties
2.2. Description of Specimens
2.3. Location and Size of Circular Openings
2.4. Test Setup and Procedure
2.5. Temperature and Displacement Measurement
3. Test Results and Discussion
3.1. Experimental Observations and Failure Modes
3.2. Temperature of the Furnace and Measuring Points
- (1)
- The temperature change was first fast and then slow; the average temperature of the furnace was in good agreement with the ISO-834 standard fire curve, indicating that the horizontal furnace of the fire-resistance laboratory of Huaqiao University met the requirements in national standard [61]. In general, the furnace heating curves measured in the fire tests can simulate real fire conditions.
- (2)
- The average furnace temperatures at the beginning of T1, T2, T3, and T4 were 38 °C, 37 °C, 42 °C, and 32 °C, respectively. Furthermore, the furnace was turned off at 127 min, 126 min, 122 min, and 103 min for T1, T2, T3, and T4, reaching maximum furnace temperatures of 1060 °C, 1057 °C, 1056 °C, and 1032 °C, respectively.
- (3)
- The temperatures of T2-2, T3-2, and T4-2 match well with the measured furnace temperature.
- (1)
- Because point 6 of T1; point 4, point 6, point 10, and point 12 of T2 and T3; point 6, point 10, and point 12 of T4 measured the temperatures of longitudinal reinforcements, they reached the highest temperature almost immediately after flameout and then cooled down at a fast rate. The temperatures of points on sections with openings were higher than those on sections without openings. The temperatures of T2-4, T2-6, T3-4, T3-6, and T3-6 were 18 °C, 5 °C, 8 °C, 6 °C, and 105 °C higher than those of T2-10, T2-12, T3-10, T3-12, and T3-12, respectively.
- (2)
- The temperatures of points such as T2-1, T3-1, and T4-3 close to the fire exposed sides dropped quickly after the flameout, while the temperatures of some points such as point 7, point 8, and point 9 in sections without openings of T2~T4, far away from the fire exposed sides, showed the thermal inertia of concrete apparently and did not fall immediately.
- (3)
- There were “temperature plateaus” [65] with different lengths around 100 °C in the temperature rise curves of the measuring points, which was due to the fact that the heat was continuously carried away by the water evaporation inside the concrete. The closer the measuring point was to the fire exposed side, the faster the concrete heated up and the moisture inside the concrete evaporated. As a result, the length of the temperature plateau was a significant positive correlation with the distance between the measuring point and the fire exposed side.
3.3. Fire Resistance and Deformation of Specimens
- (1)
- The mid-span deflections of the specimens increased with time in general. The actuator was stopped at the same time as the flame was turned off, and then the mid-span deflections of T1~T3 rebounded by 13.7 mm, 20.1 mm, and 23.4 mm, subsequently. However, the mid-span deflection of T4 could not be rebounded due to the rupture of three longitudinal reinforcements. The bending stiffness of T1~T4 decreased sequentially. The mid-span deflections of T1~T4 at the flameout moment were 285.2 mm, 285.6 mm, 285.0 mm, and 285.6 mm, respectively. The mid-span deflections of T1~T4 at the end of fire tests were 224.7 mm, 222.4 mm, 219.4 mm, and 287.8 mm, respectively. Except that no recovery happened to T4, the recovery ratios of T1~T3 were 21.2%, 22.1%, and 23.0%, respectively. The deformation recovery ability still performed well for the RC T-beam with a circular opening under the premise that the depth of the bottom chord was 220 mm.
- (2)
- The fire resistances of T1~T4 were 127 min, 126 min, 122 min, and 103 min, respectively. Compared with T1, the fire resistances of T2 and T4 were reduced by 0.8% and 18.9%, respectively. The fire resistance was reduced with the depth of the bottom chord. When the depth was 100 mm, the failure mode of the RC T-beam with a circular opening under fire changed from ductile failure to brittle failure.
- (3)
- Compared with T1, the fire resistance of T3 was reduced by 3.9%. The fire resistance was influenced by the cutting off of stirrups on the premise that the depth of the bottom chord was 220 mm, but no brittle failure occurred.
- (4)
- After the actuator load was applied, the mid-span deflections of T1~T4 were 11.8 mm, 15.2 mm, 19.4 mm, and 15.2 mm, respectively. Compared with T1, the mid-span deflections of T2~T4 after the loading increased by 28.8%, 64.4%, and 28.8%, respectively, which showed that both drilling an opening and cutting off the stirrups could increase the mid-span deflection of the RC T-beam.
- (1)
- For T1, the mid-span deflection increased rapidly after the temperature of longitudinal reinforcement at the beam bottom beyond 600 °C until the fire resistance.
- (2)
- For T2~T4, the mid-span deflection increased rapidly after the temperatures of T2-4, T2-10, T3-4, T3-10, and T4-10 reached 500 °C and the temperatures of T2-6, T2-12, T3-6, T3-12, T4-6, and T4-12 reached 600 °C until the fire-resistance, which could be since points 4 and 10 were close to the beam bottom while points 6 and 12 were close to the beam bottom and the web side.
4. Numerical Simulation
4.1. Finite Element Modeling
4.2. Validation of Numerical Modeling and Analysis
4.3. Numerical Results and Discussion
5. Simplified Calculation of the Flexural Capacity of the RC T-Beam with a Circular Opening under Fire
6. Conclusions
- (1)
- In the case of a 100 mm bottom chord and an uncut stirrup, the fire resistance of the RC T-beam with a circular opening was 18.9% lower than the normal RC T-beam, and brittle failure occurred. The fire resistance of the RC T-beams with an uncut stirrup and a cutting stirrup were 0.8% and 4.0% lower than the normal RC T-beam, respectively, both failing in ductile mode. Consequently, an opening at the appropriate location would ensure that the fire resistance of the RC T-beam remained almost constant.
- (2)
- The FE model can simultaneously simulate the temperature field of the RC T-beams with a circular opening at high temperature and the mechanical response of the beams under concentrated load, which can provide a helpful reference for applying static load levels on the RC T-beams with a circular opening under fire.
- (3)
- FE models indicated that brittle failure of the specimen under fire can be avoided with a reasonable depth of the bottom chord. For the case of the specimens in this study, the depth of the bottom chord should be ensured to be no less than 120 mm. The parametric analysis illustrated that the fire resistance decreased with the increasing design load ratio.
- (4)
- Within the parameter range of this study, the 300–800 °C isotherm method was demonstrated to be a good method for calculating the flexural capacity of the RC T-beam with a circular opening under fire. By evaluating the relationship between the flexural capacity and the internal force of the section of the RC T-beam after the creating of the opening, the suitability of the opening location can be assessed.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Material | Consumption (kg/m3) | Mix Proportion |
---|---|---|
Ordinary Portland cement (42.5R) | 305 | 1 |
Water | 163 | 0.534 |
Sand | 721 | 2.364 |
Gravel (5 mm~20 mm) | 336 | 1.102 |
Gravel (16 mm~31.5 mm) | 760 | 2.492 |
Water reducer | 6.7 | 0.022 |
Fly ash 1 | 29 | 0.095 |
Mineral powder 2 | 29 | 0.095 |
Measuring Time | Compressive Strength, fcu (MPa) | Moisture Content, ω (%) |
---|---|---|
28 days | 57.9 | 2.35 |
Fire tests (330 days) | 64.2 | 2.32 |
Reinforcement Type | ds1 (mm) | fy2 (MPa) | ɛy3 (10−3) | fu4 (MPa) | E5 (N/mm2) |
---|---|---|---|---|---|
HPB300 | 8 | 479 | 2.281 | 667 | 2.12 × 105 |
HPB300 | 10 | 503 | 2.395 | 666 | 2.11 × 105 |
HRB400 | 12 | 488 | 2.440 | 675 | 2.05 × 105 |
HRB400 | 25 | 453 | 2.265 | 660 | 2.03 × 105 |
Specimens | lc 1 (mm) | ld 2 (mm) | Cut Off the Stirrup | tm 3 (min) |
---|---|---|---|---|
T1 | – | – | – | 127 |
T2 | 310 | 220 | No | 126 |
T3 | 310 | 220 | Yes | 122 |
T4 | 190 | 100 | No | 103 |
Specimen | Measuring Point | Temperature (°C) | Measuring Point | Temperature (°C) |
---|---|---|---|---|
T1 | T1-4 | - | - | - |
T1-6 | 697 | - | - | |
T2 | T2-4 | 592 | T2-10 | 574 |
T2-6 | 670 | T2-12 | 665 | |
T3 | T3-4 | 589 | T3-10 | 581 |
T3-6 | 687 | T3-12 | 681 | |
T4 | T4-4 | - | T4-10 | 527 |
T4-6 | 717 | T4-12 | 612 |
Specimen | ld (mm) | Specimen | ld (mm) | Specimen | ld (mm) |
---|---|---|---|---|---|
T4 | 100 | T8 | 140 | T12 | 180 |
T5 | 110 | T9 | 150 | T13 | 190 |
T6 | 120 | T10 | 160 | T14 | 200 |
T7 | 130 | T11 | 170 | T15 | 210 |
T2 | 220 |
Specimen | ld (mm) | tme (min) | Specimen | ld (mm) | tme (min) | Specimen | ld (mm) | tme (min) |
---|---|---|---|---|---|---|---|---|
T4 | 100 | 109 | T8 | 140 | 118 | T12 | 180 | 122 |
T5 | 110 | 110 | T9 | 150 | 120 | T13 | 190 | 122 |
T6 | 120 | 115 | T10 | 160 | 120 | T14 | 200 | 120 |
T7 | 130 | 117 | T11 | 170 | 121 | T15 | 210 | 119 |
T2 | 220 | 118 |
Specimen | ld (mm) | tme (min) | |||
---|---|---|---|---|---|
rd 1 = 0.3 | rd = 0.5 | rd = 0.6 | rd = 0.7 | ||
T1 | - | 178 | 138 | 125 | 117 |
T2 | 220 | 166 | 129 | 118 | 108 |
T3 | 220 | 166 | 129 | 118 | 108 |
T4 | 100 | 147 | 117 | 109 | 98 |
Specimen | Location of the Longitudinal Reinforcement | Tl 1 (°C) | Location of the Longitudinal Reinforcement | Ts 2 (°C) |
---|---|---|---|---|
T1 | Middle | 650 | Middle | 580 |
Corner | 665 | Corner | 712 | |
T2 | Middle | 644 | Middle | 586 |
Corner | 659 | Corner | 693 | |
T3 | Middle | 621 | Middle | 575 |
Corner | 635 | Corner | 673 | |
T4 | Middle | 615 | Middle | 596 |
Corner | 629 | Corner | 672 |
Specimens | ld (mm) | tm (min) | Mul 1 (kN·m) | Mut 2 (kN·m) | Mul/Mut | Error | Mus 3 (kN·m) | Mus/Mut | Error |
---|---|---|---|---|---|---|---|---|---|
T1 | - | 127 | 151.4 | 168.7 | 0.90 | −0.10 | 140.2 | 0.83 | −0.17 |
T2 | 220 | 126 | 155.6 | 167.6 | 0.93 | −0.07 | 149.7 | 0.89 | −0.11 |
T3 | 220 | 122 | 171.3 | 168.1 | 1.02 | 0.02 | 161.9 | 0.96 | −0.04 |
T4 | 100 | 103 | 175.0 | 166.5 | 1.05 | 0.05 | 158.9 | 0.95 | −0.05 |
Average | 0.97 | −0.03 | 0.87 | −0.09 |
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Jin, X.; Xu, Y.; Zhu, W.; Zhang, D. Fire Resistance of Reinforced Concrete T-Beams with Circular Web Openings. Buildings 2023, 13, 436. https://doi.org/10.3390/buildings13020436
Jin X, Xu Y, Zhu W, Zhang D. Fire Resistance of Reinforced Concrete T-Beams with Circular Web Openings. Buildings. 2023; 13(2):436. https://doi.org/10.3390/buildings13020436
Chicago/Turabian StyleJin, Xianhong, Yuye Xu, Wenjun Zhu, and Dashan Zhang. 2023. "Fire Resistance of Reinforced Concrete T-Beams with Circular Web Openings" Buildings 13, no. 2: 436. https://doi.org/10.3390/buildings13020436
APA StyleJin, X., Xu, Y., Zhu, W., & Zhang, D. (2023). Fire Resistance of Reinforced Concrete T-Beams with Circular Web Openings. Buildings, 13(2), 436. https://doi.org/10.3390/buildings13020436