Determination of the combined heat transfer coefficient to simulate the fire-induced damage of a concrete tunnel lining under a severe fire condition

To avoid underestimating the severity of damage to tunnel concrete lining under the high-temperature conditions of a fire using thermal analysis, it is important to consider the cross-sectional loss of a concrete lining during heating. This study simulates the structural loss by numerical analysis using an element elimination model and a combined heat transfer coefficient. A series of fire tests was performed with fire curves that differed in the initial temperature gradient and maximum temperature. Values of the optimized combined heat transfer coefficient were obtained from the coincidence of the results of the numerical analysis with experimental data. The results reveal that an increase in both the initial temperature gradient and maximum temperature causes greater damage to the concrete structures and also gives rise to an increase in the combined heat transfer coefficient. Values of the combined heat transfer coefficient can be inferred from values of initial temperature gradient and maximum temperature in the case of structural concrete loss. Two sets of regression equations were derived from the results depending on whether or not a structural loss occurs. The proposed method of thermal analysis outperforms the conventional method in terms of accurately simulating observed results.     

Hydrothermal model for predicting fire-induced spalling in concrete structural systems

A one-dimensional numerical model to predict fire-induced spalling in concrete structures is presented. The model is based on pore pressure calculations in concrete, as a function of time. Principles of mechanics and thermodynamics are applied to predict pore pressure in concrete structures exposed to fire. An assessment of the possibility of tensile fracture is made by comparing the computed pore pressure with temperature-dependent tensile strength. The pore pressure calculations are coupled with heat transfer analysis to ensure that the loss of concrete section, resulting from spalling, is accounted for in subsequent heat transfer analysis. The validity of the numerical model is established by comparing temperature, pore pressure, and concrete spalling predictions with results from fire tests. The computer program is applied to conduct case studies to investigate the influence of concrete permeability, tensile strength of concrete, relative humidity in concrete, and heating rate on fire-induced spalling in concrete members. Through these case studies, it is shown that permeability, tensile strength of concrete, and heating rate have a significant influence on fire-induced spalling in concrete. It is also shown that relative humidity has a marginal influence on fire-induced spalling in concrete.     

混凝土高温爆裂临界温度和防爆裂纤维掺量研究综述与分析

确定无纤维或低纤维掺量的不同强度等级混凝土的爆裂临界温度,以及防止火灾时不同强度混凝土爆裂所需聚丙烯（PP）纤维或钢纤维最小掺量,对混凝土结构抗火设计具有重要意义。为此,对国内外大量高温爆裂试验研究结果进行分析,获得了爆裂临界温度与混凝土抗压强度（23~238MPa）的关系曲线,发现混凝土抗压强度越高,爆裂临界温度越低。通过大量试验数据拟合得到了防爆裂PP纤维掺量、钢纤维掺量与混凝土抗压强度的关系曲线,发现随着混凝土抗压强度的提高,所需防爆裂PP纤维掺量呈线性增长,而所需防爆裂钢纤维掺量呈指数增长。按EN 1992-1-2:2004《欧洲混凝土抗火设计规范》建议值,PP纤维掺量为0.22%的防爆裂混凝土,火灾下仍可能发生爆裂;按所提出计算式计算的掺量,则可有效降低火灾下混凝土爆裂的风险。     

Application of coupled XFEM-BCQO in the structural optimization of a circular tunnel lining subjected to a ground motion

A new structural optimization method of coupled extended finite element method and bound constrained quadratic optimization method (XFEM-BCQO) is adopted to quantify the optimum values of four design parameters for a circular tunnel lining when it is subjected to earthquakes. The parameters are: tunnel lining thickness, tunnel diameter, tunnel lining concrete modulus of elasticity and tunnel lining concrete density. Monte-Carlo sampling method is dedicated to construct the meta models so that to be used for the BCQO method using matlab codes. Numerical simulations of the tensile damage in the tunnel lining due to a real earthquake in the literature are created for three design cases. XFEM approach is used to show the cracks for the mentioned design cases. The results of the BCQO method for the maximum design case for the tunnel tensile damage was matching the results obtained from XFEM approach to a fair extent. The new coupled approach manifested a significant capability to predict the cracks and spalling of the tunnel lining concrete under the effects of dynamic earthquakes.     

Thermo-hydro-chemical couplings considered in safety assessment of shallow tunnels subjected to fire load

Fire loading of concrete tunnel linings is characterized by various physical, chemical, and mechanical processes, resulting in spalling of near-surface concrete layers and degradation of strength and stiffness of the remaining tunnel lining. In this paper, the governing transport processes taking place in concrete at elevated temperatures are considered within a recently published fire-safety assessment tool [Savov K, Lackner R, Mang HA. Stability assessment of shallow tunnels subjected to fire load. Fire Safety J 2005; 40: 745–763] for underground structures. In contrast to consideration of heat transport only [Savov et al.], a coupled thermo-hydro-chemical analysis, simulating the heat and mass transport in concrete under fire loading, is performed, giving access to more realistic temperature distributions as well as gas-pressure distributions within the tunnel lining. These data serve as input for the structural safety assessment tool considering, in addition to the temperature dependence of mechanical properties, the effect of the gas pressure on the strength properties of the heated lining concrete. The combination of the two analysis tools (coupled analysis of governing transport processes and structural safety assessment) is illustrated by the fire-safety assessment of a cross-section of the Lainzer tunnel (Austria) characterized by low overburden (shallow tunnel).     

火灾中衬砌混凝土的爆裂损伤特征

铜-黄公路某一涵洞衬砌混凝土的火灾损伤在宏观上显示了典型的逐层剥落现象,遭受多层剥落的混凝土在显微尺度下的损伤特征表明,混凝土的剥落与裂隙的扩展特征和局部爆裂现象有关.裂隙的扩展和爆裂作用的叠加是衬砌混凝土发生多层剥落的重要原因.混凝土的湿度大、火灾温度高且持续时间长是发生多层爆裂损伤的重要条件.     

Experimental study of tunnel segmental joints subjected to elevated temperature

Segmental joints present a weak link in the tunnel lining both structurally (due to its low stiffness) and non-structurally (high risk of water leakage); therefore the behaviour of the lining joints has a significant effect on the performance of the shield TBM tunnel lining. Segmental joints are thus a particular concern when the tunnel lining is exposed to high temperature in the case of a tunnel fire. This paper presents an experimental study on the behaviour of TBM tunnel joints in fire under different mechanical loading and boundary conditions, and with both the normal reinforced concrete (RC) segments and hybrid fibre reinforced concrete (HFRC) segments. Totally thirteen jointed specimens were constructed at a scale of 1:3 and tested. Eleven specimens were exposed to a HC (Hydrocarbon) curve and mechanically loaded to failure either under-fire or post-fire, while two specimens were tested in ambient temperature to provide benchmark data. The results demonstrate that the initial loading conditions have a significant effect on the jointed segments during and after fire, and this is closely related to different rate of degradation of concrete in different stress state under high temperature. In general, the resistance capacity of both RC and HFRC joints increased with axial force. The use of HFRC material provided good spalling resistance.     

Damages of the Shaohuoping road tunnel near the epicentre

Numerous infrastructures including national roads, tunnels, etc. in Sichuan Province were damaged to various extents in China's Wenchuan earthquake in 2008. The Shaohuoping road tunnel, near the epicentre of the earthquake, was the typical damaged one. The damage modes of different parts of the Shaohuoping tunnel were summarised based on the spot investigation data. It was found that various damages involving cracking and spalling of concrete lining were observed on different parts of the tunnel. The earthquake damages of critical positions, portal section and body portion, were simulated and analysed by finite element numerical model proposed in this paper. Influences of vertical motion on seismic responses of the tunnel were also studied by means of the finite element simulation. The simulation responses were compared with the spot investigations of earthquake structural damages to different parts of the tunnel. The comparisons showed a good agreement, which indicated that, the damage mechanism of the tunnel can be predicted reliably by the proposed finite element model. Furthermore, the results show that the damage of different parts of the tunnel is mainly controlled by horizontal ground motions, but the tunnel may suffer more damages with the effect of the vertical motion.     

火灾对隧道衬砌结构的损害及防范措施研究

由于隧道火灾温度高,升温速度快,会对衬砌结构造成严重的损坏,并影响衬砌结构体系的可靠性。文章以上海某地铁隧道火灾为背景,在对国内外隧道火灾案例调研的基础上,分析了火灾高温对衬砌结构的损害形式,指出了提高衬砌结构耐火性能的必要性和紧迫性,并初步给出了提高隧道衬砌结构耐火性能的思路和方法,可供类似研究参考。     

The structural behavior and simplified thermal analysis of normal-strength and high-strength concrete beams under fire

The objective of this study is to investigate the effects of concrete compressive strength and cover thickness on the structural behavior of reinforced concrete (RC) beams under fire. For this purpose, four normal-strength and high-strength concrete test beams were fabricated and tested under the ISO 834 standard fire curve to point of the failure. The test set-up was designed to evaluate the heat distribution and displacement changes of simply supported beams subjected to sustained loads under fire. Test results for normal-strength and high-strength concrete beams were compared for each of the test variables. The test results show that the relationships between time and temperature distributions in the beam sections are very similar and are unrelated to the strength of the concrete, with the exception of the upper part of the beam section. They also showed that the rates of deflection increase for both normal-strength and high-strength concrete beams is very similar before spalling but becomes remarkably high for high-strength concrete beams after spalling. A simplified model was proposed to determine the effect of spalling on the temperature gradient of a high-strength concrete beam. The results of finite difference method (FDM) analysis using this proposed model showed a section temperature gradient that was similar to that of the test results.     

足尺震损钢筋混凝土柱耐火性能试验研究

地震次生火灾具有发生概率高、灾害破坏力强、损失巨大和机理复杂等特点,为分析震后火灾环境下钢筋混凝土柱构件的抗火性能,以钢筋混凝土柱的裂缝、混凝土剥落和残余变形作为地震损伤形式,设计了7根带有预制损伤的足尺钢筋混凝土方形截面柱,进行明火试验,研究不同几何损伤形式及损伤程度对混凝土柱的火灾破坏特征、截面温度场分布、竖向变形过程及耐火极限等性能影响规律。试验结果表明,预制损伤混凝土柱构件的火灾破坏程度明显大于无损伤混凝土柱,其中混凝土剥落对震损柱内部温度场及耐火性能的影响最大。根据足尺混凝土柱的耐火试验结果,采用ABAQUS有限元软件分析震损混凝土柱的温度场分布与耐火极限,结果表明：当试件端部产生保护层剥落时,剥落区轴向等温线向柱中心凹陷,整体呈“瓶颈”状;轴压比与剥落厚度对震损混凝土柱耐火极限有较大影响;残余变形小于GB 50011—2010《建筑抗震设计规范》中的弹塑性层间位移角1/50时,残余变形对震损混凝土柱的耐火极限影响有限。通过拟合得到了混凝土柱的轴压比、残余层间位移角、剥落厚度与其耐火极限之间的量化关系式,关系式计算结果与试验结果吻合较好。     

Modeling of insulated CFRP-strengthened reinforced concrete T-beam exposed to fire

The use of Fiber Reinforced Polymer (FRP) has been established as one of the possible techniques to strengthen concrete beams in flexure and shear. Carbon Fiber Reinforced Polymer (CFRP) has been identified as the material of choice in civil infrastructure applications. The fire performance of such CFRP-strengthened members and their resistance to heat transfer and to various environmental exposure factors need to be investigated. In this paper, a detailed finite element model of a CFRP-strengthened reinforced concrete T-beam is developed. The model accounts for the variation in thermal and mechanical parameters of the beams’ constituent materials with temperature, including CFRP and insulation materials. Nonlinear time domain transient thermal-stress finite element analysis is performed using the commercial software ANSYS to study the heat transfer mechanism and deformation within the beam for fire conditions initiating at the bottom of the beam. To relate the simulation to an actual case, a reinforced concrete T-beam strengthened with CFRP and fire-tested by other investigators is modeled. The progression of temperature in the beam, CFRP, reinforcing steel, and along the CFRP–concrete interface is compared to the observed fire test data. Overall, the predicted temperature results are in good agreement with the measured ones. In addition, the mid-span deflection increases nonlinearly during the fire exposure time due to the increase in the total strain on the tension side of the beams and due to concrete cracking. Successful FE modeling of this structure provides an economical, alternative solution to expensive experimental investigations.     

寒区隧道地源热泵型供热系统取热段温度场解析

为解决寒区隧道冻害问题,将地源热泵型供热系统应用于内蒙古博牙高速林场隧道中。供热系统由取热段、加热段、热泵和分、集水管路组成,可用于隧道洞口段衬砌和排水系统加热。取热段位于隧道中部,获取围岩中的地热能。隧道取热段温度场由热交换管外围岩温度场和热交换管内流体温度场两部分组成。建立考虑衬砌结构和热源的隧道取热段热交换管外围岩传热模型,利用叠加原理、拉普拉斯变换和积分变换相结合的方法获得取热段温度场解析解。基于能量守恒原理建立热交换管内流体传热模型,根据热交换管外围岩温度场,利用迭代法计算热交换管的出口温度。采用反分析的方法确定试验段围岩的综合热物性参数,将理论解与现场试验数据对比分析,其精度满足工程要求。取热段温度场理论解为系统的设计提供理论指导。     

火灾下隧道衬砌结构力学行为的热力耦合分析

结合某水下隧道实例,采用大型三维有限元分析软件建立了二维热力耦合分析模型,得到了隧道衬砌结构在火荷载高温下的温度场、内力分布及变形情况,为隧道结构的防火设计及灾后损伤修复和加固提供了参考。     

Structural analysis of reinforced concrete chimneys subjected to uncontrolled fire

We studied the behavior and residual structural capacity of reinforced concrete chimneys subjected to an uncontrolled fire. We used a combination of a heat transfer finite element model-to obtain the temporal distributions of temperature during the fire event-and the structural model of concrete chimney design provided by the American Concrete Institute (ACI 307-08). This approach allows estimating the reduction in the vertical (axial) strength and moment strength of the chimney both during a fire and post-fire, and gives a direct estimate of the reduction in the safety factors of the concrete chimney. Using this method, we examined the impact of various design parameters on the residual structural capacity of a concrete chimney subjected to an internal fire. An iterative finite element method was also presented as an alternative to the ACI 307 calculations. Moreover, finite element calculations were used to study the role of thermal stresses on the axial strength of the chimney during fire. Our study provides insight into possible failure mechanisms of concrete chimneys damaged due to fire and could suggest possible approaches for minimizing the risk of chimney failure due to an uncontrolled fire.     

火灾后隧道衬砌混凝土力学性能及微观结构实验研究

为研究火灾后隧道衬砌结构的损伤程度和性能特征,对火灾后隧道衬砌混凝土进行抗压强度试验和超声检测,分析不同受火时间(2,3,4 h)下,隧道衬砌混凝土剩余抗压强度和超声波声速变化的规律.结果表明:200～800℃范围内,不同受火时间下,受火温度和剩余抗压强度、超声波声速和剩余抗压强度都有较好的线性关系,且随受火时间增大,...     

Progressive Collapse Analysis of a Typical Super-Tall Reinforced Concrete Frame-Core Tube Building Exposed to Extreme Fires

A number of disastrous incidents have indicated that extreme fires can act as a trigger event to initiate the progressive collapse of reinforced concrete (RC) structures. Hence, research on progressive collapse risks of RC structures under extreme fires is most important. However, limited studies have been undertaken in the fire-induced progressive collapse of tall and super-tall RC buildings. Hence, a high-performance finite element model was developed for this study to simulate the mechanical behavior of RC members in fire-induced progressive collapse. Fiber beam and multi-layer shell elements were used, in conjunction with appropriate material constitutive laws and elemental failure criteria under high temperature conditions. Extreme fire scenarios were also considered, based on the actual fire-induced progressive collapse events of the WTC towers and the Windsor Tower. The simulation results indicated that a progressive collapse of a super-tall building was triggered by the flexural failure of the peripheral columns, approximately 7 h after being exposed to fire. The bending deformations of the peripheral columns increased significantly, due to the outward thermal expansion of the upper floors and the inward contraction of the lower floors, a result of the fire-induced damage. The results also revealed that, when multiple stories are subjected to fire, the internal forces in the components are redistributed in the horizontal and vertical directions by way of the Vierendeel truss mechanism, leading to a maximum increase (of approximately 100%) of the axial forces in the columns. The present work identified the mechanisms of the fire-induced progressive collapse of a typical RC super-tall building, and provided an effective analysis framework for further research on the fire safety of tall and super-tall RC buildings.     

Fire Induced Spalling in High Strength Concrete Beams

A macroscopic finite element model is extended to account for fire induced spalling in high strength concrete (HSC) beams. The model is based on the principles of mechanics and thermodynamics and utilizes pore pressure calculations to predict fire induced spalling in concrete. For validating the model, spalling measurements were made by conducting fire resistance experiments on four normal strength and high strength concrete beams. Spalling predictions from the model are compared with the measured values of spalling at various stages of fire exposure. The validated model is applied to investigate the influence of fire scenario, concrete strength (permeability) and axial restraint on the fire induced spalling and fire response of RC beams. Results from the analysis show that fire scenario, and concrete permeability largely influence the extent of fire induced spalling in concrete beams. Further, it is also shown that the extent of spalling has significant influence on the fire resistance of RC beams.     

Position of Maximum Ceiling Temperature in a Tunnel Fire

The position of the maximum ceiling gas temperature indicates how far the fire plum could be blown away by a ventilation flow. It could be applied to estimate the activation of a detection system or a sprinkler system, or to estimate the range of damage to the tunnel structure. An equation for predicting the position of the maximum ceiling gas temperature in a tunnel fire is proposed based on a theoretical analysis and validated using both laboratory test data and full scale test data. A flame angle has been defined based on the position of the maximum ceiling temperature in a tunnel fire. The flame angle is directly related to the dimensionless ventilation velocity, and it becomes insensitive to the heat release rate for a large tunnel fire. Further, it is found that a constant critical flame angle exists, defined as the flame angle under the critical condition when the backlayering just disappears. For a given tunnel and fire source, the flame angle under critical conditions is the same value, independent of heat release rate, and the maximum ceiling temperature under critical conditions always corresponds to the same position. Generally the horizontal distance between the position of the maximum ceiling temperature and the fire source centre is around 1.5 times the effective tunnel height under the critical condition.