Fire behavior of axially loaded slender high strength concrete-filled tubular columns

This paper describes sixteen fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to concentric axial loads. The test parameters were the nominal strength of concrete (30 and 80 MPa), the infilling type (plain concrete, reinforced concrete and steel fiber reinforced concrete) and the axial load level (20% and 40%). The columns were tested under fixed-pinned boundary conditions and the relative slenderness at room temperature was higher than 0.5 in all of the cases. A numerical model was validated against the tests, in order to extend the results and understand the failure mode of such columns. It is the aim of this paper to study the influence in a fire situation of the use of high strength concrete, as opposed to normal strength concrete. The results have shown that for slender columns subjected to high temperatures, the behavior of high strength concrete was different than for stub columns, spalling not being observed in the experiments. Furthermore, the addition of steel fibers was not found very advantageous in slender columns, since no increment in terms of fire resistance was obtained for the columns which used this type of reinforcement. However, the addition of reinforcing bars seems to be the solution in some cases, where the use of external fire protection wants to be avoided in the design of HSS structures, since the reinforcing bars allow the tube to resist a higher axial load.     

Fire behavior of axially loaded slender high strength concrete-filled tubular columns

This paper describes sixteen fire tests conducted on slender circular hollow section columns filled with normal and high strength concrete, subjected to concentric axial loads. The test parameters were the nominal strength of concrete (30 and 80 MPa), the infilling type (plain concrete, reinforced concrete and steel fiber reinforced concrete) and the axial load level (20% and 40%). The columns were tested under fixed-pinned boundary conditions and the relative slenderness at room temperature was higher than 0.5 in all of the cases. A numerical model was validated against the tests, in order to extend the results and understand the failure mode of such columns. It is the aim of this paper to study the influence in a fire situation of the use of high strength concrete, as opposed to normal strength concrete. The results have shown that for slender columns subjected to high temperatures, the behavior of high strength concrete was different than for stub columns, spalling not being observed in the experiments. Furthermore, the addition of steel fibers was not found very advantageous in slender columns, since no increment in terms of fire resistance was obtained for the columns which used this type of reinforcement. However, the addition of reinforcing bars seems to be the solution in some cases, where the use of external fire protection wants to be avoided in the design of HSS structures, since the reinforcing bars allow the tube to resist a higher axial load.     

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。     

Fire resistance of concrete encased steel columns under 3- and 4-side standard heating

Results from seven fire resistance experiments on concrete encased steel (CES) columns under standard fire exposure conditions are presented. The test parameters include column size, 3- and 4-side fire exposure, load intensity and load eccentricity. Data from the tests is utilized to study the effect of the aforementioned parameters on thermal and structural response of concrete encased steel columns. Test results show that CES columns have higher fire resistance under 3-side heating than that under 4-side heating. Also, load ratio and load eccentricity have a noticeable influence on the fire resistance of CES columns. In addition, spalling of the concrete decreases the fire resistance of CES columns. A comparison of measured fire resistance of CES columns with those predicted using current code provisions indicate that the current provisions may not be conservative in some situations.     

约束受热伸长中空柱填充混凝土的耐火性能

在圆形中空柱中填充混凝土是一种很好的柱子加固方案,因为它能够增加柱子在室内和高温环境下的承载能力。然而,发生火灾时,对受热伸长的约束可能会改变其力学性能。通过对约束受热伸长中空柱进行一系列耐火试验后,得出试验结果。试验对柱子长细比﹑承载等级﹑周围结构刚度以及所占钢筋的百分比和混凝土强度等参数进行了测定。结果显示,这些柱子的极限抗火时间都小于46min。在柱子内层墙面使用混凝土环,从其性能上来讲在火灾时并没有太大优势,因为混凝土环会受到钢管过热而带来的大面积碎裂。柱子最主要的破坏方式即是整体屈曲。然而,在一些试验样本上也出现了局部屈曲。     

Fire resistance of concrete filled circular hollow columns with restrained thermal elongation

The filling of circular hollow sections (CHS) with concrete is a good solution for strengthening columns since such procedure increase their load bearing capacity at room and high temperatures. However, in the event of a fire, restraining to thermal elongation may change their mechanical behavior. This paper presents the results of a large series of fire resistance tests on CHS columns with restrained thermal elongation. Parameters such as the slenderness of the column, its load level, the stiffness of the surrounding structure, the percentage of steel reinforcement and the degree of concrete filling inside the column, were tested. The results obtained show that the critical time of the columns was less than 46 min. The use of a concrete ring around the internal surface of the column's wall is of no advantage in terms of its behavior under fire conditions because this concrete ring suffers extensive spalling and cracking due to overheating of the steel tube. The main failure mode of the columns was global buckling. However in several cases local buckling also occurred.     

Testing of self-consolidating concrete-filled double skin tubular stub columns exposed to fire

A concrete-filled double skin tube (CFDST) is an innovative steel and concrete composite construction with the potential to be used as columns in high-rise buildings. Self-consolidating concrete (SCC) can offer convenience for construction and ensure the construction quality of CFDST columns. There is very limited knowledge about the fire performance of CFDST columns, which is a key issue in the application to high-rise buildings. This paper reports an experimental investigation into the fire behaviour of CFDST stub columns filled with SCC and fibre reinforced SCC. The study aims to obtain thermal and structural responses of the stub columns through standard fire testing. Details are given in terms of the failure mode, temperature distribution, critical or limiting temperature and fire resistance. It was found that the added fibre does not affect the temperature distribution whereas the increased concrete thickness and outer tube perimeter reduces the temperature in CFDST specimens. The added steel fibre increases the fire resistance of CFDST stub columns especially for load levels less than 0.6. The critical or limiting temperature for CFDST sections was found to be higher than for CFST (concrete-filled steel tube) or unfilled tubes.     

The Influence of Pressure in the Pore System on Fire Spalling of Concrete

Fire spalling of concrete is often attributed to the pressure in the pore system. Recent results from fire tests on Self Compacting Concrete (SCC) have shown that the connection between pressure in the capillary pore system and fire spalling of concrete is not obvious [Jansson (2006) Proceedings from the 4th international workshop, Aveiro, Portugal, pp 747–756]. This issue has been investigated by performing pressure measurements on SCC and a traditional vibrated concrete designed for tunnel construction. In the tests conducted on SCC data from exposure to the hydrocarbon (HC) fire curve are presented. In the tests conducted on vibrated concrete, measurements were performed in concrete exposed to the standard fire curve (EN1363-1) and the Rijkswaterstaat (RWS) curve. The highest pressures in the two test series were measured in the concretes that did not exhibit spalling during fire. One conclusion from the tests is that pressure in the capillary system is not the driving force for spalling during fire exposure. However, pressure is involved in the redistribution of moisture during fire exposure. A new theory is proposed to explain the spalling reducing function of PP-fibres based on the presence and movement of moisture.     

Concrete for Tunnel Linings Exposed to Severe Fire Exposure

The fire resistance with respect to temperature response and spalling have been experimentally determined for several different concrete qualities. The results show that some qualities perform relatively well even without any type of passive fire protection. Tests have also been made using protection in form of addition of polypropylene fibres as well as thermal barriers. Concrete with addition of polypropylene fibres perform very well. Although, there is still a question regarding the long term durability such as frost resistance and chloride penetration when polypropylene fibres are used. The tests made on concrete with thermal barriers were unsatisfactory since the products used in the tests had documentation on the performance but failed during these tests. A reason for this may be that the previous tests did not catch eventual problems with adhesion between the thermal barrier and the concrete. In the present study three different systems were tested and all failed. Although, when using mechanical bonding and reinforcement net two of the products (spray-on systems) worked well.     

圆钢管及钢管混凝土柱的抗火设计

分别对圆钢管、钢管混凝土、中空夹层钢管混凝土柱进行了抗火设计,并对结果进行比较分析。结果表明,在较高荷载比下柱的耐火极限不能满足实际要求,必须进行防火保护。在相同条件下,耐火极限从大到小排序为:圆钢管混凝土、中空夹层钢管混凝土、钢管柱。在一级耐火等级下,钢管混凝土柱和中空夹层钢管混凝土柱需要厚涂型钢结构防火涂料的厚度可比钢管柱分别少55%和18%以上。随着荷载比的减小或截面尺寸的增加,柱的耐火极限提高,需要的保护层厚度减小。对于钢管混凝土柱,若采用水泥砂浆保护层,其厚度是防火涂料的3倍及以上。     

Fire Endurance of High Strength Concrete Columns

In buildings, fire represents one of the most severe environmental conditions, and therefore, should be properly accounted for in the design of high strength concrete (HSC) structural members. The increased use of HSC in buildings has raised concerns regarding the behavior of such concrete in fire. In particular, spalling at elevated temperatures, as identified in studies by a number of laboratories, is a main concern.In this paper, results from an experimental program on the fire resistance of HSC columns are presented. The factors that influence the thermal and structural behavior of HSC concrete columns under fire conditions are discussed. Data from this study indicate that the type of aggregate, concrete strength, load intensity, and detailing and spacing of ties have an influence on the fire resistance performance of HSC columns. Further, the test results show that tie configuration (bending of ties at 135°, ties and provision of cross ties) and closer tie spacing has a significantly beneficial effect on the fire resistance of HSC columns. The results presented will generate data on the fire resistance of HSC columns, and contribute to identifying the factors that influence the behavior of HSC columns.     

Fire Resistance and Post-fire Seismic Behavior of High Strength Concrete Shear Walls

In this study, both fire tests and low-frequency cyclic loading tests after fire were conducted on three conventional high strength concrete (HSC) shear walls and a superimposed HSC shear wall with precast recycled aggregate concrete (RAC) panels. The RAC in this paper was made with recycled concrete aggregate. When specimens suffered the fire exposure on one side for 45 min, 90 min, and 135 min separately, spalling of concrete, temperature distribution and deformation of specimens were investigated as indicators of fire response. When specimens were subjected to cyclic load after fire, hysteresis curves were obtained, based on which the secant stiffness degradation and energy dissipation capacity of walls were analyzed. The results indicated that HSC would suffer severe spalling during the fire and that fire response of the superimposed wall including spalling was smaller than that of conventional walls. Using RAC panel as a thermal barrier was found to be effective to alleviate spalling, as it reduced more than 60% of spalling of HSC compared with bare walls. Based on the seismic tests results, the fire exposure deteriorated the load bearing capacity, lateral stiffness and energy dissipation capacity of walls, whereas the application of RAC panels improved the load bearing capacity by about 10% even when the superimposed wall was exposed to the fire for a long time.     

Spalling of concrete cover of cyclically loaded columns and fire behaviour of columns with concrete spalling

Fire following an earthquake is a threat to seismically damaged structural members with spalling of concrete cover. To evaluate the fire behaviour of the damaged members, it is important to determine the spalling length and thickness reasonably. In this paper, 16 columns were cyclically loaded to check their concrete cover spalling features. Based on the test results and PEER’s database, formulas are proposed for flexure-critical rectangular columns to approximately evaluate the spalling length and thickness of the columns’ concrete cover. Then, thermal and mechanical performance of the damaged columns in fire is numerically studied. It is found that: (a) the maximum spalling length generally increases with the increasing column’s drift ratio and axial load ratio, but it does not exhibit monotonic trend with the change in the shear span ratio and volumetric transverse reinforcement ratio; (b) the ratio of the equivalent spalling length to the maximum spalling length generally increases with the increasing column’s drift ratio and axial load ratio; and (c) fire endurance of the damaged column calculated using the equivalent model is 7–18% higher than that using the realistic model, and a discounted factor of .8 is recommended for the former.     

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.     

钢管高强混凝土轴压柱耐炎极限的试验研究

通过对5个圆形截面钢管高强度混凝土（以下简称钢管高强混凝土）柱的耐火试验，研究钢管高强混凝土柱在标准温升曲线下的力学性能和耐火极限。试验研究结果表明，钢管高强混凝土柱具有较好的耐火性能，在柱子外围只需进行适应的防火涂料保护，即可达到《高层民用建筑设计防火规范》（GB50045-95）对柱结构所要求的耐火极限。     

Macroscopic FE model for tracing the fire response of reinforced concrete structures

A macroscopic finite element model for tracing the fire response of reinforced concrete (RC) structural members is presented. The model accounts for critical factors that are to be considered for performance-based fire resistance assessment of RC structural members. Fire induced spalling, various strain components, high temperature material properties, restraint effects, different fire scenarios and failure criteria are incorporated in the model. The validity of the numerical model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. Case studies are conducted to demonstrate the use of the computer program for tracing the response of RC members under standard and design fire exposures. Through the results of the case studies, it is shown that the fire scenario has a significant effect on the fire resistance of RC columns and beams. It is also shown that macroscopic finite element models are capable of predicting the fire response of RC structural members with an adequate accuracy for practical applications.     

矩形钢管混凝土柱的耐火性能和抗火设计方法

进行了8个轴心受压或偏心受压矩形钢管混凝土柱,按ISO—834和GB/T9978—1999规定的标准升温曲线升温作用下耐火极限的实验研究。实验结果表明,截面尺寸和防火保护层厚度对构件耐火极限的影响较大,而荷载偏心率的影响则相对较小。分析结果还表明,国家规范GB50045—95中钢结构柱防火保护层厚度的确定方法不适合于矩形钢管混凝土柱。在大规模参数分析结果的基础上,提出了矩形钢管混凝土柱耐火极限及防火保护层厚度的简化计算公式,公式的计算结果与数值计算结果和实验结果均吻合较好,且总体上偏于安全。本文的研究成果可为有关矩形钢管混凝土工程进行抗火设计时提供参考。     

Investigation of insulated FRP-wrapped reinforced concrete columns in fire

Research has demonstrated that fibre-reinforced polymers (FRPs) can be used efficiently and safely in strengthening and rehabilitation of reinforced concrete structures. However, the use of FRPs in buildings has been limited because relatively little is known about the behaviour in fire of reinforced concrete structural members that have been strengthened with FRP systems. This paper presents the recent results of an ongoing experimental study of the fire performance of FRP-wrapped reinforced concrete circular columns. The results of fire tests on two columns are presented, one of which was tested without supplemental fire protection, and one of which was protected by a supplemental fire protection system applied to the exterior of the FRP-strengthening system. The primary objective of these tests was to compare the fire behaviour of the two FRP-wrapped columns and to investigate the effectiveness of the supplemental insulation system. The thermal and structural behaviour of the two columns are discussed. The results show that, although FRP systems are sensitive to high temperatures, satisfactory fire endurance ratings can be achieved for reinforced concrete columns that are strengthened with FRP systems by providing adequate supplemental fire protection. In particular, the insulated FRP-strengthened column in this study was able to resist elevated temperatures during the fire tests for at least 90 min longer than the equivalent uninsulated FRP-strengthened column.     

Experimental evaluation of the fire behaviour of insulated fibre-reinforced-polymer-strengthened reinforced concrete columns

In recent years, there has been an increase in the use of fibre reinforced polymer (FRP) materials for strengthening of concrete structures. However, since FRP materials are combustible, and because they are typically applied to the exterior of structural members in these strengthening applications, concerns exist regarding the behaviour of such FRP strengthening systems in fire. There is currently little information available on the fire endurance of FRP-strengthened concrete systems. This paper presents results from full-scale fire resistance experiments on three insulated FRP-strengthened reinforced concrete (RC) columns. A comparison is made between the fire performances of FRP-strengthened RC columns and conventional unstrengthened RC columns tested previously. Data obtained during the experiments is used to show that the fire behaviour of FRP-wrapped concrete columns incorporating appropriate fire protection systems is as good as that of unstrengthened RC columns. Thus, satisfactory fire resistance ratings for FRP-wrapped concrete columns can be obtained by properly incorporating appropriate fire protection measures into the overall FRP-strengthened structural system. Fire endurance criteria and preliminary design recommendations for fire safety of FRP-strengthened RC columns are briefly discussed.     

纤维增强超高强混凝土防高温爆裂研究

对15组立方体抗压强度为116～143MPa纤维增强超高强混凝土(FRUHSC)试件,开展了ISO834火灾标准升温曲线下的高温爆裂试验,考察了水胶比、孔隙率、纤维类型及体积分数、试件尺寸对其高温爆裂的影响.结果表明:水胶比为0.15的超高强混凝土与水胶比为0.18时相比,具有更低孔隙率和更高强度,表现为爆裂程度更高;...