Fire tests on bonded post-tensioned concrete slabs

The results from eight fire tests conducted on bonded post-tensioned one-way spanning concrete slabs are presented in this paper. The fire tests were augmented with two additional tests at ambient temperature, carried out to failure on slabs with identical geometry and prestressing tendons. The different structural response between using plastic and metallic ducts, Limestone and Thames Gravel aggregates, and different axial restraint conditions to longitudinal thermal expansion, have been highlighted. Slabs with Thames Gravel aggregates were shown to have a much higher deflection compared to slabs with Limestone aggregates, with restrained slabs having a lower vertical deflection compared to equivalent unrestrained slabs. In all the fire tests, cracks directly inline and parallel to the tendons occurred due to thermal stresses at relatively low tendon temperatures, which were not observed in the ambient tests. It is shown that the use of plastic ducts resulted in slightly higher tendon temperatures due to the ease at which water migrated from the grout once the duct had melted. The fire tests have shown that the fire resistance specified in current codes of practice are generally conservative for bonded post-tensioned one-way spanning concrete slabs. The tests have provided detailed experimental data in the form of temperature distributions within the slab, vertical and horizontal displacements and strains in the tendons, which will allow validation of future computer models to predict the behaviour of bonded post-tensioned concrete slabs under fire conditions.     

Transient high-temperature stress relaxation of prestressing tendons in unbonded construction

Unbonded post-tensioned (UPT) flat plate concrete slabs are popular for modern continuous multiple bay floor assemblies due to economic and sustainability benefits (reductions in slab thickness and building self-weight) and structural advantages (decreased deflections over larger spans). Only limited research has been conducted on the performance of UPT flat plate slabs under fire conditions, yet the inherent fire endurance of these systems is sometimes quoted as a benefit of this type of construction. One concern for these structures in fire is that high-temperature stress relaxation of the unbonded prestressed reinforcement may cause considerable and irrecoverable prestress loss, with subsequent structural consequences. This paper uses a computational model which has been developed to predict the transient high-temperature stress relaxation (i.e., prestress loss) for typical UPT multiple span flat plate slabs in fire, to study the potential prestress relaxation behaviour under various plausible temperature conditions as might occur during exposure to a standard fire. The model is validated using experimental data from relaxation tests performed on locally heated unbonded seven-wire prestressing stand. The initial prestress level, concrete cover to the prestressed reinforcement, and ratio of heated length to overall tendon length are varied to investigate the potential implications for prestress loss, and subsequently for flexural and punching shear capacity. The results highlight the need for particular care in the construction of UPT slabs to ensure adequate concrete cover for structural fire safety.     

Whole-building behaviour of bonded post-tensioned concrete floor plates exposed to fire

This paper discusses the whole-building behaviour of post-tensioned concrete floor plates under fire conditions. Based on the results of eight fire tests on one-way spanning bonded post-tensioned concrete slab strips, recently conducted by the authors, a nonlinear finite element model was developed to model an entire typical concrete floor plate. The considered floor plate was post-tensioned using bonded tendons and was supported on traditional reinforced concrete columns. The overall objective of the study was to provide an understanding of the structural response and modes of failure of these floor plates under fire conditions. The mechanical and thermal material nonlinearities of the floor’s components, consisting of the concrete, grout, prestressing tendon, and the anchorages, as well as the reinforced concrete columns, have been considered in the model. The interface between the tendon and grout was also considered, allowing the bond behaviour to be modelled and the tendon to retain its profile shape during the deformation of the floor. The model has been validated against published finite element results on a floor plate under normal temperature conditions. The temperature distribution throughout the floor slab and supporting columns, together with the developed displacement and stress due to heating, and the overall fire resistance of the floor were predicted by the model. Furthermore, the variables that influence the structural behaviour comprising different natural fires, applied static load during fire, use of non-tensioned reinforcement, as well the difference between unbraced and braced frames were investigated in a parametric study. The study has shown that the failure mode of the floor under fire conditions is mainly due to tensile splitting along the tendons that extended to the top surface, while at ambient conditions the mode of failure is punching shear. The restraint provided by shear walls in the considered braced frame and the use of non-tensioned flexural reinforcement affected the vertical displacement behaviour under fire conditions, but did not affect the fire resistance due to the predicted tensile splitting failure mode. From the studies presented it is concluded that the design fire resistance of the floor specified in Eurocode BSEN1992-1-2 is acceptable, while that in the UK code BS8110 is unconservative and should be modified.     

预应力混凝土简支板抗火性能试验与分析

完成了9块无粘结和1块有粘结预应力混凝土单向简支板的抗火试验。各试验板内的热电偶在火灾下的实测结果表明,温度由迎火面至背火面逐渐递减,由迎火面至背火面的温度变化梯度随离迎火面距离的增大而减小。布置于无粘结预应力筋锚固端的力传感器和试验板上的位移传感器的测试结果表明,受火初期试验板挠曲变形增长较快,无粘结预应力筋应力呈增长趋势;受火一段时间之后试验板变形增长速度有所减缓,无粘结预应力筋应力呈减小趋势,且这种趋势一直延续到停火后一段时间;停火一段时间之后,试验板有变形恢复趋势,无粘结预应力筋应力水平又有所恢复。在受火过程中试验板背火面经历了水分溢出、水蒸气形成和发展、溢出水干涸及水蒸气消失的过程。在试验炉两侧壁的观察孔观测到有6块试验板发生了爆裂,爆裂发生在预应力度较高、荷载作用水平较低的试验板的邻近支座区域。基于试验结果,试算确定了对流换热系数、辐射换热相当发射率系数、混凝土导热系数和比热的取值,应用ANSYS     

Investigation of concrete encased steel composite columns at elevated temperatures

This paper presents a nonlinear 3-D finite element model investigating the behaviour of concrete encased steel composite columns at elevated temperatures. The composite columns were pin-ended axially loaded columns having different cross-sectional dimensions, different structural steel sections, different coarse aggregates and different load ratios during fire. The nonlinear material properties of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement at ambient and elevated temperatures were considered in the finite element models. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the column. The initial overall (out-of-straightness) geometric imperfection was carefully included in the model. The finite element model has been validated against published tests conducted at elevated temperatures. The time–temperature relationships, deformed shapes at failure, time–axial displacement relationships, failure modes and fire resistances of the columns were evaluated by the finite element model. It has been shown that the finite element model can accurately predict the behaviour of the columns at elevated temperatures. Furthermore, the variables that influence the fire resistance and behaviour of the composite columns comprising different load ratios during fire, different coarse aggregates and different slenderness ratios were investigated in parametric studies. It is shown that the fire resistance of the columns generally increases with the decrease in the column slenderness ratio as well as the increase in the structural steel ratio. It is also shown that the time–axial displacement relationship is considerably affected by the coarse aggregate. The fire resistances of the composite columns obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite columns at elevated temperatures. It is shown that the EC4 is conservative for all the concrete encased steel composite columns, except for the columns having a load ratio of 0.5 as well as the columns having a slenderness ratio of 0.69 and a load ratio of 0.4.     

无粘结预应力混凝土连续板边跨受火试验研究

本文对3块无粘结预应力混凝土连续板在边跨受火下的火灾行为进行了试验研究。试验中考虑了负弯矩筋长度和预应力度两个主要因素。得出了板的温度场分布、板的变形及支反力的变化情况。并得出了负弯矩筋长度、预应力度对无粘结预应力混凝土连续板的影响。试验结果表明负弯矩筋的长度对板的破坏形式影响明显,而预应力度的影响则不明显。为今后无粘结预应力混凝土结构的火灾行为研究提供依据。     

Structural performance of a post-tensioned concrete floor during horizontally travelling fires

This paper highlights the structural performance of a bonded post-tensioned concrete floor subject to fires that travel horizontally between zones within the floorplate. The floorplate was previously analysed by the authors based on experimental and numerical investigations on one-way spanning bonded post-tensioned concrete slab strips. In the previous studies, a nonlinear finite element model was developed for the floor that considered the mechanical and thermal material nonlinearities of the floor’s components, interfaces between the components, different natural fire severities, different applied static load during the fire and different restraint conditions. The previous studies highlighted the importance of investigating the whole-building behaviour and provided a useful insight into the temperature distribution throughout the floor slab, failure modes, comparisons with current design rules and time-displacement behaviour of the floor under fire conditions. This paper extends the previous studies and uses the validated finite element model to investigate different horizontal travelling fire scenarios between zones and different inter-zone time delays to represent fire travelling time. The time-temperature distribution throughout the floor slab was predicted at different locations in the floor subject to travelling fires. Furthermore, the time-deflection and time-axial displacement relationships were predicted at different locations in the floor. The current study has shown that horizontally travelling fire scenarios and the inter-zone time delay affect the time-deflection behaviour considerably. The change in heating/cooling scenarios between zones has resulted in a cyclic deflection pattern, which has previously not been considered when designing post-tensioned concrete floors against fire. Based on the analysis of the results presented, it is shown that the worst case in terms of maximum vertical defection or maximum residual deflection, at a given point in the floorplate, could occur either under the assumption of a uniform fire or a travelling fire. It is therefore recommended that designers should consider the integrity of floorplates using various travelling fires.     

Nonlinear behaviour of unprotected composite slim floor steel beams exposed to different fire conditions

An efficient nonlinear 3D finite element model has been developed to investigate the structural performance of composite slim floor steel beams with deep profiled steel decking under fire conditions. The composite steel beams were unprotected simply supported with different cross-sectional dimensions, structural steel sections, load ratios during fire and were subjected to different fire scenarios. The nonlinear material properties of steel, composite slim concrete floor and reinforcement bars were incorporated in the model at ambient and elevated temperatures. The interface between the structural steel section and composite slim concrete floor was also considered, allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the composite beam. Furthermore the thermal properties of the interface were included in the finite element analysis. The finite element model has been validated against published fire tests on unprotected composite slim floor steel beams. The time–temperature relationships, deformed shapes at failure, time–vertical displacement relationships, failure modes and fire resistances of the composite steel beams were evaluated by the finite element model. Comparisons between predicted behaviour and that recorded in fire tests have shown that the finite element model can accurately predict the behaviour of the composite steel beams under fire conditions. Furthermore, the variables that influence the fire resistance and behaviour of the unprotected composite slim floor steel beams, comprising different load ratios during fire, cross-section geometries, beam length and fire scenarios, were investigated in parametric studies. It is shown that the failure of the composite beams under fire conditions occurred for the standard fire curve, but did not occur for the natural fires. The use of high strength structural steel considerably limited the vertical displacements after fire exposure. It is also shown that presence of additional top reinforcement mesh is necessary for composite beams exposed to short hot natural fires. The fire resistances of the composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 predictions are generally conservative for the design of composite slim floor steel beams heated using different fire scenarios.     

两跨无粘结预应力混凝土连续板抗火性能试验与分析

完成了9块两跨无粘结预应力混凝土连续单向板抗火试验。通过布置于试验板内的热电偶,获得了试验板截面的温度场分布。通过布置于无粘结筋锚固端的力传感器和试验板上的位移传感器,获得了试验板中无粘结筋应力和试验板新增变形在火灾下的变化过程。通过布置于各试验板边支座上的压力传感器,获得了两跨连续板支座反力在火灾下的变化过程。受火过程中试验板背火面经历了水分溢出、水蒸气形成和发展、溢出水干涸及水蒸气消失的过程。基于不同试件不同时刻的无粘结钢丝应力实测值、边支座反力实测值、跨中新增变形实测值,拟合出了考虑各关键参数影响的火灾下两跨连续板中无粘结钢丝应力计算公式、边支座反力计算公式和跨中新增变形计算公式。针对在抗火试验中有4块试验板板顶负弯矩的钢筋截断处出现了贯穿裂缝的现象,提出了火灾下两跨连续板中支座两侧负弯矩钢筋的截断建议。     

Factors governing the shear response of prestressed concrete hollowcore slabs under fire conditions

This paper presents results from numerical studies on the effect of critical factors governing the shear response on prestressed concrete (PC) hollowcore slabs exposed to fire. A validated three dimensional finite element model is applied for evaluating failure of fire exposed prestressed concrete (PC) hollowcore slabs under different limiting states, including through shear. This model accounts for temperature induced property degradation in concrete and prestressing strands, cracking in concrete, varying fire exposure, loading and restraint conditions. The factors varied in the parametric study include, slab depth, load level, loading pattern, axial restraint, level of prestressing, and fire scenario. Results from parametric studies show that slab depth, load level, loading pattern, axial restraint, level of prestressing and fire scenario have significant influence on the fire response of PC hollowcore slabs, and failure under these conditions can occur through shear limiting state prior to reaching flexural limiting state. Results from parametric studies are further utilized to propose a simplified approach for evaluating shear capacity PC hollowcore slabs under fire conditions.     

无黏结预应力混凝土连续板火灾后的试验研究

对9块12.6m×1.5 m后张无黏结预应力混凝土三跨连续板在火灾后性能进行了检测.试验首先按照ISO834标准曲线升温,到一定时间后采用自然冷却方式进行降温,并维持2.0 kN/m2的使用荷载不变.得出了板的变形及支反力、预应力变化情况,从而得出了灾后内力重分布的规律.裂缝基本没有新的发展,其宽度有所减小.为今后无黏结预应力混凝土结构的灾后行为研究提供依据.     

无粘结预应力混凝土简支板的抗火试验研究

主要研究了受火无粘结预应力混凝土简支板的温度场分布和板的变形情况 ,并给出了钢筋保护层的厚度、预应力度对无粘结预应力混凝土简支板受火性能的影响。为无粘结预应力混凝土板的火灾行为的理论研究提供了依据。     

Strengthening of lightweight reinforced concrete slabs using different techniques

The present study examines the potential of using four different techniques in repairing and strengthening of preloaded, cracked, lightweight, reinforced concrete one-way solid slabs. Nineteen lightweight reinforced concrete (LWRC) slabs specimens were casted with one-third scale dimensions from prototype structure in buildings. The slabs dimensions were 1.2 m in length and cross section of (70 × 500) mm in depth and width. The slabs were reinforced with 3φ10 for the main reinforcement and tested under a four-point loading system. The tested slabs were divided into two groups according to preloading level to study the effect of cracking level. Two preloadings were selected, i.e. 60% and 80% from the ultimate load. The strengthening techniques used were ferrocement layer, steel plate, carbon fibre reinforced polymers (CFRP) sheets and CFRP strips. The slabs were loaded up to failure and the structural response of each slab specimen was predicted in terms of the onset of cracking, deflection, collapse load and failure mode. The efficiency of different repair and strengthening techniques and their effects on the structural behaviour of cracked concrete slab had been analysed. It was observed that the type of strengthening technique used affect the load carrying capacity, deflection, stiffness and toughness of the slab. All repair techniques were found to be able to restore and enhance the structural capacity of cracked concrete slabs. The enhancement ratio is found to be affected by the preloading or the cracking level.     

无粘结预应力混凝土连续板中跨受火试验研究

对无粘结预应力混凝土连续板在中跨受火下的反应进行试验研究 ,得出了板的温度场分布、板的变形情况及支座反力的变化情况 ,并得出了负弯矩钢筋长度、预应力度对无粘结预应力混凝土连续板的影响。为今后无粘结预应力混凝土板火灾行为研究提供了依据     

预应力混凝土结构非线性有限元分析

提出了能反映预应力钢筋与混凝土间相对滑移和相互作用的单元模型,此模型可模拟预应力钢筋的张拉和用于无粘结预应力混凝土结构分析。采用这一单元模型建立了预应力混凝土结构的非线性分析有限元模型,非线性方面考虑了材料和几何非线性、预应力摩擦损失、钢筋松弛、混凝土收缩和徐变等因素。该有限元模型可用于有粘结和无粘结预应力混凝土结构张拉施工和工作阶段的全过程分析,分析内容包括弹塑性变形、混凝土开裂和破坏等。最后给出了有粘结和无粘结预应力混凝土连续梁的分析实例,受力变形、破坏形态等的分析结果与试验结果吻合良好,验证了该模型的合理性和有效性。     

Repair and structural performance of initially cracked reinforced concrete slabs

Crack is one of the most common defects observed in reinforced concrete slabs and beams. Major cracks in concrete structures may occur due to overloading, corrosion of reinforcement or differential settlement of support. To restore the structural capacity of the distressed elements, retrofitting and/or strengthening are needed. There are different techniques available for retrofitting and strengthening of different reinforced concrete structural elements reported in the literature. This paper investigates the structural behaviour of cracked reinforced concrete one-way slab, which is repaired using different techniques.Five different techniques are used for the purpose of repair in the cracked concrete slab namely; cement grout, epoxy injection, ferrocement layer, carbon fibre strip and section enlargement. The slabs were loaded to failure stage and the structural response of each slab specimens have been predicted in terms of deflection, variation of strain in concrete and steel, collapse loads and the failure modes.The efficiency of different repair and strengthening techniques and their effects on the structural behaviour of cracked one-way reinforced concrete slab had been analyzed. It was observed that the type of repair technique used will affect the load carrying capacity of the slab and will lead to a redistribution of the strains and hence stresses in both concrete and steel reinforcement. All repair techniques are found to be able to restore or enhance the structural capacity of cracked concrete slabs.     

Anisotropic Curvature and Damage of Unbonded Post-tensioned Concrete Slabs During Fire Testing

Two furnace tests, using two different fire exposures, on unbonded post-tensioned concrete slabs (1700 × 1200 mm) are reported. Local curvature is measured along two lines approximately in the middle of the slabs both parallel (longitudinal) and orthogonal (transverse) to the prestressing direction. More pronounced curvature in the transverse direction is accompanied by the formation of cracks running predominantly in the longitudinal direction. While the transverse curvature relaxes back to the original state after the cooling phase the curvature in the longitudinal direction ultimately exhibits upward deflection due to the hogging moment caused by the prestress in the tendons acting on a cross section with temperature reduced mechanical properties at the fire exposed side. The effect on crack formation due to the prestressing can additionally be detected by ultrasonic pulse velocity measurements in the different directions through the depth of the slab, where a reduction of 5–25% is observed in the transverse direction compared to the longitudinal direction. The phenomenological mechanical behaviour of the slabs is captured in a finite element model which describes the evolution of stress in the prestressing tendons. This model additionally suggests that the curvature in the transverse direction is independent of the prestressing in the longitudinal direction.     

Behaviour of headed stud shear connectors for composite steel-concrete beams at elevated temperatures

The behaviour of composite steel-concrete beams at elevated temperatures is an important problem. A three-dimensional push test model is developed herein with a two-dimensional temperature distribution field based on the finite element method (FEM) and which may be applied to steel-concrete composite beams. The motivation for this paper is to increase the awareness of the structural engineering community to the concepts behind composite steel-concrete structural design for fire exposure. The behaviour of reinforced concrete slabs under fire conditions strongly depends on the interaction of the slabs with the surrounding elements which include the structural steel beam, steel reinforcing and shear connectors. This study was carried out to consider the effects of elevated temperatures on the behaviour of composite steel-concrete beams for both solid and profiled steel sheeting slabs. This investigation considers the load-slip relationship and ultimate load behaviour for push tests with a three-dimensional non-linear finite element program ABAQUS. As a result of elevated temperatures, the material properties change with temperature. The studies were compared with experimental tests under both ambient and elevated temperatures. Furthermore, for the elevated temperature study, the models were loaded progressively up to the ultimate load to illustrate the capability of the structure to withstand load during a fire. It is concluded that finite element analysis showed that the shear connector strength under fire exposure was very sensitive. It is also shown that profiled steel sheeting slabs exhibit greater fire resistance when compared with that of a solid slab as a function of their ambient temperature strength.     

The behaviour of reinforced concrete slabs in fire

In this paper a robust model is presented based on the previous layer procedure developed by the author to also take into account the effects of concrete spalling on the behaviour of concrete slabs under fire conditions. In this study, a detailed analysis of a uniformly loaded reinforced concrete slab subject to different degrees of concrete spalling under a standard fire regime is first carried out. Further, a series of analysis of floor slabs with different degrees of concrete spalling is also performed on a generic reinforced concrete building. A total of 16 cases have been analysed using different degrees of spalling on the slabs, with different extents and positions of localised fire compartments. It is clear that adjacent cool structures provide considerable thermal restraint to the floor slabs within the fire compartment. And it is evident that the compressive membrane force within the slabs is a major player in reducing the impact of concrete spalling on the structural behaviour of floor slabs in fire.     

开孔薄壁钢梁-轻骨料混凝土装配式组合楼板受火试验研究

为研究由冷弯卷边H形开孔薄壁钢梁和预制轻骨料混凝土板形成的装配式组合楼板的受火性能,对6个两端简支组合楼板试件开展了高温下试验研究,得到了试件的温度及位移响应。研究结果表明：在等效均布荷载2.0 kN/m²及最高炉温为600℃的烃类火灾作用下,以轻骨料混凝土及腹板开孔薄壁钢梁形成装配式组合楼板具有良好的抗火性能,组合楼板的跨中挠度最大为L/44（L为组合楼板跨度）时,试件仍保持良好的整体性;主钢梁腹板开孔直径与分布形式对组合楼板的整体抗火性能均有显著影响,为减小组合楼板高温下的位移响应,对主钢梁腹板的开孔宜优选孔径大、数量少的分布形式;抗剪键数量对组合楼板抗火性能的响应影响远大于对预制板类型的影响。