有单（双）向面内约束力混凝土板火灾行为机理分析#br#

为考查面内约束作用对混凝土双向板火灾行为影响,对八块面内约束混凝土试验板进行数值模拟和对比分析,研究了几何(非)线性和混凝土膨胀应变对约束试验板变形、弯矩分布和薄膜机理影响规律。同时,开展参数分析,研究约束类型、约束水平、长宽比、配筋率和板厚等对面内约束混凝土板温度场、变形、破坏模式和耐火极限等影响。结果表明,对任一约束工况,较大混凝土膨胀应变导致约束板较大变形,且几何非线性影响不可忽略;面内约束作用倾向于降低跨中弯矩和增大板边负弯矩,且不利于板大变形阶段受拉薄膜效应发展;约束板火灾行为取决于约束类型、约束水平和长宽比等相互作用;增加板厚和配筋率有助于提高约束板抗火性能。     

板角约束混凝土双向板抗火性能试验研究

为研究混凝土双向简支板板角受平面外约束下的火灾行为,进行了恒载-升温条件下的火灾试验。采用无线测温技术和红外探测技术,获得了火灾下双向板沿板厚温度场分布、钢筋温度、板平面内外变形、裂缝以及板角平面外约束力变化,并与国内外相关试验进行对比分析。测试结果表明：自行设计无线测温设备测量精确度较高,且便于使用;红外探测技术能够实时记录板面裂缝发展。分析结果表明：当板角受平面外约束时,其裂缝模式及变形规律与四边简支板明显不同;且板角区域易出现弧形裂缝,板角约束程度对板角区域弧形裂缝分布与发展有显著影响。     

整体结构中钢筋混凝土双向板火灾试验研究

利用自行研制的火灾试验炉,对足尺整体结构中钢筋混凝土双向板在火灾作用下的性能进行试验研究。介绍了整体结构足尺模型与火灾试验炉的设计、试验方案以及观测内容和方法,描述了试验现象及板的破坏特征,研究了双向板在受火过程中沿板厚的温度场分布规律以及板平面外和平面内的变形。结果表明：火灾作用下,沿板厚存在非线性温度场及较大的温度梯度;整体结构中相邻未受火构件的约束对受火双向板的火灾行为影响显著;主裂缝出现在受火板板顶靠近内板边的1/4跨度处（板顶负弯矩钢筋截断处）,由于结构连续性及相邻构件间的互相约束,与受火板相邻的未受火板顶也出现了规则裂缝;受火板顶主裂缝的形成和发展引起板内弯矩重分布,导致最大平面外位移最终发生在距板中心不远,偏向于内板边的位置;整体结构中钢筋混凝土双向板具有较好的抗火性能。     

四边简支钢筋混凝土双向板火灾试验研究

进行了2块四边简支钢筋混凝土双向板足尺试件在恒载-升温工况下的火灾试验。研究了双向板在受火过程中沿板厚混凝土的温度场分布规律、钢筋的温度变化、板平面内外的变形、板边转角随温度的改变情况。试验结果表明,在荷载和温度的耦合作用下,沿板厚存在非线性温度场,板平面外变形比常温下显著增加;四边简支钢筋混凝土双向板具有与常温下不同的破坏模式,裂缝主要出现在板顶的长边跨中、距短边支座1/4处以及角部;在火灾作用下板的荷载传递路径有明显改变。     

火灾下受约束钢筋混凝土板试验研究

<正>中国矿业大学的研究人员对火灾下板角受约束的钢筋混凝土双向板的力学性能开展了试验研究。完成了1块钢筋混凝土双向板的耐火性能试验,采用无线测温和红外探测技术,测得了沿板厚温度场分布、钢筋温度、板平面内外变形、裂缝以及板角平面外约束力变化。研究结果表明:板角约束对混凝土双向板的裂缝破坏模式及变形影响较大,特别是板角区域弧形裂缝和板中心点竖向     

钢筋混凝土双向板火灾试验研究

进行了一块四边简支与一块四边固支钢筋混凝土双向板足尺试件在恒载-升温工况下的火灾试验。介绍了试件的设计、试验方案以及观测内容和方法。主要研究了双向板在受火过程中沿板厚混凝土的温度场分布规律、钢筋的温度变化、板平面内外的变形、板边转角随温度的改变情况以及固支板支座反力随火灾作用的变化。试验结果表明,在荷载和温度的耦合作用下,沿板厚存在非线性温度场,固支板的支座反力有显著的重分布,板的面外变形比常温下显著增加;四边简支与四边固支钢筋混凝土双向板具有与常温下不同的破坏模式,四边简支板破坏主要出现在长向跨中以及吊装阶段配置在板顶的负弯矩钢筋截断处;四边固支板在板顶出现椭圆型盆状塑性铰线。     

不同跨依次受火混凝土连续板试验研究及理论分析

为研究不同跨依次受火工况下混凝土连续板力学行为,对三块混凝土连续板进行火灾试验,分析不同跨受火顺序、受火时间间隔、配筋率和配筋方式等对试验板各跨温度、变形、板角约束力和破坏模式等影响规律;在此基础上,采用6种混凝土板极限承载力理论,对连续板各跨承载力进行计算分析。试验表明,每跨炉温及板截面温度分布主要取决于自身跨受火情况;不同跨受火工况和边界条件对连续板各跨跨中变形趋势、裂缝分布和破坏机理有重要影响,进而两边跨和中跨具有完全相反的跨中变形趋势,且中跨裂缝较多,易出现完整性破坏。此外,理论表明相比边界作用和受拉薄膜影响,不同跨受火工况及其配筋率对连续板各跨极限承载力变化趋势和数值有决定性影响。总之,对于任一火灾蔓延工况,提高配筋率和采用连续配筋方式有助于提高混凝土连续板各跨抗火性能。     

无粘结预应力混凝土三跨连续板火灾试验研究

在实现按照ISO标准升温曲线升温的前提下,研究了无粘结预应力混凝土连续板在火灾作用下的火灾行为。试验考察了预应力度、不同跨受火以及负筋长度三种因素,分析结果表明不同跨受火、负筋长度对三跨连续板的破坏机构、挠度变形有决定性的影响,而预应力度对其影响不是十分明显,三种因素都对支反力的重分布有很显著的影响,预应力筋中的预张力在火灾试验过程中经历了先增加,再减小,然后突然被拉断三个阶段,构件的温度场在受火面温度梯度大,背火面小,裂缝的出现和水蒸气的蒸发影响着板内温度场的分布。     

钢框架边跨梁抗火性能试验研究和理论分析

对3层钢框架结构边跨梁在火灾作用下的性能进行足尺试验研究,试验中测量了炉温、钢梁和混凝土板沿截面高度的温度场分布以及钢梁和混凝土板的竖向位移。结果表明：钢梁下翼缘、腹板和烟气的温度基本一致;混凝土板内的温度滞后于钢梁温度,且在钢梁降温阶段,混凝土板温度持续升高,因此混凝土板限制了钢梁升温阶段的膨胀和降温阶段的收缩;钢梁在122 min的升温过程中变形较小,且在升温阶段变形就开始恢复,最终出现向上的竖向位移,这是框架结构中钢梁火灾行为区别于单个构件火灾行为的特点之一,表明框架结构中与混凝土楼板形成整体的边跨梁具有较强的抗火性能;最后钢梁上下翼缘焊缝被拉裂破坏,因此在工程应用中应尽量避免工地现场施焊的全焊接节点。利用非线性有限元软件对试验梁进行了火灾反应分析,得出了试验梁的温度场分布和位移反应的变化规律。有限元分析结果和试验结果基本一致,验证了有限元分析模型的正确性和可行性。     

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。     

Non-linear structural modelling of a fire test subject to high restraint

The computer code VULCAN has been developed for the three-dimensional structural analysis of composite and steel-framed buildings in fire. In this paper, the main features of the program are outlined, with particular emphasis on the most recent development to the layered procedure for modelling of concrete floor slabs. This development has introduced geometric non-linearity into the modelling of slabs, whose layer structure already allowed temperature distributions and change of material properties through the thickness, as well as modelling the effect of the ribs at the bottom of composite decking slabs. The capabilities of the model are firstly tested at ambient temperature for a uniformly loaded ribbed reinforced concrete slab with simply supported edges, and this is followed by a very detailed modelling of the Cardington restrained beam fire test. In both cases the development of membrane action is demonstrated and the structural behaviour is compared with the geometrically linear case. A number of studies are carried out to demonstrate the influence of the major floor slab details on the behaviour of the structure in fire conditions. These studies provide evidence that when exposed steel temperatures are less than 400°C the concrete slab has little influence, other than to play a part in generating thermal curvature to composite beams. For temperatures higher than about 500°C the effect of the slab progressively becomes much greater, and it is very important to model concrete slabs correctly. The influence of membrane action cannot be ignored, particularly when the fire compartment is subject to high restraint because it is surrounded by cool, stiff structure. At very high temperatures the floor slab becomes the main load-bearing element and the floor loads above the fire compartment are carried by the membrane forces developed in the slab, with tension being carried mainly by the steel anti-cracking mesh or reinforcing bars. However, the effect of the very high in-plane restraint to thermal expansion in the particular Cardington test considered is to enhance the peripheral zone of compressive membrane force and to reduce the extent of the central area of tensile force compared with more usual cases.     

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.     

带约束预制混凝土叠合板受火后抗弯性能的试验研究

进行带约束预制混凝土叠合板常温和受火后受弯性能的对比试验,研究升降温过程中截面温度场变化和不同受火时间自然冷却后极限荷载、初始弯曲刚度和延性系数的变化规律。结果表明：未受火预制混凝土叠合板和不同受火时间自然冷却后预制混凝土叠合板均发生弯曲破坏。带约束预制混凝土叠合板升温阶段截面温度场呈受火面温度高、背火面温度低的层状分布,降温阶段温度场呈内部温度高、外部温度低的圈状分布;随着受火时间增加,极限荷载和初始弯曲刚度均呈抛物线型下降;受火自然冷却后的残余挠度也明显增大。理论分析表明,经典塑形铰线理论适用于预制混凝土叠合板未受火试件和受火后试件极限荷载的计算;截面温度场和受火后极限荷载数值模拟结果均与试验结果吻合较好,可为预制混凝土叠合板的防火设计和火灾后鉴定评估提供依据。     

Enhancing the fire resistance of composite floor assemblies through the use of steel fiber reinforced concrete

This paper presents a strategy for achieving the required fire resistance in composite floor systems through the use of steel fiber reinforced concrete (SFRC). Both experimental and numerical studies were carried out to evaluate the fire performance of floor systems comprising unprotected steel beams and concrete/SFRC deck slabs. The results from these studies show that SFRC composite deck slabs develop significant tensile forces (through tensile membrane action) that transfer load from fire-weakened steel beams to other cooler parts of the structure. Preliminary results indicate that the combined effect of composite construction, tensile membrane action, and the improved properties of SFRC under realistic fire, loading, and restraint conditions can provide sufficient fire resistance in steel beam-concrete deck slabs without the need for external fire protection on the floor assembly.     

Behaviour of concrete floor slabs at ambient and elevated temperatures

This paper presents the latest developments of a simple design method used to predict the membrane action of unrestrained concrete, or concrete and steel composite, floors under fire conditions. The developments include the refinement of the assumed in-plane stress pattern and a prediction of when concrete crushing occurs in the proximity of the corners of the slab. The design method is compared against 44 small-scale tests carried out at ambient and elevated temperatures on horizontally unrestrained slabs with an aspect ratio of 1.0 or 1.55. The slabs were reinforced with either mild steel or stainless-steel welded mesh of different grades, ductility, sizes and bar spacing. Both the ambient and fire tests highlighted the occurrence of membrane action, either supporting loads higher than the theoretical yield-line load in the case of the ambient tests, or reaching higher failure temperatures compared to those calculated based on yield-line theory. Comparison between the developed simple design approach and test results showed good correlation both at ambient and elevated temperatures.     

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.     

火灾下钢结构和组合结构的非线性模拟

近年来Sheffield大学开发了一种计算机程序Vulcan，用来对火灾下组合结构的性能进行三维模拟。本简要叙述了程序的理论基础。1995—1996年在Cardington进行了三个组合框架的抗火试验，各试验中相邻结构所提供的平面内约束程度不同，通过模拟这三个试验来显示受热区域内约束对结构性能的影响。为了阐明薄膜作用的影响及其和边界约束的关系，所有的试验都分别用几何线性以及非线性的板单元进行了分析。为此，首先对引起这种作用的钢筋的性能做了一系列参数研究。很明显，板内的薄膜作用对隔间最终的完整性很重要，并且在模拟火灾下这种类型的结构时应加以考虑。