Modelling Localized Fracture in Composite Floors at Elevated Temperatures

钢框架结构，特别是组合结构，在英国被广泛用于多层建筑。因此，复合材料结构的抗火性能已成为设计人员关注的主要问题。在最新的设计规范中，特别是欧洲规范1、3和4中，火被视为结构的基本设计极限状态之一，这本身就是从以前强调防火的规定性要求的哲学上向前迈出的一大步。不同程度的复杂性和保守性的一系列设计程序允许设计师从规范性要求转向适当的基于性能的概念。结构构件的耐火性定义最多有三个标准：(1)结构稳定性；(2)完整性；(3)绝缘性。当其中任何一个被违反时，就会发生故障。第一个适用于所有类型的元素；其他两个仅适用于分离的元素，如楼板和墙。在过去的十年里，大量的研究已经发展了组合板在火灾条件下的结构性能的数值模型。这些都是基于连续体方法，在这种方法中，混凝土的局部破坏表现为弥散裂缝。这些模型可以相当准确地预测大挠度混凝土板的整体行为，至少在断裂开始占据主导地位之前是这样。然而，由于板厚中个别的大裂缝，他们不能预测局部失效。很明显，当这种类型的裂缝形成时，结构完整性的标准就被违反了，尽管稳定性仍然可以保持。因此，重要的是开发一种稳健的数值程序，用它来评估结构的稳定性和完整性，以便通过分析确定有意义的失效定义。因此，基于性能的设计目前可能不得不依赖任意的和不适当的限制挠度标准，例如欧洲法规1第1-2部分和BS476中使用的跨度/30、跨度/20和限制挠度速率标准来控制炉膛测试。这些偏差标准几乎没有科学依据，通常被认为是在标准火灾条件下为保护短跨度炉膛而制定的经验。这种任意的元素大大削弱了基于性能的设计方法的威力。这项拟议研究的目的是解决这一重要而困难的问题。该项目的直接目标是开发一种实用而健壮的数值程序来模拟高温下钢筋混凝土板的局部断裂。分层钢筋混凝土板单元将由三个组件组成，分别代表素混凝土板、钢筋和混凝土与钢筋之间的粘结特性。该模型应具有评估结构稳定性和楼板完整性的能力，以确定有意义的抗火故障点，并应适合于直接用于组合和混凝土建筑的结构消防工程设计。这一程序将首次使设计者能够根据结构稳定性和完整性标准直接评估建筑物的耐火性能。这将极大地改变结构消防工程设计实践，在这种设计实践中，咨询工程师目前使用的是任意的限制挠度，要么是SPAN/30，要么是SPAN/20。

Steel-framed structures, particularly of composite construction, are widely used in the UK for multi-storey buildings. The fire resistance of composite structures has therefore become of major concern to designers. In the most recent design codes, notably Eurocodes 1, 3 and 4, fire is treated as one of the basic design limit states for structures, which is in itself a major step forward in philosophy from the previous emphasis on prescriptive requirements for fire protection. A range of design procedures at different levels of complexity and conservatism allows designers to move away from prescriptive requirements towards appropriate performance-based concepts. Fire resistance of structural elements is defined according to up to three criteria: (1) Structural Stability; (2) Integrity; (3) Insulation. Failure occurs when any one of these is violated. The first of these applies to all types of elements; the other two apply only to separating elements such as floor slabs and walls. Over the past decade a significant amount of research has developed numerical models of the structural behaviour of composite slabs in fire conditions. These are all based on a continuum approach in which local concrete failure is represented by smeared cracking. These models can predict the global behaviour of concrete slabs subject to large deflection with reasonable accuracy, at least until fracture begins to dominate. However, they cannot predict localized failures due to individual large cracks through the slab thickness. It is clear that when this type of crack forms the criterion of structural integrity is violated, although stability may still be maintained. It is therefore important to develop a robust numerical procedure with which both structural stability and integrity can be assessed, so that a meaningful failure definition can be identified by the analysis. Performance-based designs may therefore currently have to rely on arbitrary and inappropriate limiting deflection criteria, such as the span/30 , span/20 and limiting deflection rate criteria used in Eurocode 1 Part 1-2 and BS476 to control furnace testing. These deflection criteria have very little scientific basis, and are generally assumed to have been developed empirically to protect short-span furnaces under standard fire conditions. This arbitrary element significantly undermines the power of the performance-based design method. It is the objective of the proposed research to tackle this important and difficult issue. The direct aim of the project is to develop a practical and robust numerical procedure to model localized fracture of reinforced concrete slabs at high temperatures. The layered reinforced concrete slab elements will consist of three components, representing the plain concrete slabs, reinforcing steel bars and the bond characteristics between concrete and reinforcement. The model should have the ability to assess both the structural stability and the integrity of floor slabs, in order to identify meaningful fire resistance failure points, and should be suitable for direct employment in structural fire engineering design of composite and concrete buildings. For the first time this procedure will enable designers to assess fire resistance of a building directly on the basis of both the structural stability and integrity criteria. This would dramatically change structural fire engineering design practice, in which arbitrary limiting deflections, of either span/30 or span/20, are currently used by consulting engineers.