Modelling Localized Fracture in Composite Floors at Elevated Temperatures

钢框架结构，特别是复合结构，在英国被广泛用于多层建筑。因此，复合材料结构的耐火性能已成为设计人员主要关注的问题。在最近的设计规范中，特别是欧洲规范1、3和4，火灾被视为结构的基本设计极限状态之一，这本身就是在哲学上向前迈出的重要一步，而不是以前强调防火的规范性要求。不同复杂程度和保守程度的一系列设计程序允许设计师从规定性需求转向适当的基于性能的概念。结构构件的耐火性是根据三个标准来定义的：(1)结构稳定性；(2)完整性;(3)绝缘。当违反其中任何一条时，就会发生故障。第一个适用于所有类型的元素；另外两个只适用于分离的元素，如楼板和墙壁。在过去的十年中，大量的研究开发了复合板在火灾条件下结构行为的数值模型。这些都是基于连续体方法，其中局部混凝土破坏由涂抹开裂表示。这些模型可以以合理的精度预测受大挠度影响的混凝土板的整体行为，至少在断裂开始占主导地位之前。然而，它们不能预测由于板坯厚度的单个大裂纹而导致的局部破坏。很明显，当这种类型的裂缝形成时，结构的完整性准则被破坏了，尽管稳定性仍然可以保持。因此，重要的是开发一个强大的数值程序，以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于任意和不适当的极限挠度标准，例如欧洲规范1第1-2部分和BS476中使用的跨度/30，跨度/20和极限挠度率标准来控制炉测试。这些挠度准则几乎没有科学依据，通常被认为是在标准火灾条件下为保护短跨度炉而根据经验制定的。这种武断的元素极大地削弱了基于性能的设计方法的力量。这是拟议的研究的目标，以解决这一重要而困难的问题。该项目的直接目的是开发一种实用和鲁棒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部分组成，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，以确定有意义的耐火失效点，并应适合直接用于复合材料和混凝土建筑的结构防火工程设计。这一程序首次使设计师能够根据结构稳定性和完整性标准直接评估建筑物的防火性。这将极大地改变结构消防工程设计实践，目前咨询工程师使用任意的极限挠度，跨度为30或跨度为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.