摘要:
火灾是钢结构安全的主要威胁之一,传统钢结构的主要防火措施为涂抹防火涂料、浇筑混凝土形成组合结构或设置防火板,其中又以防火涂料应用最为广泛。然而,室外涂抹防火涂料耐久性差,存在坠落伤人隐患。且对于防火要求较高的钢结构需厚涂防火涂料,这尤其会影响地标性建筑的建筑效果。近年来,耐火钢作为一种新型的防火保护措施逐渐兴起,其能够有效避免上述防火措施的缺点,但由于大量添加合金元素Mo导致其成本往往较高。南钢集团研发的新型低Mo耐火钢在保证优异抗火性能的同时减少了Mo元素的添加量,从而减小了综合制造成本而受到广泛关注。基于构件承载力法,依托于某具有高大空间的钢结构大雨棚,对南钢新型低Mo耐火钢在抗火设计中的应用可行性进行了研究。作为某市新建地标性建筑,设计要求大雨棚屋面梁应满足3 h耐火极限。首先探索了高温下该国产耐火钢的材性折减规律,通过与现行规范理论计算结果对比,发现该国产耐火钢满足通用耐火钢的材性折减要求且可根据规范相应公式准确预测不同温度下的材性折减。随后基于结构抗火设计原则共确定了11个火灾场景,分别分布于雨棚边跨及中跨。以上火灾场景被认为已覆盖该大雨棚实际使用状态下可能发生的所有火灾。进一步根据相关规范,分析了各火灾场景下大雨棚的升温规律。分析表明,按照设计要求经3 h升温后,边跨火灾场景下最高空气温度接近750℃,最高构件温度接近730℃;中跨火灾场景下最高空气温度接近600℃,最高构件温度接近570℃,均远低于ISO 834标准火灾升温。高大空间建筑火灾下升温低的特点表明高大空间建筑可作为未来耐火钢应用的主要场景。最终,分别采用Q355普通结构钢与Q345FR国产耐火钢对各火灾工况下各构件进行承载力和稳定性验算。结果表明,若仅采用Q355普通结构钢,高温下钢梁GL3、GL3a及GL4无法满足承载力要求;而若采用Q345FR耐火钢,屋顶钢构件的强度和整体稳定性均能满足要求。
Abstract:
Fire is one of the critical safety threats to steel structures. The main conventional fire-resistant measures include painting fireresistant coating, forming composite structures by combining steel with concrete and setting fire-resistant plates, among which the fireresistant coating is the most widely used. However, the fire-resistant coating painted outdoors performs poor durability, owning the hidden danger that the coating may fall and hurt people. In addition, the extremely thick coating is required for steel structures with strict demand on fire resistance, which will especially affect the architectural rending of landmark buildings. In recent years, fireresistant steel has been paid more attention for its ability to effectively avoid the above drawbacks, as a novel fire-resistant approach. However, the cost usually becomes higher due to the massively added high-price alloy Mo. As a result, the novel fire-resistant steel proposed by the Institute of Nanjing Iron & Steel Co. Ltd gained widespread attention owing to the much lower cost by reducing the additive quantity of alloy Mo while maintaining excellent fire-resistant properties. This paper investigates the feasibility of applying low-Mo fire-resistant steel in the fire-resistant design, based on the carrying capacity method and relying on a large steel canopy with a tall space. The large canopy should meet the 3 h fire-resistant requirement as a landmarking building. First of all, the material properties of domestic fire-resistant steel are investigated under high temperatures. It is found that the novel fire-resistant steel meets the requirements of general fire-resistant steel by comparing with the current specification and the reduction of material properties can be accurately predicted according to corresponding equations in the specification. Subsequently, a total of 11 fire scenes located at the side-span and the middle-span are identified according to the structural design criterion, which is thought to include all the possible fire scenes of the large canopy during service. Furthermore, it is analyzed that the heating laws of the large canopy under various fire scenes. The results show that the highest temperatures of air and steel beam are close to 750 ℃ and 730 ℃, respectively, when the canopy is in the side-span fire scene, and those for the canopy in the mid-span fire scene are 600 ℃ and 570 ℃, respectively. All of them are much lower than the temperature in a standard fire specified in ISO 834, which demonstrates that the tall space structure could be the main application scene of fire-resistant steel. Finally, the bearing capacity and stability of the structural members are analyzed and validated using the general structural steel Q355 and the domestic fire-resistant steel Q345FR. It is indicated that the steel beams GL3, GL3a and GL4 fail to meet the load-carrying requirements when using the general structural steel Q355, while the strength and overall stability requirements can be achieved when applying the fire-resistant steel Q345FR.
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