Modern architecture is striving for large open spaces, which has resulted in the development of design methodologies such as the Improved Travelling Fires Methodology (iTFM). This methodology is only applicable in large open spaces where flashover may not occur. This paper studies the effect of various design fires, including traditional uniform fires and for the first time iTFM on flat-plate unbonded post-tensioned (UPT) concrete, a structural system which allows long flooring spans (> 8 m). The predictions of a structural model of high temperature unbonded prestressing tendon relaxation that is available in the literature are compared for the first time with previously published fire experiments thus validating the model. A novel case study based on a real building was then analysed using the model to propose an acceptance criterion based on the longest tendon length where rupture and extreme stress relaxation is prevented. Two types of steel, fabricated according to BS 5896 and AS/NZ4672, were analysed for various unbonded lengths (between 8.15 m and 65.2 m). Emphasis is on the unique deformation mechanisms present when prestressing steel is heated locally. The relaxation model has been validated for negligible slab deflection, allowing for discussion on the difference between the impact of travelling and uniform fires on the design of UPT concrete. The tendon rupture and relaxation analysis have novelty in showing that slow moving fires are more onerous structurally than fast moving or uniform fires and therefore are important to consider when defining critical design thermal boundaries in a design context. Results also demonstrate that tendons in similar designs require consideration to be limited in unbonded length. The development of new acceptance criteria herein is the first step in producing a generalized criteria for better informed structural fire design of UPT concrete exposed to fire.

中文翻译：

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暴露于移动和传统设计火灾的无粘结后张混凝土的验收标准

现代建筑正在努力争取大型开放空间，这导致了设计方法的发展，例如改进的行火方法 (iTFM)。此方法仅适用于可能不会发生闪络的大型开放空间。本文研究了各种设计火灾的影响，包括传统的均匀火灾和首次 iTFM 对平板无粘结后张 (UPT) 混凝土的影响，UPT 混凝土是一种允许长地板跨度 (> 8 m) 的结构系统。文献中提供的高温无粘结预应力筋松弛结构模型的预测首次与先前发表的火灾实验进行了比较，从而验证了该模型。然后使用该模型分析了基于真实建筑物的新案例研究，以提出基于最长钢筋束长度的验收标准，其中防止断裂和极端应力松弛。对根据 BS 5896 和 AS/NZ4672 制造的两种钢进行了各种非粘合长度（8.15 m 和 65.2 m 之间）的分析。重点是局部加热预应力钢时存在的独特变形机制。松弛模型已针对可忽略的板挠度进行了验证，从而可以讨论行进火和均匀火对 UPT 混凝土设计的影响之间的差异。肌腱断裂和松弛分析具有新颖性，它表明缓慢移动的火灾在结构上比快速移动或均匀的火灾更加繁重，因此在设计环境中定义关键设计热边界时考虑这一点很重要。结果还表明，类似设计中的肌腱需要考虑限制非粘合长度。此处制定新的验收标准是制定通用标准的第一步，以便更好地了解暴露于火灾的 UPT 混凝土的结构防火设计。