Shape Memory Alloy retrofitting of columns for future resilient infrastructures

形状记忆效应描述了一些金属合金通过热弹性转变（即加热到奥氏体表面转变温度以上）恢复塑性变形的能力。利用这种效应的镍钛诺基形状记忆合金（SMA）也具有高强度、高能量吸收能力、高阻尼、良好的抗疲劳性、良好的耐腐蚀性和优异的再定心能力。SMA NiTiNb是一种镍、钛和铌合金，可以恢复8%以上的应变，并且具有一定的特征温度范围，使其适合于结构工程中的应用，特别是本文所介绍的应用，即通过对构件核心施加主动约束来增强桥梁支撑元件抵抗道路车辆冲击的能力。目前，混凝土或钢护套和纤维增强聚合物织物等被动损伤缓解技术最常用于改造结构元件。然而，主动约束是这项工作的重点，它在减少损伤方面更有效，因为它在事件的早期阶段起作用，在施加任何横向载荷之前。本研究的目的是研究形状记忆合金NiTiNb在钢筋混凝土桥梁柱改造中的应用。这项工作的新颖之处在于处理受到重型货车突然冲击的柱子。道路车辆撞击是美国桥梁损坏的第二大原因，比地震造成的损坏要频繁得多。本研究旨在模拟带有NiTiNb螺旋结构的桥柱的力学响应，并量化其抗侧向冲击载荷的弹性，相对于在建成条件下的柱。该项目分为三个主要任务：NiTiNb合金的热力学表征；采用全尺寸LS-DYNA有限元模型，开发了一套对比数值分析方法，分析了钢筋混凝土柱在安装了NiTiNb螺旋结构前后被重型货车撞击的情况；以及用于验证目的的小规模跌落试验，以赋予FE模型预测的可信度。这种改造技术简单地包括将一根合金线缠绕在一根柱子上，通过机械紧固件约束其两端，并使用电流或气体火炬将螺旋加热到特定温度以上。这项研究的结果将立即适用于支持桥面的圆形RC混凝土柱，也适用于广泛的结构，如地下停车场、铁路桥和靠近主要道路或容易受到恶意车辆撞击的建筑物。该项目将有助于建设关键国家基础设施的复原力，并可用于防止战略建筑的脆弱性。由于该方法依赖于混凝土主动约束的概念，它不仅可以防止未受损结构成员的脆弱性，还可以恢复严重受损柱的功能。此外，该技术也适用于紧急修复，因为它不需要对钢丝进行机械预应力，与其他改造方法相比，可以大大减少劳动力和时间。

The Shape Memory Effect describes the capacity of some metallic alloys of recovering plastic deformations through a thermo-elastic transformation, i.e. by being heated above the austenitic finish transformation temperature. Nitinol based Shape Memory Alloys (SMA) that exploit this effect also exhibit high strength, high energy absorption capacity, high damping, good fatigue resistance, good corrosion resistance and excellent re-centring ability. The SMA NiTiNb, a Nickel, Titanium and Niobium alloy, can recover above 8% strain and has a range of characteristic temperatures that make it suitable for applications in structural engineering, and in particular the one promoted here, i.e. enhancing the capacity of bridge support elements to resist impacts from road vehicles by applying active confinement to the member core. At present passive damage mitigation techniques, such as concrete or steel jacketing and fibre reinforced polymer fabrics, are most commonly used to retrofit structural elements. Active confinement, however, which is the focus of this work, is much more effective in reducing damage since it acts at an earlier stage in the event, before any lateral loads are applied. The aim of this research is to investigate the application of the shape memory alloy NiTiNb to retrofit reinforced concrete bridge columns. The novelty of this work lies in addressing columns subjected to a sudden impact by a heavy goods vehicle. Road vehicle impacts are the second cause of bridge failures in the US and much more frequent than failures due to earthquakes. This study intends to model the mechanical response of bridge columns retrofitted with NiTiNb spirals and quantify their resilience in resisting lateral impact loads with respect to columns in the as-built condition. The project is structured in three main tasks: the thermo-mechanical characterization of NiTiNb alloy; the development of a set of comparative numerical analyses, using a full scale LS-DYNA finite element model of RC columns hit by heavy goods vehicles before and after being retrofitted with NiTiNb spirals; and a small scale drop test used for validation purposes to give credibility to the FE model predictions.The retrofitting technique simply consists in winding an alloy wire around a column, constraining the two ends by means of mechanical fasteners and heating the spiral above a characteristic temperature using electrical current or a gas torch. The outcomes of this research will be immediately applicable to circular RC concrete columns supporting bridge decks, but also to a wide range of structures such as underground car parks, railway bridges and buildings located close to major roads or vulnerable to malicious vehicle impact. The project will contribute toward building resilience in critical national infrastructures and can be used to prevent vulnerability of strategic buildings. Since the method relies on the concept of active confinement of concrete it would not only prevent the vulnerability of undamaged structural members but also restore the functionality of severely damaged columns. Moreover the technique is suitable also for emergency repair since it does not need mechanical pre-stressing of the wire and can be implemented with significant reduction of labour and time with respect to other retrofitting methods.