Shape Memory Alloy retrofitting of columns for future resilient infrastructures

形状记忆效应描述了某些金属合金通过热弹转变，即在奥氏体相变温度以上加热来恢复塑性变形的能力。利用这一效应的镍基形状记忆合金(SMA)还具有高强度、高吸能能力、高阻尼性、良好的抗疲劳性能、良好的耐腐蚀性和良好的回心能力。SMA NiTiNb是一种镍、钛和铌合金，可恢复8%以上的应变，并具有一系列特征温度，使其适合于结构工程应用，特别是这里推广的SMA NiTiNb，即通过对构件核心应用主动约束来增强桥梁支撑元件抵御道路车辆冲击的能力。目前，被动减损技术，如混凝土或钢护套和纤维增强聚合物织物，最常用于结构构件的加固。然而，作为这项工作的重点，主动约束在减少损害方面要有效得多，因为它在事件的早期阶段起作用，在施加任何横向荷载之前。本研究旨在探讨形状记忆合金NiTiNb在钢筋混凝土桥柱加固中的应用。这项工作的新颖性在于解决了被重型货车突然撞击的柱子。在美国，道路车辆撞击是桥梁损坏的第二大原因，比地震造成的损坏更频繁。本研究旨在模拟NiTiNb螺旋加固桥柱的力学响应，并量化其相对于竣工状态下的柱抵抗横向冲击荷载的回弹能力。该项目分为三个主要任务：NiTiNb合金的热力学表征；开发一套对比数值分析，使用全尺寸LS-DYNA有限元模型，分析重型货车在改装NiTiNb螺线前后被击中的钢筋混凝土柱；以及用于验证目的的小规模跌落试验，以验证有限元模型预测的可靠性。改装技术仅包括将合金丝缠绕在立柱周围，通过机械紧固件约束两端，并使用电流或气炬将螺旋加热到特征温度以上。这项研究的结果将立即适用于支撑桥面的钢筋混凝土圆形柱，但也适用于各种结构，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。该项目将有助于在关键的国家基础设施中建立复原力，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，它不仅可以防止未损坏的结构构件的易损性，而且可以恢复严重损坏的柱的功能。此外，该技术还适用于紧急修复，因为它不需要对钢丝施加机械预应力，而且与其他加固方法相比，可以大大减少人力和时间。

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.