Beam strengthening by external post-tensioning: Design recommendations

Buildings and bridge structures are often in need of strengthening due to deterioration of material properties or a change in design loads. External post-tensioning, in which tendons are installed on the outside of a structural member with the aid of deviators and anchorages, is an attractive strengthening method as it does not add much weight and can be carried out with minimal disruption to the operational function of the structure. An extensive programme has been carried out on simple-span and continuous beams as well as beams in reinforced concrete frames to study the effect of tendon configuration, second-order effects, and secondary and tertiary effects due to external post-tensioning. The effect of external post-tensioning on shear capacity and the use of fibre-reinforced polymer prestressing cables were also examined. This paper highlights the main findings of the study, based on which design recommendations are given and a direct design approach is proposed.     

体外预应力混凝土桥梁的发展趋势

体外预应力混凝土桥梁有维修及更换方便、旧桥加固不中断交通等优点,被广泛应用于桥梁结构的新建及加固。阐述了体外预应力混凝土桥梁结构的未来发展方向,包括高强度和高性能混凝土材料的应用、预应力筋的材料性能改进及体外预应力结构设计理论的进展。事实证明,体外预应力桥梁结构已成为现代预应力桥梁结构技术的重大发展和新技术标志。     

体外预应力加固钢结构的研究与发展

体外预应力加固钢结构具有加固工作可在不卸载情况下进行、减小变形、降低内力峰值、扩大结构弹性受力幅度等优点。本文给出了国内外体外预应力加固钢结构的几个工程实例,综述了国内外学者在预应力加固钢结构的承载力、布索方案、预应力损失、设计方法、构造措施方面的研究成果,并指出有待进一步研究的工作。     

体外预应力混凝土梁弯曲性能分析

体外预应力混凝土技术已被广泛地应用于各种结构工程及对既有结构进行加固中。本文通过十片体外预应力混凝土梁的试验,建立了体外预应力混凝土梁受弯条件下全过程非线性分析的计算方法和计算机程序。计算结果表明,该计算方法可以较好地反映此类梁在受弯条件下的规律,为进一步深入研究体外预应力混凝土结构提供了理论分析的依据。同时,采用体外预应力技术对一座铁路桥进行了加固。     

Experimental monitoring of the strengthening construction of a segmental box girder bridge and field testing of external prestressing tendons anchorage

Prestressed concrete segmental box girder bridges are composed of short concrete segments that are either precast or cast in situ and then joined together by longitudinally post-tensioning internal, external, or mixed tendons. The objectives of this study are to monitor the construction process of the external prestressing tendons to strengthen the bridge structure and perform a field load test to measure the strain and the deflection of the anchorage devices of the external prestressing tendons to determine the state of these devices after tension forces are applied. The monitoring process of the external prestressing tendons construction includes inspecting the cracks in the diaphragm anchorage and the deviation block devices before the tension forces are applied to the external tendons; measuring the deformation of the steel deviation cross beam during the tension process; measuring the deformation of the box girder after different levels of tension forces are applied; measuring the elongation of the external tendons in each level of the tension; and measuring the natural frequency of the external tendons after the tension process is complete. The results of the monitoring process show that the measured values of the deformation, the elongation, and the natural frequency meet the requirements. Therefore, there is no damage during the construction and the tensioning of the external prestressing tendons. A field load test is performed to the anchorage beam, the steel deviation block devices, and the steel deviation cross beam. The field load test results of the anchorage devices show that the values of the strains, the stresses, and the deflection are less than the respective allowable limit values in the requirements. Therefore, the anchorage devices have sufficient strength, and the working state is good after the tension forces are applied to the external prestressing tendons.     

体外预应力加固混凝土框架梁体外筋应力增量分析

体外预应力是后张预应力体系的一个重要分支,已越来越广泛地应用于桥梁工程和建筑结构以及结构加固.应力增量和二次效应是体外预应力结构设计中两个关键因素,同时也是两个难点问题.国内外对体外预应力的研究大都集中在简支梁,而对超静定梁的研究很少,因此有必要对体外预应力超静定梁进行深入的研究.通过试验重点分析了框架粱体外筋应力增量随荷载增长的规律以及应力增量与梁跨中挠度的关系,并通过综合分析国内外研究资料及试验结果,总结出了一些有益的关于体外预应力加固框架梁应力增量的试验结论,并提出了计算超静定结构应力增量的简要计算公式,这对我们今后进行体外预应力超静定结构设计和加同都能够提供一定的帮助,在港口、水坝、桥梁等结构的建设上也都有广泛的应用前景.     

体外预应力混凝土结构的预应力损失估算

体外预应力结构的构造形式及施工方法与有粘结或无粘结预应力结构有较大差别 ,因此 ,其产生的预应力损失也不同。结合体外预应力结构自身特点 ,对其设计阶段预应力损失的估算进行了探讨 ,并提出了相关的设计计算建议     

体外预应力钢束在汽车活载作用下的应力变幅分析

利用研发的体外预应力混凝土桥梁结构弹性阶段非线性分析专用程序 ,对几种典型跨径体外预应力混凝土梁桥的体外预应力钢束在汽车活载作用下的应力变幅进行分析。研究了简支梁中结构跨高比、体外预应力钢束与结构的粘结关系、转向块间距等参数对计算结果的影响 ,以及连续梁中施工方案、体内钢束和体外钢束的配束比例、连续跨个数和钢束布置形式等参数对计算结果的影响。同时也与相同条件下的传统体内预应力结构的预应力钢束的应力变化幅度进行比较。这项研究是体外预应力钢束体系疲劳评价研究的一部分。     

桥梁加固维修体外预应力施工质量控制实例

介绍了体外预应力技术,通过工程实例,探讨了体外预应力技术在桥梁加固维修中的应用,从施工放样、梁体钻孔、梁体凿毛打磨、钢板的粘贴锚固、体外预应力张拉等多方面提出施工质量控制措施,从而推广体外预应力加固法的应用.     

混凝土连续箱梁的体外预应力次内力研究

以锡澄高速公路江阴高架桥跨东横河东幅桥加固工程为研究背景,对体外预应力次内力（次弯矩）的分析计算理论展开研究,研究结果表明,对已有预应力混凝土连续箱梁的体外预应力加固计算中必须考虑体外预应力束产生的二次内力作用,采用等效荷载法计算体外预应力次内力是可行的,计算值与实测值吻合。     

Multi-stepwise thermal prestressing using a cover-plate in steel structures

This paper introduces an innovative prestressing method for steel structures utilizing a multi-stepwise thermal prestressing method (M-TPSM). The proposed prestressing method is founded on a simple concept of thermal expansion and contraction of steel. A cover-plate, a high strength steel cover-plate used as a post-tensioning member in this method, is heated by electrical heaters before it is attached to the main member. After the cover-plate is attached to the main member by using high tension bolts at several points or by welding, the electromagnetic induction heaters are removed, and the resulting contraction force of the cover-plate induces eccentric axial forces to the main member. In this study, a fundamental investigation involving analytical and experimental study is carried out to evaluate the feasibility of the M-TPSM for steel structures. Also, static loading tests were performed on the rolled beam specimens for assessment of the behavior of the steel beam prestressed with M-TPSM.     

高效体外预应力结构锚固体系研究与应用

体外预应力技术是后张预应力体系的主要分支之一。本文进行了体外锚固系统应变测试与有限元分析对比研究，组装件静载与疲劳试验研究，体外弯折索试验研究。工程设计与施工实例表明该技术可在大跨桥梁、屋盖及既有结构加固中应用。     

Corrosion failure of post-tensioning tendons in alkaline and chloride-free segregated grout: a case study

This paper describes a case study of corrosion of post-tensioning strands in segregated grout that led to failure of one of the external tendons of a bridge after less than 2 years from the construction. Unexpectedly, severe corrosion attacks took place in areas where the ducts were filled by the injection grout, but segregation of the grout led to the presence of an unhardened whitish paste. A detailed inspection of the failed tendon, as well as other tendons removed afterwards, showed that corrosion occurred mainly in the inclined parts of the external post-tensioning tendons, in the presence of whitish grout. Analysis on samples of segregated grout showed that it was highly alkaline and had a high content of sulphate ions, but was free of chloride ions, excluding thus the normal causes of corrosion of steel in cement-based materials. Microstructural analysis of failed wires also excluded the failure for hydrogen embrittlement.     

体内外预应力混合配索桥梁设计中分阶段预应力配置设计

体内外预应力筋混合配索的桥梁设计方案,可利用体内索承担悬臂浇注施工荷载,又可利用体外索承担二期恒载和使用阶段荷载,并可发挥体外预应力筋在使用阶段易于检测与更换的优点。但在预应力筋混合配索中,不同的配索方案会导致体内体外预应力的布置和用量存在差异,也决定桥梁的应力状态。本文以实际工程为背景,以体内索承担施工阶段荷载,体外索承担二期恒载和使用荷载为原则,完成了施工过程中的不同配索优化,重点介绍了不同体内预应力索的设置方案,以及相应的体外预应力索的设计,确定了最终的桥梁设计方案与桥梁应力状态。相关设计过程可为类似混合配索的预应力连续梁或连续刚构桥提供参考。     

体外预应力技术在结构加固工程中的应用

结合工程实际,阐述了体外预应力加固技术的设计方法和施工工艺,包括体外预应力加固设计步骤、束形选择与优化、锚固节点和转向器的构造要求等。实践证明,体外预应力加固技术是一种经济适用、施工简单、效果显著的加固方法,值得推广。     

体外预应力混凝土结构研究进展

阐述了体外预应力结构的发展概况及其特点，分析了体外预应力结构承载力的研究方法与进展，介绍了桥梁修复工程中的体外预应力技术，指出了体外预应力技术在我国的发展前景。     

外张预应力钢筋混凝土结构非线性有限元分析

本文提出了一种用于外张预应力钢筋混凝土结构非线性分析的单元模型,此模型可模拟外张预应力钢筋在转向点处的滑移和摩擦作用。将这一单元模型与混凝土和钢筋单元模型相组合,建立了外张预应力钢筋混凝土结构非线性分析程序,非线性方面包括了材料和几何非线性。该有限元模型可用于外张预应力钢筋混凝土结构从张拉施工至使用阶段的全过程分析,分析内容包括弹塑性变形、混凝土开裂和破坏等。最后给出的实例表明模型计算分析结果与试验结果[１］吻合良好。     

体外预应力技术在桥梁加固中的应用

对某 5跨钢筋混凝土变截面连续箱梁桥应用体外预应力加固技术进行了加固。介绍了体外预应力技术在桥梁加固中的施工工艺以及遇到的问题 ,可供体外索桥梁加固设计施工参考     

有粘结体外预应力加固体系施工技术研究

文章提出了有粘结体外预应力的基本概念,该体系分为体外预应力和喷注复合砂浆两个施工阶段并最终形成有粘结体系。文中主要讨论了有粘结体外预应力加固体系的施工工艺和施工关键技术,详细分析了体外筋张拉施工时的控制措施及在张拉过程中可能存在的异常情况。