Being cost-effective, rapid construction, high transportability, and maintainability, post-tensioned precast concrete segmental bridges (PT-PCSBs) have been one of the most preferred choices for sites that are restrictive to access. Yet, deterioration due to aging, improper maintenance, and unaccounted shrinkage of bridge decks can cause unexpected prestress loss/gain and alter the structural behaviour of PT-PCSBs, especially with unbonded external tendons. In this regard, this paper presents the development of a rigorous and detailed three-dimensional discrete finite element model (DFEM) for a PT-PCSB, of which both global load-deflection behaviour and local responses were verified against experimental results by Takebayashi et al. (1994) and Jiang et al. (2018). The verified model was then adopted to investigate the effects of prestress loss/gain on the structural responses and failure behaviour. Based on the analysis results, a 20% increase/decrease of prestressing can lead to a 10% increase/16.8% decrease of the moment resistance but a 30.2% decrease/36.6% increase of the deflection capacity. At large prestressing level, e.g. 0.874 $f_{\mathit{pu}}$, significant crushing of concrete can occur in the bottom region of the bridge at ULS that significantly restricts the joint opening, tendon strain development, and ductility. On the contrary, at the extremely low prestressing level of 0.125 $f_{\mathit{pu}}$, the bridge decompresses under its self-weight and is very unstable under additional load, resulting in very low ductility and moment resistance. Furthermore, for a regular prestress level of around 0.6 $f_{\mathit{pu}}$, the stress change in tendons from the effective prestress to the stress in ULS matches the recommendations by ACI318 and AASHTO LRFD but is underestimated by EC2. Yet, the ultimate tendon stress which is less sensitive to the prestress change may be more suitable than the stress change in evaluating the ULS behaviour of PT-PCSBs with unbonded tendons.

后张预应力混凝土分段桥（PT-PCSB）具有成本效益，快速施工，高运输性和可维护性的特点，已成为限制出入地点的最优选选择之一。然而，由于老化，维护不当以及桥面板收缩不当引起的劣化可能会导致意外的预应力损失/增加，并且会改变PT-PCSB的结构性能，尤其是在未粘结外部筋的情况下。在这方面，本文提出了针对PT-PCSB的严格而详细的三维离散有限元模型（DFEM）的开发，Takebayashi等人针对实验结果验证了其整体载荷-挠度行为和局部响应。（1994）和Jiang等。（2018）。然后采用验证模型来研究预应力损失/增益对结构响应和破坏行为的影响。根据分析结果，预应力的增加/减少20％可以使抗弯矩增加10％/减少16.8％，但使挠曲能力减少30.2％/增加36.6％。在较大的预应力水平下，例如0.874$F_{\mathit{PU}}$，在ULS的桥梁底部可能发生混凝土的严重压碎，这严重限制了接头的开度，腱的应变发展和延展性。相反，在极低的预应力水平0.125$F_{\mathit{PU}}$，该桥在其自重作用下会减压，并且在附加负载下非常不稳定，从而导致延展性和抗力矩性非常低。此外，对于约0.6的常规预应力水平$F_{\mathit{PU}}$，从有效预应力到ULS中的应力，肌腱的应力变化与ACI318和AASHTO LRFD的建议相符，但被EC2低估了。然而，对于评估具有未粘结筋的PT-PCSB的ULS行为，对预应力变化不太敏感的最终筋应力可能比应力变化更合适。