Serviceability Performance of Prestressed Concrete Buildings Taking Into Account Long-Term Behavior

This dissertation, "Serviceability Performance of Prestressed Concrete Buildings Taking Into Account Long-term Behavior" by Hing-lun, Yip, 葉慶倫, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: A common problem faced by engineers nowadays is the restriction on structural member dimensions due to architectural and spatial concerns. Such restrictions have resulted in the use of high-strength concrete in vertical members to reduce sizes, the use of central core walls and peripheral columns to increase window areas, the use of prestressed concrete floors to increase spans, etc. Serviceability problems such as cracking may, however, arise in the long term if these problems have not received proper attention during the design stage. This paper addresses several major issues associated with this type of buildings. Firstly, the differential axial shortening between the core walls and columns caused by large differences in stress levels will induce additional stresses and strains in the horizontal structural members, which are not normally accounted for in the traditional design methods. Secondly, the post-tensioning of concrete floors gives rise to additional internal forces induced in several ways such as time-dependent effects, sequential construction, and secondary "P-δ" effects of the high-strength slender columns. Thirdly, the soil-structure interaction could induce significant additional deformations and stresses in the buildings, although they are not always taken into account properly especially when carrying out simple or preliminary designs. These issues are vital and should be carefully considered in regular structural analyses and designs. With the common practice that most of the designs of prestressed concrete building structures are sublet to prestressing specialists, common structural engineers seldom have the insight into the structural performance of these buildings. Furthermore, utilities for calculating steel relaxation, which is an important factor governing the behaviour of prestressed concrete buildings, and its interactions with other time-dependent effects of concrete are hardly found in popular commercial software packages developed for building designs. All of these problems present obstacles in the correct modelling of prestressed concrete buildings. In the light of this, a practical but accurate method of modelling steel relaxation using the equivalent creep in commercial packages, that are normally good at dealing with complicated geometry, has firstly been developed in this work. The accuracy and reliability of the method are examined by comparing the results with available numerical solutions. Good agreement is observed. Secondly, a series of studies have been carried out based on a typical prestressed concrete building to examine various effects on the structural performance. It is found that the most influential effect is the time-dependent behaviour. It induces extra column moments, differential axial shortening, losses of tendon stresses, and P-delta moments. The construction sequence and soil-structure interactions are also found to affect the structural performance but they are less critical compared with the time-dependent effects. Finally, a parametric study has been carried out to evaluate the likely ranges of time-dependent effects on the structural behaviour. DOI: 10.5353/th_b4833008 Subjects: Prestressed concrete construction