Prediction of time-dependent deformations in post-tensioned concrete suspended beams and slabs in tall buildings
Creators: name: Jayasinghe, Mudalige
Publisher: name: RMIT University
Year: 2024

description: Deflection analysis is important to ensure that the reinforced or prestressed concrete flexural member will deflect within the allowable limits during its lifetime. Prediction of deflections in reinforced or prestressed concrete members can be extremely complex due to many factors that affect the deflection such as non-linear and inelastic concrete properties, time-dependent behaviour of concrete such as creep and shrinkage, bond-slip between concrete and steel, crack development with time and time-dependent prestress losses. <br><br>The available simplified deflection models presented in design standards are mostly empirical and approximate and do not realistically account for factors such as variations in the shrinkage and creep characteristics of concrete, bond slip behaviour between concrete and steel and the effect of prestress losses over time. Furthermore, in recent years the tendency to use high strength concrete and steel, and shallower sections has resulted greater importance of predicting and controlling short-term and long-term deflections of concrete flexural members under service loads. Hence, there exists a need for research on the development of a reliable and practical solution for predicting the long-term deflection of concrete beams and slabs under sustained loads. In particular, further research into the prediction of long term deflection in post-tensioned concrete members is required. An analytical model which considers the above factors has been developed to predict long-term deformations of post-tensioned concrete flexural beams and one-way spanning slabs subjected to sustained loads in the service load range.<br><br>In the proposed analytical model, the analysis is governed by two conditions: whether the member is cracked or uncracked. The cracked region of the member is divided into a series of discrete blocks bounded by flexural cracks. The sectional stresses and strains in concrete and steel in the cracked block are computed by applying the equations governing the sectional equilibrium conditions, material laws for concrete and steel and local bond and slip laws between concrete and steel. In the cracked block the non-linear variation of strains in concrete and non-compatibility of tensile strains between steel and concrete, have been taken into account. The uncracked regions of the flexural member are analysed by applying equilibrium conditions and strain compatibility. The sectional stresses and strains at any given time are computed by incorporating the long-term effects of creep and shrinkage to cracked and uncracked regions.<br><br>A monitoring of long-term deflections of post-tensioned slabs and beams of a multi-storey office building during construction was carried out and a comparison between the monitored and predicted deflections using the developed analytical model is presented. Furthermore, a parametric study to investigate the effects of material and geometric properties on the deformations of post-tensioned flexural members has been carried out and the results are presented.<br>