End zone design and detailing of pre-tensioned concrete elements

Abstract

Optimisation of building elements towards more efficient and sustainable use of materials is an ever present drive for innovation within an industry. In order to achieve an optimised design for a building element, a detailed knowledge of the construction materials, structural behaviour, manufacturing process and design process are important. This thesis focusses on the design of end zone detailing of pre-tensioned elements. Even though pre-tensioned elements are already well established as a construction element for their great weight-to-span ratio and optimised use of the applied materials, some issues sometimes still occur concerning end zone cracking. This is mainly due to the intricate nature of the stress distribution in the anchorage zone and the many parameters influencing this behaviour. In order to provide current structural designers with the appropriate knowledge towards efficient end zone detailing, an overview of the currently available literature concerning the end zone stresses and anchorage zone reinforcement detailing is provided. This overview illustrates the difficulties concerning the design of end zone reinforcement. The main parameters influencing the anchorage zone stress distribution are identified. The most common design models (both theoretical and numerical) and the associated guidelines towards end zone reinforcement detailing are also discussed. Even though some of these models are based on similar theoretical principles, a consistent method towards proper end zone detailing is not provided in the current literature. To improve on the understanding of the intricate stress behaviour in the end zones of pre-tensioned elements, a two-stage modelling approach is presented. This numerical model allows for the full assessment of the anchorage zone stress distribution and post-cracking behaviour based on input parameters known at the design stage. In the first stage of the modelling approach, the bond behaviour and transfer length are assessed in order to properly simulate the transfer of the prestress load from the prestress strand to the concrete element. Next, this behaviour is used as an input to assess the stress distribution within the anchorage zone of the analysed element. The established numerical model provides the opportunity to further investigate the stress behaviour within the anchorage zone. A parametric study is performed in which the impact of important influencing parameters is discussed. A detailed analysis of the current end zone detailing provisions is also performed. Through the evaluation of the strain behaviour associated with these end zone detailing provisions and an additional parametric analysis of the anchorage zone reinforcement, good practice guidelines of the anchorage zone detailing towards efficient end zone crack control are developed. Lastly, a basic strut-and-tie model is created as a design tool to compute the total required end zone reinforcement. This strut-and-tie model is based on the internal force flow of the anchorage zone computed through the two-stage numerical model and takes into account several geometrical, material, and mechanical properties. It can be used as a simplified model to assess the necessary end zone reinforcement. The efficiency of the strut-and-tie model is illustrated through the analyses of ten design examples ranging from prestressed beam elements to hollow core slabs. The conservativeness of the strut-and-tie model is also analysed. Even though further refinement and optimisation is necessary, it is shown that the model can provide a possible groundwork towards a more complete design tool for efficient anchorage zone reinforcement detailing and proper end zone crack control.