Behaviour of bottle-shaped struts in non-flexural concrete members: state-of-the-art-review

The design of bottle-shaped struts for bursting forces, especially those without bursting reinforcement, has been the subject of much debate over recent years. This review article summarises the latest research into bursting failure of bottle-shaped struts and the capacity of bottle-shaped struts without bruising reinforcement to resist bursting forces. It was found that, while extensive research has been undertaken on the influence of the width of concrete members subject to concentrated loads, little attention has been given to cases where the boundaries of the concrete provide no restriction to the dispersion of the compressive stress field. Furthermore, the current two-type bottle-shaped strut model adopted by design standards fails to consider the transition between fully restricted compressive stress fields and fully unrestricted compressive stress fields. This paper suggests a three-type bottle-shaped strut model for further investigation. Limited research has also been undertaken on the influence of the ratio of the length of the strut to the width of the nodal zones (i.e. bearing area), particularly in fully unrestricted compressive stress fields. There is significant variation in the literature, however, unlike the design models adopted by design standards, studies tend to show a downwards trend in the induced bursting forces as the ratio of strut length to nodal zone width decreases. Further research is required to challenge the analytical models of design standards. Additionally, few studies have directly investigated the splitting capacity of the concrete to resist bursting without reinforcement. The majority of studies recommend minimum reinforcement for ductility and those that have provided equations for estimating the cracking capacity are limited in their applicability. Further study is recommended, particularly in struts with low strut length to nodal zone width ratios. Finally, it is shown that 3-dimensionally dispersing compressive stress fields tend to be stronger than 2D cases. Further research is required to develop a simple analytical approach to accurately predicting the bursting forces and unreinforced cracking capacity of these cases. Such a model could have great benefit, by potentially reducing the quantity of bursting reinforcement in certain non-flexural concrete structures, allowing increased structural efficiency and increased confidence in structural capacity in real world applications.