Abstract Knowledge of the initial rotational stiffness of a connection is important for the global elastic analysis of frame structures. Based on a simple force transfer mechanism and consideration of the behaviour of individual components, a method has been developed to predict the initial stiffness of composite flush endplate connections. The approach is compatible with that proposed earlier to predict moment capacity for several types of composite connection. In order to apply plastic analysis to frame structures a knowledge of the available and the required rotation capacities is necessary. A simple technique to determine the available rotation capacity of composite flush endplates is described herein. Taken together, the two methods represent key steps in the development of an approach to predict the main measures of the behaviour of composite endplate connections. Used in conjunction with the moment capacity and failure mode prediction method developed earlier by the authors, they provide a complete representation of the design properties of composite flush endplate connections.