This paper presents numerical and theoretical studies on the performance of internal diaphragm connections to box columns. The study aims to investigate the static and cyclic performances of the connections including the behavior of diaphragms. This paper contains an assessment of the numerically and theoretically parametric studies on plate-to-box column connections considering several influence parameters, such as diaphragm, plate width, and column thickness. Subsequent sections discuss the prediction and validation of strength of the connections, followed by the performance of wide flange (WF) beam-to-box column connections. The results are presented and discussed as finding that diaphragms were the key elements to enhance the performance of the connections.