Fingerplate expansion devices are commonly used to account for movements and rotations of two bridge deck slabs. Under high traffic volumes, these devices experience premature damage that affects the structural integrity of the bridge superstructure. In this paper, a group of experimental tests and 3D elastic–plastic finite element models (FEMs) were conducted to study the structural behavior and modes of failure of this type of joints. The experimental results were also used to validate the FEMs, from which additional FEM models were employed to study the effects of various design parameters. The parametric study results showed that the finger thickness did not influence the response in the absence of stiffeners. On the other hand, the presence of stiffeners increased the finger joint stiffness while decreased the device ductility. Welding zones were the sites of crack initiation in most of the studied cases. The numerical results showed that the locations of the highest stress concentrations were in the weld between the fingerplate and the supporting beam top flange and in the weld between the stiffeners and the top flange.