Many old riveted steel bridges remain operational and require retrofit to accommodate ever increasing demands. Complicating retrofit efforts, riveted steel bridges are often considered historical structures and as such structural modifications that affect the original construction are to be avoided. The presence of rivets along with preservation requirements often prevent the use of traditional retrofit methods, such as bonding of fiber reinforced composites, or the addition of supplementary steel elements. In this paper, an un-bonded post-tensioning retrofit method is numerically investigated using an existing riveted bridge geometry from Münchenstein, Switzerland. Pre-stressed un-bonded carbon fiber reinforced plastic (CFRP) plates will be considered for the strengthening elements. By using prestressed CFRP plates, a portion of the permanent loads (such as the dead-weight) will be transferred to the strengthening element. Fatigue critical regions of the bridge are identified, and the effects of the un-bonded post-tensioning method on strength and fatigue susceptibility are explored.