Strengthening steel-concrete composite girder bridges with open cross section by implementation of additional horizontal bottom bracing is one elegant solution, which has proven to be effective in reducing the normal stresses in the steel beams by sharing effectively the applied eccentric traffic loads. The paper reviews the additional effects onto the stress state of the concrete slab coming from the transformation of open conventional twin girder steel-concrete composite bridge cross section to semi-closed (box-section). These effects are investigated through the help of FE model of a bridge superstructure. Conclusions and recommendations are drawn at the end.