Side-by-side box-beam bridge superstructure system is often specified for rapid bridge construction. A common performance concern with this bridge type is the longitudinal reflective deck cracking directly above the joints between precast beams. Empirical design procedures modified frequently since the 1950's in the US and abroad were not able to abate the longitudinal reflective deck cracking. It is postulated that the cracking is due to underestimation of the load demand at the joints. The reason for the demand uncertainty is the lack of accurate analysis models. A few analysis models for side-by-side box-beam superstructure system are discussed in literature. Due to oversimplifications incorporated in those models, accurate load response could not be obtained. A simple but powerful analysis model, that has been developed based on the macromechanics concept, is presented here. The objective here is to present the use of the macromechanical analysis model in conjunction with AASHTO LRFD stipulations to calculate the transverse moment demand along the longitudinal joints between precast beams. These calculations are used to design the level of transverse posttension, assuring crack control. Numerical example presented herein is compatible with the current practice where transverse connection is established with grouted shear-keys, transverse posttension through diaphragms at discrete locations, and a 150 mm (6 in.) cast-in-place concrete deck. The transverse posttension design calculation details are also provided.