The structural system for bridge decks adopted nowadays has been the Box Girder Bridges. However, the box girders suffer from shear lag effect which causes a non-uniform axial stress along its cross-sectional direction which is not true as per the assumption of Simple Beam theory i.e. Plane sections remain plane. In this project, parametric study of three dimensional finite element analysis of a simply supported PSC box girder section for dead load and prestressing using four nodded shell elements is done to evaluate the effect of shear lag. The results obtained show, the effects of geometry of crosssection on the behavior in terms of development of longitudinal stresses in transverse direction of different box girders. The effective flange width is also calculated considering shear lag effects in the design of box girders bridges as per AASHTO LRFD Bridge design specifications (2017). It can be concluded from the present study that the Simple Beam theory is a rough assumption for analysis of box sections. The structure is modeled and analyzed in SOFiSTiK software.