flange longitudinal displacement of shear lag of box girder

in the analysis of shear lag of prestressed concrete box girder based on the energy variational method, there exists two viewpoints of the assumption of the flange longitudinal displacement :
1)U(x,y)=hi[w'(x)+f(x)u(x)];
2)U(x,y)=hi[-w'(x)+f(x)u(x)];