In this paper, Prandtl’s membrane analogy has been extended to facilitate the calculation of the properties of typical prestressed concrete sections. This method is particularly useful for computing the St. Venant torsional constant, I, of typical prestressed concrete girders or to compute all the cross-sectional properties of any irregular shaped open cross section. The membrane analogy requires the evaluation of the volume of the fictitious soap bubble. The computation of the volume of the soap bubble based on the membrane analogy is accomplished by numerically solving the governing differential equation (using the finite difference technique) for the ordinates and summing the volumes of each parallelepiped shape under an average ordinate. Application of the method is illustrated for typical bridge girders and building wall assemblies.
