This investigation includes the determination of the elastic critical summation of loading of
different varieties of portal and box frames resting on elastic foundations and subjected to
unsymmetrical loading. The critical buckling loads of these frames are obtained for practical
values of the stiffness of the elastic springs. In addition, the effect of the stiffness of the elastic
foundations on the elastic stability of these frames is thoroughly examined by changing the
stiffness of the elastic springs. Different families of curves are obtained to represent the
relationship between the critical buckling loads and the parameters affecting it. For
unsymmetrical loading P1 & P2, The elastic critical sum of loading is found to be nearly
constant and is irrespective of the load ratio p1/p2
even for the most severe case when
P = zero 1 and the whole load 2 P acts on the other column. In general, there was a complete
analogy between frames subjected to unsymmetrical loads and resting on elastic springs and
those resting on rigid supports for both portal and box frames.