The article deals with the calculation of plane frames each bar of which is able to withstand flexion perpendicularly to the main plane. After having shown that the stress corresponding to the loads parallel to that plane is independent of the stress corresponding to the perpendicular loads, the author develops for this latter a method of resolution based on Cross’ principle. Fictitious supports are added to each knot and then successively removed, till the structure reaches the true form corresponding to the loading. A very important improvement of the original method—formerly not applied to a three-dimensional construction— consists in the use of adjustments, that is appropriate movements of parts of the structure as a whole.

The method proved to have many advantages: Difficulties independent of the degree of redundancy, no loss of accuracy between the beginning and the end of calculations, large number of verifications in the course of resolution, reliability of the results, easy understanding of the successive steps, possibility of simultaneous work for many contributors, and relative shortness of the amount of work required.