Clearly, for a plane, two-bay portal frame we would have six compatibility equations so that the solution would then become laborious further additions to the frame would make a hand method of solution impracticable. In this example there would be three compatibility equations requiring solution, two of translation and one of rotation. 16.23(b) we then have two releases of force and one of moment and the primary structure is, in effect, a cranked cantilever. These may be obtained by removing the foundation at the foot of one of the vertical legs as shown in Fig. Therefore we require 3 releases to obtain the statically determinate primary structure. From Section 16.2 we see that the frame, together with its foundation, form a single two-dimensional ring and is therefore three times statically indeterminate.
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