TABLE OF CONTENTS In Section 4.4 we briefly discussed the statical indeterminacy of trusses and established a condition, not always applicable, for a truss to be stable and statically determinate. This condition, which related the number of members and the number of joints, did not involve the support reactions which themselves could be either statically determinate or indeterminate. The condition was therefore one of internal statical determinacy; clearly the determinacy, or otherwise, of the support reactions is one of external statical determinacy.
Consider the portal frame shown in Fig. 16.1. The frame carries loads, P and W, in its own plane so that the system is two-dimensional. Since the vertical members AB and FD of the frame are fixed at A and F, the applied loads will generate a total of six reactions of force and moment as shown. For a two-dimensional system there are three possible equations of statical equilibrium (Eq. (2.10)) so that the frame is externally statically indeterminate to the third degree. The situation is not improved by taking a section through one of the members since this procedure, although eliminating one of the sets of reactive forces, would introduce three internal stress resultants. If, however, three of the support reactions were known or, alternatively, if the three internal stress resultants were known, the remaining three unknowns could be determined from the equations of statical equilibrium and the solution completed.
A different...
