Exact truss topology optimization for external loads and friction forces

In replying to a valuable Discussion by Mariano Vazquez Espi, the authors show that the problem of friction forces in general can be handled by the Prager-Rozvany layout theory, and the optimal Michell layout does not always correspond to the maximum value of the static friction force. Moreover, it is explained that discontinuities in the specific cost function can be accommodated by an extended version of the Prager-Shield optimality criteria, which was already demonstrated in the second author's first (1976) book.