From whoever's derivation you look to for bends Von Karman, Beskin, Vignesse and Pardue. The bend flexibility is due to it's ovalising and the stress concentration comes from the shell stresses associated with this. Stick a trunnion on it and it cannot ovalise. In this case the flex factor is one as for straight pipe but also those associated shell stresses will not be there. I surmise Winkler's curved beam theory should apply. I once calculated the SIF for this and it was around 1.03. Of course the trunnion is taking load so there is a local load effect with a SIF since the trunnion physically connects with only a part of the bend cross section. I presume the Hankin paper referred to in the Caesar manual correctly gives the SIF for this which should be quite localised. I suppose the bend could be split in two by the trunnion intersection and assumed to be stiffened in each part by an imaginary flange so allowing restricted ovality and accompanying flexibility and associated shell stresses elsewhere in the two parts of the bend.

Michael