In respect to H-shaped steel stiffened by concrete, the followings have been comprehended from examining plastic mechanical behavior of stub columns under compression. 1. Concrete-crash does not much influence the stiffening effect if b/t_f is relatively small. 2. The stiffening effect can be examined by the proposed analytical method including confinement of out-of-plane deformation of the web. 3. Collapse mechanism is influenced by the web stretching according the flange local buckling. 4. Plastic deformation capacity can be quantitatively evaluated by α(=√<(t_f/b)・(t_w/d)> derived from the proposed collapse model. 5. From the evaluation equation, plastic deformation capacity is much affected by d/t_w as b/t_f is larger.
