With　shaking　table　tests　of　steel　braced　concrete　frames,　an　issue　of　suddenly　increasing　torsion　duo　to　steel　brace　buckling　or　other　elements　damage,　i.　e.　inelastic　torsion,　is　studied.　Test　results　indicate　that　steel　brace　buckling　or　failure　would　be　the　main　cause　of　the　inelastic　torsion　in　this　kind　of　buildings.　Based　on　the　test　results,　the　mechanism　of　the　suddenly　increasing　inelastic　torsion　is　studied,　some　FEM　models　of　calculation　examples　with　preseting　occasional　eccentricity　are　established,　and　nonlinear　analysis　are　conducted.　With　the　analytical　models,　the　sudden　increasing　torsion　issue　is　verified　still　existed　while　the　number　of　elements　in　symmetric　structure　is　increased　but　the　brace　number　remains　unchanged.　It　is　cleared　that　even　if　it　is　a　symmetrical　structure,　both　routings　of　brace　buckling　or　its　re-straightening　will　induce　the　inelastic　torsion　and　its　sudden　increase.　Although　there　are　inelastic　torsion　in　pure　concrete　frames,　but　it　perform　a　smaller　torsional　amplitude　increment　than　steel　braced　one　under　same　loading　and　eccentricity　conditions,　and　not　only　tests　but　also　analytical　models　reveal　the　similar　phenomenon.　With　installed　buckling-restrained　braces(BRB)in　concrete　frames,　it　has　been　proved　significantly　effective　to　resist　nonlinear　torsion,　superior　to　steel　braced　concrete　frame　or　pure　concrete　frame.　There　are　suggestions　to　structural　engineers　as　follows:　installing　BRB　is　suggested　to　effectively　minimize　and　control　the　inelastic　torsion　of　concrete　frames,　it　is　recommended　that　a　non-linear　FEM　analysis　with　preset　occasional　floor　mass　eccentricity　to　get　inelastic　torsion　under　strong　earthquakes,　and　the　maximum　torsional　response　should　be　limited　to　a　reasonable　value　in　seismic　design　stage.　©　2018,　Editorial　Office　of　Journal　of　Building　Structures.　All　right　reserved.