General report on local ductility

Abstract

A series of factors contributes to the poor local ductility of steel structures, which was observed during recent earthquakes in the USA and Japan. These factors are explained. The factors on the resistance side are the discrepancies between real and design yield stress, the value of through thickness resistance of steel, the need for requirements on toughness of the base and weld material and the effect of strain rate. On the action effect side, other factors contributed to a bad ductility: the past underestimates of needed plastic rotations, the existence of 3D stress states created in welded connections of high beams, the consideration of wrong stress distributions in beam ends, a bad design of connections and the influence of the composite character of beams. The recent trends aiming at an improved local ductility are explained. They consist in forcing the development of plastic hinges in beams of moment frames to take place away from the columns, by either a connection strengthening or a beam weakening. Qualified connections are being developed. The other connections should go through a severe qualification process. However, future research work could relax this request by developing explicit requirements on the connections themselves.