Influence of Local Inertia on Seismic Response of Offshore Jackets

Abstract

The effect of brace local inertia (LI) on seismic response of short to medium rise diagonally braced offshore jackets is studied using conventional beam-column finite elements. A range of variables including brace slenderness and structural periods in the practical range and seismic input direction are considered. The results show reductions in the global ductility demand due to LI arising from changes in the brace axial load-displacement hysteresis. This is further scrutinized by analyzing isolated braces under conditions experienced within vibrating jackets and by including a wider range of parameters. Initial increases in the buckling capacity and the overall improvement in energy absorption of brace hysteresis are attributed to “delayed hysteretic behavior” that depends on the fundamental period of brace lateral vibrations and the vibrating period of structure. The procedures for incorporating LI in phenomenological modeling practice are also examined and are found to be unrealistic, particularly with an increase in brace slenderness.