Seismic response of composite frames - II. Calculation of behaviour factors

The response criteria and earthquake ground motions selected in a companion paper are applied to the evaluation of the actual behaviour factors of a number of moment-resisting composite frames designed to the requirements of the structural Eurocodes. These behaviour factors are obtained through the use of nonlinear dynamic analysis by comparing the ground motion intensities at which yield and failure occurs in each frame. The principal features affecting the seismic response of composite frames are identified, in which special emphasis is placed on those features which differ significantly in the response of steel frames. It is concluded that the present design guidance on behaviour factors for composite frames is excessively conservative and improved recommendations are suggested. Other areas in which design guidance needs to be amended to reflect the seismic behaviour of composite frames are also discussed.