This paper provides background material to the loadings section
of the model code recently published by the Society's Discussion Group
on Bridge Design, and presents a preliminary re-evaluation of the design spectra given in the proposed code. The basis for the proposed zoning scheme, in which the present uniform Zone B is replaced by a transition zone, is discussed. Arguments are given underlying the return period coefficients, and the force reduction factor used in generating the inelastic response spectra of the code. It is likely that the design spectra and the values of the other coefficients determining base shear forces will need to be revised as further research results become available; however, the form of the base shear expression, and the loadings section
as a whole, should remain unchanged. Re-evaluated spectra suggest that
the seismic coefficient values given in the proposed code may be too large by about 25 percent in Zone A, and too low by as much as 40 percent in
 Zone C. While the reassessed values should be more reliable than the original ones, they are based on a Japanese attenuation model, which has
not yet been calibrated against New Zealand data. Further research is required to establish an appropriate attenuation model for New Zealand;
 to avoid undue proliferation of design loadings it is preferable to defer revision of the various coefficients in the proposed code until such a
model is available. Until this is done, the proposed spectra should be viewed with caution, particularly in Zone C.