Reply to the discussion by R.D. Watts on "Overview of seismic provisions of the proposed 2005 edition of the National Building Code of Canada"

Abstract

The discussion by R. Watts raises several questions that are concerned with the changes in seismic force levels and the increased complexity in the proposed 2005 seismic provisions. Watts uses examples of large changes in the seismic hazard parameters to question the validity of the design forces themselves. The reasons for such large changes are due primarily to better knowledge of the geographical distribution of seismic hazard and improved methodology for estimating that hazard, as described by Adams et al. (1999). While the nature of earthquakes and their effects as well as the limited amount of historical data result in considerable uncertainty, the median values of seismic hazard parameters, which are used in the proposed 2005 seismic provisions for the determination of design forces, are robust values that are expected to remain stable over some period of time. Of course, the experience of future earthquakes (locations, magnitudes, and resulting ground motions) will have an effect on estimates of seismic hazard that will be calculated in years to come. Given the potentially catastrophic effects of large earthquake ground motions, the design of buildings must take these rare events into account, which is precisely the approach taken in treating earthquake loads in the National Building Code of Canada. Watts also raises several questions arising from the increased complexity of the proposed 2005 seismic provisions. The question of the justification of increased building design and construction costs are certainly relevant to the development of seismic design provisions in a building code. The cost associated with increased design effort is, in the author’s view, largely one of educating designers to enable them to use the code easily and effectively as a design tool. The increase in the total cost of a building, even in the cases in which the seismic design forces have increased significantly, is likely to be relatively small. Both costs are certainly justified in the interests of improved life safety to the occupants of buildings; significant increases in forces arise primarily because the improved knowledge of seismic hazard has demonstrated that previous seismic hazard estimates were too low. Watts’questions and comments on code enforcement and site supervision are not issues that can or should be addressed in a seismic design code, nor are they germaine to the content of the paper.