Discussion of "Overview of seismic provisions of the proposed 2005 edition of the National Building Code of Canada"

Abstract

First, do we really know what the forces are? After all, if the input is dubious, no amount of sophisticated analysis will produce an accurate result, i.e., garbage in – garbage out. The open files (Adams et al. 1996, 1999) on the seismicity in Canada by the Geological Survey of Canada show very different results from the values in the current code. Comparing the spectra (Adams et al. 1996) for Prince Rupert, Tofino, and Queen Charlotte City to Vancouver, the spectra for the first two are less than Vancouver over the entire range of periods and Queen Charlotte City is only higher in the long period range, i.e., greater than 1.0 s. Under the current code Prince Rupert and Tofino are in Zone 5 and Queen Charlotte City is in Zone 6, while Vancouver is only Zone 4. This means in a relative sense that the forces are changing by more than 50% in many cases from the current code. The recent charts (Adams et al. 1999) show that the spectra for Tofino is now higher than that for Vancouver, so the values have changed significantly in just 3 to 4 years. In the case of Comox, the proposed values have changed from the current code in a relative sense to Vancouver by a factor of three. Do we really know what the seismic forces are? What are the costs of precision, and are they justified? The most obvious cost is the extra effort required to design a building. For some buildings, the owner might actually benefit by lower building costs, but in many cases costs will increase. Finally, does it make sense to spend significantly more resources on design and analysis when the level of site supervision on many jobs is minimal?