Determination of seismic design forces by equivalent static load method

In the proposed 2005 edition of the National Building Code of Canada (NBCC), the seismic hazard will be represented by uniform hazard spectra corresponding to a 2% probability of being exceeded in 50 years. The seismic design base shear for use in an equivalent static load method of design will be obtained from the uniform hazard spectrum for the site corresponding to the first mode period of the building. Because this procedure ignores the effect of higher modes, the base shear so derived must be suitably adjusted. A procedure for deriving the base shear adjustment factors for different types of structural systems is described and the adjustment factor values proposed for the 2005 NBCC are presented. The adjusted base shear will be distributed across the height of the building in accordance with the provisions in the current version of the code. Since the code-specified distribution is primarily based on the first mode vibration shape, it leads to an overestimation of the overturning moments, which should therefore be suitably adjusted. Adjustment factors that must be applied to the overturning moments at the base and across the height are derived for different structural shapes, and the empirical values for use in the 2005 NBCC are presented.