Discussion of "Making use of brace overstrength to improve the seismic response of multistorey split-X concentrically braced steel frames"

Abstract

Redwood and Paultre 687 The authors have pointed out inconsistencies among the design and analysis assumptions used in Redwood et al. (1991). These inconsistencies originated from the collaboration among the first two authors of this reference, Redwood and Lu. In the paper under discussion, the first series of structures studied (the 2 to 12 storey L, R, and S buildings in Victoria and Quebec City) were designed without inclusion of P-delta effects that were included in the subsequent analyses. As pointed out in the paper, the lower base shear forces mandated by the 2005 National Building Code of Canada (NBCC) (NRCC 2005) compared with those in the 1990 NBCC (NRCC 1990) make P-delta effects more critical than earlier requirements. These forces are not insignificant. For example, the paper indicates that for the eight-storey L and R buildings in Victoria, the P-delta shear forces in the lowest storey would have increased the design shear forces by 20% and 30%, respectively. The enhanced storey ductility redistribution capacity (EDSRC) designed frames did have P-delta effects included; therefore, the improved performance of these buildings compared with the first series must be attributed to both the EDSRC approach and the impact of the P-delta forces included in these designs. The significant difference in design base shear forces as reported by the authors for the 8-storey split-X braced frame in Victoria (Vf/W is equal to 0.089 for NBCC 1990 (NRCC 1990) and 0.043 for NBCC 2005 (NRCC 2005)) can be partly explained by the difference in soil properties used in the two studies. Redwood et al. (1991) used a foundation factor of 1.0 corresponding to rock, dense and very dense coarse-grained soils, very stiff and hard fine-grained soils, etc., whereas the authors used accelerationand velocitybased site coefficients Fa = 1.0 and Fv = 0.78, and Fa = 0.836 and Fv = 0.640, corresponding to site class B for Victoria and Quebec City, respectively. However, site class C would appear to better represent the conditions used by Redwood et al. (1991). The difference is significant, as can be seen from the values of the design spectral acceleration function S(T) in the following table (NRCC 2005):