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.Key words: uniform hazard spectrum, seismic design base shear, equivalent static load procedure, higher mode effects, base shear adjustment factors, distribution of base shear, overturning moment adjustment factors.
In a building structure subjected to seismic forces, the gravity loads acting through the lateral displacements lead to additional shears and moments. This is generally referred to as the P–Δ effect; it tends to reduce the capacity of the structure to resist the seismic forces and may lead to instability. It has been suggested that an increase in structural strength, in stiffness, or in both would mitigate the P–Δ effect and ensure stability of the structure. It is shown here that instability results when the P–Δ effect causes the stiffness of the structure to become negative in the post-yield range, in which case increasing the strength, the stiffness, or both does not ensure stability. In a single-storey structure, stability can be ensured if there is sufficient strain hardening that the post-yield stiffness is positive even in the presence of the P–Δ effect. For a multistorey building the vulnerability of the structure to P–Δ instability can be judged by obtaining a pushover curve. It is shown that as long as the maximum displacement produced by the design earthquake lies in the region of positive slope of the pushover curve, the structure will remain stable.Key words: seismic response, P–Δ effect, dynamic instability, stability coefficient, amplification factor, pushover analysis, nonlinear analysis.
The current study investigates the performance of concrete incorporating ground granulated blast-furnace (GGBF) slag in the presence of colloidal nano-silica. A control group of concrete mixtures is compared with a group of mixtures with 50% slag replacement, with each group examined at two different ratios of colloidal nano-silica (3% and 6% of the total cementitious material). Subsequently, the relative performance of the two groups is compared with ordinary Portland cement concrete in relation to strength and durability properties. Evaluation included experimental examination of compressive and tensile strength, rapid chloride penetration, and porosimetry using mercury intrusion tests. Furthermore, the microstructure of the cementitious matrix was evaluated using scanning electron microscopy imaging. Results of the tested concrete mixtures indicated that nano-silica particles can improve the properties of concrete containing GGBF slag. Improvement in high early strength as well as reduction in permeability are observed. Furthermore, nano-silica caused a refinement of the pore structure and an improvement to the interfacial transition zone (ITZ) as seen through mercury intrusion porosimetry (MIP) results and scanning electron microscopy imaging, respectively.
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