Abstract
Determination of seismic design forces of structures is performed by the building codes usually using response reduction (or behaviour) factors that incorporate indeterminacy and ductility capacity of lateral bearing systems. In this procedure story drifts are checked as a final design step approximately preventing stories from assuming excessive ductility demands, or seismic damage. If this procedure is reversed, a more logical seismic design approach may be developed by starting with a ductility-controlled procedure. It is the incentive of this research in which by using a large number of earthquakes, first nonlinear acceleration spectra are developed for different levels of ductility demand. Then an energy-based modal procedure is developed in which the system ductility demand is distributed between the important vibration modes based on their contribution. Finally, the developed method is applied to seismic design of several buildings selected from both regular and irregular structural systems. Comparison with a sample code design establishes success of the method in developing a more rational seismic design.
Highlights
Design of structures in seismic prone areas has been traditionally performed by determining the equivalent lateral forces as a first step
In calculation of the forces, the plastic indeterminacy and the ductility capacity of the lateral load bearing system of building are inherently taken into account, collectively within a response modification or R factor in most building codes
The ductility demand is the ratio of the maximum displacement Δp to the yield displacement Δy in Figure 1, while both of the mentioned deformations are unknown under a given earthquake
Summary
Design of structures in seismic prone areas has been traditionally performed by determining the equivalent lateral forces as a first step. The R-factor is reduced by the code directly or indirectly in two cases, for important buildings and for buildings with less-than-usual indeterminacy to account approximately for a smaller level of allowable seismic damage. After a first round of member proportioning, the story drifts are checked to see again approximately if the story ductility demand, or seismic damage, is within the accepted limits. In all, this is a procedure that tries to balance the required stiffness/strength of members with their ductility demand, where reduction of one results in increase of the other. The fact that if the story drift is small the member strengths can be reduced without violating the required gravity strength and story drift limit (and the serviceability level earthquake shaking demands, where applicable), is neglected in this procedure
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