New optimum distribution of lateral strength of shear-type buildings for uniform damage

The seismic design of conventional frame structures is meant to enhance plastic deformations at beam ends and prevent yielding in columns. To this end, columns are made stronger than beams. Yet yielding in columns cannot be avoided with the column-to-beam strength ratios (about 1.3) prescribed by seismic codes. Preventing plastic deformations in columns calls for ratios close to 4, which is not feasible for economic reasons. Furthermore, material properties and the rearrangement of geometric shapes inevitably make the distribution of damage among stories uneven. Damage in the i-th story can be characterized as the accumulated plastic strain energy (W-pi) normalized by the product of the story shear force (Q(yi)) and drift (delta(yi)) at yielding. Past studies showed that the distribution of the plastic strain energy dissipation demand, W-pi /EWpj, can be evaluated from the deviation of Q(yi) with respect to an "optimum value" that would make the ratio W-pi/(Q(yi) delta(yi)) -i.e. the damage-equal in all stories. This paper investigates how the soil type and ductility demand affect the optimum lateral strength distribution. New optimum lateral strength distributions are put forth and compared with others proposed in the literature.