A reliability function β(Q) is defined as the ratio of the expected value of the natural logarithm of the safety margin of the structural system divided by its standard deviation. The safety margin is the minimum value of the ratio of the story deformation capacity to the corresponding response-associated demand, and Q is the peak ductility demand of a simplified reference system subjected to the design earthquake. A discussion is presented about the limitations of available criteria and knowledge applicable to the determination of the story deformation capacities. An exploratory study is made about the influence of several mechanical properties, and of their spatial distribution throughout the system, on the values of β(Q) for systems subjected to sets of narrow-band earthquake accelerograms similar to those recorded on soft soil in Mexico City. For the systems studied in the paper, a linearly decreasing relation is found between β and the natural logarithm of Q. The parameters of that relation are not sensitive to the fundamental period of the system for frames whose nonlinear behavior occurs concentrated at plastic hinges with bilinear constitutive moment-curvature functions. Dependence with period is found for more general types of constitutive functions.
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