Displacement demand equations for the non-linear static analysis of short-period masonry structures

This paper discusses the accuracy of four established methods and proposes two improved formulations for calculating earthquake-induced displacement de-mands, to be used for the assessment of masonry structures in conjunction with non-linear static analysis procedures. All predictions make use of codified elastic displacement spectra, which are modified to account for inelastic effects follow-ing different approaches. Focus is placed on systems with fundamental periods between 0.1 and 0.5 s, for which the inelastic seismic displacement amplification is usually more pronounced. The accuracy of the predictive equations is assessed based on the results from nonlinear time-history analyses, carried out on single-degree-of-freedom oscillators with hysteretic force-displacement relationships representative of masonry structures; the same analyses are also used to calibrate the proposed formulations. Two independent sets of ground-motion records are employed to evaluate the methods under tectonic and induced-seismicity scenari-os. First, the study demonstrates some limitations of two established approaches based on the equivalent linearization concept: the capacity-spectrum method of the Dutch guidelines NPR 9998-18, and its version outlined in FEMA 440, both of which overpredict the maximum displacement. A non-iterative procedure, making use of an equivalent linear system with an optimal stiffness, is then pro-posed to overcome some of the problems identified for the two established meth-ods. Two codified formulations relying on inelastic displacement spectra are also evaluated: the N2 method of Eurocode 8 and the displacement-coefficient meth-od of ASCE 41-17. The former proves to be significantly unconservative, while the latter is affected by excessive dispersion. A recently developed modified N2 formulation is shown to improve the accuracy while limiting the dispersion of the predictions for both suites of ground motions.