Displacement-Based Seismic Design of Frames At Soft Soils of the Valley of Mexico Considering P-Delta Effects

Abstract eng:
The amplification of demands of a structure subjected to lateral displacements due to the action of vertical loads over its deformed shape, the so-called second order effects or P-Delta, is an issue that has received considerable attention in earthquake engineering for many years, as it has been recognized that it may lead to undesirable structural behaviour and even collapse. To this date, the most extended approach followed in several national and international building codes to consider P-Delta effects in seismic design are based on either the amplification of demands via factors which are the product of extrapolation of elastic behavior under monotonic loading, or by the direct use of second order elastic analysis. Several studies show that such approach is not consistent with actual behavior of inelastic structures subjected to earthquake induced ground motions. Moreover, as it is well known, several displacement based methods aimed at performance oriented design applications that rely on a more rational basis than code prescribed force-based design have been developed in the last two decades. However, few efforts have been carried out towards the development of criteria to account for P-Delta effects in this type of methods, and some of them rely on similar assumptions to that of the aforementioned approach. For this reason, the authors of this paper proposed a new design method that allows the design of structures with a P-Delta induced instability condition for either a near-collapse limit state, i.e., exceedance of a code prescribed interstorey drift, or a sidesway-collapse limit state, i.e., failure due to dynamic instability. The method proposed was presented in a previous paper where it was shown that it provided good results when used for the design of framed structures subjected to far-field ground motions of hard soil sites. It is the purpose of the present paper to show the validity of the method proposed to design structures subjected to seismic demands characteristic of soft soil sites in the Valley of Mexico. It illustrates an overview of the response of unstable SDOF systems due to P-Delta effects subjected to such type of demands, and the results of design applications of the method proposed of 8-, 12-, 16- and 20-storey frames, along with the results of incremental dynamic analysis of such frames. The seismic demands considered were a set of 100 real earthquake ground motions recorded at a few soft soil sites of Mexico City. From the comparison of the expected performance vs. that attained from incremental dynamic analysis, the authors conclude that the method proposed allows the design of instability prone structures with severe P-Delta effects subjected to demands characteristic of the lake-bed zone of the Mexico Valley for either near-collapse or sidesway-collapse limit states.

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