Jorge Ruiz-García

Influence of stiffness degradation on strength demands of structures built on soft soil sites

The effect of stiffness degradation on the lateral strength demands of inelastic single-degree-of-freedom systems subjected to soft soil records is investigated. The modified-Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastic-perfectly-plastic model is used to represent non-degrading structures. The study is based on two sets of ground motions. The first set includes 100 accelerograms recorded in various recent earthquakes on soft soil sites of Mexico City. The second set includes 16 ground motions recorded on bay mud sites in the San Francisco Bay Area during the 1989 Loma Prieta earthquake. A special emphasis is placed on the effect of stiffness degradation on strength reduction factors that permit the estimation of inelastic strength demands from elastic strength demands. Mean ratios of lateral strength demands of stiffness degrading systems to lateral strength demands of non-degrading systems for both sets of ground motions are presented. It is concluded that structures with stiffness degradation, and with periods of vibration shorter than the predominant period of vibration of the ground motion, can experience lateral strength demands larger than those of non-degrading structures in the same period range. Whereas for structures with periods of vibration equal or longer than the predominant period of the ground motion, the lateral strength demands are typically smaller for stiffness-degrading structures than those of non-degrading structures.

Inelastic Displacement Ratios for Seismic Assessment of Structures Subjected to Forward-Directivity Near-Fault Ground Motions

This article presents results of a statistical study focused on evaluating inelastic displacement ratios (i.e., ratio of maximum inelastic displacement with respect to maximum elastic displacement demand) of degrading and non degrading single-degree-of-freedom (SDOF) systems subjected to forward-directivity near-fault ground motions. CR spectra are computed for normalized periods of vibration with respect to the predominant period of the ground motion to provide a better ground motion characterization. This period normalization allows reducing the record-to-record variability in the estimation of CR. An equation to obtain estimates of CR for the seismic assessment of structures exposed to forward-directivity near-fault ground motions is proposed.

Mainshock-Aftershock Ground Motion Features and Their Influence in Building's Seismic Response

This article presents the results of examining ground motion characteristics in 184 real mainshock-aftershock earthquake ground motions. It is shown that the predominant period (a measure of the frequency content) of the set of mainshocks tends to be longer than that of the corresponding aftershocks. It is highlighted that the response of structures under artificial sequences is very different from that of real sequences, particularly when the approach of repeating the real mainshock with identical ground motion features as an artificial aftershock is employed. It is also demonstrated that the predominant period of the aftershock significantly influences the post-mainshock response.

Displacement-Based Seismic Assessment of Low-Height Confined Masonry Buildings

This paper presents a practical displacement-based evaluation procedure for the seismic assessment of low-height regular confined masonry buildings. First, the so-called Coefficient Method established in several FEMA documents is adapted to obtain rapid estimates of inelastic roof displacement demands for regular confined masonry buildings. For that purpose, a statistical study of constant relative strength inelastic displacement ratios of single-degree-of-freedom systems representing confined masonry buildings is carried out. Second, a nonlinear simplified model is introduced to perform pushover analysis of regular confined masonry buildings whose global and local behavior is dominated by shear deformations in the masonry walls. The model, which can be applied through the use of commercial software, can be used to establish the capacity curve of such buildings. Finally, the evaluation procedure is applied to a three-story building tested at a shaking table testing facility.

A Simplified Drift-Based Assessment Procedure for Regular Confined Masonry Buildings in Seismic Regions

This article presents a simplified procedure for assessing the seismic performance of existing low-to-medium rise confined masonry (CM) buildings, which are a typical construction type in Latin-America. The procedure consists of the estimation of the peak roof and first-story inelastic drift demand of CM buildings. The expected peak inelastic displacement demand is related to drift-based fragility curves, which express the probability of being or exceeding two key damage states in the masonry panels, developed from a relatively large experimental database. The proposed procedure could be very useful for obtaining rapid estimates of expected performance during future earthquake events and for assessing the seismic vulnerability of regular confined masonry structures.

Flexible Frames as Self-Centering Mechanism for Buildings Having Buckling-Restrained Braces

Buckling-restrained braces have been found to be an efficient way to provide seismic-resistance to buildings. In spite of the many structural advantages they offer, several studies have found a lack of ability of buckling-restrained braces to promote a self-centering behavior. Under this circumstance, it has been suggested that the use of this type of braces should be complemented with robust moment-resisting frames with the purpose of reducing residual (permanent) drifts at the end of the main-shock excitation, and thus, the vulnerability of braced frames to aftershocks. Unlike previous studies, the results presented in this paper demonstrate that design strategies can be developed to achieve adequate self-centering behavior of buildings whose structural system is composed of flexible moment-resisting frames and buckling-restrained braces. Particularly, it is concluded that if the flexible frames provide at least one-sixth of the lateral stiffness of the dual structural system while remaining practically undamaged (operational) after the ground motion, the system will exhibit adequate self-centering behavior in spite of the fact that the bracing system may develop significant plastic behavior.

Evaluation of Coefficient Method for Seismic Assessment of Existing Buildings Built on Soft Soil Sites

This paper presents the results of an ongoing investigation aimed at evaluating the Coefficient Method (CM) for the seismic assessment of existing buildings built on soft soil conditions. In the first part of this investigation, error statistics of target roof displacement demand predicted by the CM with respect to peak inelastic roof displacement demand of two frame models representative of exterior steel momentresisting frames when subjected to 18 earthquake ground motion recorded on soft soil sites found in the San Francisco Bay Area were obtained. In the second part of this study, a statistical study of the ratio of peak inelastic displacement demand to peak elastic displacement demand (i.e. coefficient �� 1) of elastoplastic single-degree-offreedom systems when subjected to 118 soft-soil records is reported. Error statistics of coefficient �� 1 were also obtained from the statistical results. A functional form that takes into account the frequency content of the ground motion is proposed to estimate coefficient �� 1 when evaluating existing buildings built on soft soil sites.