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Soil response analyses require that the soil strata involved be characterized in terms of their dynamic material properties. Two important material properties are the shear modulus and a measure of energy dissipation characteristics, usually expressed as the damping ratio. Both can be obtained from standard laboratory tests, most commonly resonant columns or cyclic triaxial devices which provide strain-dependent values. Alternative experimental procedures are now available to estimate shear moduli at very small strains from piezoelectric transducer tests (bender elements); procedures have also been put forth to obtain the energy dissipation properties from this test. In this paper, we present a modified version of the spectral-ratio method using the wavelet transform to measure the damping ratio at small strains from waveforms acquired from bender element tests. Synthetic bender element signals were first used to test the procedure and validate the approach. Then, we analyzed signals obtained from a large-size oedometer equipped with bender elements on very soft Texcoco Clays (near Mexico City), and the waveforms were propagated along different paths. Our results show that the range of values of D min obtained from bender element tests agrees well with those obtained from resonant column tests performed on very soft clayey soil specimens retrieved from the same area at different depths. • This research is aimed to study the damping ratio on very soft soil Texcoco Clays (near Mexico City) using a large-size oedometer equipped with piezoelectric transducers. • We propose a modified version of the spectral ratio method for assessing the damping ratio at very small strains (elastic behavior). • It is calculated synthetic waveforms which input data are similar to the soil deposits present in the area where the specimens were retrieved. • We compare the data obtained with the modified version of the spectral ratio concerning to resonant column tests. [ABSTRACT FROM AUTHOR]

In this paper, the authors propose and validate a code-oriented, resilient design methodology for reinforced concrete buildings with intermediate moment frames (RC-IMFs) and hysteretic energy dissipation devices (HEDDs) mounted in chevron steel bracing design as structural fuses, considering soil-structure effects. The design methodology is based on global lateral stiffness balances among different structural components (frames, bracings and HEDDs). Global seismic design parameters were specifically assessed for such stiffness ratios through extensive parametric studies for the subject structural system under study. In order to achieve an immediate occupancy seismic performance, the design sequence using capacity-design principles is clearly established and emphasized for the design of main structural elements. Soil-structure interaction (SSI) effects are included in the formal design process following the recommendations available in the seismic code for Mexico City. In order to fully test the efficacy of the proposed design methodology under a demanding seismic scenario, three buildings of 8, 15 and 24 stories were designed in soft soil sites of Mexico City close to resonant responses. Different types or HEDDs were considered and evaluated. Pushover analyses were performed in order to verify that subject buildings may achieve the objective resilient deformation mechanism (immediate occupancy) at the maximum useful ductility capacity assumed for the HEDDs. Nonlinear dynamic analyses were performed for the complete 3D designs according to the code under the action of eight pairs of acceleration records, representative of the site where the buildings were located. This was done in order to assess maximum inelastic demands, residual drifts, and fully verify and validate the proposed resilient, structural fuse code-oriented design. From the obtained results, it is confirmed that a resilient structural performance is feasible for RC-IMFs with HEDDs under the proposed code-oriented design methodology. • A code-oriented, resilient seismic design procedure for RC-IMRFs with HEEDs is presented. • An immediate occupancy category is achieved for RC-IMRFs with HEEDs for the MCE scenario. • SSI effects were included in the design procedure and evaluated with real soft soil properties of Mexico City. • Peak displacements and base shears of buildings were amplified (up to 21%) when considering SSI effects. • Practical values for assessed global design parameters are proposed for their insertion in Mexican and international codes. [ABSTRACT FROM AUTHOR]

In this paper, the authors discuss the extensive structural pounding observed in Mexico City during the September 19, 2017 earthquake. A critical review of the recommendations available for required separations between adjacent buildings in the seismic codes of Mexico City since 1966 is offered. The reasons why these recommendations have not been effectively enforced since then are also addressed. A parametric study was conducted to understand better the structural pounding phenomena for strong ground motions which have been recorded in soft soils in Mexico City during the 1985 and 2017 earthquakes. For this purpose, equivalent frame models for existing reinforced concrete and structural steel flatslab buildings with nonlinear pounding contacts were used. Some well-known proposed equations for building separations were evaluated based upon the results of these parametric studies for pounding. The evaluated equations were the ABS, SRSS and DDC rules, as well as the absolute difference combination (ADC) rule proposed by the authors. From the obtained results, it was found that the minimum building separation between adjacent buildings established in the seismic code of Mexico City should be raised to avoid strong pounding on soft soils. Also, it was found that both the DDC and ADC rules are promising in defining building separations closer to the one obtained from dynamic simulations. These equations are less conservative than the version of the absolute sum rule currently established in Mexican seismic codes. • 98.9% of the observed pounding in Mexico City occurred in the soft soils gained to the former lakes. • The height difference between adjacent structures where more pounding was observed was ±4 stories. • Gap distances required to avoid pounding in the soft soils of Mexico City could be as large as 170 cm. • The minimum distance between adjacent structures to enforce for the soft soils of Mexico City should be raised to 15 cm. • The DDC and the ADC rules could be used with confidence to assess gap distances to avoid pounding in soft soils. [ABSTRACT FROM AUTHOR]

Weak first-story buildings are susceptible to experience severe structural damage, and even collapse, during strong earthquakes as was evidenced during the September 19, 2017 (M w = 7.1) intraslab earthquake that hit Mexico City. Such type of buildings are prone to exhibit a negative slope in their capacity curve due to weak first-story mechanism that might lead to dynamic instability during earthquake excitation. Therefore, this paper presents the main results of an investigation focused on evaluating the collapse strength ratios, R c (i.e., the normalized lateral strength ratio in the onset of dynamic instability) of single-degree-of-freedom (SDOF) systems whose force-displacement nonlinear envelope (backbone) behavior is representative of that observed in existing multistory weak first-story buildings. Particularly, their envelope exhibit several levels of negative slope after a post-yield displacement ductility capacity. Median and dispersion of R c ratios were obtained for degrading SDOF systems subjected to a set of 136 earthquake ground motions recorded in soft soil sites of Mexico City from instraslab earthquakes. Results of this investigation showed that R c ratios depend on the normalized period of vibration with respect to the predominant period of the ground motion, T/T g , the displacement ductility capacity, μ c , and the negative slope after peak strength, α c. Additionally, the spectral trend of R c ratios at soft soil sites is different to that reported for firm soil sites. A continuous functional form is introduced to obtain estimates of median R c ratios for the preliminary seismic assessment of weak first-story buildings in this seismic environment. • Dynamic instability in buildings can be evaluated through collapse strength ratios Rc. • Rc ratios follows a different spectral trend for soft soil sites than for firm sites. • Displacement ductility capacity of structures is beneficial to increase Rc. • Robust equation to estimate median Rc is proposed based on statistical results. [ABSTRACT FROM AUTHOR]