Sonia E. Ruiz

Influence of ground motion intensity on the effectiveness of tuned mass dampers

The effectiveness of Tuned Mass Dampers (TMD) on buildings subjected to moderate and high-intensity motions is analysed. First, the response of a 22-storey four-bay reinforced concrete non-linear frame with a TMD is studied for motions with different intensities. Several values of the relevant parameters are assumed in the analyses. Then, equivalent single-degree-of-freedom systems with TMDs and without them are defined and analysed under the action of ground motions with intensities associated with different return intervals at the site where the structures are located. Vulnerability curves for the systems are obtained based on the probabilities of reaching two different performance limit states. The expected annual rate of exceedance of each limit state is calculated. The results show that the effectiveness of TMDs is higher for systems with small non-linearity produced by small and moderate earthquakes, than for systems with high non-linear behaviour, generally associated with high-intensity motions. Some recommendations about the applicability of TMD are given. Copyright

Evaluation of Structural Reliability of Steel Frames: Interstory Drift versus Plastic Hysteretic Energy

The structural reliability in terms of maximum interstory drift—and, alternatively, in terms of plastic hysteretic energy—is evaluated for six regular moment-resisting steel frames designed according to the Mexico City Building Code, and located in the Lake Zone of that city. While the maximum interstory drift was used because of its relevance within the format of current seismic design codes, the plastic hysteretic energy was considered due to its importance for the performance of structures when subjected to severe cumulative plastic deformation demands. The demand hazard curves of the frames in terms of drift and energy are compared to provide a general idea of the reliability levels associated to the models, and to provide insights into which response parameter dominates their dynamic behavior and structural performance. In some cases, large differences are observed in the reliabilities computed by measure of the two different response parameters under consideration.

Reliability index for offshore piles subjected to bending

Abstract A first order second moment formulation is developed for analyzing the reliability of individual cross sections of piles driven on soft soils and subjected to random lateral static loads. The soil properties are expressed in terms of random p–y curves for a set of non-linear Winkler springs. The stiffness of the pile is taken as deterministic, but uncertainty about bending strength is accounted for. An algorithm is proposed and applied for determining probabilistic second moment estimates of internal forces and lateral displacements of a pile, taking into account both non-linear behavior and uncertainty about the parameters of the p–y curves. The cases studied include hypothetical piles driven on identical soft clays, and the loads correspond to the design conditions representative of several locations throughout the world. Very wide variations are found for the reliability index β of piles designed in accordance with ordinarily accepted regulations.

Simplified closed-form expressions for the mean failure rate of structures considering structural deterioration

Two closed-form mathematical expressions are presented for the evaluation of the mean annual failure rate of deteriorating structures at the end of a given time interval. It is assumed that the structural capacity deteriorates linearly in time in both expressions. The assumption that the structural demand varies as a function of time is added to one of them. The approach is based on the probabilistic seismic demand hazard analysis used in SAC-FEMA project. The expressions are applied to a 10-storey building. The difference in expected number of failures after 150 years of constructing the building results about 16.3% higher with respect to the case when the structural deterioration is neglected. Attention is called to the importance of considering not only the time variation of the structural capacity, but also that of the structural demand in the evaluation of time-dependent failure probabilities of deteriorating structures.

Performance‐Based Design Approach for Seismic Rehabilitation of Buildings with Displacement‐Dependent Dissipators

A performance-based approach for seismic retrofitting of buildings with energy dissipating devices is presented. The approach may be seen as an algorithm useful for converging to a preliminary design of a system to be analyzed according to the time history approach recommended in the NEHRP Guidelines for the Seismic Rehabilitation of Buildings (FEMA 273). The algorithm is based on the analysis of equivalent single-degree-of-freedom models with added parallel elements that represent the dissipating devices. The combined systems are analyzed under sets of accelerograms associated with different return intervals. The acceptance criteria are intended to control the peak drift of the rehabilitated structure and the maximum ductility demand of the dissipating devices. It is required that these maximum structural responses, for a given seismic intensity, be equal to or smaller than those associated with a given probability of exceedance. The approach is successfully applied to a ten-story, three-bay frame rehabilitated with U-shaped steel dissipators.

Design Algorithm Based on Probabilistic Seismic Demands for Buildings Rehabilitated with Hysteretic Energy- Dissipating Devices

A design algorithm based on a probabilistic seismic demand analysis is proposed for the rehabilitation of buildings with hysteretic energy dissipating devices (EDDs). The acceptance conditions imposed are referred to the maximum story drift developed by the building under rehabilitation, and to the maximum ductility demanded by the EDDs. The algorithm is successfully applied to a ten-story three-bay reinforced concrete building that is rehabilitated with steel U-type EDDs.

Calibration of the equivalent linearization gaussian approach applied to simple hysteretic systems subjected to narrow band seismic motions

A calibration of the equivalent linearization approach (EL) applied to simple hysteretic systems subjected to narrow band seismic motions is reported. The results obtained through this method are compared with those derived using the Monte Carlo simulation technique. An evaluation is presented of the influence of the ductility demand of the system, as well as of the degradation parameters, hardening ratio and smoothness of the hysteretic behavior on the accuracy of the EL approach. The computational efficiency of the approach is estimated for the cases analyzed.

A New Spectral Shape-Based Record Selection Approach Using and Genetic Algorithms

With the aim to improve code-based real records selection criteria, an approach inspired in a parameter proxy of spectral shape, named , is analyzed. The procedure is based on several objectives aimed to minimize the record-to-record variability of the ground motions selected for seismic structural assessment. In order to select the best ground motion set of records to be used as an input for nonlinear dynamic analysis, an optimization approach is applied using genetic algorithms focuse on finding the set of records more compatible with a target spectrum and target values. The results of the new -based approach suggest that the real accelerograms obtained with this procedure, reduce the scatter of the response spectra as compared with the traditional approach; furthermore, the mean spectrum of the set of records is very similar to the target seismic design spectrum in the range of interest periods, and at the same time, similar values are obtained for the selected records and the target spectrum.

Prediction of Inelastic Response Spectra Using Artificial Neural Networks

Several studies have been oriented to develop methodologies for estimating inelastic response of structures; however, the estimation of inelastic seismic response spectra requires complex analyses, in such a way that traditional methods can hardly get an acceptable error. In this paper, an Artificial Neural Network (ANN) model is presented as an alternative to estimate inelastic response spectra for earthquake ground motion records. The moment magnitude (), fault mechanism (), Joyner-Boore distance (), shear-wave velocity (), fundamental period of the structure (), and the maximum ductility () were selected as inputs of the ANN model. Fifty earthquake ground motions taken from the NGA database and recorded at sites with different types of soils are used during the training phase of the Feedforward Multilayer Perceptron model. The Backpropagation algorithm was selected to train the network. The ANN results present an acceptable concordance with the real seismic response spectra preserving the spectral shape between the actual and the estimated spectra.

Design Approach Based on UAFR Spectra for Structures with Displacement- Dependent Dissipating Elements

In the first part of this paper an algorithm is proposed to obtain demand hazard curves and, from them, seismic-response spectra with uniform annual failure rates for single-degree-of-freedom (SDOF) systems with energy-dissipating devices (EDDs). In the second part, a performance-based dual-level approach is proposed for the design of buildings with EDDs. The criterion establishes acceptance conditions (related to serviceability—and to ultimate limit-states) that correspond to the main system and to the dissipating elements. The uncertainties implicit in the transformation between the response of SDOF and MDOF systems with EDDs are taken into account by means of their demand hazard curves. The reliability-based dual-level design criterion proposed is illustrated by means of a ten-story reinforced concrete building rehabilitated with steel energy-dissipating devices.