Amador Teran-Gilmore

A Damage Model for Practical Seismic Design That Accounts for Low Cycle Fatigue

The structural properties of a structure deteriorate when deformations reach the range of inelastic behavior. A possible consequence of deterioration of the hysteretic behavior of a structure is failure of critical elements at deformation levels that are significantly smaller than its ultimate deformation capacity. Seismic design methodologies that account for low cycle fatigue can be formulated using the concept of target ductility. The practical use of one such methodology requires the consideration of simple low cycle fatigue models that consider the severity of repeated loading through a normalized plastic energy parameter. The inconsistencies inherent to the use of such indices can be corrected through simple empirical rules derived from an understanding of the effect of the history of energy dissipation in the assessment of the level of structural damage.

On the Use of Spectra to Establish Damage Control in Regular Frames during Global Predesign

A reliable seismic design requires the formulation of explicit design requirements for structural and nonstructural damage control. Recently, several methods for structural damage control have been established. While the majority of them take into consideration the maximum deformation demand, a few of them account for the effect of cumulative plastic deformation demands. From the comparison of the spectral values of displacement and plastic energy evaluated at the fundamental period of vibration and their corresponding demands in regular frames, it can be concluded that reliable estimates of these demands can be obtained for regular frames through the use of response spectra. Furthermore, within the context of performance-based design that accounts for low cycle fatigue, response spectra can be used for the conception of regular frames and, through the use of the static method of analysis, for their global predesign. Any reduction in lateral strength (base shear) that may be obtained from the use of dynamic methods of analysis should be carefully assessed.

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.

Strength Reduction Factors for Ductile Structures with Passive Energy Dissipating Devices

In recent years, current seismic codes started contemplating the design of structures with passive energy dissipating devices. One important issue for the rational seismic design of these devices and the structure that contains them is the formulation of numerical methods to estimate their design seismic forces. From the study of the dynamic response of single-degree-of-freedom systems subjected to accelerograms recorded in Mexico during the last two decades, expressions to estimate the strength reduction factor that should be used to reduce the elastic design strength spectra for 5 percent damping, to establish the design seismic forces for structures having different combinations of plastic and viscous energy dissipating capacities, are formulated.

Cumulative Ductility Spectra for Seismic Design of Ductile Structures Subjected to Long Duration Motions: Concept and Theoretical Background

A seismic design procedure that does not take into account the maximum and cumulative plastic deformation demands that a structure will likely undergo during severe ground motion could lead to unreliable performance. Damage models that quantify the severity of repeated plastic cycling through plastic energy are simple tools that can be used for practical seismic design. The concept of constant cumulative ductility strength spectra, developed from one such model, is a useful tool for performance-based seismic design. Particularly, constant cumulative ductility strength spectra can be used to identify cases in which low-cycle fatigue may become a design issue, and provides quantitative means to estimate the design lateral strength that should be provided to a structure to adequately control its cumulative plastic deformation demands during seismic response. Design expressions can be offered to estimate the strength reduction factors associated to the practical use of constant cumulative ductility strength spectra.

A parametric approach to performance-based numerical seismic design

The economic losses during recent seismic events, partially a product of the inadequate performance of modern buildings, have outlined the need to improve our current seismic design procedures. Current procedures lay out the design problem considering explicitly the demand-supply pairs of strength and stiffness. These two mechanical characteristics are estimated from the same design parameter: the base shear. That is, current procedures are based on a parametric approach, and in particular, on a one-parameter approach. Although it may be desirable to keep the parametric approach, it is necessary to establish design procedures that contemplate the use of more than one design parameter, in such a way that all relevant mechanical characteristics (e.g., strength, stiffness and deformation) can be designed rationally. Although currently it is possible to formulate a parametric approach to performance-based seismic design, ambitious educational and research programs should be undertaken to make possible its practical implementation.

Preliminary Design of Low-Rise Buildings Stiffened with Buckling-Restrained Braces by a Displacement-Based Approach

A displacement-based methodology for the preliminary design of a system of buckling-restrained braces is introduced. The methodology applies to the case of low-rise buildings, whose dynamic response is not significantly influenced by global flexural behavior or higher modes. The methodology is applied to the preliminary design of a five-story building located in the Lake Zone of Mexico City. From the evaluation of the global mechanical characteristics of the building and of its seismic performance when subjected to ground motions generated in that zone, it is concluded that the proposed methodology yields an adequate level of seismic design.

On the use of plastic energy to establish strength requirements in ductile structures

Although there is a consensus that low cycle fatigue should be avoided, there are different opinions regarding how to account for it during seismic design. Different approaches have been used to formulate structural damage indices; in such a way that it is important to clarify if the number and amplitude of plastic incursions have a significant influence during the assessment of structural damage. Under the assumption of proportionality between the demand and supply of plastic incursions as a function of their amplitude, low cycle fatigue can be assessed exclusively through the normalized plastic energy demand. The response of simple systems to motions recorded in different soil conditions suggest that plastic energy tends to be dissipated in a well defined manner in earthquake-resistant structures. Particularly, energy dissipation follows similar tendencies as those exhibited by typical cycle capacity curves, in such way that the plastic energy dissipated during the design ground motion provides reliable information for assessment of low cycle fatigue. In the case of soft soil, the use of the plastic energy demand to characterize plastic cycling may result in an important underestimation of the level of structural damage.

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.

Displacement-Based Preliminary Design of Tall Buildings Stiffened with a System of Buckling-Restrained Braces

A displacement-based methodology for the preliminary design of a system of buckling-restrained braces is introduced. The methodology applies to the case of tall buildings, whose dynamic response is significantly influenced by global flexural drift mode and higher modes. The methodology is applied to the preliminary design of a 24-story building located in the Lake Zone of Mexico City. From the evaluation of the global mechanical characteristics of the building and of its seismic performance, it is concluded that the proposed methodology yields tall buildings that adequately satisfy predefined deterministic performance levels.