Rafael Riddell

Response modification factors for earthquake resistant design of short period buildings

Most recent seismic codes include response modification factors in the definition of the equivalent lateral forces that are used for the design of earthquake resistant buildings. The response modification factors (R) are used to reduce the linear elastic design spectrum to account for the energy dissipation capacity of the structure. The evaluation of these response modification factors for various sets of earthquake records and ductility factors is presented herein. Special attention is given to the short period range where the reduction of linear elastic response spectra is smaller than the values for intermediate and long period structures. An idealized and simple variation of the response modification factor as a function of the period of vibration, suitable for seismic codes formulation, is also presented.

On Ground Motion Intensity Indices

The characterization of strength of earthquake demands for seismic analysis or design requires the specification of a level of intensity. Numerous ground motion intensity indices that have been proposed over the years are being used for normalizing or scaling earthquake records regardless of their efficiency. An essential point of this study is that a ground motion index is appropriate, or efficient, as long as it can predict the level of structural response. This study presents correlations between 23 ground motion intensity indices and four response variables: elastic and inelastic deformation demands, and input energy and hysteretic energy; nonlinear responses are computed using elastoplastic, bilinear, and bilinear with stiffness degradation models. As expected, no index is found to be satisfactory over the entire frequency range. Indeed, indices related to ground acceleration rank better in the acceleration-sensitive region of the spectrum; indices based on ground velocity are better in the velocity-sensitive region and, correspondingly, generally occur in the displacement-controlled region. Despite frequent criticism, the peak ground motion parameters passed the test successfully. A ranking of indices is presented, thus providing a choice of the most appropriate one for a particular application in the frequency range of interest.

Inelastic response of one-storey asymmetric-plan systems subjected to bi-directional earthquake motions

The inelastic response of one-storey systems with one axis of asymmetry subjected to bi-directional base motion is studied in this paper. The effect of the system parameters on response is also evaluated: uncoupled torsional-to-lateral frequency ratio, stiffness eccentricity, and yield strength of the lateral resisting elements. The ensemble of earthquake records used consists of 15 two-component strong ground motions. The response to uni-directional excitation is considered first to examine the influence of the system parameters and to serve as a basis to examine the results of the bi-directional case, which are presented in terms of average spectra for bi- over uni-directional lateral-deformation ratios. It is shown that the effect of inelastic behaviour is, on the average, noteworthy for stiff structures, in turn, the same structures are the most affected by the action of bi-directional ground motions. The effect of the relative intensity of the two orthogonal ground motion components is also studied. Copyright

An integrated model for earthquake risk assessment of buildings

This study is devoted to the formulation and construction of an integrated model for earthquake risk assessment of buildings in seismic regions. The model developed has five stages: i) characterization of ground motion, ii) construction of the building model, iii) evaluation of the inelastic building response, iv) structural damage assessment, and v) risk evaluation. Stages iii) and iv) are emphasized in the current presentation, because they include some newer aspects such as the use of story-shear and torque surfaces for inelastic building analysis and fuzzy logic for assessing the earthquake performance of a structure. Examples including a large building inventory and two individual structures are developed to show the potential use of the model. Results show that the model is capable of discriminating different foundation soils, earthquake performance of shear-wall and frame buildings, asymmetries in height and plan, and between conventional and seismically isolated structures. Such features may be useful to engineers working in city planning, emergency and risk management, and the insurance industry.

Analysis and interpretation of the seismic response of RC buildings in Concepción during the February 27, 2010, Chile earthquake

Observed trends in the seismic performance of eight severely damaged reinforced concrete (RC) structures after the February 27, 2010, Chile earthquake are presented in this article. After a reconnaissance and surveying process conducted immediately after the earthquake, several aspects not conventionally considered in building design were observed in the field. Most of the considered structures showed extensive localized damage in walls of lower stories and first basements. Several factors indicate that damage was brittle, and occurred mainly in recent RC structures supported on soft soils with some degree of vertical and/or horizontal irregularity. Non-ductile behavior has been inferred due to the lack of evidence of spread damage in the structure, and the fact that very similar structural configurations existed nearby without apparent damage. Some key aspects in understanding the observed damage are: geographical orientation of the building, presence of vertical and horizontal irregularities, wall thickness and reinforcement detailing, and lack of sources for energy dissipation. Additionally, results of a building-code type analysis are presented for the 4 most critical buildings, and Demand/Capacity ratios are calculated and compared with the observed behavior. It is concluded that the design codes must be revised relative to wall design provisions.

Optimal Strut-and-Tie Models Using Full Homogenization Optimization Method

An optimization method based on homogenization is proposed for finding optimal strut-and-tie (ST) models for reinforced concrete elements. The method uses a layout that minimizes displacement for a given loading state in a linearly elastic regime by mixing two materials. Although this optimal layout might contain fine mixtures, one can still obtain a strongly resembling ST model without mixtures that performs closely to the optimal configuration through a penalization procedure. Two examples from the ST literature are used to illustrate the application of the method: the dapped beam and the beam on beam. The reinforcement layouts obtained make the element more efficient in terms of ultimate load divided by the weight of the steel used and having smaller deflections and crack widths. It is remarkable that the used optimization method, which considers the structure in the linearly elastic regime, gives a very good performance for the nonlinear regime.

A study of site amplification effects on ground motions in Santiago, Chile

Abstract The smasch array for the observation of strong ground motion has been operating in the Santiago Valley since 1989. One of the objectives of this instrumentation project is to study the influence of local geological conditions on earthquake ground motions. The array consists of seven free-field sites located on different soil conditions ranging from a reference rock outcrop site to a site located on soft silty ground. Using data from low intensity events which occurred in 1989 and 1990, a preliminary evaluation of site amplification effects has been carried out. Fourier Amplitude spectra of the accelerograms for each site, for several different events, were computed. For this purpose only a limited portion of the record was used: that containing predominantly S waves. The spectral shapes were smoothed using a Parzens lag window of 0.7 Hz and amplitude ratios with respect to the rock site were calculated. For each site mean amplification ratios were computed. From these results it is apparent that there is a noticeable influence of the site soil conditions on the amplification value and also that this amplification can be quite different depending on the period range considered.

Discussion of: “Simplified R-Factor Relationships for Strong Ground Motions”

While agreeing with much of the approach presented by the authors, there are several aspects that the writer would like to discuss. First, the authors propose R-m-T relations independent of damping (Equation 15), which seem to be recommended for systems having from 2 to 10% damping (since such damping range was considered in their response calculations). The writer finds it difficult to reconcile this proposal with the findings of Riddell and Newmark (1979a, 1980), who were the first to propose R-m relationships based on statistical analyses of responses to a number of earthquakes. Riddell and Newmark proposed the following expression: