ALBERT

Description: A new, passive, vibroprotective device of the rolling-pendulum tuned mass damper type is presented that, relying on a proper three-dimensional guiding surface, can simultaneously control the response of the supporting structure in two mutually orthogonal horizontal directions. Unlike existing examples of ball vibration absorbers, mounted on spherical recesses and effective for axial-symmetrical structures, the new device is bidirectionally tuneable, by virtue of the optimum shape of the rolling cavity, to both fundamental structural modes, even when the corresponding natural frequencies are different, in such a case recurring to an innovative non-axial-symmetrical rolling guide. Based on Appell's non-holonomic mechanics, a non-linear dynamic model is first derived for the bidirectional absorber mounted on a 1-storey 3-degrees-of-freedom linear structure translating under the effect of both imparted base motion and applied dynamic forces. A laboratory-scaled prototype of the device is then tested to experimentally demonstrate the bidirectional tuning capability and to validate the mathematical model. The design procedure and the seismic performance of the absorber are finally exemplified through numerical simulation. Copyright © 2009 John Wiley & Sons, Ltd.

Description: Simple formulas are derived for the dynamic stiffness of pile group foundations subjected to horizontal and rocking dynamic loads. The formulations are based on the construction of a general model of impedance matrices as the condensation of matrices of mass, damping, and stiffness, and on the identification of the values of these matrices on an extensive database of numerical experiments computed using coupled finite element-boundary element models. The formulations obtained can be readily used for the design of both floating piles on homogeneous half-space and end-bearing piles and are applicable for a wide range of mechanical and geometrical parameters of the soil and piles, in particular for large pile groups. For the seismic design of a building, the use of the simple formulas rather than a full computational model is shown to induce little error on the evaluation of the response spectra and time histories. Copyright © 2009 John Wiley & Sons, Ltd.

Description: Risk assessment of spatially distributed building portfolios or infrastructure systems requires quantification of the joint occurrence of ground-motion intensities at several sites, during the same earthquake. The ground-motion models that are used for site-specific hazard analysis do not provide information on the spatial correlation between ground-motion intensities, which is required for the joint prediction of intensities at multiple sites. Moreover, researchers who have previously computed these correlations using observed ground-motion recordings differ in their estimates of spatial correlation. In this paper, ground motions observed during seven past earthquakes are used to estimate correlations between spatially distributed spectral accelerations at various spectral periods. Geostatistical tools are used to quantify and express the observed correlations in a standard format. The estimated correlation model is also compared with previously published results, and apparent discrepancies among the previous results are explained. The analysis shows that the spatial correlation reduces with increasing separation between the sites of interest. The rate of decay of correlation typically decreases with increasing spectral acceleration period. At periods longer than 2 s, the correlations were similar for all the earthquake ground motions considered. At shorter periods, however, the correlations were found to be related to the local-site conditions (as indicated by site Vs30 values) at the ground-motion recording stations. The research work also investigates the assumption of isotropy used in developing the spatial correlation models. It is seen using the Northridge and Chi-Chi earthquake time histories that the isotropy assumption is reasonable at both long and short periods. Based on the factors identified as influencing the spatial correlation, a model is developed that can be used to select appropriate correlation estimates for use in practical risk assessment problems. Copyright © 2009 John Wiley & Sons, Ltd.

Description: A method is presented for determining the goodness-of-fit between measured and calculated structural response signals. The method is termed as the frequency domain error (FDE) index, which is a value between 0 and 1 where zero indicates a perfect correlation. The FDE is calculated from the Fourier spectra of the measured and calculated waveforms, using a relatively simple calculation procedure, readily implemented using standard fast Fourier transform (FFT) libraries. The FDE is demonstrated herein using three different types of problems: simple harmonic waves, a large number of earthquake response simulations, and a set of limited simulations. Along with the FDE concept, effective methods for displaying the results are presented. Copyright © 2009 John Wiley & Sons, Ltd.

Description: Probabilistically controlled design values of the nonlinear seismic response of reinforced concrete frames are obtained using a method previously proposed by the authors. The method allows to calculate conservative design values characterized by a predefined non-exceedance probability, using a limited number of spectrum-fitting generated accelerograms. Herein the method is applied to elastic-strain hardening single degree of freedom systems representative of RC framed structures and is then assessed with reference to four reinforced concrete model frames designed according to EC8. The frames are characterized by different natural periods and aspect ratios. The results, compared with those obtained applying current EC8 recommendations, show the effectiveness of the proposed method. EC8 provides for design values of the seismic response of a structure with a nonlinear behavior computed as the mean value of the responses to seven accelerograms or as the maximum value of the responses to three accelerograms. These two criteria lead to design values characterized by very different and uncontrolled non-exceedance probability levels, while the proposed method allows the analyst to directly control the non-exceedance probability level of the calculated design values. Copyright © 2009 John Wiley & Sons, Ltd.

Description: In the conventional structural seismic analysis, the rigid base model is usually adopted without considering the flexibility of the ground, leading to inaccurate estimation of the vibration characteristics and the seismic response of the structure. In 2007, several in situ tests were conducted by the National Center for Research on Earthquake Engineering (NCREE) on the school buildings in the Guanmiao Elementary School in Tainan, Taiwan. For the study of soil-structure interaction (SSI) effects, the forced vibration test (FVT) was performed, and the deformation of the foundation system was measured during the pushover test. In this paper, the results of these in situ tests are presented and discussed, and the finite element models of the school buildings were generated for the simulation of the FVT and for the pushover analysis in order to investigate the difference between the rigid base model and the flexible base model. Results show that the mechanical properties of the structure and the foundation could be demonstrated in these in situ tests. Additionally, the introduction of the flexibility of the foundation has a considerable influence on the results of structural analysis. © 2009 John Wiley & Sons, Ltd.

Description: The available substructure method and computer program for earthquake response analysis of arch dams, including the effects of dam-water-foundation rock interaction and recognizing the semi-unbounded size of the foundation rock and fluid domains, are extended to consider spatial variations in ground motions around the canyon. The response of Mauvoisin Dam in Switzerland to spatially varying ground motion recorded during a small earthquake is analyzed to illustrate the results from this analysis procedure. Copyright © 2009 John Wiley & Sons, Ltd.

Description: This paper studies the stability of the central difference method (CDM) for real-time substructure test considering specimen mass. Because the standard CDM is implicit in terms of acceleration, to avoid iteration, an explicit acceleration formulation is assumed for its implementation in real-time dynamic substructure testing. The analytical work shows that the stability of the algorithm decreases with increasing specimen mass if the experimental substructure is a pure inertia specimen. The algorithm becomes unstable however small the time integration interval is, when the mass of specimen equal or greater than that of its numerical counterpart. For the case of dynamic specimen, the algorithm is unstable when there is no damping in the whole test structure; a damping will make the algorithm stable conditionally. Part of the analytical results is validated through an actual test. © 2009 John Wiley & Sons, Ltd.

Description: A summary of the development of a new coupled shear-bending model for analysis of stacked wood shear walls and multi-story wood-frame buildings is presented in this paper. The model focuses on dynamic response of light-frame wood structures under seismic excitation. The formulation is intended to provide a more versatile option than present pure shear models in that the new model is capable of accurately capturing the overall lateral response of each story diaphragm and separates the inter-story shear deformation and the deformation associated with rotation of the diaphragm due to rod elongation, which is an analogue to the bending deformation in an Euler-Bernoulli beam model. Modeling the coupling of bending and shear deformation is shown to provide more accurate representation of stacked shear wall system behavior than a pure shear model, particularly for the upper stories in the assembly. The formulation is coupled with the newly developed evolutionary parameter hysteretic model for wood shear walls. Existing data from a shake table test of an isolated three-story wood shear wall were used to verify the accuracy of the model prediction. The numerical results agreed very well with shake table test measurements. The influence of a continuous rod hold-down system on the dynamic behavior of the three-story stacked wood shear wall was also successfully simulated. © 2009 John Wiley & Sons, Ltd.

Description: This paper demonstrates the applicability of response history analysis based on rigid-plastic models for the seismic assessment and design of steel buildings. The rigid-plastic force-deformation relationship as applied in steel moment-resisting frames (MRF) is re-examined and new rigid-plastic models are developed for concentrically-braced frames and dual structural systems consisting of MRF coupled with braced systems. This paper demonstrates that such rigid-plastic models are able to predict global seismic demands with reasonable accuracy. It is also shown that, the direct relationship that exists between peak displacement and the plastic capacity of rigid-plastic oscillators can be used to define the level of seismic demand for a given performance target. Copyright q 2009 John Wiley & Sons, Ltd.