ALBERT

Description: Two models are tested on a shake-table. One of the models is a normal reinforced concrete megaframe structure and the other is a multifunctional vibration-absorption reinforced concrete megaframe structure in which the laminated rubber bearings are placed between the major frame and the minor frames. Two earthquake motions (the El Centro wave and the Taft wave) are used during the test. This paper presents the dynamic characteristic, the seismic responses and the failure mechanism of these two models under varying peak acceleration levels for each of the earthquake motions. The test results demonstrate that the aseismic behavior of a multifunctional vibration-absorption reinforced concrete megaframe structure is much better than that of a normal reinforced concrete megaframe structure. © 2003 John Wiley and Sons, Ltd.

Description: The low-cycle fatigue model presented in the companion paper is employed for developing hysteresis and damage models for deteriorating systems. The hysteresis model performs strength reduction at a current displacement cycle by evaluating the loss in the energy dissipation capacity along the completed displacement path. Hence it is completely memory dependent. Pinching is accounted for implicitly by a reduced energy dissipation capacity in a displacement cycle. The model predicts the experimental results obtained from variable-amplitude tests reasonably well. Response analysis under earthquake excitations reveals that both the maximum displacements and the number of large-amplitude displacement response cycles increase significantly with the reduction in energy dissipation capacity, resulting in higher damage. Damage is defined as the deterioration in the effective stiffness of a displacement cycle, which is in turn related to the reduction in the energy dissipation capacity. A simple damage function is developed accordingly, consisting of displacement and fatigue components. It is observed that the fatigue component of damage is more significant than the displacement component for deteriorating systems under ground motions with significant effective durations. © 2003 John Wiley and Sons, Ltd.

Description: This paper proposes a non-iterative time integration (NITI) scheme for non-linear dynamic FEM analysis. The NITI scheme is constructed by combining explicit and implicit schemes, taking advantage of their merits, and enables stable computation without an iteration process for convergence even when used for non-linear dynamic problems. Formulation of the NITI scheme is presented and its stability is studied. Although the NITI scheme is not unconditionally stable when applied to non-linear problems, it is stable in most cases unless stiffness hardening occurs or the problem has a large velocity-dependent term. The NITI scheme is applied to dynamic analysis of the non-linear soil-structure system and computation results are compared with those by the central difference method (CDM). Comparison shows that the stability of the NITI scheme is superior to that of the CDM. Accuracy of the NITI scheme is verified because its results are identical with those by the CDM in which the time step is set as 1/10 of that for the NITI scheme. The application of the NITI scheme to the mesh-partitioned FEM is also proposed. It is applied to dynamic analysis of the linear soil-structure system. It yields the same results as a conventional single-domain FEM analysis using the Newmark β method. This result verifies the usability of mesh-partitioned FEM analysis using the NITI scheme. © 2003 John Wiley and Sons, Ltd.

Description: For the performance-based seismic design of buildings, both the displacement coefficient method used by FEMA-273 and the capacity spectrum method adopted by ATC-40 are non-linear static procedures. The pushover curves of structures need to be established during processing of these two methods. They are applied to evaluation and rehabilitation of existing structures. This paper is concerned with experimental studies on the accuracy of both methods. Through carrying out the pseudo-dynamic tests, cyclic loading tests and pushover tests on three reinforced concrete (RC) columns, the maximum inelastic deformation demands (target displacements) determined by the coefficient method of FEMA-273 and the capacity spectrum method of ATC-40 are compared. In addition, a modified capacity spectrum method which is based on the use of inelastic design response spectra is also included in this study. It is shown from the test specimens that the coefficient method overestimates the peak test displacements with an average error of +28% while the capacity spectrum method underestimates them with an average error of -20%. If the Kowalsky hysteretic damping model is used in the capacity spectrum method instead of the original damping model, the average errors become - 11% by ignoring the effect of stiffness degrading and - 1.2% by slightly including the effect of stiffness degrading. Furthermore, if the Newmark-Hall inelastic design spectrum is implemented in the capacity spectrum method instead of the elastic design spectrum, the average error decreases to -6.6% which undervalues, but is close to, the experimental results. © 2003 John Wiley and Sons, Ltd.

Description: The dynamic behaviour of two curved cable-stayed bridges, recently constructed in northern Italy, has been investigated by full-scale testing and theoretical models. Two different excitation techniques were employed in the dynamic tests: traffic-induced ambient vibrations and free vibrations. Since the modal behaviour identified from the two types of test are very well correlated and a greater number of normal modes was detected during ambient vibration tests, the validity of the ambient vibration survey is assessed in view of future monitoring. For both bridges, 11 vibration modes were identified in the frequency range of 0-10Hz, being a one-to-one correspondence between the observed modes of the two bridges. Successively, the information obtained from the field tests was used to validate and improve 3D finite elements so that the dynamic performance of the two systems were assessed and compared based on both the experimental results and the updated theoretical models. © 2003 John Wiley and Sons, Ltd.

Description: An absorbing boundary for saturated porous media is developed that can be used for transient analyses in the time domain. The elastic constitutive equations for the saturated porous media follow Bowen's formulation. The method consists of applying viscous tractions along the artificial boundary. The absorbing boundary behaviour is assumed linear and isotropic. Hadamard's conditions provide the speeds of the dilatational and shear waves that propagate in saturated porous media. Since these expressions are frequency independent, the intensities of the viscous tractions are evaluated in the time domain, and the two dilatational waves are accounted for. The viscous tractions are defined from the drained characteristics, assuming an infinite permeability, at variance with the traditional 'undrained' method based on undrained characteristics and a null permeability. Solid media and materials with low permeability are also retrieved as subcases. The results show that, at no additional cost, this 'drained' method is more accurate for all permeabilities than the 'undrained' method, which disregards the existence of the second dilatational wave. © 2003 John Wiley and Sons, Ltd.

Description: Energy dissipation characteristics of structural members which exhibit both strength and stiffness deterioration under imposed displacement reversals are investigated. In the experimental part, 17 reinforced concrete beam specimens were tested under constant and variable amplitude inelastic displacement cycles. The constant-amplitude tests were employed to determine the low-cycle fatigue behaviour of specimens where the imposed displacement amplitude was the major variable. A two-parameter fatigue model was developed in order to express the variation of dissipated energy with the number of displacement cycles. This model was then used to predict the energy dissipation of test specimens subjected to variable-amplitude displacement cycles simulating severe seismic excitations. It has been demonstrated that the remaining energy dissipation capacity in a forthcoming displacement cycle is dependent on the energy dissipated along the completed displacement path. Moreover, it is observed that total energy dissipation is dependent on the length of the displacement path. © 2003 John Wiley and Sons, Ltd.

Description: This paper presents a new identification technique for the extraction of modal parameters of structural systems subjected to base excitation. The technique uses output-only measurements of the structural response. A combined subspace-maximum likelihood algorithm is developed and applied to a three-degree-of-freedom simulation model. Five ensembles of synthetically generated input signals, representing varying input characteristics, are employed in Monte Carlo simulations to illustrate the applicability of the method. The technique is able to circumvent some of the difficulties arising from short data sets by employing the Expectation Maximization (EM) algorithm to refine the subspace state estimates. This approach is motivated by successful application by previous authors on speech signals. Results indicate that, for certain system characteristics, more accurate pole estimates can be identified using the combined subspace-EM formulation. In general, the damping ratios of the system are difficult to identify accurately due to limitations on data set length. The applicability of the technique to structural vibration signals is illustrated through the identification of seismic response data from the Vincent Thomas Bridge. © 2003 John Wiley and Sons, Ltd.

Description: The effects of soil-structure interaction in yielding systems are evaluated, including both kinematic and inertial interaction. The concepts developed previously for interacting elastic systems are extended to include the non-linear behavior of the structure. A simple soil-structure system representative of code-designed buildings is investigated. The replacement oscillator approach used in practice to account for the elastic interaction effects is adjusted to consider the inelastic interaction effects. This is done by means of a non-linear replacement oscillator defined by an effective ductility together with the known effective period and damping of the system for the elastic condition. To demonstrate the efficiency of this simplified approach, extensive numerical evaluations are conducted for elastoplastic structures with embedded foundation in a soil layer over elastic bedrock, excited by vertically propagating shear waves. Both strength and displacement demands are computed with and without regard to the effect of foundation flexibility, taking as control motion the great 1985 Michoacan earthquake recorded at a site representative of the soft zone in Mexico City. Results are properly interpreted to show the relative effects of interaction for elastic and yielding systems. Finally, it is demonstrated how to implement this information in the context of code design of buildings. © 2003 John Wiley and Sons, Ltd.

Description: The authors apply soil response estimation techniques employing accelerograms for fifteen earthquakes recorded at the Yokohama Strong Motion Network and its vertical array of nine sites, plus microtremor data recorded at all 150 sites. Assessment of the reliability of surface to reference site spectral ratios, and horizontal to vertical spectral ratios of S-waves, coda, and microtremors, relied on cross-validation with both surface to borehole spectral ratios, and theoretical soil response functions for vertically incident SH plane waves calculated from the vertical array recordings and logging data, respectively. The results indicate the following: (1) the reference site method provides more accurate soil response estimations; (2) the H/V of microtremors provides relatively better soil response estimations than its S-wave and coda counterparts; and (3) the H/V of microtremors provides coincident soil response estimations at two-thirds of the sites when considering no more than ±30% mismatch in the determination of the predominant periods. The matching of parameters obtained from the analysis of microtremors, combined with their relatively low economic cost and the possibility of recording without strict spatial or time restrictions turns microtremors into a particularly appealing approach. © 2003 John Wiley and Sons, Ltd.