Showing papers in "Earthquake Engineering & Structural Dynamics in 1991"

Estimation of structural parameters in time domain: A substructure approach

Abstract: A substructure approach is used to estimate the stiffness and damping coefficients of structures from measurement of dynamic responses. The structures are decomposed into smaller subsystems for which state and observation equations are formulated and solved by the method of extended Kalman filter with a weighted global iteration algorithm. Substructural identification methods with and without overlapping members are proposed. In both methods, the convergence of the structural parameters to the optimal values is improved significantly with less computation time as compared to a complete structural approach. Numerical simulation studies are performed for three types of structures, namely a shear building, a plane frame building and a plane truss bridge. The effects of measurement noise and response observations required for identification of system parameters are also investigated.

Seismic‐response‐controlled structure with active mass driver system. Part 1: Design

Abstract: An Active Mass Driver (AMD) system is proposed to suppress actively the response of a building to irregular external excitations such as earthquakes and typhoons. This system has been introduced to an actual ten-storey office building for the first time in the world. The system controls the motions of a structure by means of an external energy supply. It consists of an auxiliary mass installed in a building and an actuator that operates the mass and produces a control force which counters disturbances to the building. The design method of the AMD system, including the location of the installation and the capacity and stability of the system, is proposed. Simplification of the control algorithm is also described.

Modelling of a cable-stayed bridge for dynamic analysis

Abstract: The subject of dynamic response of cable-stayed bridges has received increasing attention in recent years as this type of bridge becomes ever more popular. In most publications on the dynamic behaviour of cable-stayed bridges, only cursory treatment is given to many of the details of the actual development of a suitable finite element model. This paper provides a detailed description of the development of one class of linear elastic finite element model for the dynamic analysis of a cable-stayed bridge. The bridge modelled in this study is the Quincy Bayview Bridge in Illinois. The translation of structural data into a form useful for formulation of a three-dimensional finite element model is presented in some detail, including evaluation of translational and rotational mass and stiffnesses for the deck, modelling of the towers and cables, modelling of the bearings and considerations for accurate geometric representation between the model and the full-scale structure. In a companion paper, the modal behaviour predicted by the finite element model is compared to measured ambient vibration properties of the full-scale structure.

The attenuation of ground accelerations in Europe

Abstract: SUMMARY The paper presents predictive relationships for horizontal and vertical peak ground accelerations derived from 529 triaxial strong-motion records generated by 219 shallow earthquakes in the European area. If no account is taken of the focal depth, the attenuations of peak horizontal and vertical accelerations, in g, are given by log (ah) = - 1.09 + 0*238M, - log(r) - 0@0050r + 028P and log (a,) = - 1-34 + 0.230Ms - log (r) + 0‘27P where r = (8 + 6*0’)*, d is the source distance in km and M, is the surface-wave magnitude; P is 0 for 50-percentile values, 1 for 84-percentiles. If we allow for tlie effect of the focal depth h, in km, the corresponding equations for ah and a, are found to be lOg(0,) = - 0.87 + 0.217MS - log(r) - 0.00117r + 026P and log (a,) = - 1-10 + 0.200M, - log(r) - 0-OOO15r + 0.26P where r = (d’ + h’)”’. distance, and equal to 0.5. The mean ratio of the peak vertical to horizontal acceleration is found to be almost independent of magnitude and

Dynamic pile-soil-pile interaction. part i: analysis of axial vibration

Abstract: SUMMARY Simple methods of analysis are developed for computing the dynamic steady-state axial response of floating pile groups embedded in homogeneous and non-homogeneous soil deposits. Physically-motivated approximations are introduced to account for the interaction between two individual piles. It is found that such an interaction arises chiefly from the ‘interference’ of wave fields originating along each pile shaft and spreading outward. For homogeneous deposits the wave fronts originating at an individual pile are cylindrical and the interaction is essentially independent of pile flexibility and slenderness. For non-homogeneous deposits the wave fronts are non-cylindrical and ray-theory approximations are invoked to derive pile flexibility-dependent interaction functions. Results are presented for the dynamic stiffness and damping of several pile groups, as well as for distribution of the applied load among individual piles. For deposits with modulus proportional to depth, the agreement with the few rigorous solutions available is encouraging. A comprehensive parameter study focuses on the effects of soil inhomogeneity and pile-group configuration. It is demonstrated that the ‘dynamic group efficiency’ may far exceed unity at certain frequencies. Increasing soil inhomogeneity tends to reduce the respective resonant peaks and lead to smoother interaction functions, in qualitative agreement with field evidence.

Implicit time integration for pseudodynamic tests

Abstract: The use of unconditionally stable implicit time integration techniques for pseudodynamic tests has been recently proposed and advanced by several researchers. Inspired by such developments, a pseudodynamic test scheme based on an unconditionally stable implicit time integration algorithm and dual displacement control is presented in this paper. The accuracy of the proposed scheme is proved with error-propagation analysis. It is shown by numerical examples and verification tests that the error-correction method incorporated can eliminate the spurious higher-mode response, which can often be excited by experimental errors. The practicality of the proposed scheme lies in the fact that the implementation is as easy as that of explicit schemes and that the convergence criteria required are compatible with the accuracy limits of ordinary test apparatus.

A simple error estimator and adaptive time stepping procedure for dynamic analysis

Abstract: A simple local error estimator is presented for time integration schemes in dynamic analysis This error estimator involves only a small computational cost The time step size is adaptively adjusted so that the local error at each time step is within a prescribed accuracy It is found that the estimator performs well under various circumstances and provides an economical adaptive process Attempts to estimate the global time integration error are also reproted

Consistent lumped‐parameter models for unbounded soil: Physical representation

Abstract: Note: [112] Reference LCH-ARTICLE-1991-004View record in Web of Science Record created on 2007-04-24, modified on 2016-08-08

Dynamic behaviour of R/C highway bridges under the combined effect of vertical and horizontal earthquake motions

Abstract: Measurements of ground motions during past earthquakes indicate that the vertical acceleration can reach values comparable to horizontal accelerations or may even exceed these accelerations. Furthermore, measurements of structural response show the possibility of significant amplification in the response of bridges in the vertical direction that can be attributed to the vertical component of ground motion. In this study, the relative importance of the vertical component of ground motion on the inelastic response of R/C highway bridges is investigated. Particular emphasis is placed on modelling of the deck and piers to account for complex loading histories under combined vertical and horizontal earthquake motions. Analyses of actual bridges indicate that, in general, the vertical motion will increase the level of response and the amount of damage sustained by a highway bridge. Vertical motion generates fluctuating axial forces in the columns, which cause unstability of the hysteresis loops and increase the ductility demand. Furthermore, vertical motion can generate forces of high magnitude in the abutments and foundations that are not accounted for by the current seismic design guidelines. Thus, it is important to consider this component of the ground motion in the design of highway bridges, especially for those located in regions near seismic faults.

Seismic‐response‐controlled structure with active mass driver system. Part 2: Verification

Abstract: An Active Mass Driver (AMD) system is proposed to suppress actively the response of a building to irregular external excitations such as earthquakes and typhoons.1 This system has been introduced to an actual ten-storey office building constructed in Tokyo in August, 1989. The proposed analytical methods utilize circuits of the system and mechanical characteristics to understand the real control effect of the system. Simulation analyses are also performed to verify the analytical model and the control effect during observed earthquakes.

Analytical prediction of the collapse earthquake for R/C buildings: Suggested methodology

Abstract: Criteria to be used for the analytical prediction of collapse of R/C buildings subjected to earthquake excitation are suggested. It is attempted to include in the criteria as many of the parameters involved as possible, while keeping the complexity, as well as the cost of analysis reasonably low. Two combined collapse criteria are finally suggested, corresponding to member failure and storey failure. Using these criteria, quantitative estimations can be obtained of the actual values of behaviour factors (q-factors) used in modern seismic codes. The ground motion dependence of these factors, as well as the sensitivity of their calculated values to the assumption made about the stiffness of R/C members, is illustrated with reference to two typical structures.

Ambient vibration measurements on a cable-stayed bridge

Abstract: An extensive programme of full-scale ambient vibration tests has been conducted to measure the dynamic response of a 542 m (centre span of 274 m) cable-stayed bridge—the Quincy Bayview Bridge in Illinois. A microcomputer-based system was used to collect and analyse the ambient vibration data. A total of 25 modal frequencies and associated mode shapes were identified for the deck structure within the frequency range of 0–2 Hz. Also, estimations were made for damping ratios. The experimental data clearly indicated the occurrence of many closely spaced modal frequencies and spatially complicated mode shapes. Most tower modes were found to be associated with the deck modes, implying a considerable interaction between the deck and tower structure. No detectable levels of motion were evident at the foundation support of the pier. The results of the ambient vibration survey were compared to modal frequencies and mode shapes computed using a three-dimensional finite element model of the bridge. For most modes, the analytic and experimental modal frequencies and mode shapes compare quite well, especially for the vertical modes. Based on the findings of this study, a linear elastic finite element model appears to be capable of capturing much of the complex dynamic behaviour of the bridge with very good accuracy, when compared to the low-level dynamic responses induced by ambient wind and traffic excitations.

A Lagrange multiplier solution method for pounding of buildings during earthquakes

Abstract: An effective solution method for studying the pounding response of two or more adjacent buildings during earthquakes is presented. The method is based on the Lagrange multiplier approach by which the geometric compatibility conditions due to contact are enforced. The energy and momentum balance criteria for the contacting bodies are satisfied according to the laws of impact with proper post-impact conditions, which can also take into account local energy absorption phenomena during impact. A solution scheme is proposed which can be incorporated easily into existing computer programs for static and dynamic analysis of structures. The applicability of the algorithm is illustrated in a number of selected sample problems and the correlation of numerical results to analytical solutions, whenever possible, is very satisfactory.

Consistent lumped‐parameter models for unbounded soil: Frequency‐independent stiffness, damping and mass matrices

Abstract: Note: [113] Reference LCH-ARTICLE-1991-005View record in Web of Science Record created on 2007-04-24, modified on 2016-08-08

Three‐dimensional analysis of spatially varying ground motions around a uniform canyon in a homogeneous half‐space

Abstract: Department of Civil Engineering, University of California, Berkeley, CA 94720, U.S.A. A direct boundary element method to determine the three-dimensional seismic response of an infinitely-long canyon of arbitrary but uniform cross-section cut in a homogeneous viscoelastic half-space is presented. The seismic excitation is represented by P, SV, SH or Rayleigh waves at arbitrary angles with respect to the axis of the canyon. The accuracy of the procedure and implementing computer program is demonstrated by comparison with previous solutions for the limiting case of two-dimensional response, recently obtained three-dimensional response results for infinitely-long canyons, and three-dimensional boundary method solutions presented in this paper for finite canyons.

Evaluation of the earthquake performance of anchored wine tanks during the San Juan, Argentina, 1977 earthquakexs

Abstract: This paper details the damage sustained by a number of wine tanks during the San Juan, Argentina, 1977 earthquake. The available ground motion information is used to compare the actual tank performance with that predicted. Two of the tanks examined were located quite near the two accelerographs that recorded the ground motion in the city of San Juan for the main shock. An empirical attenuation formula which describes the intensity of the shaking was derived for this earthquake and used in a quantitative evaluation of the performance of a number of other tanks

Time-harmonic boussinesq problem for a continuously non-homogeneous soil

Abstract: An analytical solution is presented for the response of a non-homogeneous, compressible, elastic half-space to a time-harmonic vertical point load on its surface. The shear modulus is assumed to increase continuously with depth. The model is chosen so as to describe uniformly deposited cohesionless soils. Expressions for displacements and stresses in the interior of the half-space medium are derived by means of Hankel transforms and contour integration.

Dynamic response of buildings due to trains in underground tunnels

Abstract: The interaction of a tunnel-soil-building system due to trains is investigated by a substructure technnique. The soil medium is assumed to be a viscoelastic halfspace. The method of wave function expansion is used to construct the displacement fields in terms of potentials. The total soil-structure interaction problem is decomposed into a foundation radiation problem and a tunnel radiation problem. The impedance matrices for the corresponding substructure problems are obtained using a collocation technique. The steady state response of buildings for a given tunnel-foundation geometry is determined using the impedance matrix. Hence, the response of the building to train loading at different speeds is evaluated and compared with allowable vibration limits.

Vibrations of timoshenko beam‐columns on two‐parameter elastic foundations

Abstract: A finite element procedure is developed for analysing the flexural vibrations of a uniform Timoshenko beam-column on a two-parameter elastic foundation. The beam-column is discretized into a number of simple elements with four degrees of freedom each. The governing matrix equation for small-amplitude, free vibrations of the beam-column on the elastic foundation is derived from Hamilton's principle. Several numerical examples are provided to show the effects of axial force, foundation stiffness parameters, partial elastic foundation, shear deformation and rotatory inertia on the natural frequencies of the beam-column.

A modal superposition pseudo‐force method for dynamic analysis of structural systems with non‐proportional damping

Abstract: A modal superposition pseudo-force method for the dynamic analysis of structural systems with non-proportional damping is presented. The method combines the advantages of the classical modal superposition method and the pseudo-force method. When the system damping is non-proportional, the dynamic equilibrium equations in generalized coordinates are coupled through the damping terms. In the present method, these coupled equations are solved by an iterative process in which the coupling terms are treated as pseudo-forces. A proof of the convergence of the iterative process is given. Numerical examples show the good convergence characteristics of the process and the good accuracy of the obtained results.

Impact interactions between two vibration systems under random excitation

Abstract: The exact stationary solutions of the random response of two special vibration systems with impact interactions are formulated in this paper. Between the two systems, the Hertz contact law is used to model the contact process during vibration. A clearance is also introduced. The excitation is assumed to be a stationary white Gaussian process with zero mean and acting on the two systems independently. By assuming the ratio of the excitation and damping parameter of each system to be the same, the exact solutions can be found through solving the time-independent Fokker-Planck equation. The effects of contact stiffness, clearance and the system properties on the response are discussed probabilistically. From this study, it is found that, under some cases, the contact phenomena still play an important role on the response even when the clearance is larger than three times the root mean square response of the corresponding linear systems.

Evaluation of factors influencing the inelastic seismic performance of torsionally asymmetric buildings

Abstract: This paper addresses some fundamentally contradictory conclusions drawn by Tso and Ying1 and the authors2 regarding the additional seismic ductility demand in asymmetric building structures and the adequacy of certain code torsional provisions. It also clarifies a number of issues arising from the different approaches employed in the two studies. The Mexico 76 and 87 code torsional provisions are taken as examples. Results show that the structural element at the stiff edge is the more critical and that the Mexico 76 code torsional provisions (among others) are inadequate, substantially underestimating the strength demand of this element. On the other hand, the Mexico 87 code torsional provisions are found to be over-conservative. Recommendations are also given for improving the form and effectiveness of these code torsional provisions.

A case study of mimo system identification applied to building seismic records

Abstract: A large number of high quality strong-motion records of building response are obtained from recent earthquake events in California. The accelerometers are typically deployed at several levels from the basement up. In order to learn as much as possible about the building behaviour from these records, a multiple input-multiple output (MIMO) system identification procedure is implemented to analyse these records in a systematic way. The procedure is an extension of the least-square-output-error method applied to a classically damped linear second order system. The time varying behaviour is modelled through a time window approach. The procedure includes (i) preliminary record analysis, (ii) input and model selection, (iii) parameter identification and (iv) drift analysis. The records of a 15-storey reinforced concrete building obtained during the Whittier earthquake are analysed. It is found that the fundamental period is much longer than that of a regular building. The torsional response is significant and is caused by both the translational and the torsional motions at the ground level. The maximum drift occurs at the ground storey. The second and the third translational modes in each direction are as important as the first modes in making up the ground storey drift. When the maximum drift occurs in one direction the corresponding drift in the orthogonal direction can be as high as 30 per cent of the maximum drift.

Dynamics of rigid strip foundations embedded in orthotropic elastic soils

Abstract: This study is concerned with the dynamic response of an arbitrary shaped rigid strip foundation embedded in an orthotropic elastic soil. The foundation is subjected to time-harmonic vertical, horizontal and moment loadings. The boundary-value problem related to an embedded foundation is analysed by using the indirect boundary integral equation method. The kernel functions of the integral equations are displacement and traction Green's functions of an anisotropic elastic half plane

An improved response spectrum method for calculating seismic design response. Part 2: Non‐classically damped structures

Abstract: A response spectrum method which combines the analytical advantage of the mode acceleration formulation and the practical advantage of the mode displacement formulation is developed for seismic response calculation of non-classically damped structures. It reduces the error associated with the truncation of the high frequency modes without explicitly using them in the analysis. The method is especially effective for calculating the response of stiff structural systems and also for calculating the response quantities which are strongly affected by high frequency modes. Even with flexible structures, it is shown to provide more accurate response results than the results obtained with the mode displacement approach.

Shaking table study of concrete gravity dam monoliths

Abstract: A series of shaking table tests was performed on three small-scale models of a monolith of a concrete gravity dam in order to simulate earthquake shaking. The purpose of the tests was to examine the nature of crack formation in a gravity dam and the stability of the dam in the presence of cracks. No failures occurred even though the levels of shaking employed were unrealistically high. The good performance owed to the development of crack profiles which had favourable orientations to resist sliding failures in each case. However, the development of an unfavourable crack profile, which cannot be ruled out, and the possibility of water intrusion into open cracks, something not included in the experiments, could lead to failure under significantly lower levels of excitation than those employed.

A method to estimate intensity occurrence probabilities in low seismic activity regions

Abstract: The estimation of site intensity occurrence probabilities in low seismic activity regions has been studied from different points of view. However, no method has been definitively established because several problems arise when macroseismic historical data are incomplete and the active zones are not well determined. The purpose of this paper is to present a method that estimates site occurrence probabilities and at the same time measures the uncertainties inherent in these probabilities in low activity regions. The region to be studied is divided into very broad seismic zones. An exponential intensity probability law is adjusted for each zone and the degree of uncertainty in the assumed incompleteness of the catalogue is evaluated for each intensity. These probabilities are used to establish what may be termed ‘prior site occurrence models’. A Bayesian method is used to improve ‘prior models’ and to obtain the ‘posterior site occurrence models’. Epicentre locations are used to recover spatial information lost in the prior broad zoning. This Bayesian correction permits the use of specific attenuation for different events and may take into account, by means of conservative criteria, epicentre location errors. Following Bayesian methods, probabilities are assumed to be random variables and their distribution may be used to estimate the degree of uncertainty arising from (a) the statistical variance of estimators, (b) catalogue incompleteness and (c) mismatch of data to prior assumptions such as Poisson distribution for events and exponential distribution for intensities. The results are maps of probability and uncertainty for each intensity. These maps exhibit better spatial definition than those obtained by means of simple, broad zones. Some results for Catalonia (NE of Iberian Peninsula) are shown.

Controlling seismic response of eccentric structures by friction dampers

Abstract: Previous studies have demonstrated the good performance of friction dampers in symmetric frame structures subjected to earthquake excitation. This paper examines their effectiveness in asymmetric structures where lateral-torsional coupling characterizes the behaviour. A parametric study is first performed employing an idealized single-storey structure; this is followed by the example of a three-dimensional 5-storey prototype structure equipped with friction dampers. The parametric results show that it is necessary to tune the friction damped braces with respect to both the stiffness of the braces and the slip load of the devices. For properly tuned structures, maximum response for all magnitudes of eccentricity between the centres of stiffness and mass is reduced to levels equal to or less than that of the corresponding symmetric structure. Compared to this prediction, the prototype structure with friction damped bracing exhibits the desired improvement in performance; namely, the devices slip at all storey levels while the frames remain elastic.

Earthquake induced hydrodynamic pressure on axisymmetric offshore structures

Abstract: The horizontal earthquake induced hydrodynamic pressure acting on the surface of axisymmetric offshore and coastal structures is explored. A semi-analytical and semi-numerical approach based on the use of a complete and non-singular set of Trefftz functions is developed. Using this method, one can model accurately the compressibility of the sea water and gravity waves on the water surface. The proposed method shows a relatively simple and efficient approach. This is because the number of degrees of freedom in the matrix equation depends only on the number of selected Trefftz functions and because the discretization of the fluid domain into boundary elements is restricted to the structural surface only. This method is also easy to apply in engineering analysis of hydrodynamic pressure induced by an actual earthquake acceleration. Numerical examples are presented to illustrate the results obtained from this method. Several special interesting topics, such as the effects of the water compressibility, the gravity waves on the water surface and the geometrical shape of the structural surface, are also discussed.