An algorithm is proposed for the subspace identification of a generalized discrete linear time invariant model structure, composing of a deterministic and a stochastic subsystem that are only partially coupled. In a first step, the impulse response of the deterministic subsystem and the correlation function of the stochastic subsystem are estimated in a statistically consistent way. The second step consists of system realization, where a specific choice of the state-space basis, that reveals the deterministic and the stochastic dynamics and their coupling, is imposed. A simulation example illustrates the identification of a system that is excited by a known input and by colored noise.
This paper describes an efficient technique for the selection of relevant modes resulting from a vector autoregressive (ARV) model. The technique is based on plotting the deterministic and stochastic contributions of each mode separately. It helps to identify visually how strong a mode is present in the measured data of the system under consideration. The method is simple to program and applicable to any model based on the data-dependent system (DDS) methodology. In order to demonstrate the applicability of the technique, two examples are presented. In the first example, simulated data of a two-degree-of-freedom system are used, while in the second example, real data on a prestressed concrete bridge are considered.
No abstract is provided for this article.
No abstract is provided for this article.
The authors would like to express their gratitude for the Fundacao para a Ciencia e Tecnologia, from Portugal, for providing a doctoral scholarship to the first Author, Contract SFRH/BD/24688/2005.
An integrated train–track–subsoil dynamic interaction model of moving-train induced ground vibration is developed on the basis of vehicle dynamics, track dynamics and the Green's functions of subsoil. The model takes account of the vibrations of vehicle components, the quasi-static axle loads and the dynamic excitations between the wheels and track. The analyzed results from an example show that the ground vibration characteristics have a close relationship with train speed and soil properties; the dynamic responses excited by wheel–track irregularity have big influence on the high frequency components of ground vibration; with the increase of distance to the track, the ground acceleration has the tendency of decrease, and the relevance of acceleration curves and train excitation becomes less obvious; the intersections of moving load speed-lines and subsoil dispersion curves are some resonance frequencies that cause the amplification of ground vibrations; there exists a critical speed for moving train that is close to the minimum velocity of the Rayleigh's wave in the subsoil.
The knowledge of tensile forces in cables of cable-stayed bridges or in external tendons is required for regular inspection and safety assessment of these structures. Force determination by lift-off tests with hydraulic jacks is connected with considerable expenditures as well as the danger of damages. But also vibration measurements conducted up to now have shown that the accuracies achieved sometimes were not satisfying. Above all for high cable forces and short cables, errors of up to ± 10% could be determined. Therefore fast, accurate and non-destructive methods for assessing cable forces are required. Main target of this paper is the presentation of a recently developed, accurate and practically applicable procedure to determine cable forces for civil engineering constructions by means of vibration measurements. By taking into account the identified natural frequencies, the bending stiffness as well as the boundary conditions an accuracy in the range of ± 1% may be achieved. The application of the described method is shown on one selected cable stayed bridge crossing the Danube-Channel in Vienna.
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The use of threaded connection is a valuable alternative to conventional welding in tubularconstructions, e.g. pipelines, drill pipes and deep water risers. Those applications are normally exposed toenvironmental hazards – wave induced vibrations, temperature changes, etc. – as well as subjected tosevere service conditions. A classical way to determine fatigue strength in many engineering situations is tocalibrate material models by means of ad hoc designed experiments. Unfortunately, it is very difficult in thiscase even under laboratory conditions due to complicated stress and strain states in the contacting taperedhelical thread surfaces of the connection. Therefore, a classical four-point bending fatigue test setup hasbeen built with a real pipe specimen of 3.75 meter long, consisting of two standard API pipes connected bya threaded coupling, under unsymmetric (non-zero average) load control cycles. A complete vibration studyhas been carried out based on input-output modal tests for the entire period of the fatigue experiment. Inputexcitation is due to hammer impact and responses are recorded by accelerometers and by reusabledynamic strain gauges. The measured modal strains from the dynamic strain gauges allow for directcalculation of the modal curvatures, rather than deriving approximately from acceleration information. Bycomparing the measured modal parameters with those of a numerical model of the same structure inundamaged condition, damage detection, localization in the coupling and quantification are possible. Thisstudy leads to the following conclusion of practical use: the recent advancement in modal analysis, i.e. thereference based input-output combined deterministic-stochastic subspace identification, makes it possibleto identify the structural modal properties from in-situ modal tests, which are performed while the fatiguetest is ongoing. In this way the fatigue test is uninterrupted to avoid the problem of stress and straindisturbances happened in un-reversing load cycles test.
A new finite element family which contains a λ singularity is developed. Three elements are derived; namely three-, four- and five-noded elements which are compatible with linear, quadratic and cubic standard isoparametric finite elements, respectively. The elements are tested on two different examples. In the first example, an edge crack problem is analysed using two different meshes and different integration orders. The second example is a crack perpendicular to the interface problem which is solved for different material properties and, in turn, different singularity order λ. The results of those examples illustrate the efficiency of the proposed elements.
