C.Y. Liaw

A hybrid computational strategy for identification of structural parameters

By identifying parameters such as stiffness values of a structural system, the numerical model can be updated to give more accurate response prediction or to monitor the state of the structure. Considerable progress has been made in this subject area, but most research works have considered only small systems. A major challenge lies in obtaining good identification results for systems with many unknown parameters. In this study, a non-classical approach is adopted involving the use of genetic algorithms (GA). Nevertheless, direct application of GA does not necessarily work, particularly with regards to computational efficiency in fine-tuning when the solution approaches the optimal value. A hybrid computational strategy is thus proposed, combining GA with a compatible local search operator. Two hybrid methods are formulated and illustrated by numerical simulation studies to perform significantly better than the GA method without local search. A fairly large structural system with 52 unknown parameters is identified with good results, taking into consideration the effects of incomplete measurement and noisy data.

Substructural and progressive structural identification methods

Abstract While it is possible in principle to determine unknown structural parameters by system identification techniques, a major challenge lies in the numerical difficulty in obtaining reasonably accurate results when the system size is large. Adopting the strategy of “divide-and-conquer” to address this issue, substructural identification and progressive structural identification methods are formulated. The main idea is to divide the structure into substructures such that the number of unknown parameters is within manageable size in each stage of identification. A non-classical approach of genetic algorithms is employed as the search tool for its several advantages including ease of implementation and desirable characteristics of global search. Numerical simulation study is presented, including a fairly large system of 50 degrees of freedom, to illustrate the identification accuracy and efficiency. The methods are tested for known-mass and unknown-mass systems with up to 102 unknown parameters, accounting for the effects of incomplete and noisy measurements.

Preliminary studies into the behaviour of knee braced frames subject to seismic loading

Abstract A recently proposed structural system for earthquake-resistant steel-structures is investigated. This new framing system, called the knee braced frame (KBF), dissipates energy during severe seismic excitations through flexural yielding of the knee element. The lateral stiffness is provided by a conventional diagonal brace with at least one end connected to the knee element. As the performance of this framing system depends on the ductile behaviour of the knee element, this critical element was tested to confirm its behaviour under cyclic loading. An analytical model for the moment-rotation relationship of the knee element, derived from the bilinear stress-strain relationship obtained through tensile test, is found to predict the cyclic response of the knee element accurately. Using this analytical model, the energy dissipation characteristics of the KBF are presented.

Earthquake-resistant steel frames with energy dissipating knee elements

Moment-resisting and concentrically braced frames have been widely used for earthquake-resistant steel buildings. However, neither of these structural systems alone, can efficiently provide the required stiffness and ductility simultaneously, which are required for structures subjected to severe seismic excitations. This paper presents the results of an investigation into a new alternative structural system for earthquake-resistant steel buildings, which aims to overcome the limitations of existing systems. This system employs a diagonal brace with one end anchored to a knee element. Stiffness is derived from the nonbuckling brace, while ductility under severe lateral loading is achieved through flexural yielding of the knee element. Large-scale model tests were conducted to assess the performance of this system. A nonlinear analytical model of the knee element has been presented for application in the dynamic analysis of the new structural system. Using this analytical model, the dynamic response of a multistorey frame with energy dissipating knee elements is compared with the corresponding eccentric braced frames.

Design of earthquake-resistant steel frames with knee bracing

Abstract A new structural system, called the knee-brace-frame (KBF), is investigated here for its suitability for seismic resistant steel structures. In this framing system, one end of the diagonal brace is connected to a knee anchor instead of the beam-column joint. The diagonal brace provides the required stiffness during moderate earthquakes, while the knee anchor yields in flexure to dissipate energy during a severe earthquake, whereby buckling of the brace or yielding of beam or column is prevented. Large scale dynamic tests were carried out to assess the ductility capacity of the KBF, which is shown to be suitable for seismic design. A set of design charts are thus produced, using the program DRAIN-2D, for proportioning the member sizes to meet the seismic design requirements.

Bifurcations of subharmonics and chaotic motions of articulated towers

Abstract The dynamic responses of articulated towers, subjected to regular waves and large motions, are computed numerically using the governing nonlinear differential equations. In certain frequency and system parameter ranges, the motions of the towers are found to exhibit subharmonics and chaotic behaviour which are identified to be the type of intermittency with random alternations of chaotic and regular behaviour in time evolution. The identification, characterization and evaluation of the chaotic motions are performed numerically by studying the discrete trajectories and Poincare points of the responses. Such chaotic behaviour, if with significant magnitude, can make the responses of the deterministic system random and unpredictable in a practical sense, because the system is extremely sensitive to the initial conditions assumed.

Structural monitoring by system identification in time domain

A system identification (SI) procedure to identify structural parameters is proposed in this paper. Instead of identifying parameters in the physical domain, which involves search in a large-dimensional parameter space, the search is made easier computationally in sub-domains orders. Modal decomposition technique is employed. The objective function is the minimization of error of modal response history. Physical parameters are then recovered by making use of modal orthogonality properties. The genetic algorithm is employed based on the principle of natural selection whereby better genes survive and propagate and therefore increase the change of converging to the best parameter set. For numerical study, a 10-story shear building is considered to illustrate the efficiency of the proposed procedure.

Chaotic and periodic responses of a coupled wave-force and structure system

Abstract Numerical results of chaotic and subharmonic responses of a cylinder subjected to the coupled interaction of wave-force and structure are presented in the forms of phase plane plots and Poincare maps. The system studied is deterministic and the excitations are regular, but in addition to the expected periodic responses, including harmonics and subharmonics, the system is found to behave irregularly in certain parameter and initial condition regions. The irregular motion and the subharmonics accompanying it indicate possible chaos via period-doubling bifurcations. The chaotic nature of the responses is confirmed by numerical divergence tests.

Bifurcation behaviour and wave slamming force on a compliant horizontal cylinder

Abstract The effect of a horizontal wave slamming force on the dynamic behaviour of a horizontal compliant cylinder is studied from the point of view of bifurcation analysis. It is found that, even though the time history responses of the system may change significantly, the patterns of bifurcations are not very sensitive to the slamming force.Therefore, the parameter ranges for the bifurcations can be reasonably determined without a complete knowledge of the exact values of the parameters. An understanding of the bifurcation behaviour of the system may have important practical relevance, e.g. in forecasting the possible parameter ranges in which the nonlinear system can be approximated by an equivalent linear system.