K.C.S. Kwok

Optimization of tuned liquid column dampers

Abstract The effectiveness of a tuned liquid column damper (TLCD) in controlling structural vibration is studied. A numerical method is adopted to account for nonlinearity of the governing equation. Optimum parameters of the TLCD for maximum reduction of peak structural response to harmonic excitations in a wide frequency range are presented for a wide range of flexible structures. A variation of U-shaped TLCD with different cross-sectional areas in its vertical and horizontal sections is investigated. It is found that an increase in cross-sectional area ratio (vertical/horizontal) can greatly reduce the length requirement of a TLCD making it more attractive to flexible structures. A new type V-shaped TLCD is also investigated. This type of TLCD can suppress stronger vibrations such as those caused by strong wind. It is found that, like the tuned mass damper (TIVID), there is an optimum tuning ratio (damper frequency/structure frequency) for maximum reduction in response of a structure system which is independent of the excitation. The optimum coefficient of head loss will depend on the intensity of the excitation with smaller coefficient of head loss associated with stronger excitation.

Physical and numerical modelling of thunderstorm downbursts

Abstract In this study, a static continuous impinging jet has been used to simulate a thunderstorm downburst. Velocity characteristics have been investigated at various positions in the flow over flat terrain, and over simple topographic features, thereby yielding topographical speed-up factors. A generic empirical equation describing the development of the velocity profile over a flat board has been developed. Comparison with previous works with impinging jets and conventional boundary layer flow speed-up factors has been completed. Speed-up factors were similar at the crest of the embankment, but dropped off rapidly behind the crest compared with boundary layer flow. Preliminary studies using computational fluid dynamics to predict the flow regime over topographical features has been shown to give reasonable agreement.

Performance of tuned mass dampers under wind loads

The performance of both passive and active tuned mass damper (TMD) systems can be readily assessed by parametric studies which have been the subject of numerous research. The results of those parametric studies are invaluable in the selection of optimum TMD parameters. Few experimental verifications of TMD theory have been carried out, particularly those involving active control, but the results of those experiments generally compared well with those obtained by parametric studies. Despite some serious design constraints, a number of passive and active tuned mass damper systems have been successfully installed in tall buildings and other structures to reduce the dynamic response due to wind and earthquakes. The results of extensive full-scale measurement programs conducted on a significant number of these clearly show the effectiveness of the systems.

Interference excitation of twin tall buildings

Abstract The enhanced dynamic response of a tall square building under interference excitation from neighbouring tall buildings has been studied in a series of wind-tunnel model tests. In a low-turbulence wind environment and under normal strong wind conditions, the dynamic loads on the upstream of an identical pair of tall buildings may increase by a factor of up to 4.4. The dynamic loads on the downstream building of the pair may increase by a factor of up to 3.2 due to “resonant buffeting”. Measurements of along-wind and cross-wind force spectra and a number of wake spectra provide an explanation for the observed behaviour. Possible excitation mechanisms are discussed and critical building arrangements presented. The large interference loads found in this study indicate that interference excitation should be carefully considered in the design of tall buildings.

Use of viscoelastic dampers in reducing wind- and earthquake-induced motion of building structures

Viscoelastic (VE) dampers have been successfully incorporated in a number of tall buildings as a viable energy dissipating system to suppress wind- and earthquake-induced motion of building structures. This type of damper dissipates the buildings mechanical energy by converting it into heat. Several factors such as ambient temperature and the loading frequency will affect the performance and hence the effectiveness of the damper system. VE dampers have been able to increase the overall damping of the structure significantly, hence improving the overall performance of dynamically sensitive structures. The effectiveness of VE dampers has been demonstrated both experimentally and analytically by many researchers over the past 25 years. The twin towers of the World Trade Center Buildings in New York City and the Columbia SeaFirst Building in Seattle, Washington, are among the first buildings which benefited from the installation of VE dampers. In seismic applications, the VE dampers can be incorporated either into new construction or as a viable candidate for the retrofit of existing buildings which adds to the versatility of VE dampers.

Active control of along wind response of tall building using a fuzzy controller

Abstract This paper focuses on the benchmark problem application regarding the vibration control of tall buildings under along wind excitation. The building under consideration is the 76-story, 306 meter tall reinforced concrete office tower proposed for the city of Melbourne, Australia. The adopted control scheme consists of an Active Tuned Mass Damper (ATMD) where the control action is achieved by a Fuzzy Logic Controller (FLC). The main advantage of the fuzzy controller is its inherent robustness and ability to handle any non-linear behaviour of the structure. This benchmark study is based on specified design constraints for the ATMD to be considered in the design of the proposed control scheme. The robustness of the controller has been demonstrated through the uncertainty in stiffness (+15% and −15% variation from initial stiffness) of the building. The results of the simulation show a good performance by the fuzzy controller for all cases tested. Also the results show that the fuzzy controller performs slightly better than the LQG controller, while possessing several advantages over the LQG controller.

Eigenvector modes of fluctuating pressures on low-rise building models

Proper orthogonal decomposition, i.e. determination of the eigenvectors and eigenvalues of the covariance matrix, is applied to fluctuating wind pressure fields on low-rise building models. Two cases are considered: (A) centre-line pressures, representing the tributary area for a building frame, and (B) pressures along a transverse line on a roof, representing pressures on a purlin. In both cases, the pressures have been measured on panels using pneumatic area averaging. The main conclusions are that the mathematical constraint of orthogonality has a dominant effect on the mode shapes, and association of the modes with physical causes may be fictitious in many cases.

EFFECT OF BUILDING SHAPE ON WIND-INDUCED RESPONSE OF TALL BUILDING

Abstract Wind tunnel model tests were conducted to investigate the effect of building shape on the wind-induced response of a tall building with a rectangular cross-section. It was found that horizontal slots, slotted corners and chamfered corners caused significant reductions in both the along-wind and cross-wind responses. The wake spectra, probability distributions of the responses, and the along-wind and cross-wind force spectra were examined to determine the cause for these changes in response characteristics.

Effect of edge configuration on wind-induced response of tall buildings

Abstract Wind tunnel model tests were conducted to investigate the effect of edge configuration on the wind-induced response of a tall building with rectangular cross-section. It was found that slotted corners and chamfered corners, and combinations of these, caused significant reductions in both the along-wind and cross-wind responses.

Mode shape corrections for wind tunnel tests of tall buildings

Abstract General mode shape correction factors are suggested to adjust aeroelastic model test results to prototype values of tall buildings for alongwind, crosswind and torsional vibrations. These factors were found to agree well with previous experimental and analytical results in the literature.