|Title:||Lateral and torsional vibration control of long span bridge deck using novel tuned liquid column dampers|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Bridges, Long-span -- Vibration
|Department:||Department of Civil and Structural Engineering|
|Pages:||1 v. (various pagings) : ill. ; 30 cm|
|Abstract:||Long span cable-supported bridges are rapidly increasing nowadays not only in number but also in the length of central-span because of their inherent economical and technical advantages. However, these long span bridges are very flexible and lightly damped and vulnerable to wind-induced vibration. In particular, during the erection, long span bridge deck lacks continuity from pylon to pylon and its rigidity is much lower than that of a completed bridge. Serious buffeting vibration has been observed from a few long span cable-supported bridges during construction. Reviews of the existing literature, however, show that few studies have examined the suppression of lateral and torsional vibration of long span bridges. This thesis thus focuses on the development and application of novel tuned liquid column dampers for suppressing lateral and torsional vibration of long span cable-supported bridges during construction and at completion stage. The first part of the thesis, consisting of Chapters 3 and 4, presents a combined experimental and theoretical investigation on the performance of multiple tuned liquid column damper (MTLCD) for reducing torsional vibration of structures in comparison with single-tuned liquid column damper (STLCD), with particular focus on the sensitivity of its performance to frequency tuning. A large structure model simulating the torsional vibration of a bridge deck and several STLCDs and MTLCDs of different configurations are designed and constructed. A series of harmonically forced vibration tests are conducted to evaluate the effectiveness of MTLCD in reducing torsional vibration of the structure. An averaging method is also developed to identify the head loss coefficients of STLCD and MTLCD in conjunction with free vibration test technique. An analytical model for the torsional vibration of the structure with an MTLCD under either harmonic excitation or white noise excitation is then developed and verified using the obtained experimental results together with the identified head-loss coefficient. The performance of MTLCD in reducing torsional displacement is further investigated through extensive parametric studies using the verified theoretical model. It is found that the sensitivity of an optimized MTLCD to the frequency tuning ratio is less than that of an optimized STLCD and it can be further improved by increasing the bandwidth but at the cost of smaller torsional vibration reduction. The frequency of STLCD or MTLCD depends solely on the length of liquid column, which imposes certain restrictions on its application to long span bridges. The short period of torsional vibration of long span bridges may require short liquid column length to have a proper frequency tuning. Consequently, a large number of such small TLCD containers are required, which leads to a higher cost of installation and maintenance. Multiple pressurized tuned liquid column dampers (MPTLCD) are thus studied in Chapter 5 to facilitate torsional vibration reduction of a structure and to improve the sensitivity under mistuning. The MPTLCD container is sealed with an air chamber at its two ends. The frequency tuning can be adjusted by manipulating static pressure inside the air chamber while the length of liquid column is fixed. An analytical model is developed for torsional vibration of a structure with a MPTLCD under either harmonic or white noise excitation. The nonlinear damping due to orifice and the nonlinear restoring force due to air pressure in the MPTLCD are linearised in the frequency domain. After such linearization is proved to be satisfactory through a comparison with a nonlinear analysis in the time domain, extensive parametric studies are finally carried out in the frequency domain to find the beneficial parameters by which the maximum torsional vibration reduction can be achieved. The investigations demonstrate that MPTLCD can provide a greater flexibility for application in practice and achieve a high degree of vibration reduction. To control vibration of a structure with high natural frequency, the MPTLCD with a longer liquid column length can be used to replace the STLCD or MTLCD with a shorter liquid column length. However, for a long span cable-supported bridge during construction, its very low natural frequency may require a STLCD or a MTLCD with very long liquid column length, which may not be possible in practice. Moreover, natural frequencies of a long span bridge vary during its construction stage. Hence, tuned liquid column dampers with adaptive frequency tuning capacity are developed in Chapter 6 for suppressing lateral or torsional vibration of a structure using a semi-active control technology. The natural frequency of the semi-active tuned liquid column dampers (SATLCD) studied herein can be adjusted by active control of air pressures at the two chambers of a PTLCD. Analytical models are developed for lateral vibration of a structure with SATLCD and torsional vibration of a structure with SATLCD, respectively, under either harmonic or white noise excitation. The nonlinear damping property of SATLCD is linearized using an equivalent linearization technique. Extensive parametric studies are carried out in the frequency domain to find the beneficial parameters by which the maximum vibration reduction can be achieved. The investigations demonstrate that the SATLCD can provide a greater flexibility for its application in practice and achieve a high degree of vibration reduction. The sensitivity of SATLCD to the frequency offset between the damper and the structure can be improved by adapting its frequency precisely to the measured structural frequency. Wind-induced vibration of a long span bridge involves many modes of vibration. Large vibration may result from coupling of different modes of vibration. Most of previous studies pertaining to the suppression of wind-induced vibration of a bridge focused on the coupling of vertical and torsional vibrations. With an increase in the span length and complexity of a bridge deck, significant mechanical and aerodynamic coupling may exist between the first lateral and torsional vibration under turbulent winds. However, little information is available on this topic. The use of MTLCD for reducing the coupled lateral and torsional vibration of a bridge deck is therefore explored using mode-by-mode spectral approach in Chapter 7. The equations of motion for coupled lateral and torsional vibration of the bridge deck are formulated. The efficiency of MTLCD in reducing the coupled lateral and torsional vibration is investigated through extensive parametric studies. The results show that the MTLCD can reduce both the lateral and torsional vibrations of the bridge deck effectively if the parameters are properly selected. The aeroelastic effects due to the interaction between turbulent wind and bridge motion is a crucial factor which affects the performance of MTLCD in reducing buffeting responses of the bridge deck. The flexibility of MPTLCD in frequency tuning offers wider choice of container configurations, which makes it easier to be installed in a real long span bridge in its completion stage. The use of MPTLCD for reducing the coupled lateral and torsional vibration might be an alternative solution for some long span cable-supported bridges with relatively high lateral or torsional frequency. The performance of MPTLCD for the suppression of lateral and torsional vibration of a long span bridge deck at the completion stage is investigated using finite element based approach in Chapter 8. The prediction of buffeting response of long span bridge is usually done by the finite element method in addition with the aerodynamic characteristics of the bridge obtained from wind tunnel tests. A finite element model of MPTLCD is thus developed and incorporated into the finite element model of a long span bridge for predicting the buffeting response of the coupled MPTLCD-bridge system in the time domain. The investigations show that the MPTLCD not only provides great flexibility for selecting liquid column length but also significantly reduces the lateral and torsional displacement response of a long span bridge under wind excitation. The configuration of a long span bridge varies from different construction stages and so do its natural frequencies. It is thus difficult to apply TLCD with a fixed configuration to the bridge during construction or it is not economical to design a series of TLCD with different liquid column length to suit for various construction stages. The use of SATLCD with a fixed container configuration is thus studied in Chapter 9 for the suppression of the lateral and torsional vibration of a long span cable-supported bridge during construction. The finite element model of SATLCD is also developed for the prediction of buffeting response of the coupled SATLCD-bridge system and the assessment of the control performance of SATLCD in the time domain. Five different construction stages of the bridge are selected for the study of the SATLCD performance and adaptability. It is found that with a fixed container configuration, SATLCD can effectively reduce the lateral and torsional vibration of the bridge deck under all the five construction stages.|
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