To model dissipation of energy in vibrating civil structures, existence of viscous damping is commonly assumed primarily for mathematical convenience. In such a classical damper, the damping force is assumed to depend only on the velocity of deformation. Fluid viscous dampers that provide this type of damping have been manufactured to provide supplementary damping in civil and mechanical systems to enhance their performance. Some fluid dampers, however, exhibit stiffening characteristics at higher frequencies of deformation. The force deformation relationship of such dampers can be better represented by the Maxwell model of visco-elasticity. This model consists of a viscous dashpot in series with a spring, the latter element providing the stiffening characteristics. This study is concerned with the optimal utilization of such Maxwell dampers for seismic performance improvement of civil structures. The force deformation relationship of Maxwell dampers is described by a first order differential equation. Earlier studies dealing with these dampers, used an unsymmetric set of equations for combined structure and damper system. The solution of such equations for response analysis or for optimization calculation by a modal analysis approach would require the pair of the left and right eigenvectors. In this study, an auxiliary variable is introduced in the representation of a Maxwell damper to obtain symmetric equations of motion for combined structure and damper system. This eliminates the need for working with two sets of eigenvectors and their derivatives, required for optimal analysis. Since the main objective of installing these dampers is to reduce the structural response in an optimal manner, the optimization problem is defined in terms of the minimization of some response-based performance indices. To calculate the optimal parameters of dampers placed at different location in the structure, Rosen's gradient projection method is employed. For numerical illustration, a 24-story shear building is considered. Numerical results are obtained for seismic input defined by a spectral density function; however, the formulation permits direct utilization of response spectrum-based description of design earthquake. Three different performance indices -- inter story drift-based, floor acceleration-based, and base shear-based performance indices-- have been considered to calculate the numerical results. A computational scheme is presented to calculate the amount of total damping required to achieve a desired level of response reduction. The effect of ignoring the stiffening effect at higher frequencies in the Maxwell model on the optimal performance is evaluated by parametric variation of relaxation time coefficient. It is observed that the models with higher relaxation time parameter show a decreased response reducing damping effect. Thus ignoring the stiffening effect when it is, indeed, present would provide an unconservative estimation of the damping effect. The effect of brace flexibilities on different performance indices is also investigated. It is observed that flexibility in a brace reduces the effectiveness of the damper. / Master of Science

Steel-deck composite floor systems are being increasingly used in high-rise building construction, especially in Australia, as they are economical and easy to construct. These composite floor systems use high strength materials to achieve longer spans and are thus slender. As a result, they are vulnerable to vibration induced under service loads. These floors are normally designed using static methods which will not reveal the true behaviour and miss the dynamic amplifications resulting in inappropriate designs, which ultimately cause vibration and discomfort to occupants. At present there is no adequate design guidance to address the vibration in these composite floors, due to a lack of research information, resulting in wasteful post event retrofits. To address this gap in knowledge, a comprehensive research project is presented in this thesis, which investigated the dynamic performance of composite floors under various human induced loads. A popular type of composite floor system was selected for this investigation and subjected to load models representing different human activities. These load models have variable parameters such as load intensity, activity type (contact ratio), activity frequency and damping and are applied as pattern loads to capture the maximum responses in terms of deflections and accelerations. Computer models calibrated against experimental results are used in the analysis to generate the required information. The dynamic responses of deflections and accelerations are compared with the serviceability deflection limits and human comfort levels (of accelerations) to assess these floor types. This thesis also treats the use of visco-elastic (VE) dampers to mitigate excessive vibrations in steel-deck composite floors. VE damper properties have been presented and their performances in reducing the excessive vibrations have been assessed this thesis. The results identified possible occupancies under different loading conditions that can be used in planning, design and evaluation. The findings can also be used to plan retrofitting measures in problematic floor systems.