Modal analysis and buckling effects on steel structures under dynamic loading

Dynamic loading of steel structures may result in overstressing the structural elements beyond the elastic limit. This research investigated a few problems associated with the determination of the critical buckling stress for cylindrical shells under lateral dynamic loadings. In addition, steel braced towers often used to support elevated tanks were analyzed to investigate their inelastic dynamic performance.; The first phase of research involved the experimental determination of the modal properties (natural frequencies, mode shapes, and damping ratios) of three tank models and a steel braced tower using forced vibrations. Random signals within a given frequency range were used as input to a small shaker and the structural response was measured by sensitive accelerometers. The experimental values obtained were compared to the theoretical characteristics resulting from available finite element analyses of fluid-structure systems. Experimental results were also compared to analytical solutions for the identification of the natural frequencies. This comparison can be used to assess the effects of out-of-roundness imperfections on the structure response and to indicate whether the knockdown factor used in design is sufficient to assess the safety of the actual structure.; The second phase of research involved studying the buckling criterion of cylindrical shells. The buckling of cylindrical shells under lateral loading is completely different from the conditions of ideal, axially-loaded cylinders which are used to specify the critical buckling stress. Critical horizontal shear and critical hydrodynamic pressures required to buckle a cantilever circular cylindrical shell were calculated using a semi-analytical method assuming membrane pre-buckling deformation. A series solution for the critical load was obtained and the convergence of the numerical solution was tested. The effects of axial loading and hydrostatic pressure on the critical lateral load were also examined.; The final phase of research dealt with the response of braced towers under dynamic loading. The inelastic analysis included the calculation of the response of the bracing members if the stresses exceed the yielding limit in tension or the buckling limit in compression. An inelastic model was used to investigate the behavior of individual members as well as the whole structure.