Seismic performance evaluation of concentrically braced and fiction damped braced frames through full-scale testing
Creators: name: Kullmann, Harald Georg
Publisher: name: University of British Columbia
Year: 2012

description: The study describes the analyses of a six storey concentrically braced frame (CBF) and an equivalent frame retrofitted with friction-damped bracing (FDBF). The CBF was designed according to the ductile braced frame requirements of the Canadian steel building standard CAN/CSA-S16.1-M89 (S16.1) and the National Building Code of Canada 1990. The optimum slip load for the friction dampers in the FDBF was determined on the basis of energy principles and its distribution was assumed to be uniform throughout the structure. Full-scale quasi-static cyclic and earthquake simulation testing was conducted on the storey predicted with the most severe damage. The analytical predictions of these two systems show that their overall response, in terms of peak interstorey drift, is similar. Full-scale tests conducted on the CBF using hollow steel structural sections for the cross-bracing show that the bracing may develop twice the energy dissipation ability predicted by computer analysis. This appears to be due to the coplanar interaction of the braces. Significant additional stresses are induced in the bracing due to this interaction, which the computer cannot model. Hollow structural section cross-bracing appears to provide significantly better energy dissipating properties than HSS single diagonal bracing or multilevel cross-bracing; however, this may be at the expense of early fatigue failure. The tests show that, when compared to other bracing configurations, significantly fewer load cycles can be carried by such braces prior to fatigue fracture. The test results indicate that the slenderness requirements set out in S16.1 appear to be adequate for the design of cross-bracing if based on the half length, as opposed to the full length of the brace diagonal. The width-to-thickness ratio requirements were inadequate to effectively delay fatigue fracture of the braces. The optimum slip-load study using the optimizing program FDBFAP for the six storey FDBF under investigation shows that the best response is obtainable with a frame shear resistance of 80 kN. A more detailed analysis using the program DRAIN-2D showed that a local optimum existed at 80kN, but that the global optimum was 320kN. The testing of the FDBF showed that the friction damper can be a reliable method of dissipating energy. The locally installed friction pads used in the damper were, however, unreliable for the slip-load level required. Significant fade in the slip-load occurred during cyclic loading. The friction pads failed on two occasions, and at a panel shear slip-load (base shear) of approximately 130 kN. The tests show that the compression brace remains in compression during an entire half cycle of loading. This implies that a buckled brace is not restraightened with the initiation of slip, but rather with load reversal, resulting in a slack in the hysteresis during loading and unloading, and therefore a reduction in the energy dissipation. Out-of-plane vibrations occurred during the testing. These vibrations appeared to be the result of a prestress which is induced in the system in the deformed position. The tests also indicated that significant brace bending occurred. This bending was found to be a function of the ratio of the frame panel size to damper size. It is expected that the combination of bending and tensile forces due to the prestressing effect can lead to brace yielding.