Low Cost, Light-Weight Buckling Restrained Braces for Low Rise Buildings

Abstract

Buckling restrained braces are becoming an increasing attractive alternative to conventional concentric bracing in seismic loading, due to their ability to have equal compression and tension capacities in the inelastic range. However, they are currently required to be designed and verified by testing, making them an expensive solution. This thesis investigates a novel form of light-weight, low cost buckling restrained brace for low rise buildings in developed and developing countries. The brace composes of a steel bar core as the primary yielding element surrounded by low density expandable polyurethane foam, providing full lateral restraint against buckling in compression, thus allowing the inelastic action in compression. These are encased in a bamboo culm which acts elastically to develop the required lateral restraint. Three designs were investigated, all achieving the tension member capacities. However the compression capacities were significantly less, due principally to the difficulty in centrally drilling the bamboo nodes, resulting in a compression member with significant as cast out of straightness. This resulted in induced P-delta effects which resulted detrimentally in low compression member capacities. The Endurathane 3245-100R expandable polyurethane foam was insufficient in providing significant lateral restraint. The second design achieved member compression capacity, however the higher density, Endurathane 245-60 expandable polyurethane foam and bamboo capacities were not optimised, using surplus restraint to counteract the natural camber of the bamboo which resulted in induced P-delta effects and Euler buckling forces. A final design with full node removal of the bamboo and use of a flat bar to oppose any natural camber within the bamboo was carried out. It was found that there was a significant loss in stiffness due to the removal of the nodes. The Endurathane 245-60 and steel core bound together acting as one unit. The total member capacity achieved equalled the combined member capacities of the Endurathane 245-60 and steel. Future testing has been recommended. In conjunction with physical testing, mechanical properties of the bamboo were determined revealing significant strength and elastic characteristics. It has been recommended that future investigation into a bamboo V brace system be undertaken. Finite element modelling was carried out in ABAQUS 6.11, however this was unsuccessful in determining a valid model due to the sensitivity of the crushable foam element and the resulting time taken to achieve workable results.