Catenary action to prevent progressive collapse in multistorey timber buildings : An experimental study of tube joints

Sammanfattning: In the modern building industry wood as a building material has been looked on as one of the solutions to lower the environmentally impact of the building industry. Building types that normally have been reserved for concrete and steel are today being built with wood as main load bearing component. However, the structural robustness of high-rise wood building has been questioned. The concern relates to wood’s brittle failure-mode and therefore the capacity of wood buildings to withstand catastrophic events where a loadbearing structural part its removed from the construction. To counteract a brittle failure mode in wood and create a ductile failure mode that allows alternative load paths, ductile steel connectors are seen as a solution. With inspiration from the concrete and steel industry, catenary action is brought forward as the most efficient method to create an alternative load path in high-rise wood buildings. To create catenary action, a tube connector that allows excessive deformation with increasing strength was believed to have sufficient capacity. A test method for testing tube connector capacity in a catenary event was developed with a main focus on tube capacity and forces created on the surrounding structure in a catastrophic event. To achieve this, a long stroke reversed 4-point bending test was formed and testing of traditional fasteners and the tube connector was conducted. Test result from catenary capacity testing showed low to no capacity for traditional fastening methods, however, traditional fastening method created arcing action in the beginning of the test cycles. The traditional fasteners showed a consistency in failure mode with arching and wood crushing before connection failure in form of screw withdrawal. Tube connectors had low to no consistency in failure modes, but higher consistency in capacity. Failure mode regarding tubes consisted of steel failure in tube and wood failure in the test specimen. In general, the tubes connector showed promising capacity and ductility to create alternative load paths with the help of catenary action. The project has shown that some design development for the tube connector and lager scale testing are needed to completely understand tube connector capacity and failure mode.