Load transfer mechanism in punched metal plate timber connections
The load capacity of the punched metal plate timber joints is established, in general, by empirical means as a result of destructive testing in accordance with relevant national standards. The basis of tests is tensile or compressive loading applied parallel and perpendicular to the grain of the timber. In general, the design-analyses of trusses are based on the assumption that joints behave as pins due to the concentration of fasteners in a small area limiting the moment arm. A number of testing methods and apparatus were developed to determine the behaviour of the punched metal plate timber joints under different types of loading (tension,compression and moment). A combined programme of experimental and analytical work was carried out to evaluate the semi-rigid characteristics of the punched metal plate timber joints with respect to the level of translation and rotational rigidity under short term loading. The effects of different parameters such as load and deformation rates, number and length of bites, thickness of the plates and the orientation of the plates and timber grains were considered. The load-displacement and moment-rotation characteristics were studied and empirical models were developed to simulate displacements up to failure loads. The study results show that the strength and stiffness of the joints can be expressed in terms of connector parameters. Increasing deformation rate, number of bites, length of bites, thickness of the plates and decreasing plate and grain orientations would increase the strength and stiffness of the joints. Also, the results show that the punched metal plate connections can possess a considerable moment capacity. A statistical technique was used to classify the level of importance of parameters such as number of bites, length of bites and grain direction on the performance of the punched metal plate timber joints. All the specimens were tested under both tension and compression loads up to failure. From the tests and analysis carried out it was found that the grain direction had significant effect on the performance of the joints under tensile loading and the effectiveness of the grain direction was less when joints were subjected to compressive loading. There was a strong indication that the effect of the number of bites was dominant when joints were subjected to compressive loading. A design flowchart for punched metal plate timber joints is provided incorporating the research findings into a design/analysis process.