Timber based building systems - a laboratory experiment
A laboratory experiment for the master students
When studying “Timber based building systems” the students have to carry out four different projects. One of the projects concerns design calculations and testing of three different connections for timber structures.
In the laboratory
The students gathered in the laboratory for a run-through of the test set-up before actually testing three connections with different types of loads. The connections were made with a slotted-in steel plate fastened with steel dowels in the wood.
The aim of the laboratory experiment was to compare the calculated maximum load according to the Eurocode 5 with the reality. The first connection was loaded in pure tension with the load in the same direction as the wood fibers. The first small signs of the load could be seen as a small crushing of the wood fibres directly above the dowels. At the maximum load a crack formed in the wood above the dowels which caused the load level to drop. During subsequent loading more cracks in the wood developed around the dowels.
What happens with different loads?
Two more connections were loaded but the pure tension load was changed to different combinations of tension and moment load. The changed loading mode resulted in loads both parallel to the fibre direction and perpendicular to the fibre direction. The load direction perpendicular to the fibre direction causes a force that tries to split the wood fibres and will therefore help with the creation of cracks in the wood. The results from these tests showed that the maximum load was lower than for pure tension.
Two more connections were loaded but the pure tension load was changed to different combinations of tension and moment load. The changed loading mode resulted in loads both parallel to the fibre direction and perpendicular to the fibre direction. The load direction perpendicular to the fibre direction causes a force that tries to split the wood fibres and will therefore help with the creation of cracks in the wood. The results from these tests showed that the maximum load was lower than for pure tension.
Study 3D-images
The experiments were also monitored with the optical measurement system Aramis. This system creates the opportunity to study 3D-images of all local deformations within the tested object. With this system it is possible to see which dowel is most heavily loaded and what the major direction of the resulting force is.
The results from the experiment were compared with the results from the design calculations. An analysis of the results showed that the real failure loads were between 40-60% higher than the design failure load according to the calculations. Designing according to the codes will give results on the safe side.
The laboratory experiment was a part of the course Timber based building systems in the master program in Civil engineering with a specialization in wood building technology. The programme includes advanced courses on structural design and finite element analysis. The courses try to combine practical studies about the development of new products and technology with a thorough theoretical base, preparing for work in industry as well as for further studies towards a PhD.
