An experimental study of timber beams reversibly flexural strengthened with screwed steel plates

Since it is desirable that methods for strengthening historic structures especially architectural heritages are reversible, one of such methods for strengthening timber structures was proposed to use screwed external steel plates, which can be easily removed. An experimental study was reported, in which 11 timber beams were tested, including three reference unstrengthened beams and eight beams strengthened with steel plates screwed on their tension face. The diameter and distance of screws were varied. Some of the steel plates and screws were used repeatedly in different specimens. The test results showed that the ultimate flexural strength of the timber beams was increased by 16%~79% (with an average of 50%) due to strengthening. The initial stiffness of the strengthened beams was also significantly increased by 21%~38% (with an average of 30%). A plan section remained almost plan for the timber beam, but noticeable difference in strain existed between the bottom fibre of the timber beam and the steel plate unless the latter was attached onto the former with closely spaced screws. Despite of this difference, the moment capacity based on the normal section analysis was in reasonably good agreement with the test results. A finite element analysis was also conducted and its predictions were also in good agreement with the test results. It was concluded that strengthening timber beams with screwed steel plates is a reversible and effective method which can be implemented with ease in practice.