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The paper focuses on historic timber trusses to characterize the original failure mode and identify suitable repair techniques. Four full-scale trusses with different spans (6 and 12 m) and assembly modes for the king-post to tie beam connection (close or open node) were subjected to symmetric load carrying test up to failure. For all assemblies the collapse advantageously occurred because of heel shear failure in the rafter to tie beam node with subsequent structure capability to support additional loads thank to the connecting metal devices. The king post to tie beam node configuration was found to influence the ultimate load value and the overall stiffness of the structure without changing the original mode of collapse. The type of truss most common in the surveyed Italian areas was then selected (6 m, open node) to test up to failure four different heel repair techniques: screws, bolts, lateral steel plates with screws, internal steel plates with epoxy resin. The results show that only one technique, the one with internal glued plates, enhances the overall load-carrying capacity but proper measures to avoid the sudden failure of the tie beam end should be taken. The introduction of lateral plates with screws restores the original load-carrying capacity, improves the node ductility and reduces the upper ridge lowering. |
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