Experimentation on historic timber trusses to identify repair techniques compliant with the original structural–constructive conception

The paper focuses on historic timber trusses to characterize the original failure mode and identify suitable repair techniques. 4 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 submitted to symmetric load carrying test up to failure. For all the assemblies the failure mechanism advantageously occurred for sliding shear in the tie beam toe (the heel) with subsequent structure capability to support additional loads. The king post to tie beam node configuration was found to influence the ultimate load value and the overall structure deformation without changing the original mode of collapse. The most common type of truss (in surveyed Italian areas) was then selected (6m, 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. Only one technique, the one with internal glued plates, enhanced the overall load carrying capacity but proper measures should be taken to avoid the sudden failure of the tie beam end. The introduction of lateral plates with screws restored the original load carrying capacity, improved the node ductility and reduced the upper ridge lowering.