Timber spatial trusses using laminated veneer lumber

A simple and innovative preloaded dowel-nut connection is proposed for high-performance spatial structures made of Laminated Veneer Lumber (LVL). To demonstrate the connection feasibility, three prototypes were built with spruce LVL trusses connected at steel spheres: non-preloaded connections were used in prototype 1; conversely, a tightening torque was applied to prototypes 2 and 3, with the latter featuring a thicker washer in contact with the truss end sections to make the compressive stress distribution more uniform. First, the three structures were subjected to cyclic load tests under vertical service loads, and connection efficiency was validated by comparison with an all-steel spatial truss. All LVL prototypes showed linear elastic behaviour, but the response of trusses with preloaded connections was stiffer. Prototypes 2 and 3 also showed very narrow loading cycles and absence of residual deformation at the unloading. Then, strength tests driven to final collapse were performed for the timber structures. Premature connection failure at one end of the top strut was achieved for LVL prototype 1. The load-carrying capacity achieved by prototypes 2 and 3 was 38 % and 43 % larger, respectively. For these trusses, preloading the connections resulted in a more efficient compression behaviour, which led the top strut to buckle prior to collapse by end connection failure. Apart from the ruptured joint, the other strut end sections of prototype 2 exhibited evident plastic deformation, whereas, for prototype 3, they arrived apparently undamaged at the end of test. A load-deflection data analysis, including Southwell plots, for the top struts of prototypes 2 and 3 confirmed that the buckling loads of these structures was reached in the tests.