Fatigue behavior of threaded rods bonded into glued-laminated timber for application in bridge construction

Due to the difficult climatic conditions and the fatigue stresses caused by traffic loads, timber is used comparatively little in bridge construction. Connections with bonded in rods (BiR) are of particular interest for structures with fatigue loads, as they allow a very uniform force transfer. So far, most of the fatigue strength tests have been carried out on BiR connections in dry wood. Transferability to applications in bridge construction, where higher moisture contents prevail, is limited. Focusing on the potential of BiR connections in bridge construction, fatigue tests with 2 million load cycles, and an R -value of 0.1 (pulsating tensile domain) were conducted on BiR parallel and perpendicular to grain in European ash and Norway spruce glued-laminated timber (GLT) with varying moisture content (MC). The tests showed that threaded rods bonded into ash and spruce GLT can be used to create strong and very stiff connections in components with MC of up to 20 %. In the fatigue tests, at least 3 specimens achieved the targeted 2 million load cycles in all 4 series with 3 – 4 experiments in each series. The adhesive joints were not the weakest component in any of the specimens. In the fatigue tests with BiR parallel to grain in specimens made of ash GLT, a high nominal bond line shear stress was generated, and the sufficient fatigue strength of the connection was demonstrated. In all test series (dry wood and wood at elevated MC, static and fatigue loading) with BiR perpendicular to grain in spruce GLT, the failure of the timber zone between the rows of rods in rolling shear, was found to be critical.