Pull-out performance of double glued-in rods in novel InorgBam loaded parallel-to-grain

Although glued-in rods (GiRs) connections are widely used in contemporary bamboo/timber constructions for their superior strength and stiffness, the group effect in multi-rod configurations can compromise individual load capacity. The pull-out behavior of double GiRs in a novel inorganic-bonded bamboo composite (InorgBam) loaded parallel-to-grain was investigated in this study. A total of 48 pull-out tests were examined for the effects of rod spacing and anchorage length (la) on load capacity and failure modes, using Grade 4.8 threaded rods with a nominal diameter of dr = 14 mm. Results demonstrated that the theoretical critical anchorage length (la,critical) for double GiRs was approximately 10 dr, matching single-GiR values. For la ≤10 dr, the ultimate load increased with threaded rod spacing, characterized by shear failure of InorgBam following threaded rod pull-out. For la >10 dr, failure was governed by threaded rod fracture after yielding, rendering spacing effects negligible. An empirical formula for the effective number of threaded rods and a predictive capacity model of double GiRs in InorgBam were developed based on single and double GiRs data. The proposed model showed good agreement with experimental results, with an average relative error within 10%, providing valuable design guidance for double GiRs in InorgBam material.