Pull-out performance of multiple glued-in rods in sustainable inorganic-bonded bamboo composite under perpendicular-to-grain loading

As a sustainable bio-based material derived from bamboo crops, inorganic-bonded bamboo composite (InorgBam) exhibits excellent environmental sustainability, fire resistance, and long-term durability. To enable its broader adoption in structural applications, robust and efficient connection methods are essential. Glued-in rod (GiR) connections, characterized by high strength, stiffness, and superior fire performance, are especially well-suited for modern engineered bamboo and timber connections. The pull-out behavior of multiple GiRs in InorgBam material under perpendicular-to-grain loading was experimentally investigated using double-, triple-, and quadruple-rod specimens. The influence of anchorage length and spacing on failure modes and load-carrying capacity was systematically analyzed. Results demonstrate that increasing the number of rods, spacing, and anchorage length enhances both stiffness and pull-out capacity. The pull-out capacity of individual rod is reduced by group effects in multiple GiRs configurations compared to that of isolated single GiR. Based on experimental results, an empirical expression for the effective number of rods was developed, along with a predictive model for pull-out capacity. These findings provide an experimentally grounded basis to inform the potential engineering application of multiple GiRs connections in InorgBam material.