Tensile properties and load distribution of multi-bolted pultruded basalt FRP joints with multi-directional fiber lay-ups

Pultruded fiber-reinforced polymer (FRP) composites have gained significant popularity in truss structures in recent decades. However, the inherent brittle feature of pultruded FRP plates results in uneven load distribution among bolts, weakening the composite material's strength advantage. This study focuses on experimental investigations of single-column, multi-row bolted joints in pultruded fiber-reinforced composites. The aim is to assess the tensile properties of these joints and analyze their load distribution and failure characteristics. Studied parameters include hole location, row counts, bolt pre-tightening forces, plate thicknesses, and bolt sizes. To evaluate the uneven load distribution between bolts in multi-bolted composite joints, the concept of the bolt row load distribution factor ($U_e$) is introduced. The research findings highlight the transition in failure modes from brittle shear-out failure to ductile pin-bearing failure when the end-to-diameter ratio was more than 3.5 and pitch-to-diameter ratio was over four. The bolts in the first row experienced the earliest failure due to their higher load share. However, increasing the pre-tightening force or diameter of bolts in the middle row effectively mitigated load distribution unevenness. Moreover, a calculation method for determining the joint strength of multi-bolt composite joints is presented.