Making a non-corrosive connection
A non-metallic, non-glue, connection system for timber structures could find use in corrosive environments, such as swimming pools and grit salt storage facilities, as well as the renovation or restoration of historic frames.
The system, developed at Bath University, UK, uses glass fibre reinforced polymer (GFRP) dowels and densified veneer wood (DVW) flitch plates.
These materials can be cut and machined using ‘non-specialist tools’, says lead researcher and PhD student Andrew Thompson.
The provision of non-corrosive connections reduces the need for specialist metallic fasteners in corrosive environments and provides a compatibility with the corrosive extractives found in hardwood timber. ‘They also offer the potential for improved fire resistance, [whereas] metallic fasteners lose strength in fire and conduct heat into the middle of a connection’, adds Thompson.
The aim has been to investigate the structural performance of a non-ferrous connection that is suitable for onsite fabrication and assembly. The research builds upon previous work, carried out at Bath University, on the structural performance of all-timber, traditional mortice and tenon connections.
Initially, work focused on the use of oak dowels and birch plywood plates, ‘however, the use of GFRP dowels and a DVW plate has demonstrated a significant improvement in terms of load capacity and robustness, and the use of these materials has not been previously reported’, claims Thompson.
The DVW plate can be drilled in a single operation with the timber member, allowing companies to manufacture connections in-house. The engineers at Bath also believe the technology will be useful for hardwoods to make structural timber members, as the system provides a cost effective alternative to galvanised or stainless steel connections.
Fifty GFRP-DVW connections have been tested in the investigation phase and pull-out loadings of connections made with multiple dowels and a single central flitch plate have been explored. ‘The tests were completed for parallel and perpendicular to grain load orientations. In particular, the parallel to grain, tests investigated inline spacing of GFRP dowels,’ outlines Thompson.
The load carrying efficiency of GFRP-DVW connection dowels has been further improved by positioning the dowels closer together than metallic dowels. ‘Comparative tests loaded parallel to grain have demonstrated that, for the same connected area of timber, a GFRP-DVW connection can reach a load capacity approximately equal to the Eurocode 5 design strength of an equivalent metallic connection. Experimentally, the GFRP-DVW connections provide approximately 50-60% of the strength of a metallic equivalent.’
However, he notes that the use of GFRP-DVW connections is still in its infancy. Further work is required to fully characterise the long-term performance of the connections in addition to fire testing.
Raymond Gilfillan, a former Structural Engineer, comments, ‘The connection technique offers positive advantages in relation to corrosion resistance, fire performance and buildability, as well as improved joint aesthetics. A concern may be the reduction in strength relative to metallic equivalents, since the size of members in a timber structure is often dependent on the connection design.’