Glulam structures with large spans – halls, bridges, sports arenas – are today joined using slotted-in steel plates and dowels. The steel leaves a significant climate footprint, and the pre-drilled holes in plate and timber demand high precision in assembly.
There are examples of projects where steel has been successfully replaced with various types of timber connections, including in architect Shigeru Ban's acclaimed seven-storey office building Tamedia in Zurich. But transferring the technique to an industrial working method has not yet been done successfully.
This was one reason why KTH's Department of Building Materials chose to take on the challenge. “Connections are the most important element in a timber structure. But you can't weld timber in this case. We thought it was a shame to have to use so much steel in these nodal points – both from a climate perspective and because the steel causes precision problems during assembly,” says Magnus Wålinder, professor and head of the research group.
Birch plywood similar to metal
The KTH team has discovered that birch plywood has properties more similar to metal than to timber. And Magnus Wålinder champions birch, which he considers underutilised as a building material.
“Among sheet materials, birch is in a class of its own. It has much higher tensile, compressive, flexural, shear and embedment strength than both spruce and pine. It's higher than you might expect. Oak may have higher density, but birch generally has higher tensile strength and modulus of elasticity anyway,” he says.
The research has been driven with modest funds, financed by the Swedish Forest Industries, Swedish Wood and the Swedish Timber Building Council. Two Chinese doctoral students, Yue Wang and Tianxiang Wang, have been the project's backbone, under the leadership of Roberto Crocetti, adjunct professor at KTH and expert in timber construction and particularly connection technology.
Their research has shown that birch plywood performs its task both with screws and dowels, and that connections can be designed so failure becomes ductile and predictable, rather than the brittle failure that is otherwise common for timber.
“According to our test results from both laboratory and full-scale trials, birch plywood can be designed so it doesn't constitute the weakest link in timber connections. It has great potential as a replacement for the slotted-in steel plates that currently dominate in large-scale timber structures,” says Tianxiang Wang.
From research results to practical application
In the short term, the research group hopes the technique can be tested industrially in halls and bridges, where spans are longest and steel's climate footprint greatest. Glulam manufacturer Moelven has already trial-built with birch plywood connections, and the industry's initial calculations suggest significant cost savings compared to steel.
The long-term goal is for the technique to be used in practice, something that would be facilitated by approval under Eurocode 5 – the European standard for design of buildings and structures in timber. When this might happen is unclear, according to Tianxiang Wang.
“Structurally, I'm convinced that birch plywood can be sufficiently strong. But further work is required to establish the most effective methods for protecting the material against moisture and improving its durability in outdoor applications. Fire resistance is expected to meet the requirements in current standards, but this must be verified and quantified through fire tests,” he says.
Text: Mattias Boström AI-generated illustration: Intellecta
