Bonds have been severely tested at the Structural Timber Innovation Company (STIC) in recent months. Bonds of engineered timber (LVL and glulam) that is - all in the name of producing best practice guidelines to help fabricators and designers of EXPAN components.
Due out later this year, the guidelines will include observations and recommendations on glue lines and surface preparation prior to gluing, as well as surface finish to help fabricators and architects specify a desired appearance grade.
The guidelines come out of many months of industry consultation and laboratory testing at Auckland University and The University of Canterbury. Research Project Manager Johann Betz, says with laminated veneer lumber (LVL) being a relatively new material in New Zealand, it’s important to get information out to designers and industry about how the material behaves to enable better design solutions.
So what went on in the lab?
Firstly, the research team tested the strength and performance of the glue bonds, using shear block testing – transferring load by applying pressure to two pieces of LVL glued together. Small lab-produced samples of LVL were tested, then larger members fabricated by industry. The tests showed the glue line was indeed strong, holding up to intense stresses.
Glulaminated timber needs an adequately thin glue line to perform well, especially when bonding together large timber members. The researchers delved into this, and came up with an ideal range for glue line thickness, included in the guidelines.
They also looked at the durability of glue lines when exposed to moisture. This testing put the bond through its paces well beyond what the product would encounter in real world conditions – enduring cycles of soaking and drying - and proved itself extremely durable.
And to give fabricators more options of adhesives, the work tested the performance of resorcinol, a traditional adhesive used in the glulam industry for decades, and a polyurethane adhesive. Both met requirements and produced good results. It also looked at glulaminating LOSP (light organic solvent preservative) treated LVL, again with positive results.
There’s been a lot of talk within the industry about surface preparation – and whether LVL timber can be successfully re-laminated in the form it comes in from the supplier. It was widely believed that the surface first needed to be planed in order to achieve maximum strength and durability of glue bonds. Lab testing on gluing without surface preparation showed no difference in the performance of the glue bonds, on small laboratory samples at least - findings that could bring significant benefits to the fabrication process by eliminating the need for surface preparation in certain situations.
STIC has also received a lot of feedback from the industry about the need for common terminology for standards of finish of LVL. So the fabricators guidelines will define three appearance grades, and bring a common language around surface quality, evenness and finish to fabricators and architects.
Phase three of the project is currently underway, looking at the option of screw-gluing LVL together, to get around the need for large, expensive pressing equipment, and enable more fabricators to produce EXPAN components.
Guidelines will then be developed on which types of screws to use, and recommended screw spacing for best performance.
Johann says it was vital the guidelines had industry input.
“The guidelines are the result of a team effort – with academics consulting with fabricators, suppliers, architects and designers throughout the entire process. Ultimately, the guidelines are to help all the different parties better understand the material they are working with, but also understand each other’s challenges, to help smooth any problems along the supply chain.”
AT A GLANCE:
Tests have been carried out on bonds on LVL and glulam engineered timber.
This has been done to further product development and respond to industry requests.
Guidelines will be out shortly with full details.
Highlights of the research;
The glue line is strong.
It is extremely durable
Overall highly positive results.
Bonds of engineered timber (LVL and glulam) have been put through their paces.