This document aims to summarise the results of the recent finger joint tensile tests (EN 408) and delamination tests (EN 14080) conducted at Bern University of Applied Sciences, as detailed on report 42HV-015636-L-01-PB-01, dated 10th June 2024. These tests were performed on three American hardwood species: Tulipwood, white oak and red oak (in both its natural and thermally modified form), as well as European oak, provided by Neue Holzbau at their request.

The primary objective of these tests were to evaluate and compare the performance of these timber species in two critical properties relevant for glue-laminated timber construction: bonding adhesion of finger joints and lamellas. These properties are essential for the successful manufacture of glue-laminated beams.

The testing was initiated by AHEC to demonstrate the suitability of American red oak for glue- laminated beam manufacture in a structural project for London Design Festival 2024, with a view to its potential for future commercial applications. Given that the structure is intended as an outdoor installation, it was necessary to consider a durable timber species. For this reason, thermally modified red oak, known for its enhanced durability, was included in the testing programme, despite concerns about potential reductions in strength due to brittleness.

The test specimens were manufactured under industrial conditions at Neue Holzbau AG’s factory in Lungern, Switzerland. The finger joints used for tensile strength testing were bonded with Loctite HB S109 adhesive, and the lamellas were bonded using Loctite HB S709.

Results 

A summary of the results shows that, in the finger joint tensile strength tests, tulipwood, red oak, white oak and European oak all fulfilled the test requirements for tensile strength in accordance with EN 408. However, thermally modified red oak did not meet the requirements, with 11 of 30 test specimens failing significantly below the minimum test time. Tulipwood achieved the highest tensile strength at 39.5 N/mm2, followed by red oak at 34.6 N/mm2. American white oak achieved 31.9 N/mm2, and European oak recorded 30.4 N/mm2.

In the delamination tests, tulipwood was the only species for which all test specimens passed. Both red oak and thermally modified red oak had one failure at the shorter closed time of 30 minutes but passed the longer 60-minute test. European oak passed only one test at the longer time, while white oak failed comprehensively with not one test specimen fulfilling the test requirements even at the longer time period. All species, except white oak, demonstrated better resistance with the extended closed assembly time of 60 minutes.

Summary

The results from these two tests are highly encouraging from an American hardwood perspective, proving that tulipwood and red oak have significant potential for future commercial glue-laminated timber (Glulam) production. Both Bern University and Neue Holzbau were surprised by the exceptional performance of these two species, which they had not previously worked with. Equally surprising was the performance of European oak and American White oak in the delamination tests, where both species underperformed. This failure was attributed to the density of the timber, but this must give rise to some concern when these two timbers have been used in commercial hardwood glulam projects in the past.

While the delamination tests for thermally modified red oak showed reasonable results, the disappointing performance in the finger joint tests eliminates it as an option for manufacturing commercial beams for exterior applications.

It is widely acknowledged within the timber construction industry that there is limited information regarding the performance of hardwoods for glulam production. This is largely due to the higher cost of the raw material and comprehensive testing required to meet rigorous standards, such as European Technical Approvals (ETAs). However, securing ETA approval is not necessarily a barrier to manufacture, as only Germany seems to strictly require ETAs for structural timber design. Other European countries, including Spain, France and those in Scandinavia, appear to be less insistent on this requirement.

The advantages of using hardwoods in glulam beam production include stronger beams with smaller cross-sections, delivering higher performance with less material compared to softwoods, and particularly spruce, which is the main material currently used for glulam beams. To reduce reliance on a single species such as spruce, architects and designers need to be made more aware of the structural and sustainability benefits hardwoods can offer.

The results outlined above clearly demonstrate that red oak and tulipwood provide the timber construction industry with a viable alternative to other hardwoods currently used in glulam production, such as American and European white oak, ash and beech. These two species are abundant in the U.S. temperate hardwood forests, are economically priced, and offer consistent quality and grading, with a consistent moisture content between 8-10% making them highly suitable for manufacturing. They are at least as strong as other hardwoods commonly used in glulam, such as Beech and ash.

It is hoped that these results will be compiled into a formal report and circulated to the construction industry, with potential for presentation at the next Mass Timber Conference in Portland, Oregon, next year.

Further Work

Looking ahead, if further testing is feasible, it might be beneficial to test entire beams structurally to confirm their structural strength in a real-world . This would allow for the classification of the beams into a GL structural strength class, which could then be correlated with the acoustic strength class achieved by individual boards used for the manufacture of VERT.

Martin Lehman raised an important point regarding the different species of red oak and whether variations in regional growth might influence the results. He also highlighted that Neue Holzbau’s manufacturing process -notably their priming and gluing techniques- is likely superior to most other glulam producers, which might have contributed to the better test outcomes.

Find out more about LDF project Vert’s use of red oak glulam here