- LocationVancouver + Prince George, BC
- TypeTesting Program
- FundingNSERC, GCWood NRCAN, CleanBC Building Innovation Fund
After 6 months of experimentation in Concept Lab, the team has completed our punching shear testing of point-supported CLT floor and roof systems in January 2024. Point-supported CLT stands out as one of the most efficient mass timber building systems, and we are glad to contribute to the advancement and widespread adoption of this structural system.
Six repetitions of many variables for the point-supported CLT floor system were tested, including:
In total there were 30 different configurations for a total of 180 tests conducted in our lab, and additional full-scale panel testing is scheduled to be completed at UNBC, Prince George.
The data from this testing program will be shared publicly and sent to code-review committee members to get point-supported CLT recognized as an engineered mass timber building system in the future Canadian building codes. This will help other engineers to understand the state-of-the-art methods for designing, analyzing, and detailing point-supported CLT panels, and in turn will help to maximize the use of our important timber resource to create sustainable and efficient structures.
Can a damaged CLT panel be repaired?
After completing our point-supported CLT testing program with the Concept Lab team, the CL team set out to answer this important question. When testing point-supported CLT panels in our Concept Lab, observers were impressed by how much these panels could be pushed before reaching their peak strength and the substantial elastic spring back when the load was removed. The panels exhibited clear warning signs, such as visible displacement and noticeable cracking noises, when nearing overload. After reaching peak strength, the load was supported with significant additional displacement, showcasing a non-brittle behavior. In practice, a heavy load would likely be removed before causing the panel to fail, underscoring the importance of this non-brittle behavior for structural safety.
However, a lingering question remained: Is a CLT panel repairable after being damaged? To explore this, we conducted tests on dozens of unreinforced 5-ply CLT panels, as well as several arrangements with fully threaded reinforcing screws. Thus, we aimed to investigate the concept of reparability. We took previously tested unreinforced panels and added reinforcing screws, allowing us to retest them and determine if the original panel strength could be restored.
The typical screw reinforcing pattern increased the punching shear capacity by 30% compared to unreinforced SPF (S7.X) and DFir (S8.X) CLT samples.
Retesting w/o screw reinforcing: Two panels, S7.4 and S8.1 were retested without adding reinforcing, and the specimen was softer and weaker than the original test (orange dashed lines in the attached plots). The strength was 85% (SPF) and 73% (DFir) respectively compared to original panels.
Repair and retesting with screw reinforcing: The retested samples with added reinforcing were also softer than the original tests (see the green curves). The repaired panels strength was about 107% (SPF) and 97% (DFir) respectively compared to original panels.
The reinforced retested samples were therefore increased by 30% from the unreinforced retest. The reinforcing screw pattern worked just as well on a panel that was already overloaded and broken.
This testing shows that point-supported CLT panels can be repaired with fully-threaded reinforcing screws to meet or exceed their original design strength in the event that a panel was overloaded and significantly damaged.
It also demonstrates that an unreinforced panel can be practically reinforced in-situ if there is a change in building use that requires increased strength. This is a remarkable demonstration of the resiliency and adaptability of mass timber!