Page 52 - FDMAsia Mar/Apr 2026

50 MATERIALS MAR/APR 2026 FDM ASIA | www.fdmasia.com

Findings Glulam residual strength predictions were reliable across
Based on the comparative analyses and discussions presented, all standards except AS/NZS 1720.4, which underestimated
this study provides a comprehensive evaluation of the fire residual strength by 20 percent to 30 percent.
performance of Glulam and CLT structural elements benchmarked CLT residual strength exhibited high scatter, with standard
against international design standards. deviations exceeding 50 percent, caused by non-uniform
The results demonstrate that Glulam generally exhibits charring and interlaminar shear loss.

consistent and predictable behaviour, while CLT displays Residual stiffness predictions for both Glulam and CLT
greater variability due to its layered composition and adhesive were inconsistent across all standards, generally overestimating
sensitivity under fire exposure. stiffness recovery. CLT results exhibited variability approaching
Most international standards predicted Glulam charring 100 percent, driven by delamination and adhesive softening.
rates within ±20 percent of experimental data. AS/NZS 1720.4 In conclusion, the current suite of international design
consistently overpredicted charring rates and effective charring standards remains adequate for Glulam but insufficient for
depths by approximately 40 percent, resulting in conservative accurately predicting CLT fire performance.
yet less accurate outcomes. This study highlights the need for continued experimental
CLT exhibited higher variability in effective charring depth, research and refinement of design models, particularly within

with standard deviations between 30 percent and 40 percent, the Australian context where specific CLT fire provisions are
primarily due to adhesive degradation and delamination. currently lacking.
For Glulam, all considered design codes predicted charring Incorporating adhesive performance, lamella configuration,
rates within ±20 percent for timber densities greater than 450 and delamination mechanisms into future standards will be
kg/m³, with the lowest standard deviations. essential to improve predictive reliability and support the
For timber densities below 450 kg/m³, charring rate development of safe, efficient, and performance-based fire
predictions were within ±8 percent except for AS/NZS 1720.4. design for mass timber structures.

CLT charring rate predictions were within ±20 percent across By directly comparing Glulam and CLT within a unified
all densities, but scatter was higher, with standard deviations analytical framework, this review clarifies where current design
of up to 35 percent. standards are dependable and where they are not.
For CLT panels, all codes overpredicted charring rates Practically, the results support confident use of simplified
for MUF adhesives, with the largest overprediction around methods for Glulam, while recommending adhesive-sensitive,
40 percent for AWC TR10 and standard deviations below 10 time-dependent models for CLT to account for delamination
percent, while 1C-PUR adhesives were underpredicted by and char loss.
about 20 percent, with higher variability. For regulators, the outcomes provide evidence-based priorities
Under standard fire, all codes overpredicted CLT charring for near-term updates, particularly in AS/NZS 1720.4, to incorporate

rates, with a maximum of 20 percent for AWC TR10 and adhesive qualification, lamella configuration, and ZSL evolution.
standard deviations around 30 percent. For practice, the review underpins risk-consistent performance-
Under natural fire, charring rates were underpredicted by based fire design, encouraging targeted testing and advanced
40 percent, with variability of 10 percent. The ZSL for Glulam analyses where CLT is left exposed. These steps will help
was accurately captured by fixed value models, while CLT realise the safety, sustainability, and constructability benefits
displayed significantly larger and more variable ZSLs due to of mass timber at scale. FDM
interlaminar failure and adhesive degradation. ENQUIRY NO. 2401