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

rawpixel.com values around 1.4, signifying an overestimation of approximately
40 percent compared with the normalised reference depth.
For CLT elements, the effective charring depth exhibited
wider variation than Glulam, with experimental results ranging
between 25 mm and 90 mm depending on panel thickness
and lamella configuration.

The mean normalised effective charring depth for all standards
ranged from 1.3 to 1.4, comparable to those for Glulam.
However, the standard deviation for CLT was substantially
higher, ranging from 30 percent to 40 percent, nearly double
the variability observed for Glulam.
This elevated variability was primarily attributed to CLT's
layered construction and the complex interaction between
rate predictions relatively close to the measured data, typically lamella orientation, adhesive degradation, and heat transfer.
within about 10–20 percent. However, the AS/NZS 1720.4 The ZSL represents the depth of timber immediately behind

standard consistently predicted higher charring rates than the char front that has lost all effective strength due to thermal
both the experimental results and other standards, indicating degradation. For Glulam beams, the mean ZSL thickness
a conservative bias of roughly 20 percent on average. averaged approximately 7 mm across codes, demonstrating
Conversely, Eurocode 5, Swedish, and CSA O86 standards a relatively stable and predictable response.
exhibited close agreement, clustered within ±10 percent of The AWC TR10, which defines ZSL thickness as 20
tests across datasets. percent of the charring depth, overestimated the mean value
For CLT elements, all three available standards (Swedish, by approximately 20 percent and exhibited a standard deviation

AWC TR10, and CSA O86) exhibited noticeable discrepancies roughly 40 percent higher than the other codes.
in charring rates when compared with experimental test data. For CLT elements, ZSL thickness varied substantially,
Both underestimation and overestimation of charring rates ranging from approximately 6 mm to 44 mm. All international
were observed across datasets, indicating that none of the codes underpredicted the ZSL thickness when compared to
existing standards provided a fully reliable prediction for the the experimental results.
charring behaviour of CLT elements. The mean ZSL derived from the test data was approximately
The observed variation can be attributed to the complex 2.5 times the 7 mm reference value, while the standard deviation
thermo-physical properties of CLT, including adhesive bond line exceeded 140 percent, indicating substantial variability. These
performance, layer orientation, and delamination tendencies. results confirmed that the constant ZSL assumption, while

The effective charring depth represents the combined adequate for Glulam, was not appropriate for CLT design.
thickness of the charred layer and the adjacent ZSL, signifying For Glulam beams, most standards produced values
the depth to which timber loses its structural capacity under fire comparable to experimental data, suggesting that their predictive
exposure. For Glulam beams, most codes yielded predictions models provided reasonable estimations of post-fire performance.
within ±20 percent of the reference data, suggesting reasonable However, AS/NZS 1720.4 consistently predicted lower
global agreement. residual strengths than the experimental results, indicating a
However, AS/NZS 1720.4 consistently produced mean conservative bias with normalised values typically 30 percent