Condition assessments after a fire often involve determining the load capacity of the remaining structural elements.  Are you familiar with the equations used to determine the effective depth of char and the remaining cross-sectional dimensions as prescribed in the NDS for mass timber structural elements?

In the July 2025 SEU session, Erica Fischer, PhD, PE, from Oregon State University presented Hot Topic: The Fire Performance of Mass Timber.  Erica reviewed the current code and standard guidance for the fire protection design for mass timber, and noted current research on mass timber and fire to enable performance-based design.  She also summarized some challenges with mass timber and fire.

Erica walked through the following example to determine the depth of char and the effective depth of char for a glulam beam exposed to fire.  

To hear Erica explain this procedure, watch this short video:

As Erica explained, it is necessary to use the effective depth of char to account for the zero stiffness layer beneath the char.  This effective char depth should be used to calculate section properties on reduced cross-sectional dimensions for structural calculations.  These equations for the char depth and effective char depths for sawn lumber and mass timber, as well as CLT can be found in the National Design Specification (NDS) in Chapter 16.  While both char depth equations are non-linear and proportional to the nominal char rate, CLT also includes factors to account for the lamination thickness, the number of laminations charred, and the time for the char front to reach the glulam interface, as shown below.

This specific equation used for CLT char depths accounts for known gaps in the layers of laminations which accelerates the char rate and gives additional pathways for the fire to propagate.  Engineers should be aware of the difference when using CLT members to account for this acceleration in the char rate, to ensure accurate remaining section properties can be established.