TBT: Superior Fire Resistance

Recent years have seen a lot of research and tests on the fire performance of Mass Timber, particularly focused on CLT structures. But, the basics of reliable fire performance for large structural wood components have been understood for a long time now, as you can see from this AITC brochure from 24  years ago. 

For the average person, it can be easy to think of "Combustible" materials like wood being less safe than "Noncombustible" structural materials. Most buildings are full of Combustible contents, so the key for safety is actually how the structural components perform in the heat of a fire.

All structural materials are affected by heat - at higher temperatures they lose strength. Steel is particularly susceptible, so in most structures all steel must be protected by other insulating materials ("fireproofing") to prevent rapid failure during a fire. Wood is an excellent insulator by itself, So, for large wooden components the outer surfaces can char, while the interior maintains its strength.

Because wood members are insulating themselves, they preform predictably during a fire. There's no question about how well fireproofing was applied or whether it has been damaged, the fire resistance is part of the large wooden component itself. This understanding is the basis for Type IV "Heavy Timber" construction, which is basically a recognition that structures composed of large wood members have safe and predictable performance in fires.