Fire Rating: Wednesday's Word Of The Week

A Fire Rating is the time a structural component or wall is calculated to maintain integrity during a fire. 1 Hour is a common rating.

Last week we covered Type IV Construction (Heavy Timber), which does not require specific fire ratings for glulam parts, just minimum sizes. However, glulam can also be used in almost every other construction type. For roof construction, Heavy Timber is allowed where 1 hour or less fire rating is required, so there are no other special considerations. For other framing members, a Fire Rating might be required.

A glulam member can be checked for a 1 hour fire rating by calculating the depth of char, assuming that wood is lost on all exposed sides, and analyzing the remaining wood section under load. For this calculation, realistic loading and expected strengths can be used (rather than more conservative values typically used for design.) For large wood members, frequently no size change is required.

Although the overall size may not need to change, the bottom Tension Ply of the beam has been lost, so Fire Rated beams add one or more additional Tension Ply (by removing low strength core plies.) A Fire Rated design also needs to consider steel connections, which will lose their strength faster than wood. They can be protected with intumescent paint, or by covering in wood for protection - using knife plates and plugging bolt holes.

*Wednesday's Word Of The Week is a feature on Unalam's Wood Times Blog. Each Wednesday our structural engineer, Rik Vandermeulen, will discuss a new term associated with glulam manufacturing. He will do this until we run out of words. If there is a timber or glulam term that you have heard of and want to know more about, let us know in the comments.