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Fire Impacts on Point Supported Mass Timber

16 April 2024

Ricky McLain: More and more attention is being paid to point supported CLT over the past couple of years as it has proven to be a viable solution for materially-efficient structural framing systems. 

Some of the most notable high-rise mass timber projects in North America have used it:

– Brock Commons in BC, really the catalyst for tall timber in America
– 1510 Webster in Oakland, CA, the modern demonstration that the speed of install promises for mass timber are real (47 days to install 16 stories of timber)

But looking a little deeper, one thing is common with both of these projects:

They both use multiple layers of gypsum wallboard to fully encapsulate the timber. 

Now to be clear, I’m not at all saying that these solutions aren’t valid uses of mass timber. The speed of install and thin structure depth resulted in real cost savings. 

But understandably, many more projects want to leverage the structural benefits of point supported CLT while exposing most or all of the timber. 

This is where we need to dig a little deeper. A scenario that I often hear developers or design teams want to replicate includes these common factors:

– Point supported CLT
– 2 hour fire-rating (for IV-C or IV-B)
– Pushing grids to 12×15 ft
– Exposing most of the CLT ceilings

Oh, and make that a 5-ply panel, because volume of fiber drives up costs.

Unfortunately, this isn’t a realistic option.

But Brock Commons and 1510 Webster used 5-ply (or similar MPP thicknesses) with similar grids?

Yes, but remember that the mass timber was fully encapsulated in GWB.

In a one-way CLT condition, exposed for fire-conditions, the primary design criteria that drives panel thickness is vibration or deflection. And in a post-fire scenario it’s bending. 

With a two-way CLT condition, punching shear usually controls the panel specs. With both of the projects noted above, the GWB encapsulating the mass timber benefited the design in the post-fire scenario, where the structural requirements were impacted only a little, if at all, by fire design since the GWB took care of most of the fire resistance needed.

So what can you do? A couple of feasible solutions:

1. Use strips of GWB on the underside of the mass timber, centered on the column lines. This minimizes the fire impacts on punching shear (since the panels would not need to be designed for full fire exposure PLUS max. punching shear). This is a great reference on the topic: https://shorturl.at/aefH2

2. Use a thicker panel. I co-presented in a webinar last week with Eric McDonnell of Holmes US who shared details of a project where they are evaluating a 7-ply V rated panel and a custom 6-ply E rated panel, for full exposure in a point supported system.

P.S. in a point supported system, the floor panels will usually be part of the primary structural frame (rather than floor construction) due to their direct connection to columns. This can drive up required FRR. Plan accordingly.

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