IBC 2009 - Concrete assemblies

[pmarx]

Specifically, I'm having a bit of trouble understanding 721.2.1.4.

I need to increase the fire-resistance rating of a existing 5" thick pre-cast concrete panel construction from 2 HR to 3 HR. This is making the presumption that the worst case concrete (siliceous) of that thickness is worth 2 HR per Table 721.2.1.1.

Going to 721.2.1.4, it looks like I have three choices if I want to increase the panel rating by adding layers of 5/8" Type X GWB.

721.2.1.4.1-Nonfire-exposed side

721.2.1.4.2 Fire-exposed side

721.2.1.4.3 Nonsymmetrical assemblies

From a practicality standpoint, we would like to add layers of GWB to one side only (Nonsymmetrical assembly). I've read the commentary but I'm still not sure how to apply 721.2.1.4.1 and 721.2.1.4.2 as required by 721.2.1.4.3.

I would be very grateful if someone could explain this to me.

 

[RLGA]

Is the assembly an exterior wall? If so, what is the fire separation distance?

 

[pmarx]

It's an interior condition where the potential for fire exposure is at both sides. From a construction standpoint, no demolition is required on one side of the concrete panels so we are trying to use the nonsymmetrical approach. The opposite side has some cold formed furring and GWB that we don't propose to use for any fire-resistance contribution.

 

[JBI]

Have you tried looking at the UL designs? or the Gypsum Association materials?

For your project it appears the 40 minute rating won't be adequate, and I'm not sure if you can add multiple layers using only 721.

Also, consider having the material evaluated for actual content or adding GWB to both sides of the concrete wall.

 

[Francis Vineyard]

pmarx,

This calculation is the same for other wall construction types as applicable. The fire-resistive rating is the least or weakest fire-resistive side. From the calculations done in 721.2.1.4.1 – nonfire exposed side and 721.2.1.4.2 - fire-exposed side; the least resistive of these two is the rating for the wall.

The nonfire-exposed calculation measures the thickness of the wall for total rating; the fire-exposed calculation adds the time assigned to the existing wall. A quick look at this formula; combining 1.2" (two-5/8's) of gypsum wallboard (multiplier of 1.0) assumes 3 hr. nonfire exposed side and whereas the fire exposed side the gypsum assumes adding 80 min. to the wall. The wall is 3 hrs. based on the weakest side.

Aware that precast concrete panels are typically an engineered product where the manufacturer may have proprietary guidelines.