Wood Diaphragm Stiffness with Gypcrete 5

[brlee]

Hi All,

I am evaluating a 3/4" wood diaphragm with 3/4" of gypcrete. Most sources I find suggest that the gypcrete has a significant stiffening effect. However, it's hard to find resources that quantify that change in stiffness for calculating diaphragm deflections and for semirigid modeling.

Any recommendations or resources would be much appreciated.

Thanks

 

[jayrod12]

Ignore it.

A couple of reasons, number 1: who knows what the bond is like between the gypcrete and the plywood. number 2: when people spill stuff and it goes on long enough the gypcrete loses strength and bond. Number 3: Some flooring manufacturer's are beginning refuse to warranty their floors if installed on gypcrete, if it's no good for supporting flooring, why would I want to count on it to resist lateral loads.

In the end, the bulk of the stiffness is from the connection of the sheathing to the supporting members and lateral resisting systems. 3/4" T&G plywood is plenty strong and stiff enough for most applications. If you're at a stage where that is no longer enough, maybe you need to consider additional vertical braces/shear walls to allow the diaphragm to not work so hard.

 

[ChorasDen]

I would generally call 3/4" gypcrete non-structural. As such ASCE 7-16 Section 12.3.1.1 would call this a flexible diaphragm for analysis purposes. I know this isn't exactly what you are asking, but it's a reference to consider.

What's the attachment method between the gypcrete and decking? What was the surface roughness of the decking prior to gypcrete application?

 

[phamENG]

I wouldn't bother accounting for it - gypcrete is brittle and I question the bond between it and the floor sheathing. It's not like we design them as composite in bending.

Diaphragms constructed of untopped steel decking or wood structural panels are permitted to be idealized as flexible if any of the following conditions exist:

c. In structures of light-frame construction where all of the following conditions are met:
1. Topping of concrete or similar materials is not placed over wood structural panel diaphragms except for non-structural topping no greater than 1 1/2 in. (38 mm)thick.
2. Each line of vertical elements of the seismic force-resisting system complies with the allowable story driftof Table 12.12-1.

 

[phamENG]

jayrod and ChorasDen beat me too it. Took too long looking up the direct reference!

 

[Brad805]

If you ever look at gypcrete during a reno you will see it is best to ignore it. Depending on your contractor it may not turn out like you think, but the trades following can fill problems easily with different products.

 

[XR250]

Yea, it pretty much just ends up in loose chunks over time.
Especially because it is usually on 24" joist spacing so the subfloor is not stiff enough to prevent cracking.
Do you actually need that extra stiffness?

 

[brlee]

Thanks everyone! I'll definitely be ignoring it after these responses.

XR250, I don't need it. It would have helped with shear wall fastener spacing on the exterior walls.