Gyp Board Distress At Ceiling Peak - Prefab Truss 2

[bigmig]

We design residential wood framed houses frequently with vaulted interior pitch (known as a scissor truss).
Does anyone have a reference explaning or experience dealing with how far the truss can kickout (horizontal deflection)
before the gyp board ceiling seem rips? In the attached example, the truss deflects -0.45 inches. Which is a LONG
ways and in my opinion impractical to expect a gyp board tape seem to stretch that far.
The goal is to avoid callbacks (Hey Mr Engineer, I have a split in my ceiling) and just overall better quality designs.

 

[bigmig]

@DoubleSTud you are right, I can see that now.

 

[XR250]

When I stand in that court, your opinions will serve as zero
reference for the crack in my clients ceiling.

Guaranteeing your client's sheetrock won't crack is a fool's errand. I would just design the trusses for L/480 Live load and call it a day. You are dbl. code at that point and it would be hard for anyone to argue against in court as std. practice is less than that.

 

[dauwerda]

In the attached example, the truss deflects -0.45 inches. Which is a LONG
ways and in my opinion impractical to expect a gyp board tape seem to stretch that far.

Lets clear up one thing. The tape seam in no way needs to stretch that far. The horizontal deflection is due to rotations, not the bottom chord/gyp board getting longer. Your peak deflects downwards, causing small rotations at the peak and where the truss meets the wall. It's a change in geometry, not a change in length.
In the image below the red and blue lines are the exact same length, the red lines have just been rotated 1 degree. The only movement the tape will see will be the rotation itself and a distance that is a function of the sin of the angle it rotates and the thickness of the drywall.

 

[EdStainless]

A good drywaller knows to leave a little slack in the tape at joints like this.
It is more likely to crack from variations in temp and humidity in the house.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Tmoose]

Will the drywall be applied directly to the trusses' lower chords ?

Here in Massachusetts it was fairly popular to attach residential ceiling drywall to ?x3 furring strips, at least it was 30 years ago.

In other areas ceiling furring/strapping was considered WRONG. The Internet suggests it still is.

For rooms with a room above I used a very flexible USG (?) meshed steel furring/strapping system for much reduced sound transmission.

https://sp-ao.shortpixel.ai/client/...2011/06/furring_channel_resilent_channel5.jpg

The flexibility seemed to let the drywall be as flat as it wanted to be even when the ceiling joist had other ideas.

Internet legend has it US Gypsum originally developed a metal resilient furring channel to help prevent drywall cracking.

 

[gte447f]

bigmig, I get your frustration with some responses that sorta kinda come across sounding like hand waving away an issue and saying just don't worry about it. But on the other hand, if you want code references, I am pretty sure that you won't find any code requirements for interior drywall finishes to remain uncracked, so on those grounds, you could maybe not worry about it, because its not a thing. In reality, I would worry about satisfying the code required vertical deflection limit, and since you know these types of ceiling are more susceptible to deflection, maybe use a tighter deflection limit than is required by code. Say, use L/480 instead of L/240. The design you posted is less than L/999 for live load, so you are more than good there.

For horizontal deflection, the nearest thing to a code requirement is for exterior walls exposed to wind loads. The IRC limit varies from H/120 to H/180 to H/240 to H/360 depending on the flexibility/brittleness of the wall finish materials. These lateral deflection limits are probably meant to be evaluated at mid-height for walls simply supported in the vertical direction between diaphragms, which is not the same as your situation. For your situation you could look to drift limits. These are not codified, but are included in the IBC appendices for reference. Typical limits range between H/400 and H/700, if memory serves. Realizing that half of your horizontal deflection/translation will occur at each end of the truss, 0.225 inch would be in the ballpark of H/500 for a 10 feet tall wall.