Drywall – How to store between 30 and 50 sheets of QuietRock ES

It's difficult to model the situation with rational analysis, there's too many intangible factors. You could do an empirical test. You need to support 20 lbs per fastener. We can apply a safety factor of 3 for ultimate strength, so the fastener should support 60 lbs without actually breaking. So you would need 2-4 fasteners to support your weight. Round down to the closest whole number. Install the clips as you did in the wall, except now install a metal strap between the screw head and clip. Arrange the straps so you can step into them to weight the system. Arrange the straps such that your weight is distributed evenly to each fastener.

Weight the system and see if they break. If you live in a seismic area, bounce on them a bit and see if they break. You'll either be able to sleep better or you'll know what to do next, depending on the outcome. Obviously there are better ways to set up an empirical test, I chose to illustrate a quick and dirty method just as an example. Be sure you are protected from flying shards of metal.

Regarding an increaser for the number of fasteners. No, you can't do that. It is a valid concept though, for example you can use a higher allowable bending stress in multiple floor joists than you can in a single use situation such as a header. The concept is not generally applied to fasteners.

Response to OP's Update

Shear strength in relation to fasteners partly depends on what the fastener is holding. In this case it's known as a metal side plate condition, meaning the expected failure mode will either be the top of the screw failing through the shank (shear) or the wood collapsing under the compression from the screw. It's rare in reality to have a perfect shear condition, there is usually some bending and tension components as well.

A true shear condition would something like a metal strap screwed to the wood surface and all the force was parallel to the wood surface, exactly perpendicular to the screw shank. In your test, you mostly have the vertical shear component, but there is a tension component as the center of mass is away from the wall surface. We can safely ignore the tension component in calculating a working load since 80# in pure shear is more conservative than 80# shear and, oh... say 15# tension combined.

A picture of the clip was helpful, I imagined a much worse condition. Either way, the ultimate strength will not be proportional to shear alone, there are other factors difficult to model, thus testing is the best approach. The failure mode you experienced is a bending failure, but your actual installation, while having a bending component, is in fact mostly a shear condition.

The duration of load is a factor. The usual allowable stresses specified in construction are for permanently applied loads. The allowable stresses can be increased for shorter durations, 15% for a few months, 25% for a few weeks, 33% for a few minutes. Meaning we should reduce the allowable load determined through short term tests accordingly. But we also don't know the ultimate load since you didn't achieve failure. Just as well, uncontrolled destructive testing can be a little too exciting. You also haven't run multiple tests (I assume) to confirm you are getting consistent results.

Let's say you did run multiple tests and they all actually failed at 80#. When you apply the 3x safety factor, then adjust for duration of load, you end up with a working load of 20#, exactly what you need. Considering there was no failure experienced, and the installation does appear to be predominantly shear, I think your installation is safe. Barely. Next time around, use heavy ordinary wood screws ;)

The answer is that it depends on the exact method of pressure treatment (did the chemical penetrate to the center of the wood or not) and the current state of deterioration. The main thing to protect the wood from is water; you want to keep it as dry as possible, by both preventing large amounts of water washing over it, and also small amounts of water remaining in contact long-term. Water washes away the bug-repelling copper compounds used to treat the wood. So, stacking the wood in a sheltered area (like your shed) with some spacing between boards (you can stack them in a loose grid if you have the floor space, or cut the worst pieces up into roughly square pieces you can use as spacers) should allow the wood the best chance.

Now, this is if the wood is in good enough shape to save. You may think so, but if the wood has already been penetrated by WDIs, the protection offered by the PT chemicals is likely past its expiration date. This is normal for fences; they're out in the elements, and unfortunately the copper compounds used are water soluble, which is how they get them into the wood in the first place (kiln-dry it, then immerse it in a bath of copper arsenide and ramp up the pressure to force the chemical-laden water into the grain). If the wood was well-treated, with green all the way to the core, the parts that haven't already been savaged by WDIs should stay good, but if you cut a board in half and see white wood in the center, termites and carpenter ants can eat that board from the inside out. If you see grey-brown all the way in, rot has taken hold and the board is garbage.

One last thing; whatever you do, DO NOT BURN PRESSURE-TREATED LUMBER. Not in your fireplace, not in your wood stove, not in an outdoor fire pit, not in a bonfire, not anywhere. Burning PT lumber, no matter how weathered and degraded, will release toxic, polluting smoke; the primary chemical used in PT lumber is copper arsenide, which oxidizes when burned to produce arsenic. In addition to the normal harmful effects of wood smoke inhalation, the amount of arsenic in 12 ft of 2x6" PT can kill 250 adult humans. A single 6' 1-by fence slat would still have enough to kill you and your family about 10 times over. If you don't breathe it in, it'll dissolve in the moisture in the air and be carried up to the clouds where it will fall as acid rain.